Healthy HeadWaters
Coal Seam Gas Water Feasibility Study
Spatial Analysis of Coal Seam Gas Water
Chemistry
Final report
301001-01210 – 00-EN-REP-0002
02 December 2011
Prepared for the Department of Environment and Resource Management
Prepared for the Department of Environment and Resource Management
February 2012
SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
This document presents the outcomes of Activity 1.2 (Spatial Analysis of CSG Water Chemistry) of the
Healthy HeadWaters Coal Seam Gas Water Feasibility Study.
The Healthy HeadWaters Coal Seam Gas Water Feasibility Study is analysing the opportunities for, and
the risks and practicability of, using coal seam gas water to address water sustainability and adjustment
issues in the Queensland section of the Murray-Darling Basin.
The study is being funded with $5 million from the Commonwealth Government, with support from the
Queensland Government, as part of the Healthy HeadWaters Program, which is Queensland’s priority
project funded through the Commonwealth Government’s Water for the Future initiative. The study is
being managed by the Queensland Department of Environment and Resource Management (DERM), and
is due to finish in 2012.
This report was prepared by WorleyParsons for the State of Queensland (Department of Environment and
Resource Management).
Disclaimers
This document was prepared exclusively for the State of Queensland (Department of Environment &
Resource Management) and is not to be relied upon by any other person. WorleyParsons has made every
effort to ensure that the information provided is accurate but errors and omissions can occur and
circumstances can change from the time that the report or document was prepared. Therefore, except for
any liability that cannot be excluded by law, WorleyParsons excludes any liability for loss or damage,
direct or indirect, from any person relying (directly or indirectly) on opinions, forecasts, conclusions,
recommendations or other information in this report or document.
© State of Queensland (Department of Environment and Resource Management) 2012.
The State gives no warranty in relation to the contents of this document (including accuracy, reliability,
completeness, currency or suitability) and accepts no liability (including without limitation, liability in
negligence) for any loss, damage or costs (including consequential damage) relating to any use of the
contents of this document.
301001-01210-CSG Water Chemistry
SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
E X EC UT IV E SUM M AR Y
Over the last decade, the coal seam gas (CSG) industry has expanded rapidly in Queensland, and even
greater expansion is expected in the future as the major CSG companies look to export CSG as liquefied
natural gas (LNG).
In order to capture CSG, groundwater is abstracted from coal seams in order to reduce the pressure that
keeps the gas in position. However, as a result of the depressurisation of coal seams there is the potential
for negative effects. For example, lowering of water levels in surrounding aquifers may affect the
availability of groundwater which supports important economic activity and groundwater-dependent
ecosystems. Being able to predict and monitor such impacts requires an understanding of the origins and
flow regimes of groundwater in the coal seams and surrounding aquifers, and of the hydraulic connectivity
between these formations. However, the current knowledge of these dynamics is somewhat limited.
The objective of this study, the Spatial Analysis of CSG Water Chemistry, was to collate and interpret
groundwater chemistry data from aquifers of the Bowen and Surat basins in order to:
•
characterise and determine the origin, recharge and flow regimes of coal seam waters; and
•
identify potential indicators of connectivity between coal seams and surrounding aquifers.
Such knowledge is fundamental to understanding the timing and extent of isolation of the coal measures
from surrounding aquifers, and whether the coal measures function as relatively continuous or
compartmentalised reservoirs.
The study consolidated existing information regarding the mineralogical, hydrogeological and
hydrochemical properties of formations in the Bowen and Surat basins, and conducted various supporting
analyses. These analyses included:
•
Assessment of residual head conditions to identify areas where the potential for hydraulic
interaction between formations may be higher than otherwise suspected. The information generated
by this analysis confirmed results from previous studies on certain formations, and provided new
information on other formations not previously assessed; and
•
Spatial assessment of key water quality constituents, including multivariate statistical analyses of
the data that had not been conducted previously. This approach was used to identify major
hydrochemical groupings and their spatial distribution across each of the basins, thus helping to
clarify the origins of the water in the various formations
Although the completeness of data available for this study was variable, interpretations based on
statistical analyses using mineralogical, hydrogeological and hydrochemical data has shown that there is
potential for aquifer connectivity between some formations in the Surat and Bowen basins. The following
presents a synopsis of the findings of the present study as they relate to the Bowen and Surat basins
respectively.
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
Bowen Basin
Hydrochemical results from this study indicate that there is mixing of pore waters between the Clematis
Sandstone and the Rewan Group, and possibly the Upper Permian Coal and Sandstones in the southern
part of the basin. This evidence is provided by the occurrence of anomalous water quality conditions
(expressed by various constituents and ionic ratios) at the southern end of a data cluster associated with
2+
the Clematis Sandstone. For example, notable influences from Na , Cl and SO4 suggest a marine
provenance likely derived from waters trapped during deposition in marginal-marine setting. Secondary to
2+
2+
that, is a correlation between Ca , Mg , SiO2 , CO2 , and pH consistent with the weathering of
aluminosilicate minerals. Analysis of k-means clustering for the Lower and Upper Permian Sandstones,
respectively, indicates a highly variable and somewhat random distribution of major water-types across
the basin. Much of this variability is situated near known geological structures (e.g. faults). However, the
importance of the presence of faults and pore water interactions cannot be resolved due to the lack of
supporting hydraulic data.
Groundwater surface elevations were limited; however, where available, these indicate a general regional
flow pattern from upland areas on the western side of the basin (near the Nebine Ridge) towards the
adjacent low-lying areas to the east and south. This flow pattern is consistent with the local terrain, thus
indicating topographic control on flow patterns.
The lack of refined water level and water quality data through the majority of the Bowen Basin limits the
ability to resolve potential interactions between CSG units of interest (the Bandanna and Baralaba Coal
Measures and overlying formations, particularly the Clematis Sandstone). Additional information will be
needed to further refine knowledge and understanding of the hydrogeological conditions, and properly
frame the risks to these formations as a result of CSG development (i.e., water level drawdown and
potential gas migration).
Surat Basin
The difference in water quality from groundwater bores located in recharge areas suggests a link to
variable recharge processes, groundwater flow rates and resulting geochemical reactions. Water types
+
are generally dominated by Na and HCO3 ions, with variable concentrations of Cl , while recharge to the
formations generally occurs along the northern and eastern edges of the basin. This recharge occurs into
formations or sediments that are exposed at surface and are in hydraulic connection with major streams
and rivers flowing through the area.
Groundwater movement is generally from topographically elevated areas in the north and east along the
flanking areas of the Great Dividing Range towards adjacent low-lying areas to the south and west. Flow
on the western side of the basin occurs in a parallel to sub-parallel direction along the Eulo-Nebine Ridge,
and represents a hydraulic divide between the Eromanga and Surat basins. Estimates of groundwater flow
velocities in the Surat Basin are similar to those for the Bowen Basin (i.e., of the order of 1 to 3 metres per
annum.
Residual hydraulic head results between the various formations suggest possible interaction between
otherwise discrete aquifers in areas where differences are neutral to near-neutral, and where intervening
aquitards thin out. The transfer of associated pore waters may be direct in some cases, or more diffuse
across thinner layers of intervening mudstone and siltstone.
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
Based on modelled residual head conditions, areas of potential hydraulic connection are noted for the
Walloon Subgroup and Springbok Sandstone; particularly where the upper mudstone intervals of the
Walloon Subgroup are absent or thin (i.e., in the middle of the Walloons fairway). Given that the Walloon
Coal Measure is the target for CSG production in the Surat Basin, this particular situation represents the
highest risk to any Surat Basin aquifer from CSG operations (i.e., drawdown and potential gas migration).
Data Limitations:
Although some interpretations were drawn from the present study, there are certain limitations of the
dataset used, thus impeding further knowledge and understanding of hydrogeological and geochemical
conditions in the study area. Among these limitations is the sparse and clustered nature of the available
data for water chemistry and water level measurements in many formations, and a lack of trace element
and isotope data to refine understanding of the origins and interactions of aquifer pore waters. For
example, although there is some evidence to suggest that there is the potential for hydraulic connectivity
between units of interest in the Bowen Basin, there is a general lack of refined water level and water
quality data through the majority of the basin, limiting the ability to resolve potential interactions between
the CSG units of interest (i.e. Bandanna and Baralaba Coal Measures) and overlying formations,
particularly the Clematis Sandstone.
These data limitations mean that it is not possible to fully characterise and determine the origin, recharge
and flow regimes of coal seam waters at this time. Although some chemical indicators for aquifer
connectivity between coal seams and some aquifers were identified in this study, access to additional
data, including data held by the CSG industry and that obtained through a regional monitoring program,
could be used to build a more robust understanding of the hydrogeological systems in the Bowen and
Surat basins.
Despite the limitations regarding the data and knowledge gaps, this study has generated valuable
information which contributes to an enriched understanding of baseline conditions in the Permian and
Jurassic coal seams and aquifer systems within the Bowen and Surat basins. Results from previous
studies have been strengthened and confirmed, and new knowledge has been gained from the analyses
conducted. The database and study results provide a solid basis for subsequent investigations, such as
vulnerability and risk assessment of aquifers to CSG development in the basins, and will be invaluable in
the design and interpretation of future groundwater monitoring activities.
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
CONTENTS
1
INTRODUCTION ................................................................................................................ 1
1.1
Project Background .................................................................................................. 1
1.2
Approach .................................................................................................................. 2
1.3
Structure of this Document ...................................................................................... 2
2
PROJECT METHODOLOGY ............................................................................................. 4
2.1
Unified Database...................................................................................................... 4
2.1.1
Data Sources ......................................................................................................... 4
2.1.2
Assignment of Formations and Surface Elevations ............................................... 6
2.2
Hydrogeology and Hydraulics .................................................................................. 7
2.2.1
Selecting a Suitable Time Period ........................................................................... 7
2.2.2
Potentiometric Surface Maps ................................................................................. 8
2.2.3
Residual Head Mapping ........................................................................................ 9
2.3
3
Water Characterisation (Hydrochemistry) ................................................................ 9
2.3.1
Data Preparation .................................................................................................... 9
2.3.2
Hydrochemical Characterisation Methods ........................................................... 11
THE STUDY AREA ........................................................................................................... 15
3.1
Major Formations of the Bowen and Surat basins ................................................. 15
3.1.1
Bowen Basin ........................................................................................................ 15
3.1.2
Surat Basin .......................................................................................................... 19
3.1.3
Alluvium and Tertiary Volcanics ........................................................................... 28
3.2
4
Geological Structures ............................................................................................. 29
RESULTS – BOWEN BASIN ............................................................................................ 31
4.1
Permian Volcanics ................................................................................................. 31
4.1.1
Hydrogeology and Hydraulics .............................................................................. 31
4.1.2
Hydrochemistry .................................................................................................... 31
4.2
4.3
Lower Permian Sandstones ................................................................................... 34
4.2.1
Hydrogeology and Hydraulics .............................................................................. 34
4.2.2
Hydrochemistry .................................................................................................... 34
Upper Permian Coal............................................................................................... 37
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
4.3.1
Hydrogeology and Hydraulics .............................................................................. 37
4.3.2
Hydrochemistry .................................................................................................... 37
4.4
Upper Permian Sandstones ................................................................................... 38
4.4.1
Hydrogeology and Hydraulics .............................................................................. 38
4.4.2
Hydrochemistry .................................................................................................... 38
4.5
Rewan Group ......................................................................................................... 42
4.5.1
Hydrogeology and Hydraulics .............................................................................. 42
4.5.2
Hydrochemistry .................................................................................................... 42
4.6
5
Clematis Sandstone ............................................................................................... 44
4.6.1
Hydrogeology and Hydraulics .............................................................................. 44
4.6.2
Hydrochemistry .................................................................................................... 45
RESULTS – SURAT BASIN ............................................................................................. 49
5.1
Precipice Sandstone .............................................................................................. 49
5.1.1
Hydrogeology and Hydraulics .............................................................................. 49
5.1.2
Hydrochemistry .................................................................................................... 50
5.2
Evergreen Formation ............................................................................................. 53
5.2.1
Hydrogeology and Hydraulics .............................................................................. 53
5.2.2
Hydrochemistry .................................................................................................... 54
5.3
Hutton Sandstone .................................................................................................. 59
5.3.1
Hydrogeology and Hydraulics .............................................................................. 59
5.3.2
Hydrochemistry .................................................................................................... 60
5.4
Eurombah Formation ............................................................................................. 63
5.4.1
Hydrogeology and Hydraulics .............................................................................. 63
5.4.2
Hydrochemistry .................................................................................................... 63
5.5
Walloons Subgroup ................................................................................................ 64
5.5.1
Hydrogeology and Hydraulics .............................................................................. 64
5.5.2
Hydrochemistry .................................................................................................... 65
5.6
Springbok Sandstone ............................................................................................. 70
5.6.1
Hydrogeology and Hydraulics .............................................................................. 70
5.6.2
Hydrochemistry .................................................................................................... 71
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
5.7
Westbourne Formation .......................................................................................... 74
5.7.1
Hydrogeology and Hydraulics .............................................................................. 74
5.7.2
Hydrochemistry .................................................................................................... 74
5.8
Gubberamunda Sandstone .................................................................................... 75
5.8.1
Hydrogeology and Hydraulics .............................................................................. 75
5.8.2
Hydrochemistry .................................................................................................... 76
5.9
BMO Group ............................................................................................................ 79
5.9.1
Hydrogeology and Hydraulics .............................................................................. 79
5.9.2
Hydrochemistry .................................................................................................... 79
5.10
Rolling Downs Group ............................................................................................. 83
5.10.1 Hydrogeology and Hydraulics .............................................................................. 83
5.10.2 Hydrochemistry .................................................................................................... 84
5.11
Cainozoic Cover (Alluvium and Tertiary Volcanics) ............................................... 86
5.11.1 Hydrogeology and Hydraulics .............................................................................. 86
5.11.2 Hydrochemistry .................................................................................................... 87
6
GROUNDWATER FLOW REGIMES ................................................................................ 92
6.1
Recharge and Discharge Phenomena ................................................................... 92
6.2
General Groundwater Flow Patterns ..................................................................... 94
7
SPATIAL VARIABILITY OF GROUNDWATER CHEMISTRY........................................ 101
7.1
General Weathering Reactions ............................................................................ 101
7.2
Spatial Analysis of Hydrochemical Parameters ................................................... 102
7.2.1
Temperature ...................................................................................................... 103
7.2.2
Total Dissolved Solids........................................................................................ 105
7.2.3
Alkalinity ............................................................................................................. 107
7.2.4
Sulfate ................................................................................................................ 108
7.2.5
Silica................................................................................................................... 110
7.2.6
Fluoride .............................................................................................................. 111
7.2.7
Boron.................................................................................................................. 112
7.2.8
HCO3 to Cl ratios .............................................................................................. 113
7.2.9
Isotopes ............................................................................................................. 117
-
301001-01210-CSG Water Chemistry
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
7.2.10 Dissolved Methane ............................................................................................ 121
8
INDICATORS OF POTENTIAL HYDRAULIC CONNECTIVITY..................................... 124
8.1
8.2
Inter-Aquifer ......................................................................................................... 124
8.1.1
Bowen Basin ...................................................................................................... 124
8.1.2
Surat Basin ........................................................................................................ 125
Inter-Basin ............................................................................................................ 129
9
IMPROVING KNOWLEDGE AND UNDERSTANDING ................................................. 130
10
SUMMARY OF FINDINGS ............................................................................................. 132
10.1
Bowen Basin ........................................................................................................ 132
10.1.1 Hydrochemistry .................................................................................................. 132
10.2
Surat Basin ........................................................................................................... 135
10.2.1 Hydrochemistry .................................................................................................. 135
11
CONCLUSION ................................................................................................................ 139
12
REFERENCES ............................................................................................................... 141
13
ABBREVIATIONS ........................................................................................................... 144
14
GLOSSARY .................................................................................................................... 146
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
Tables
Table 1
List of data obtained for the study ............................................................................... 5
Table 2
Summary of data used for the baseline hydrogeological and hydrochemical
assessments in the Bowen and Surat basins .............................................................. 7
Table 3
Fluctuations in groundwater levels between 1960 and 1965; Surat Basin
formations .................................................................................................................... 8
Table 4
Hydrostratigraphy of the Bowen Basin ...................................................................... 16
Table 5
Stratigraphy of the Surat Basin.................................................................................. 19
Table 6
Surficial deposits within the Study Area .................................................................... 29
Table 7
Principal Component Analysis results for the Permian Volcanics ............................. 32
Table 8
k-means Cluster Results for Permian Volcanics ....................................................... 33
Table 9
Principal Component Analysis Results for the Lower Permian Sandstones ............. 35
Table 10
k-means Cluster Results for Lower Permian Sandstones ......................................... 35
Table 11
Summary of Selected Parameters and Ion Ratios for the Upper Permian Coal ....... 38
Table 12
Principal Component Analysis Results for the Upper Permian Sandstones ............ 39
Table 13
k-means Cluster Results for Upper Permian Sandstones ......................................... 40
Table 14
Principal Component Analysis Results for the Rewan Group ................................... 43
Table 15
k-means Cluster Results for Rewan Group ............................................................... 43
Table 16
Principal Component Analysis Results for the Clematis Sandstone ......................... 46
Table 17
k-means Cluster Results for Clematis Sandstone ..................................................... 47
Table 18
Principal Component Analysis Results for the Precipice Sandstone ........................ 51
Table 19
k-means Cluster Results for the Precipice Sandstone .............................................. 52
Table 20
Principal Component Analysis Results for the Evergreen Aquitard and Sandstone . 55
Table 21
k-means Cluster Results for Evergreen Formations ................................................. 56
Table 22
Principal Component Analysis results for the Hutton Sandstone .............................. 61
Table 23
k-means Cluster Results for Hutton Sandstone ........................................................ 61
Table 24
Summary of Selected Parameters and Ion Ratios for the Eurombah Aquitard ......... 64
Table 25
Principal Component Analysis Results for the Walloons Subgroup .......................... 66
Table 26
k-means Cluster Results for Walloon Subgroup ....................................................... 67
Table 27
Principal Component Analysis Results for the Springbok Sandstone ....................... 72
Table 28
k-means Cluster Results for the Springbok Sandstone............................................. 73
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
Table 29
Summary of Selected Parameters and Ion Ratios for the Westbourne Formation ... 74
Table 30
Principal Component Analysis Results for the Gubberamunda Sandstone .............. 77
Table 31
k-means Cluster Results for the Gubberamunda Sandstone .................................... 77
Table 32
Principal Component Analysis Results for the BMO Group ...................................... 80
Table 33
k-means Cluster Results for the BMO Group ............................................................ 81
Table 34
Principal Component Analysis Results for the Rolling Downs Group ....................... 85
Table 35
k-means Cluster Results for the Rolling Downs Group ............................................. 85
Table 36
Principal Component Analysis Results for the Alluvium and Tertiary volcanics ....... 88
Table 37
k-means Cluster Results for the Alluvium and Tertiary Volcanics ............................. 90
Table 38
Summary of mineral saturation conditions (SI values) in Bowen and Surat
formations for calcite, quartz and albite. .................................................................. 103
Table 39
δ CDIC Values for Cretaceous, Jurassic and Permian Formations ......................... 120
Table 40
Data acquisition recommendations ......................................................................... 130
Table 41
Distinguishing water quality features of the Bowen Basin formations ..................... 134
Table 42
Distinguishing water quality features of the Surat Basin formations ....................... 137
13
Figures within Text
Figure 1 Mineralogy of the Back Creek Group: n = 2 (Grigorescu, 2011a).................................... 17
Figure 2 Mineralogy of the Rewan Group: n = 2 (Grigorescu, 2011a) ........................................... 17
Figure 3 Mineralogy of the Moolayember Aquitard: n = 2 (Grigorescu, 2011a) ............................. 18
Figure 4 Mineralogy of the Precipice Sandstone: n = 44 (Grigorescu, 2011a) .............................. 20
Figure 5 Mineralogy of the Evergreen Formation (upper pie chart; n = 26) and associated
Boxvale Sandstone member (lower pie chart; n = 3) (Grigorescu, 2011a) ............... 21
Figure 6 Mineralogy of the Hutton Sandstone: n = 73 (Grigorescu, 2011a) .................................. 22
Figure 7 Mineralogy of the Walloon Subgroup: n = 132 (Grigorescu, 2011a)................................ 24
Figure 8 Mineralogy of the Springbok Sandstone: n = 21 (Grigorescu, 2011a) ............................. 24
Figure 9 Mineralogy of the Westbourne Formation: n = 32 (Grigorescu, 2011a) ......................... 25
Figure 10 Mineralogy of the Gubberamunda Sandstone: n = 16 (Grigorescu, 2011a) ................. 26
Figure 11 Mineralogy of the Orallo Formation: n = 26 (Grigorescu, 2011a) ................................. 27
Figure 12 Mineralogy of the Bungil Formation: n = 3 (Grigorescu, 2011a) ................................... 27
Figure 13 Mineralogy of the Wallumbilla Formation: n = 2 (Grigorescu, 2011a) ........................... 28
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
Figure 14 Location of major and minor fault features in the Bowen (left panel) and Surat (right
panel) basins (after SRK, 2008) ................................................................................ 30
Figure 15 Spatial Distribution of the k-means Clusters for the Permian Volcanics ........................ 33
Figure 16 Spatial distribution of the k-means Clusters for the Lower Permian Sandstones .......... 36
Figure 17 Spatial Distribution of the k-means Clusters for the Upper Permian Sandstones ........ 41
Figure 18 Spatial Distribution of the k-means Clusters for the Rewan Group ............................... 44
Figure 19 Modelled residual head condition for Clematis and Permian Coal Measures ............... 45
Figure 20 Spatial Distribution of the k-means Clusters for the Clematis Sandstone ..................... 48
Figure 21 Modelled residual head conditions for the Precipice and Hutton Sandstones ............... 50
Figure 22 Spatial Distribution of the k-means Clusters for the Precipice Sandstone..................... 53
Figure 23 Spatial Distribution of the k-means Clusters for the Evergreen Formation aquitard ...... 57
Figure 24 Spatial Distribution of the k-means Clusters for the Evergreen Formation sandstones 58
Figure 25 Modelled residual head condition for Hutton Sandstone and Walloon Subgroup ......... 60
Figure 26 Spatial Distribution of the k-means Clusters for the Hutton Sandstone ......................... 62
Figure 27 Modelled residual head condition for Walloon Subgroup and Springbok Sandstone .... 65
Figure 28 Spatial Distribution of the k-means Clusters for the Walloon Subgroup coals .............. 68
Figure 29 Spatial Distribution of the k-means Clusters for the Walloon Subgroup sandstones .... 69
Figure 30 Modelled residual head condition for Springbok Sandstone and Gubberamunda
Sandstone.................................................................................................................. 71
Figure 31 Spatial Distribution of the k-means Clusters for the Springbok Sandstone ................... 73
Figure 32 Modelled residual head conditions between Gubberamunda Sandstones and BMO
Group ......................................................................................................................... 76
Figure 33 Spatial Distribution of the k-means Clusters for the Gubberamunda Sandstone .......... 78
Figure 34 Spatial Distribution of the k-means Clusters for the BMO Group .................................. 82
Figure 35 Spatial Distribution of the k-means Clusters for the Rolling Downs Group Aquitard ..... 86
Figure 36 Spatial Distribution of the k-means Clusters for the Alluvium ........................................ 89
Figure 37 Spatial Distribution of the k-means Clusters for the Tertiary Volcanics ......................... 91
Figure 38 Location of Major Spring Groups and Potential River Baseflow Interaction (Dept. of
Natural Resources and Mines, 2005) (red box indicates the location of the Bowen
and Surat basins and grey areas indicate the location of major intake beds) ........... 93
Figure 39 General Groundwater Flow Patterns in the Upper Jurassic and Cretaceous Intervals
of the Surat Basin and Adjacent basins (Radke et. al.,2000) .................................... 94
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
Figure 40 Location of Major Recharge Areas for Surat Basin (Radke et. al., 2000) ...................... 96
Figure 41 Locations along the eastern edge of the Surat Basin where recharge via riverbed
losses is suspected (AGE, 2005) .............................................................................. 97
Figure 42 Locations along the northern edge of the Surat Basin where recharge via riverbed
losses is suspected (AGE, 2005) .............................................................................. 98
Figure 43 General Vertical Flow Conditions in the Cretaceous Formations of the Surat Basin
before 1880 (Radke et al., 2000) ............................................................................... 99
Figure 44 Groundwater Temperatures (in ºC) for the Permian (P) to Jurassic (J) formations (left
panel) and Late Jurassic (Late J) to Tertiary (T) formations (right panel): brown
and purple lines indicate locations of identified faults; major recharge areas have
been provided for reference. ................................................................................... 104
Figure 45 Total Dissolved Solids (TDS) (in mg/L) for the Permian (P) to Jurassic (J) formations
(left panel) and Late Jurassic (Late J) to Tertiary (T) formations (right panel):
brown and purple lines indicate locations of identified faults; major recharge areas
have been provided for reference............................................................................ 106
-
Figure 46 Total Alkalinity (in mg/L as CaCO3 ) for the Permian (P) to Jurassic (J) formations
(left panel) and for Late Jurassic (Late J) to Tertiary (T) formations (right panel):
brown and purple lines indicate locations of identified faults, major recharge areas
have been provided for reference............................................................................ 108
Figure 47 Sulfate Concentrations (in mg/L) for the Permian (P) to Jurassic (J) formations (left
panel) and Late Jurassic (Late J) to Tertiary (T) formations (right panel): brown
and purple lines indicate locations of identified faults; major recharge areas have
been provided for reference. ................................................................................... 109
Figure 48 Dissolved Silica (in mg/L) for the Permian (P) to Jurassic (J) formations (left panel)
and Late Jurassic (Late J) to Tertiary (T) formations (right panel): brown and
purple lines indicate locations of identified faults; major recharge areas have been
provided for reference. ............................................................................................ 110
Figure 49 Fluoride Concentrations (in mg/L) for the Permian (P) to Jurassic (J) formations (left
panel) and Late Jurassic (Late J) to Tertiary (T) formations (right panel): brown
and purple lines indicate locations of identified faults; major recharge areas have
been provided for reference. ................................................................................... 112
Figure 50 Boron Concentrations (in mg/L) for the Permian (P) to Jurassic formations (left panel)
and in Later Jurassic (Late J) to Tertiary (T) formations (right panel): brown and
purple lines indicate locations of identified faults; major recharge areas have been
provided for reference. ............................................................................................ 113
Figure 51 HCO3/Cl ratios for Permian (P) to Jurassic (J) formations (left panel) and Late
Jurassic (Late J) to Tertiary (T) formations (right panel): brown and purple lines
indicate locations of identified faults; major recharge areas have been provided for
reference.Sodium Adsorption Ratios (SAR) ............................................................ 115
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
Figure 52 SAR Values for Permian (P) to Jurassic (J) formations (left panel) and Late Jurassic
(Late J) to Tertiary (T) formations (right panel): brown and purple lines indicate
locations of identified faults; major recharge areas have been provided for
reference.................................................................................................................. 117
Figure 53 Oxygen and Deuterium Isotopes for Groundwater Samples in the Study Areas
(arrows represent expected trajectory of data points in response to various
fractionation processes (as described in Clark and Fritz, 1997). ............................ 118
13
Figure 54 Distribution of δ CDIC values (as ‰ PDB) in the Jurassic and Cretaceous formations121
13
Figure 55 δ C Isotopes Values for Dissolved Methane of the Various Coal Measures (shaded
areas indicate dominant processes leading to methane productions (after
Whiticar, 1999)......................................................................................................... 122
Figure 56 Thickness and extent of Westbourne Formation Aquitard showing thinning towards
Eulo-Nebine Ridge (from Radke et al., 2000) ......................................................... 126
Figure 57 Thickness of Rolling Downs Group sediments showing thinning over the Eulo-Nebine
Ridge and near the (stippled) recharge area (from Radke et al., 2000).................. 127
Figure 58 Summary of Potential Inter-aquifer Connectivity for the Study Area ........................... 128
Appendices
Appendix 1 Database development (Task 2 of Activity 1.2)
Appendix 2 Trilinear Diagrams (Piper Diagrams) for major formations within the Surat and
Bowen Basins
Appendix 3 Statistical analysis for the Bowen Basin
Appendix 4 Statistical analysis for the Surat Basin
Appendix 5 Dissolved methane and isotope data for the Bowen and Surat basins
Appendix 6 Water quality data for the Bowen and Surat basins
Appendix 7 Hydrogeological figures
Appendix 8 Reactions affecting groundwater quality in the Bowen and Surat basin
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1
INTRODUCTION
Over the last decade, the coal seam gas (CSG) industry has expanded rapidly in Queensland, emerging
as a competitive source of energy. At December 2011, eight Liquefied Natural Gas (LNG) projects are
proposed: three have been approved, with a fourth in the application stage. In the event that all these
projects proceed to full capacity, approximately 50 million tonnes per annum of LNG could be generated
from the production of CSG from deeply buried coal seams in the Bowen (Permian coal measures) and
Surat (Jurassic coal measures) basins (DEEDI, 2010).
The removal of CSG requires the abstraction of substantial volumes of often poor quality groundwater
from coal seams and adjacent rock layers. With the projected expansion of the CSG industry in
Queensland, significant volumes and rates of groundwater extraction are expected. Pressure differentials
created between the depressurised CSG coal intervals and overlying and underlying geological units, as a
result of groundwater drawdown, have the potential to negatively affect groundwater storage and water
levels in key confined aquifer intervals of the Great Artesian Basin (GAB). Development of CSG may also
have the potential to affect the near-surface unconsolidated alluvial aquifer systems where the key coal
measures underlie these deposits. The potential for such effects to occur may have significant implications
for groundwater users and environmental receptors in and near CSG development areas, including
surface water courses, spring complexes and groundwater dependent ecosystems (GDEs). Additionally,
the sustainability of groundwater resources over the long term may be affected.
1.1
Project Background
The origins and history of coal seam waters, and their relationship with other aquifers, can be inferred
through their chemical composition, which varies depending on the source of the water and the
geochemical system. Evidence in the literature suggests that the degree of connectivity between coal
seams and other aquifers in the Bowen and Surat basins is variable. In deeper parts of the Bowen Basin
it is likely that formation waters in the coal measures are very ancient, reflecting a lack of through-flow in
recent times. In shallower parts of the Bowen and Surat basins, there is evidence of recent recharge of
meteoric water from shallower depths (Draper and Boreham, 2006).
A more detailed understanding of the chemical and physical relationships between key aquifers and coal
measures is needed to better our understanding of the risks associated with depressurising coal seams.
Distinguishing the baseline chemistry of groundwater from different coal measures and aquifers will also
enable more effective monitoring and identification of any impacts caused to groundwater as a result of
CSG activities.
The objective of this study was to collate and interpret water chemistry data in the Surat and Bowen
basins in order to:
•
characterise and determine the origins, recharge and flow regimes of coal seam waters; and
•
identify potential indicators of connectivity between coal seams and other aquifers.
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This Study was delivered through the following principal tasks:
1.
Literature review
Relevant scientific and technical literature were reviewed to establish the state of existing knowledge and
theories about the origin, recharge and flow regimes of groundwater in coal seams and surrounding
aquifers. The literature review summarises background information including regional physiography and
drainage, climate, and geologic information relating to the Bowen and Surat basins. The literature review
is available as a separate report.
2.
Collation of groundwater chemistry data
A unified groundwater database was developed from chemical and physical data from government
databases and other sources for the state of Queensland. All data were subjected to a data review and
integrity process.
3.
Interpretation of groundwater chemistry data
Parameters used in the interpretation of these data included major, minor and trace element compositions
and relationships, total salts, stable and radiogenic isotopes, hydrocarbons, metals/metalloids, and
dissolved silica. The outputs of this task are presented in this report.
1.2
Approach
To achieve the objectives of this study, Tasks 2 and 3 (above) were divided into the following subtasks:
Task 2.1:
Collate relevant data from government databases and other sources.
Task 2.2:
Assemble the data into a unified database which allows for analysis of temporal and
spatial relationships among water quality characteristics.
Task 3.1:
Examine local and basin-wide spatial and temporal variations in major, minor and trace
element compositions& relationships, total salts (TDS and electrical conductivity (EC)),
stable and radiogenic isotopes, hydrocarbons, dissolved silica, and any other relevant
hydrochemical parameters.
Task 3.2:
Compare groundwater chemistry of coal measure waters with water from other
aquifers to basin geology, hydrodynamics and hydrochemistry. This approach was
undertaken to better understand the origins, recharge phenomena and flow regimes
within the study areas.
Task 3.3:
Identify specific hydrodynamic conditions or hydrochemical parameters that appear to
serve as potential indicators for identifying hydraulic connectivity between the coal
measures of CSG interest and adjacent aquifers.
1.3
Structure of this Document
This document is structured as follows:
Section 1:
Introduction (this section).
Section 2:
Provides the methodology utilised, including the screening processes used to generate
a unified database for assessment in the hydrodynamic and hydrochemical conditions
(addresses Tasks 2.1 and 2.2 which are also addressed in Appendix 1).
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
Section 3:
Provides a summary of the general characteristics of the Bowen and Surat basins as
well as the Cainozoic cover (i.e. alluvials), including discussion of major formations,
aquitards and aquifers, mineralogy, geological structures, and recharge and discharge
processes, as well as the variability in natural inputs and outputs to each system.
Sections 4 and 5: Document baseline hydrodynamic and hydrochemical conditions in the Bowen and
Surat basins, as well as formations comprising the Cainozoic units (addresses
Task 3.1).
Section 6:
Characterises the recharge phenomena and flow regimes (addresses Task 3.2).
Section 7:
Characterises groundwater origins (addresses Task 3.2).
Section 8:
Discusses potential indicators of groundwater connectivity between the coal seams
and adjacent aquifers. This discussion includes both hydrodynamic conditions
(characterised by modelled residual head conditions) and hydrochemical parameters
(addresses Task 3.3).
Section 9:
Summarises data needed to improve knowledge and understanding.
Section 10:
Summary of findings.
Section 11:
Conclusions.
Appendices:
Supporting documentation, data analysis results and figures.
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2
PROJECT METHODOLOGY
The following section presents the methods used to address the project objective of assessing the origins,
flow regimes and potential connectivity of groundwaters from coal seams and other aquifers in the Bowen
and Surat basins. Central to the methodology was an assessment of baseline hydrodynamics and
hydrochemistry of the Bowen and Surat basins. This task was undertaken through the use of
potentiometric surface mapping, water level hydrographs and residual head mapping to assess
hydrodynamics; and a range of analytical techniques including multivariate statistics to characterise the
hydrochemistry of waters from different geological formations.
The results of these assessments were then interpreted spatially in conjunction with mineralogical
information to characterise the spatial variability of hydrochemical parameters of waters in the Bowen and
Surat basins, and to make inferences about the origins and recharge of these waters. The results were
also interpreted to identify distinguishing hydrochemical characteristics of individual formations, and to
assess the potential for hydraulic connectivity between them.
The analyses for this project were underpinned by a unified database that integrated all publicly available
groundwater level and quality data for Queensland. In addition, an interactive ArcGIS tool was developed
to assist in spatial assessment of the data and the presentation of results.
The sections below detail the methods used in the baseline hydrodynamic and hydrochemistry
assessments. Also presented is a description of the data that was incorporated into the unified database
that supported these assessments.
2.1
Unified Database
2.1.1
Data Sources
The unified database developed for this study represents information for the entire state of Queensland,
even though this study is focussed on the Bowen and Surat basins only. Information for 160,428 bores
was collated during the construction of this database, with 26,543 located within the Bowen and Surat
basins. The data contained in the database incorporated water quality parameters, including major, minor
and trace element compositions, total salts (i.e. TDS and conductivity), stable and radiogenic isotopes,
hydrocarbons, metals/metalloids, and dissolved silica; information about the stratigraphy and depth of
extraction of groundwater.
Two primary datasets were used to construct the unified database. These datasets were:
•
The Queensland Government Groundwater Database, containing bore registry information for
138,534 registered bores within the state of Queensland (with a cut-off date of 15 November 2010);
and
•
The Geological Survey of Queensland Petroleum Exploration Database, containing similar data for
7,362 petroleum wells (with a cut-off date of 31 July 2010).
Both datasets provided distinct types of bore information, and when combined, generate a comprehensive
and up-to-date listing of publically-available bores within the state of Queensland. These primary datasets
were supplemented by secondary datasets from several sources, as detailed in Table 1. Further detail
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
about the content of the database and the procedures through which it was developed can be found in
Appendix 1.
Table 1 List of data obtained for the study
Source
Data set name
Description
DERM
DERM Groundwater
Database
Contains bore registry information for 138,534 registered
bores within the state of Queensland with a cut-off date of
November 15 2010.
GSQ
Queensland Petroleum
Exploration Database
(QPED)
Contains bore registry information for 7,362petroleum wells
within the state of Queensland with a cut-off date of 31 July
2010.
GSQ
GSQ Legacy Water
Bore Database (pre1987)
Contains 11,097 well bores. Includes legacy (pre-1987) data
from the Geological Survey of Queensland water bore
database.
GSQ
Department Drilled Coal
Holes
Contains 5,669 well bores.
GSQ
Tally CSG Wells
Contains 3,475 well bores.
GSQ
GSQ QCD QDNR
Queensland Water
Chemistry
Contains 4,762 well bores which match the DERM
Groundwater Database. However, this data set contains
additional formation details which are not available in the
DERM Groundwater Database.
GSQ
Queensland Water
Chemistry 70s and 80s
Dataset from the 1970s and 1980s. Contains 976 well bores.
Contains field pH measurements. Worksheets are broken
down into geological formations.
GSQ and GA
Recent GSQ GA
groundwater surveys
Denison Trough 2009
Contains 20 well bores. Includes a comprehensive analysis
of groundwater, including isotopes, gas analysis and trace
metals. Although focussed primarily in the Denison Trough,
some samples are in the recharge beds of the Great Artesian
Basin. Work conducted under a joint GA-GSQ collaborative
project.
GSQ and GA
Recent GSQ GA
groundwater surveys
Surat 2010
Contains 20 well bores. Includes a comprehensive analysis
of groundwater, including isotopes, gas analysis, trace
metals and trace organics. Work conducted under a joint GAGSQ collaborative project.
GA
Recent GSQ GA
groundwater surveys
Goondiwindi 2008
Contains 4 well bores. Includes an analysis of groundwater
including field pH and temperature.
Bureau of Rural
Sciences (BRS)
GAB hydrochemistry
BRS
Included in Radke et al (2000) report. “Hydrogeochemistry
and implied hydrodynamics of the Cadna-owie – Hooray
Aquifer Great Artesian Basin”. Includes hydrochemical
analysis for the period 1974–1996.
BRS
BRS Recharge
Contains laboratory, and aquifer information for 94 bores.
BRS, GSQ and
Department of
Primary Industries
and Resources of
South Australia
(PIRSA)
BRS Alkalinity
Represents the full dataset for maps generated in the Radke
et al (2000) report “Hydrogeochemistry and implied
hydrodynamics of the Cadna-owie – Hooray Aquifer Great
Artesian Basin”. The data set is more extensive in size to the
GAB Hydrochemistry BRS data, and includes PIRSA data.
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
2.1.2
Assignment of For mations and Surface Elevations
A key step in preparing the unified database to support the required analyses was to assign a
standardised set of formation names to bores in the study area. While a significant amount of interpreted
and un-interpreted lithological and stratigraphic information was available throughout the datasets, the
variations in the interpreted values were significant. For example, within the study area alone 566 different
geological names were available reflecting the completion zones of wells. This list was reduced to 22
formation names. A total of 9,366 groundwater bores were identified and assigned to the various
formations in the Bowen and Surat basins. A breakdown of how these assignments were made can be
found in Table 2.
Of the groundwater bores to which geological formations were assigned, surveyed ground elevations were
determined using a Digital Elevation Model (DEM) using a Geographic Information System (GIS). The
DEM was obtained from the Queensland Government with ground elevations at a 25 m resolution. Given
the low topographic relief in the Study area, this resolution was considered to be sufficient. In order to
assess the accuracy of this process, DEM values were compared at bores where ground surface
elevations were measured or estimated in the field. The difference between the DEM values and those
measured in the field ranged from +556.2 m (58.3%) to -209.9 m (84%) with a standard deviation of 34 m.
In total, 36% of the bore elevation values from the database were found to differ by more than 5% from the
DEM elevation value. Further investigation found that in the case where the database elevation value was
known to be reliable (i.e. a surveyor name and date was included) the equivalent DEM elevation value
was generally found to be within a range of ±0.5 to ±2 m.
Using interpreted DEM ground elevation, WorleyParsons produced a dataset of groundwater levels
referenced to the Australian Height Datum (AHD); which provided the basis for assessment of
groundwater flow conditions (presented in the following sections). It should be noted that groundwater
levels used could not be adjusted to account for the height of the top of bore casing above ground surface,
as this data was not always available from the collated datasets. This height is, in most cases, less than
1.0 m and is unlikely to influence the interpretation of groundwater flow directions and vertical hydraulic
gradients at a regional scale.
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
Table 2 Summary of data used for the baseline hydrogeological and hydrochemical assessments
in the Bowen and Surat basins
Study Area Data
Count
Number of Bores with Formation Details
9,865
Formation Assignment through Geoscience Australia
2,199
Formation Assignment through DERM
4,462
Formation Assignment through Completion Interval and Stratigraphy
1,667
Formation Assignment through Sampling Interval and Stratigraphy
889
Number of Formation Assignments in Original Data Set
601
Number of Wells with 2 or more formation assignments
5,050
Number of Final Formation Assignments Used in this Study
Number of Bores with Assigned Formations
22
9,425
Number of Water Levels
85,666
Number of Analytical Results (Field)
10,385
Number of Analytical Results (Lab)
179,378
Number of Analytical Results (Field and Lab)
189,763
2.2
Hydrogeology and Hydraulics
2.2.1
Selecting a Suitable Time Period
The ability to assess regional groundwater flow patterns within the Bowen and Surat basins was controlled
by the spatial and temporal distribution of groundwater level data. The period between 1960 and 1965
offered the most extensive spatial coverage of water level data across all aquifers. This reflects the period
when the majority of bores were drilled and installed. In the vast majority of cases, only one water level
record was provided for each bore during this period.
The more contemporary (post 2000) dataset offered significantly less spatial coverage. When reviewing
the contemporary data set, the greatest spatial coverage of data for the key hydrogeological units was
found to occur during 2006. With CSG extraction commencing around 2000, these two periods of 1960 to
1965 and 2006 represent the pre-development and post-CSG extraction periods with the most extensive
spatial data coverage, respectively.
For the purpose of the hydraulic analyses, it was deemed important to ensure that the data being
compared was representative of the groundwater system. Data integrity is normally ensured by comparing
data collected during a single, short monitoring period. When incorporating data collected over longer time
periods the potential to develop false trends, caused by temporal variations in the data (i.e. seasonal
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
fluctuations, influence of pumping etc.), is introduced. This aspect was taken into consideration during
selection of an appropriate time period for water level assessment.
Table 3 summarises the variability in the data at individual monitoring locations in the Surat Basin between
1960 and 1965. The number of monitoring locations where groundwater level fluctuated by more than
10 m represented less than 5% of the total data set for each aquifer, with the exception of the Rolling
Downs Group Aquitard (9.5%) and Springbok Sandstone (6.4%).
Table 3 Fluctuations in groundwater levels between 1960 and 1965; Surat Basin formations
Fluctuation Range
Aquifer
Total
Locations
Locations with
more than 1
measurement
Less
than
1m
2m
to 5
m
5 m to
10 m
Greater
than
10 m
Rolling Downs Group Aquitard
317
122
61
13
16
30
BMO Group Sandstones
560
40
10
4
6
20
Gubberamunda Sandstone
354
60
25
12
9
14
Westbourne Aquitard
44
0
0
0
0
0
Springbok Sandstone
47
10
1
3
3
3
Walloons Subgroup
356
21
7
2
6
6
Eurombah Aquitard
38
3
0
0
1
2
Hutton Sandstone
335
23
5
2
5
11
Evergreen Aquitard
63
6
2
2
0
2
Evergreen Sandstone
42
8
2
4
2
0
Precipice Sandstone
66
18
13
1
2
2
When applying a 20 m contour interval to the dataset, groundwater level variation of less than 10 m was
not considered a significant issue when assessing regional groundwater flow directions over the large
regional study area.
The selection of water level data for the Bowen Basin was reduced to a 12-month period, as no increased
data coverage was realised if the period was expanded to a five-year interval.
2.2.2
Potentiometric Surf ace Maps
®
®
Potentiometric surface maps were produced using the ESRI GIS software ArcGIS Version
9.3.Groundwater elevation data between the selected time periods was extracted from the database and
imported into ArcGIS. The point data was interpolated to a raster using the kriging method (in the
ArcGIS 3D Analyst extension), with a search radius of 100 points and a pixel size of approximately 350 m.
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
After attempting various means, the kriging method was found to be the most representative due to the
limited number of data points and the large distances between points. The raster surface was then
converted to a contour layer. The resulting data points and contour plots generated were then reviewed to
identify erroneous data based on the average observed groundwater levels measured at nearby bores.
2.2.3
Residual Head Mapping
The term ‘residual head’ refers to the difference in hydraulic head between two aquifers at the same
location. Residual head maps, while not as accurate as direct measurement of groundwater levels from
bores completed in different formations at the same location, provide an effective method of estimating the
vertical hydraulic gradients, and in turn, groundwater flow potential between two units at a regional scale.
Where there is a difference between two formations in the groundwater elevation at a given location, or
potentiometric surfaces in a given area, this can be interpreted as an indication of an effective hydraulic
seal between the two formations. This type of arrangement may only allow the transmission of
groundwater over geological time (i.e. diffusion controlled) in the direction from higher hydraulic potential
to lower hydraulic potential (Hodgkinson et al., 2010). Conversely, where no difference exists between
groundwater elevations or potentiometric surfaces, this can indicate hydraulic connectivity between the
two intervals. However, in the presence of an effective seal between the formations, this may only
represent a fortuitous situation due to intersections of discrete potentiometric surfaces. Therefore, an
assessment of additional information, like groundwater quality, can be used to resolve the question of
cross-formational flow.
Information from the Queensland Water Commission’s (QWC) numerical groundwater model of the Bowen
and Surat basins was used to assess residual head conditions between the various aquifers(QWC, 2011).
Steady state groundwater elevations for each layer was used to calculate the residual values of hydraulic
head by subtracting one layer’s set of values from the other. The absolute difference in water levels (the
residual head) between these two surfaces was calculated at each of the grid points using ArcGIS and the
Spatial Analyst extension. This involved simple subtraction of water level elevations at a given grid point
to provide a mapped layer. The data was then visually represented as a colour range to identify areas of
neutral or zero head difference to greater than 20m difference (either positive or negative).
It should be noted that the residual head maps used modelled potentiometric surfaces from two aquifers
and therefore represent an approximation of actual conditions based on the inherent assumptions used to
constrain the groundwater model.
2.3
Water Characterisation (Hydrochemistr y)
2.3.1
Data Pr epar ation
Once the unified database had been prepared, the following screening criteria were applied to the
hydrochemical data for quality control purposes:
+
•
The available data must have measureable concentrations of all major cations (sodium (Na ),
2+
2+
2+
potassium (K ), calcium (Ca ) and magnesium (Mg ))and anions (sulfate (SO4 ), chloride (Cl ),
2bicarbonate (HCO3 ) and carbonate (CO3 ));
•
pH values of the available data must be within a range of 6 to 9 standard units; and
•
Ion balance of the available data must be within ± 5%.
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
Any data not fulfilling these criteria was removed from further analysis.
Prior to the screening process, the total number of data records was 9,366. This starting point in the
screening process reflected all records for which there was data for at least one of the major ions and a
recorded pH value. For some samples, both field and laboratory pH values were available. Where
available, field-measured pH values were used in the screening process and retained in the final dataset.
Of the total records, 3,942 were rejected because some major ion data was not available. Of these, an
additional 145 were removed because the pH value was outside of the acceptable range considered to
reflect natural representative pH conditions in the Study area. Finally, of the 5,279 records remaining, an
additional 181 records were screened out because ion balances fell outside of the ± 5% criterion.
Following the screening process a total of 5,098 records remained. This formed the dataset used for all
subsequent hydrochemical analyses.
M iner alo g y
Information pertaining to the mineralogy of the geological units was accessed from studies conducted by
the Geological Survey of Queensland in the Jurassic and Cretaceous formations beneath the south-east
portion of the Bowen basins and the north-east portion of the Surat Basin (Grigorescu, 2011a). Sediment
cores extracted from various locations and depths in the two basins were subjected to powder X-ray
Diffraction (XRD) to identify bulk mineralogy and assessment of clay fractions.
Additional details pertaining to the methodology used to perform the mineralogical assessment of the
various formations is provided in by Grigorescu (2011a).
T rac e E l em en t s
Insufficient trace element data was available in the final unified database to assess differences or
similarities between major formation porewaters. Detection frequencies (i.e. values greater than the
respective method detection limits) were examined for the 5,098 records remaining. Only 3,194 of those
records possessed trace element data that was usable in this study.
Of the available data with trace measurements, detection frequencies were less than 20% for aluminum
(Al), barium (Ba), and lead (Pb), approximately 35% for copper (Cu), and approximately 54% for zinc (Zn).
Boron (B), iron (Fe) and manganese (Mn) yielded the highest detection frequencies at approximately 89%,
63% and 67%, respectively.
H yd ro c a rb o n s
Concentrations of dissolved methane were identified for a number of formations within the Bowen and
Surat basins (Appendix 5). Although much of the available data was related to the Jurassic-aged Walloon
Subgroup beneath the Surat Basin, there were a small number of measurements available for the
Permian Coal Measures in the Bowen Basin (i.e. Bandanna and Baralaba).
Dissolved methane data residing in the unified database were supplemented by data provided by Draper
and Boreham (2006).
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
Iso t op es
Data pertaining to certain stable and radiogenic isotopes was available for use in this study (Appendix 5).
18
2
This data included stable isotopes of oxygen and hydrogen in the pore water phase (δ OH2O and δ HH2O),
13
13
stable carbon in dissolved inorganic carbon (δ CDIC) and dissolved methane (δ CCH4). A limited number
14
of measurements for radiogenic carbon-14 ( C activity measured as % modern carbon) and related
groundwater ages were also available for certain formations within the study area. This data was used to
provide further context to the hydrogeological setting.
2.3.2
Hydr ochemical Characterisation Methods
A variety of methods were carried out to examine the hydrochemistry of groundwater in the various
formations in the Bowen and Surat basins. For each hydrostratigraphic unit, where sufficient data was
available, the assessment process included:
•
Compilation of basic statistics;
•
Determination of major hydrochemical water types (i.e. facies);
•
Preparation of Piper diagrams (Trilinear diagrams used to identify water quality (using major ions)
from geologic units and to define the evolution in water chemistry along a flow path);
•
Calculation of saturation indices for characteristics minerals;
•
Calculation of molar ratios for all major ions (Na + K , Ca , Mg , SO4 and HCO3 + CO3 ) with
respect to chloride (Cl ); and
•
Completion of multivariate statistics (Spearman correlation coefficients, Principal Component
Analysis and clustering methods including Hierarchical Cluster Analysis and k-means Cluster
Analysis).
+
+
2+
2+
2-
-
2-
The method of assessment determined to best meet the objectives of this activity was the use of
multivariate statistics. Calculation of ion molar ratios with respect to Cl were also assessed and found to
provide an interpretive value. However, the value in these methods was best realised when applied to
clustered data (resulting from the multivariate statistical methods) in each hydrostratigraphic unit. Piper
diagrams, where prepared, have been included as Appendix 2; however, their utility in achieving the
objective of this assessment was limited.
During the course of this investigation, a decision was made to separate permeable intervals within certain
hydrostratigraphic units (i.e., Evergreen Formation and Walloon Subgroup) from less permeable intervals.
This was done to assess the likely differences between the associated pore waters (due to inherent
differing hydraulic properties). Although this approach may be considered an unnecessary level of
refinement, especially given the limited data available with which to make the distinction, it did identify
some notable differences, even if these did not materially affect the findings of this study.
G en e ra l Ch ar a ct e r is a t io n
Basic statistics to assess the general characteristics of water types in each major formation or interval
th
th
th
th
included the median value plus 10 , 25 , 75 and 90 percentiles of recorded concentrations (in mg/L) for
major ions and other selected parameters (pH and TDS). Additional statistics were generated for other
less frequently-measured elements including Al, B, Cu, and Zn.
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
+
+
2+
2+
2-
-
2-
Select ion ratios for all major cations and anions (Na + K , Ca , Mg , SO4 , and HCO3 +CO3 ) were
also assessed for each hydrostratigraphic unit. Ratios were calculated with respect to Cl because of its
conservative nature as an anion (i.e. it is rarely involved in mineral dissolution/precipitation reactions
within an aquifer). Therefore, using Cl in this manner limits its source to recharge and mixed waters, thus,
potentially identifying a source. Also, the chemical inertness of Cl means that its concentration in an
aquifer is mainly influenced by dissolution of resident salts or dilution processes. Therefore, by calculating
major ion to Cl ratios, groundwater chemistries can be easily compared since the processes of salt
concentration and dilution within an aquifer have been nullified.
+
Sodium Adsorption Ratio (SAR) values were also assessed. The SAR is a ratio of the monovalent Na ion
2+
2+
to the divalent ions of Ca and Mg , as shown in the following equation:
Co rr el at io n a n d St at i st i c al An a l ysi s
All multivariate analysis was carried out using SYSTAT version 12.0 (SYSTAT Software Inc.). The
techniques used are summarised below. Results of the correlation and statistical analyses are
summarized in relevant format (figures and tables output by SYSTAT version 12.0) in Appendix 3 (Bowen
Basin) and Appendix 4 (Surat Basin) of this document. Detailed theory underlying the multivariate
methods has not been provided as part of this report but can be found in the supporting documentation for
the SYSTAT software or other relevant statistical references.
Sp e ar ma n Co rr e lat io n M at r ix
Pairwise Spearman rank correlation coefficients were drawn for the major ions and selected analytes (pH,
TDS, silica [as SiO2 ], fluoride [F ] and nitrate [NO3 ]) for hydrostratigraphic units where sufficient data was
available. The purpose of this approach is to identify alignments or differences between the various
parameters to indicate possible interactions between otherwise discrete water-bearing intervals based on
similar patterns of parameter correlations. Furthermore, review of correlation patterns can provide some
2+
indication of groundwater provenance (e.g. Na correlating with Cl and SO4 can indicate pore water from
a marine-source).
P rin c ip al Co mp o n en t An al ys i s
Principal Component Analysis (PCA) is used to describe an original dataset using a number of linear
combinations of the original variables, called components. PCA is a variable reduction procedure, and is
used to reduce redundancies in the dataset and define the smallest number of components needed to
adequately describe most of the variance observed in the original dataset (Hatcher, 1994). The first step in
PCA is to derive a matrix of correlation for an entire dataset. This matrix is derived using the Pearson’s
product moment correlation, which assumes the data is distributed as a normal population (i.e. bellshaped distribution).
The product of a PCA analysis is a factor-pattern matrix. In a factor-pattern matrix the rows represent
parameters being analysed, and the columns represent components. Within the matrix are values called
“factor loadings” which are equivalent to the bivariate correlation coefficient between the parameter and
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
the component. Rotated factor-pattern matrices are provided for each of the aquifers in Appendix3 Bowen
Basin) &Appendix 4 (Surat Basin). The number of components extracted in PCA is always equal to the
number of variables. However, typically, only the first few components will account for a meaningful
amount of the variance in a dataset. Here, the number of meaningful components retained for
interpretation of the PCA results was decided on the basis of three criteria:
•
The Kaiser Criterion;
•
The scree test; and
•
The cumulative proportion of data set variance accounted for.
The Kaiser Criterion states that only those components with eigenvalues greater than 1 are retained
(Hatcher, 1994). In a scree plot, the eigenvalues for each component are plotted, and there typically exists
a visible “break” in slope of the line between components that have large eigenvalues and those that do
not. Components before the visual break are retained. In the case of this study, the cumulative proportion
of the variance in the original dataset accounted for by the components retained had to amount to greater
than 70%. However, whilst these three criteria were used to guide the selection of the number of PCA
components, the components retained also had to be interpretable., In this assessment, two or three
components were typically retained for each aquifer.
Prior to interpretation of the PCA results, a Varimax (orthogonal) rotation was carried out. Rotation
involves linear transformation of the data and makes interpretation more straightforward. As such, rotation
of the data maximizes the variation amongst the variables. The rotated factor-matrix pattern for each
formation assessed, and the percentage of the total variance in the original dataset accounted for by the
retained components are provided in Appendix 3 (Bowen Basin) and Appendix 4 (Surat Basin).
Parameters used for interpretation of hydrochemical conditions in the aquifers had loadings greater than
0.6 in one component (in the un-rotated matrix) and small values in other components.
Throughout this document, tables are provided that summarise the PCA results for each
hydrostratigraphic unit. Typically results for the first two rotated components are shown; however, if the
first two rotated components accounted for ≤50% of the total variance, or if the second and third rotated
components account for equal amounts of variance then results for the first three components were
summarised.
Clu st er An a l ys is
Cluster analysis is a method used to explore natural groupings within a dataset. K-mean clustering is a
partitioning method in which data points are split into the number of clusters specified by the user. The
method used to split the data is driven by the desire for minimisation of within-cluster (sum of squares)
variation and maximisation of between-cluster variation. In the algorithm, initial mean values are selected
(in a number of user-specified ways), and the data for each monitoring event is classified into a cluster by
the distance to the cluster centre. A new cluster mean is then computed and this process continues until
all monitoring events in the dataset have been classified (Norušis, 2005).
The number of clusters used was initially set to the default value of 2, and additional third or fourth clusters
were added if deemed important for interpretation. Distance between clusters was determined in three
dimensions (x, y z space) using the Euclidean method. The maximum number of iterations used was 20,
and initial seeds for clustering were determined by the first k-method. The optimal number of clusters for
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
each hydrostratigraphic unit was determined using an approach similar to a scree plot (described above).
The total “within SS” value was plotted versus the number of clusters and where a break in the slope of
the line was identified, the number of clusters before the break were retained. This approach yielded 3 or
4 clusters for each hydrostratigraphic unit.
Hierarchical cluster analysis (HCA) was also carried out to assess parameter groupings. Dendrograms for
each aquifer are provided in Appendix 3 (Bowen Basin) and Appendix 4 (Surat Basin). Columns were
joined, with the linkage set to Ward’s method and the distance set to Pearson’s method. HCA output
effectively generates the same results as the PCA analysis; however, the benefit of this method is in its
visual presentation as opposed to tabular output generated by PCA. For those more attuned to pattern
recognition, this can be helpful. Similarly, the utility of the k-means cluster analysis is to provide a visual
and spatial representation of similar groupings of water types, and thereby the ability to identify particular
areas of a study domain that have similar conditions.
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
3
3.1
THE STUDY ARE A
Major Formations of the Bow en and Surat basins
In the context of this report, the term ‘aquifer’ refers to an interval with sufficient permeability and yield to
provide useful and economical quantities of water from a bore constructed within it. In turn, an aquitard is
defined as a sediment or rock layer that possesses significantly lower permeability than an aquifer
(several orders of magnitude). An aquitard may yield or transmit water over large regional areas, but
cannot provide useful or economical rates to a bore constructed within it.
In the following sections, references will be made to these terms, and in some cases water level and water
quality information will be provided for intervals generally referred to as aquitards. Although this may
seem counter-intuitive, it is a known fact that the aquitard formations in both the Bowen and Surat basins
do have intervals that possess higher water yield capability than the surrounding sediments (i.e., siltstone
versus mudstone); they just may not be that extensive either laterally and vertically (from a regional
perspective). As such, water level and water quality information provided for formations identified as
‘aquitards’ relates to the more permeable sections contained within those intervals.
3.1.1 Bow en Basin
The rocks which existed before sedimentation commenced in the Bowen Basin (during the Early Permian)
are regarded as ‘basement’. These basement rocks consists of indurated and deformed sediments, lowgrade meta-sediments, acidic igneous complexes and volcanic rocks ranging in age from Devonian to
Early Permian.
By the Early Permian, the area comprised an exposed stable western block, which formed the floor of a
shallow sea. As a result of subsidence during the Permian, as much as 9,000 m of continental and
shallow-marine (largely clastic) sediments were deposited. Within these deposits, substantial
accumulations of organic material were deposited in low-energy swamp environments resulting in the
formation of extensive coal deposits (Cadman et. al, 1998; Dickins and Malone, 1973). The thickest
sequences are situated within the Taroom and Denison Troughs, which are separated by the Comet
Ridge and in the smaller Arbroath Trough.
As a result of differential rates of uplift and subsidence around the Bowen Basin, periods of sedimentation
were not always consistent across the area. This has resulted in a complex arrangement of geological
units that are not always laterally extensive. For the purposes of this review, only the major units of the
Bowen Basin, for which data were available, have been summarised below. These units, and their
equivalent units elsewhere in the basin, are presented in a stratigraphic profile in Table 4. The name
ascribed to each formation or group of formation in the database is provided in the second column of
Table 4. Further background information on each formation is provided in the supporting literature review
for this study (WorleyParsons, 2010).
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Table 4 Hydrostratigraphy of the Bowen Basin
Age
Group
Geologic Units
Hydraulic Properties
Moolayember
Aquitard
Clematis Sandstone, Glenidal
Formation, Expedition Sandstone,
Showgrounds Sandstone, Wandoan
Formation
Aquifers
Rewan Group
Arcadia Formation, Sagittarius
Sandstone, Rewan Group Basal
Sands
Aquitards and
subordinate aquifer
intervals
Late (Upper)
Permian
Upper Permian
Sandstones and
Coal Measures
Moranbah, Fort Cooper, Rangal
German Creek (northern and central
regions), Bandanna and Baralaba
Coal (southern region)
Aquitards and minor
aquifer intervals
Early
(Lower)
Permian
Lower Permian
Sandstones
Tiverton Formation, Gebbie
Formation, Collinsville Coal
Measures, Cattle Creek Formation,
Freitag Formation, Ingelara
Formation, Catherine Sandstone and
the Peawaddy Formation, Aldebaran
Sandstone, Moonlight Sandstone
Aquitards and
subordinate aquifer
intervals
Lizzie Creek, Bulgonunna, Connors
Aquifers
Triassic
(Back Creek Group)
Permian Volcanics
Major Aquifers and Aquitar ds
The geological formations within the Bowen Basin may be simplified into the following major water-bearing
or hydrostratigraphic units encountered; from the bottom of the section (oldest) upwards in the
stratigraphic profile (youngest).
Low er a n d Up p e r P e r mi an ( aq uif e rs)
All of the water-bearing units including, and below, the Rewan Group exist as confined water-bearing units
of fluvial, lacustrine and aeolian in origin which contain isolated pore waters of generally poor quality.
These units also display dissimilar hydraulic characteristics, variable hydrochemistries and high salinities,
indicating a distinctly different hydrogeological system compared to the Great Artesian Basin (GAB)
system (GABCC, 1998). The deeper water-bearing units of Bowen Basin, namely, the Late Permian coal
measures and sandstone intervals within the Back Creek Group, are isolated from the GAB aquifers by
low permeability units within the younger (Triassic) Rewan Group. Additionally, some of the structural
features in the basin have offset more contiguous permeable intervals with lower permeability layers, in
essence fragmenting otherwise laterally continuous aquifer intervals. These deeper geologic units can be
grouped together, and collectively referred to as ‘deep reservoirs’.
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The only mineralogical information available for Late to Early Permian sediments relates to the formation
known as the Back Creek Group. The average mineralogical composition of the sediments is dominated
by quartz, followed by plagioclase feldspar (Figure 1). Mixed-layer silicates represent the dominant clay
mineral at 59%, followed by kaolinite (4.3%) and chlorite (3.7%). The presence of a trace amount of pyrite
is a distinguishing feature of this formation.
Quartz
1.3
4.3
5.9
3.7
Plagioclase
8.1
45.2
9.0
K-feldspar
Chlorite
Kaolinite
22.7
Mixed layer
Mica
Pyrite
Figure 1 Mineralogy of the Back Creek Group: n = 2 (Grigorescu, 2011a)
Rew an G ro u p ( aq u it a rd)
The Rewan Group generally comprises mudstones and siltstones of low porosity and permeability
(Butcher, 1984). The upper section is mostly comprised of shale and considered to represent a regional
seal for the Permian intervals (Henning et al., 2006). The Rewan Group is widely recognised as the base
of the GAB.
Quartz
0.9
7.1
45.6
18.2
K-feldspar
Chlorite
Kaolinite
5.45
13.8
0.8
Plagioclase
8.2
Mixed-layer
Mica
Hematite
Figure 2 Mineralogy of the Rewan Group: n = 2 (Grigorescu, 2011a)
The upper units of the Bowen Basin (Rewan Group, Clematis Sandstone and the Moolayember
Formation), while generally hydraulically separated from the overlying Surat Basin, do display similar
hydrochemical characteristics to that basin, and are suspected to be in hydraulic connectivity with the
overlying and adjoining aquifers of the GAB (Habermehl, 1980).
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
The average mineralogical composition of the Rewan Group sediments is similarly dominated by quartz
and plagioclase feldspar, but when compared to the Back Creek Group, there is a dominant proportion of
kaolinite (18.2%) and lower amount of chlorite (0.8%) (Figure 2). A distinguishing feature of this formation
is the occurrence of hematite.
Cl em at i s S an d st o n e ( aq u if e r)
The Clematis Sandstone is a confined aquifer consisting of sandstones; mainly derived from sediments
deposited in a fluvial environment. It also includes the laterally equivalent Showgrounds Sandstone, where
it exists, and the lower sandstones of the Wandoan Formation. Habermehl (1980) describes the combined
Clematis / Showgrounds Sandstone aquifer as the lowermost aquifer in the GAB with good yields and
good water quality. The thickness of the aquifer ranges from 5 m to 2,000 m (Habermehl, 1980) with an
average thickness of 115 m in the northern areas of the basin. The Showgrounds Sandstone is generally
thinner than the Clematis Sandstone.
M oola yemb e r F o rm at io n ( a q u it a rd )
The Moolayember Formation comprises a low permeability, confining unit that has a thickness, where
present, ranging between 300 m to over 1,500 m (Habermehl, 1980). This confining unit consists of
predominantly siltstone and mudstone and is responsible for the development of artesian conditions in the
underlying Clematis Sandstone and Showgrounds Sandstone aquifers. Over most of the basin, the
Moolayember Formation separates the Clematis Sandstone from the overlying aquifers of the Surat Basin.
On the western margin of the basin (near the Nebine Ridge), the Moolayember Formation pinches out
facilitating hydraulic connection between the Clematis Sandstone and Precipice Sandstone of the Surat
Basin. Further west, outside the Bowen Basin, the Moolayember Formation re-appears and the adjacent
units are again hydraulically separated.
The average mineralogical composition of the Moolayember Aquitard is dominated by quartz, K-feldspar
and kaolinite (Figure 3). Distinguishing features of this formation include the presence of siderite and
calcite.
Quartz
1.3
1.8
7.1
Plagioclase
5.4
K-feldspar
18.2
52.3
Chlorite
Kaolinite
8.6
1.6
3.9
Mixed-layer
Mica
Siderite
Calcite
Figure 3 Mineralogy of the Moolayember Aquitard: n = 2 (Grigorescu, 2011a)
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
3.1.2
Surat Basin
The Surat Basin may be described as a multi-layered system of confined aquifers and intervening
aquitards, and one of the major sub-basins of the GAB. The sedimentary sequence comprises
predominantly fluvially-deposited sandstone units interspersed with marginal-marine mudstone and
siltstone units. The sandstone units generally form aquifers, with intake beds to the GAB occurring on the
northern margin of the basin where they are exposed or interact with surface water features. The units of
marine origin generally form the intervening confining beds, or aquitard units.
Most of the aquifers in the Surat Basin either inter-finger with units in the Eromanga Basin (for example,
the Gubberamunda, Orallo and Mooga (BMO Group) intervals), or extend across the Nebine Ridge (e.g.,
Rolling Downs Group and Hutton Sandstone). These units (designated as major hydrostratigraphic units)
are summarised in Table 5, along with their hydrogeological classification.
Table 5 Stratigraphy of the Surat Basin
Age
Group
Geological Units
Hydraulic Properties
Rolling Downs
Group
Griman Creek Formation, Surat Siltstone,
Wallumbilla Formation
Aquitard with subordinate
aquifers
BMO Group
Bungil Formation Mooga Sandstone,
Orallo Formation
Aquifer and aquitard
sequence
Gubberamunda Sandstone
Aquifer
Westbourne Formation, Southlands
Formation
Aquitard
Springbok Sandstone
Aquifer
Walloon Coal Measures (including
Macalister Coal Seams, Lower Juandah
Coal Measures, Taroom Coal Measures),
Juandah Sandstone, Tangalooma
Sandstone, Pilliga Sandstone
Aquifer and aquitard
sequence
Eurombah Formation
Aquitard
Hutton Sandstone
Aquifer
Evergreen Formation, Marburg
Sandstone, Boxvale Sandstone Member
Aquitard with subordinate
aquifer intervals
Precipice Sandstone
Aquifer
Cretaceous
Jurassic
Walloon
Subgroup
Major Aquifers and Aquitar ds
The main geological formations making up the sequence of sedimentary units contained within the Surat
Basin are described below.
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
P re c ipi c e S an d st o n e ( aq u if er)
The Precipice Sandstone consists of quartzitic sandstone throughout, and is coarser-grained nearer its
base (DME, 1997). The outcrop of the Precipice Sandstone defines the northern boundary of the Surat
Basin where it forms a sinuous east-west band. This unit extends to the Auburn Complex in the northeast
and terminates against the Texas High and the St. George/Bollon Slope in the southeast and southwest,
respectively. It eventually transitions into the Helidon Sandstone within the Clarence-Moreton Basin and
continues into the Eromanga Basin across the Nebine Ridge. It is thickest in the Mimosa Syncline
adjacent to the Chinchilla-Goondiwindi and Moonie Faults (Cadman et al., 1998). Deposition of the
Precipice Sandstone marks the start of a widespread period of predominantly continental-fluvial
deposition.
This sandstone unit rests un-conformably on rocks ranging from Devonian to Triassic age (Exon, 1976),
and although greater in lateral extent than the Taroom Trough, is not ubiquitous across the Surat Basin
(DME, 1997). Underlying the Precipice Sandstone is the Wandoan Formation, a fluvial deposit of
sandstone, siltstone, shale and conglomerate, and the Moolayember Formation aquitard.
The average mineralogical composition of the Precipice Sandstone is relatively distinct from the overlying
Jurassic units by its dominance of quartz and kaolinite, lesser amounts of plagioclase, potassium feldspar
and mica, and lack of mixed layer silicates (Figure 4). Trace amounts of siderite and hematite also exist.
0.2
0.1
0.3
16.8
Quartz
0.2
Plagioclase
4.0
K-feldspar
4.4
2.8
Chlorite
71.2
Kaolinite
Mixed layer
Mica
Siderite
Hematite
Figure 4 Mineralogy of the Precipice Sandstone: n = 44 (Grigorescu, 2011a)
Ev e rg r ee n F o r ma t io n ( a q u it a rd)
The Evergreen Formation resides above the Precipice Sandstone and forms a conformable confining
layer. In part, it lies unconformably over the Bowen Basin sequence (DME, 1997). The formation is mainly
comprised of siltstone; however, there is a dominance of shale and notable sandstone intervals in some
horizons which occurs in the western part of the Surat Basin (i.e., the Boxvale Sandstone Member).
The Westgrove Ironstone Member, directly overlying the Boxvale Sandstone (or its equivalent) is a
persistent marker horizon extending across most of the Surat Basin. With the exception of the two
members, the Evergreen Formation is reasonably consistent in its fine-grained nature and corresponding
low permeability.
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
The depositional environment of this unit was fluvial and marginal to shallow marine. The maximum
thickness of the formation is of the order of 300 m (DME, 1997). The boundary between the Evergreen
Formation and the underlying Precipice Sandstone can be difficult to determine in places, as the upper
part of the Precipice Sandstone tends to be fine-grained.
The average mineralogical composition of the Evergreen Formation is dominated by quartz, plagioclase
feldspar, and kaolinite (Figure 5). Traces of calcite, siderite and hematite exist as distinguishing features
for this interval. As for the Boxvale Sandstone member, quartz comprises the main mineral (79.1%)
followed by kaolinite, and only minor amounts (about 3%) of the feldspar minerals.
1.0 1.1
4.6
0.8
Quartz
Plagioclase
6.0
46.3
19.8
K-feldspar
Chlorite
Kaolinite
Mixed layer
8.1
11.7
0.7
Mica
Siderite
Calcite
Hematite
2.5
0.7
13.8
3.9
Quartz
Plagioclase
K-feldspar
79.1
Kaolinite
Mica
Figure 5 Mineralogy of the Evergreen Formation (upper pie chart; n = 26) and
associated Boxvale Sandstone member (lower pie chart; n = 3) (Grigorescu, 2011a)
Hut t on S an d st o n e ( a q u if e r)
The Hutton Sandstone conformably overlies the Evergreen Formation (Exon, 1976). This permeable
interval crops out to the north of Injune and on the eastern margin of the basin where it grades into the
Marburg Sandstone of the Clarence-Moreton Basin across the Kumbarilla Ridge. The Hutton Sandstone
was deposited by meandering streams, and is predominantly comprised of coarser-grained sediments
with lesser amounts of siltstone and mudstone.
The unit is generally between 120 m and 180 m thick and is more variable (60 m to 240 m) along the
eastern margin of the Surat Basin. The Hutton Sandstone is the most extensive Jurassic-aged unit in the
Great Artesian Basin (DME, 1997).
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
The average mineralogical composition of the Hutton Sandstone is dominated by quartz, plagioclase
feldspar and kaolinite (Figure 6) – very similar to the Evergreen Formation (Figure 5) – even in the
proportion of trace minerals.
3.5
0.5
0.3
Quartz
0.5
Plagioclase
7.7
K-feldspar
14.3
0.7
53.2
7.2
12.0
Chlorite
Kaolinite
Mixed layer
Mica
Siderite
Calcite
Hematite
Figure 6 Mineralogy of the Hutton Sandstone: n = 73 (Grigorescu, 2011a)
Eu ro mb ah F o rm at io n ( a q u it a rd)
The Eurombah Formation lies conformably in contact with the Hutton Sandstone and the overlying
Walloon Subgroup. This confining unit is thickest, at approximately 100 m, in the north part of the study
area near Injune, and thins to the west, south and east until it diminishes completely (Exon, 1976; Scott et
al. 2004). DME (1997) included the Eurombah Formation in the Walloon Subgroup based in its lithological
descriptions, as it is not possible to always differentiate in geophysical logs. Scott et al. (2004 and 2007)
include the Eurombah Formation in the Durabilla Formation, which is associated with mudstone of the
Taroom Coal Measures.
Mineralogy of the Eurombah Formation was not available for assessment during the course of this study.
W al loon Su b g r o u p ( a q u it a rd s an d a q u if e rs )
The Walloon Subgroup is a thick sequence of sediments deposited in a low-energy environment. The
deposits extend across the Clarence-Moreton and Surat basins, and almost to the Nebine Ridge. The
entire subgroup contains deposits of light grey mudstone, siltstone, fine-grained sandstone and relatively
thin seams of coal (1 to 3 m) and reaches up to a maximum thickness of approximately 500 m (DME,
1997). The deposits dip to the south and west and outcrop, albeit poorly, across the northern and
southeast edges of the Surat Basin. In contrast, the Walloon Subgroup outcrops extensively in the
Clarence-Moreton Basin (Exon, 1976). Coal deposits are concentrated in the north-eastern margin of the
basin.
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
The deposits of the Walloon Subgroup can be readily subdivided into three sub-units (from bottom up):
•
Taroom Coal Measures;
•
Tangalooma Sandstone; and
•
Juandah Coal Measures.
Scott et al. (2004) suggest a fourth sub-unit at the base, the Durabilla Formation, which includes the
Eurombah Formation.
Within the Walloon Subgroup are accumulations of comparatively thick, laminated sandstone interbedded
sequence. Six coal intervals have been identified in the Juandah Coal Measures. From the base up, these
coal intervals include the:
•
Argyle;
•
Iona;
•
Wambo;
•
Nangram;
•
Macalister; and
•
Kogan.
Three identifiable coal seams have been noted in the Taroom Coal Measures. From base up these
include the:
•
Condamine;
•
Bulwer; and
•
Auburn.
The Juandah Coal Measures are informally divided into upper and lower units, with the divide occurring
between the Macalister and Nangram seams. The Macalister and Bulwer seams of the Juandah and
Taroom intervals, respectively, are thick and relatively clean; however, the remaining coal seams
comprise numerous thin stringers (< 1 m up to 3 m) separated by thin to thick bands of mudstone,
siltstone or sandstone (Scott et al., 2004). In parts of the north-eastern portion of the Surat Basin, the
Kogan seam was completely eroded away prior to the deposition of the Springbok Sandstone, with the
sandstone lying immediately above and in hydraulic connection with the Macalister Seam (Scott et al.,
2007). The contact between this interval and the overlying Springbok Formation represents an
unconformity in the region (Scott et al., 2004).
The Birkhead Formation, recognisable in the western part of the Surat Basin and in the Eromanga Basin,
is laterally continuous with the Walloon Subgroup (Exon, 1976).
The average mineralogical composition of the Walloon Subgroup is dominated by quartz, plagioclase
feldspar and kaolinite (Figure 7). One distinguishing feature of this formation is the comparatively higher
proportion of siderite, calcite, and traces of hematite and goethite.
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
1.1
0.1
2.7
5.3
Quartz
0.5
Plagioclase
K-feldspar
5.6
6.4
37.5
Chlorite
Kaolinite
Mixed-layer
17.4
Mica
4.8
16.6
Siderite
Calcite
1.8
MgCO3
Hematite
Goethite
Figure 7 Mineralogy of the Walloon Subgroup: n = 132 (Grigorescu, 2011a)
Sp ri ngb o k San d st o n e ( a q u if e r)
The Springbok Sandstone outcrops on the northern and eastern sides of the Surat Basin; however, its
occurrence as outcrop is poor. The unit predominantly comprises sandstone with minor interbedded
siltstone and mudstone. Calcite cement is present in some intervals. This unit was deposited mainly by
fluvial processes, and the fining-upward sequence suggests a decrease in flow energy over time. The
Springbok Sandstone is thickest in the eastern part of the Surat Basin, reaching up to 200 m. The
equivalent of this unit is the Adori Sandstone in the Eromanga Basin. It also inter-fingers with the Pilliga
Sandstone, which is predominantly situated beneath New South Wales (Exon, 1976).
The average mineralogical composition of the Springbok Sandstone is dominated by quartz, plagioclase
feldspar and kaolinite (Figure 8). Of note is the proportion of hematite, which is the highest of all of the
Surat Basin formations.
2.2
4.3
Plagioclase
3.7
12.9
0.3
Quartz
2.1 1.0
K-feldspar
45.8
Chlorite
Kaolinite
6.2
Mixed layer
21.6
Mica
Siderite
Calcite
Hematite
Figure 8 Mineralogy of the Springbok Sandstone: n = 21 (Grigorescu, 2011a)
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
W est bou rn e F o rm at i o n ( aq u it a rd )
Confining the Springbok Sandstone from above is the Westbourne Formation. This formation comprises
alternating sequences of mudstone and lithic sandstone with minor siltstone and coal in the upper
portions, and thinly-bedded siltstone and low permeability sandstone in the lower portions. The
Westbourne Formation reaches a thickness of over 250 m in the eastern portion of the study area and
thins to the west and southwest towards the Eulo-Nebine Ridge. This formation was deposited in a low
energy fluvial and marginal-marine environment. Sediments of the Westbourne Formation are exposed at
surface in the northern part of the Surat Basin (Exon, 1976).
The grouping of the Eurombah Formation, Walloon Subgroup, Springbok Sandstone and Westbourne
Formation is often referred to as the Injune Creek Group. In total, this group can reach up to 1,000 m in
aggregate thickness.
The average mineralogical composition of the Westbourne Formation is again dominated by quartz,
plagioclase feldspar and kaolinite, with a notable proportion of siderite (Figure 9).
3.2 0.7
6.4
Quartz
0.2
Plagioclase
5.5
K-feldspar
45.6
16.0
Chlorite
Kaolinite
6.7
0.3
Mixed layer
15.5
Mica
Siderite
Calcite
Hematite
Figure 9 Mineralogy of the Westbourne Formation: n = 32 (Grigorescu, 2011a)
G ubb er am u n d a S an d st o n e ( aq uif e r)
Overlying the Westbourne Formation is the Gubberamunda Sandstone, which consists of fine- to
coarse-grained sandstone with minor interbedded siltstone and mudstone. Although the finer-grained
sediments are minor in occurrence, they can form up to half of the total unit thickness. This sandstone unit
was deposited by braided and meandering streams and is up to 200 m thick in the centre of the basin, but
is generally around 100 m thick or less on the basin margins. The unit is exposed at surface in the north of
the basin, from the western margin of the study area to the area around Roma (Gray, 1972).
The average mineralogical composition of the Gubberamunda Sandstone is similar to many of the other
formations, in particular the Hutton Sandstone, and is dominated by quartz, plagioclase feldspar and
kaolinite. Of note is the relative proportion of mica and siderite (Figure 10).
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
0.3
2.7
1.6
Plagioclase
7.0
15.4
K-feldspar
50.3
6.9
0.2
Quartz
1.9
Chlorite
Kaolinite
Mixed layer
13.6
Mica
Siderite
MgCO3
Hematite
Figure 10 Mineralogy of the Gubberamunda Sandstone: n = 16 (Grigorescu, 2011a)
BM O G ro u p : Bu n g il F o rm at io n , M o o g a Sa n d st o n e a n d O r al lo F o rm at i o n ( aq uif e rs
and a qu it a rd s)
The BMO Group represents a hydrostratigraphic interval defined by anticipated similarity in
hydrogeological properties. Another grouping of formations called the Kumbarilla Beds incorporates the
Bungil Formation, Mooga Sandstone and Orallo Formation (BMO Group), as well as the Springbok
Sandstone, Westbourne Formation, and Gubberamunda Sandstone at locations where they are all
exposed at surface. Although each of these formations can be differentiated in outcrop, extensive
weathering on the eastern margin of the Surat Basin makes mapping of the individual units challenging
(Exon, 1976 and Habermehl, 1980).
The Orallo Formation, which comprises thinly-bedded siltstone and mudstone and thickly-bedded
calcareous sandstone deposits, was deposited in a fluvial stream setting. Fossil wood and minor coal
accumulations are common. This formation has been mapped in the northern part of the basin, but its low
resistance to weathering makes identification of outcrop difficult. The Orallo Formation rests in
conformable contact with the underlying Gubberamunda Sandstone, and given its comparatively
finer-grained character, acts as a confining layer.
The average mineralogical composition of the Orallo Formation is dominated by quartz and plagioclase
feldspar (Figure 11). Compared to the other formations, the low contribution from kaolinite is a
distinguishing feature, as well as the notable siderite content.
The Mooga Sandstone conformably overlies the Orallo Formation and outcrops in the northern part of the
basin. It is rarely over 100 m thick near the basin margins, but reaches thicknesses up to 200 m in the
central part of the basin. This interval was deposited in a high-energy fluvial environment and
predominantly comprises sandstones near the margins of the basin (braided streams), but siltstone and
mudstone become more common in the central portions (meandering streams) indicating a shift to a
lower-energy depositional environment. The Mooga Sandstone is considered to be consistent with the
Hooray Sandstone of the adjacent Eromanga Basin to the west (Radke et al. 2000). Unfortunately, no
mineralogical data was available for this formation at the time of preparing this document.
The Bungil Formation resides above the Mooga Sandstone and mainly consists of fine-grained sandstone,
siltstone and mudstone in roughly equal proportions. This unit was deposited in a near-shore environment.
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
The formation has been mapped where outcroppings can be positively identified in the north and northeastern parts of the Surat Basin. In the southeast portion of the Surat Basin, the Bungil Formation is
included in the Kumbarilla Beds, and represents the Surat Basin equivalent of the Cadna-owie Formation
located beneath the Eromanga Basin (Radke et al., 2000).
The average mineralogical composition of the Bungil Formation is similarly dominated by quartz,
plagioclase feldspar and kaolinite (Figure 12).
Quartz
0.1
3.3
4.2
3.9
Plagioclase
2.7 2.1
K-feldspar
5.9
41.8
7.3
Chlorite
Kaolinite
Mixed layer
5.9
Mica
0.3
Siderite
22.6
Calcite
MgCO3
Hematite
Goethite
Figure 11 Mineralogy of the Orallo Formation: n = 26 (Grigorescu, 2011a)
1.2
1.9
Quartz
7.2
17.8
Plagioclase
42.4
K-feldspar
Kaolinite
8.2
18.6
Mixed-layer
Mica
Siderite
Figure 12 Mineralogy of the Bungil Formation: n = 3 (Grigorescu, 2011a)
Rol li ng Do w n s G ro u p ( a q u it a rd )
The Wallumbilla Formation, Surat Siltstone and Griman Creek Formation are often referred to as the
Rolling Downs Group. These formations are characteristically fine-grained; indicating deposition in a
lower-energy environment (Habermehl, 1980).The Wallumbilla Formation, conformably underlying the
Surat Siltstone, is divided into two members:
•
Doncaster Member; and
•
Coreena Member.
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
The Doncaster Member predominantly consists of mudstone (blue-grey or glauconitic) with subordinate
siltstone and sandstone beds, and is characterised by inter-laminated fine- and coarse-grained
mudstones. The Wallumbilla Formation outcrops in a discontinuous arc from the town of Morven, in the far
west of the study area, to the town of Miles in the middle of the study area. It also extends westward into
the Eromanga Basin and is therefore laterally continuous and regionally extensive (Exon, 1976).
The Coreena Member of the Wallumbilla Formation comprises interbedded mudstone and siltstone,
grading into sandstone, deposited in an environment ranging from shallow marine to lagoon or swamp.
The Coreena Member is generally about 100 m thick, and is thickest in the east where it has been found
to reach in excess of 150 m.
Like most of the other formations, the average mineralogical composition of the Wallumbilla Formation is
dominated by quartz and plagioclase feldspar (Figure 13). However, this interval is somewhat unique in
its mixed layer clay and mica composition, which are the highest of all the Cretaceous and Jurassic
intervals assessed.
Conformably overlying the Wallumbilla Formation is the Surat Siltstone, which predominantly consists of
interbedded siltstones and mudstones (Gray, 1972). This formation is generally finer-grained than the
underlying Coreena Member and forms scattered outcrops in the western part of the basin (Exon, 1976).
An average thickness of approximately 110 m is documented (Gray, 1972).
Above the Surat Siltstone unit is the Griman Creek Formation. This unit was deposited during a marine
regression sequence and primarily consists of micaceous sandstone and siltstone. It is only found in the
Surat Basin and is predominantly identified in the area covered by the Surat 1:250,000 Geological Map
Sheet (Thomas and Reiser, 1971). Thicknesses of up to 350 m have been noted (Gray, 1972).
10.8
39.3
19.1
Quartz
Plagioclase
K-feldspar
5.1
Kaolinite
7.4
18.6
Mixed-layer
Mica
Figure 13 Mineralogy of the Wallumbilla Formation: n = 2 (Grigorescu, 2011a)
3.1.3
Alluvium and Tertiar y Volcanics
Overlying the Jurassic and Cretaceous strata are extrusive volcanic rocks of Tertiary age, and surficial
deposits of alluvium, comprising the Cainozoic Units. The associated geological unit names are
summarized in Table 6. The lateral distribution of these rocks and sediments occurs across both the
Bowen and Surat basins.
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
Table 6 Surficial deposits within the Study Area
Age
Group
Geological Units
Hydraulic Properties
Tertiary /
Cainozoic
Cainozoic
Units
Alluvium (including Condamine Alluvium,
Colluvium, Chinchilla Sands and Main
Range Volcanics)
Aquifers and aquitards
Unfortunately mineralogical data for these intervals was not available at the time of writing. However,
these Tertiary Volcanics have been described as consisting of olivine-rich basalt (KCB, 2010).
As for the Alluvium deposits, it is suspected that a similar quartz/plagioclase/kaolinite mineral composition
dominates. The reasoning behind this assumption is based on the fact that the general pattern for the
majority of underlying Cretaceous and Jurassic formations is this combination, and that the Alluvium is
sourced from these same formations. Distinguishing features may however be associated with the lesser
dominant minerals considering its relative age, proximity to surface and exposure to different weathering
conditions and processes.
3.2
Geological Structures
A number of geological and tectonic structures are known to exist within the Bowen and Surat basins that
are responsible for the disposition of formations and hydrocarbon resource potential(i.e. oil, gas, coal).
The occurrence of these structural features also has implications for the potential hydraulic connectivity of
otherwise discrete water-bearing formations if connected faults extend through otherwise competent
aquitard layers and are hydraulically active. Figure 14 shows the major structural features and provinces
of both basins. Additional information on the genesis of these features is provided in WorleyParsons
(2010).
Exon (1976) indicates that displacement on major faults within the Taroom Trough has been generally
less than 200 m at the commencement of deposition of the Surat Basin sequence, and less than 100 m by
the end of deposition of the Rolling Downs Group. Other more recent evidence supports the conclusion
that propagation of faults has only occurred a short distance into the formations of the Surat Basin, and
has been primarily restricted to the Precipice up to Walloon Subgroup interval.
Overall, the majority of faulting in the study area has occurred in rock formations associated with the
Bowen Basin, with some minimal thrust plane movement in the strata of the Surat Basin. Given these
findings, the potential for movement of groundwater between discrete aquifers separated by extensive
aquitards is likely more restricted to the Permian formations and deeper Jurassic formations as opposed
to the shallower Late Jurassic to Tertiary formations.
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
HuttonWallumbilla
Arbroath
BurungaLeichardt
MoonieGoondiwindi
Figure 14 Location of major and minor fault features in the Bowen (left panel) and Surat (right
panel) basins (after SRK, 2008)
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
4
RESULTS – BOWEN BASIN
The following sections highlight the various aspects of the Bowen Basin in relation to groundwater flow
and hydrochemical characteristics. For the purposes of this review, only the major formations and groups,
for which data was available, have been summarised.
Permian Volcanics
4.1
4.1.1
Hydr ogeology and Hydraulics
No information regarding groundwater levels is available for this interval due to the lack of monitoring data.
Therefore, an assessment of flow conditions was not possible.
4.1.2
Hydr ochemistr y
Tabulated data and output of water quality statistical analyses supporting the following discussion of
hydrochemistry (and likewise for all other hydrostratigraphic units) are found in Appendix 6.
G en e ra l Ch ar a ct e r ist i cs
Based on the data compiled for this study, groundwater in the Permian Volcanics displays a range in pH
from 6.7 to 8.6, but is generally considered to be slightly alkaline (median pH of 7.9). The TDS ranges
from 514 to 10,040 mg/L, but is generally brackish in character (median TDS = 1,383 mg/L) (Appendix 6).
Sodium represents the dominant cation (range = 72 mg/L to 1,900 mg/L; median = 237 mg/L). Notable
2+
2+
concentrations of Ca and Mg exist as well (median values = 92 mg/L and 114 mg/L, respectively) but
+
are subordinate to Na . Bicarbonate is the dominant anion (range = 245 to 1,399 mg/L; median = 768
mg/L), with Cl surpassing it in certain locations and at certain depths (range = 42 to 5,640 mg/L; median =
465 mg/L). Fluoride concentrations as high as 6.0 mg/L have been noted in this interval; however 90% of
the values measured exist at, or below, 1.0 mg/L. Groundwater associated with this interval is dominated
+
by a Na -HCO3 hydrochemical type.
One distinguishing feature of this interval is its silica concentrations (range = 11 to 84 mg/L; median =
42 mg/L). The occurrence of elevated silica (as SiO2) concentrations is consistent with the presence of
more easily weathered mafic minerals typically associated with these types of extrusive igneous rocks.
Co rr el at io n a n d St at i st i c al An a l ysi s
Spatial coverage of sampling locations and associated data records for the Permian Volcanics is low, and
is generally associated with outcropping areas along the eastern margin of the basin (Figure 15).
Spearman correlation coefficients (Appendix 3) determined for this interval indicate strong positive
associations for:
-
2+
•
Cl with TDS, Ba, Mg
•
TDS with Na ;
•
Mg
•
Na with SO4 .
2+
and Ca ;
+
2+
+
2-
with SO4 ; and
2-
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
Principal Component Analysis (PCA) results, presented in Table 7, indicate that three components are
required to adequately describe the total variance in the dataset. These are attributed to the major ions
2+
+
22+
(Cl , Mg , Na , SO4 , and Ca ; first component), and the general hydrochemical evolution of groundwater
2along a flow path resulting in increased pH conditions (strong loadings for pH and CO3 ; second
component). The third component yields strong loadings (hereafter defined as a factor loading greater
than 0.6) associated with HCO3 , F , SiO2 and NO3 . Some of the more important mineral weathering
reactions are described in Appendix 8.
Table 7 Principal Component Analysis results for the Permian Volcanics
Principle Component (% of Permian Volcanics
total variance explained by
1 (38%)
2 (20%)
rotated components)
TDS
pH
CO3
Parameters loading onto Cl
each PCA component (in Mg
order of descending loading Na
value)
SO4
Ca
3 (20%)
HCO3
F
SiO2
NO3
Results for k-means cluster analysis indicate that groundwater types associated with the Permian
Volcanics are typically brackish, with mean TDS values for the majority of clusters being greater than
1,000 mg/L (Table 8). This result is consistent with the GABCC (1998) study that suggested that
groundwater from these igneous rocks (and the Lower and Upper Permian Sandstones) is typically of poor
chemical quality (i.e. for human consumption). Because the data is clustered along the eastern margin of
the Bowen Basin, and no data is available to help establish groundwater flow directions, it is not possible
to comment on changes to the groundwater composition along inferred flow lines (i.e. hydrochemical
evolution).
Cluster 1 has a composition consistent with pore water source from, or interacting with, more saline
marine-type sediments. Groundwater associated with this cluster has a high TDS value (mean value
+
greater than 5,300 mg/L), a relatively low Na /Cl ratio (mean = 0.63 compared to a value of 1.0 for normal
2seawater), an intermediate Sodium Absorption Ration (SAR) value (mean = 10), a low (CO3 +HCO3 )/Cl
-4
ratio (mean = 0.05) and a low F /Cl ratio value (mean = 4.5 x 10 ). On the opposite end of the spectrum is
groundwater associated with cluster 4, with a relatively low TDS value (mean = 661 mg/L). This cluster
suggests groundwater that is intermediate along an evolutionary pathway from recent recharge(typically
2+
2+
+
higher relative Ca , Mg , and Cl ) towards a more evolved Na -HCO3 -Cl type due to water-rock
interaction and mineral weathering.
Groundwater hydrochemistries of clusters 2 and 3 suggest localised mixing of groundwater with high to
intermediate TDS values, while the hydrochemistry of Cluster 5 represents a more evolved groundwater
2based on the comparatively high CO3 +HCO3 /Cl ratio (mean = 1.1) and mean SAR value of 17. The
spatial distribution of k-mean clusters is shown in Figure 15.
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
Table 8 k-means Cluster Results for Permian Volcanics
Permian Volcanics
Cluster
N
1
13
2
16
3
14
4
10
5
6
All
59
Stats
Mean
1σ
Mean
1σ
Mean
1σ
Mean
1σ
Mean
1σ
Mean
1σ
TDS
pH
5383
2392
1444
507
1302
328
661
158
1879
569
2151
2067
7.6
0.5
7.7
0.3
7.9
0.3
8.2
0.2
8.4
0.2
7.9
0.4
Na/Cl SAR
0.63
0.15
0.89
0.27
1.06
0.40
1.82
0.75
1.56
0.46
1.10
0.58
10
4
4
2
4
2
3
2
17
11
5
6
(CO3+
HCO3)/Cl SO4/Cl F/Cl
0.2
0.1
1.0
0.4
2.0
1.0
3.9
2.9
1.1
0.4
1.6
1.8
0.05
0.04
0.18
0.20
0.09
0.05
0.08
0.04
0.05
0.04
0.10
0.12
4.5E-04
3.2E-04
2.5E-03
2.0E-03
4.1E-03
2.4E-03
6.0E-03
4.5E-03
9.6E-03
9.4E-03
3.9E-03
4.6E-03
NO3
SiO2 K
(mg/L) (mg/L) (mg/L)
4.3
4.4
1.1
1.3
6.5
7.3
1.9
1.7
4.1
4.0
3.7
5.2
37
12
32
10
60
12
30
7
37
23
42
17
3.5
2.5
3.6
4.3
1.1
0.8
2.2
1.2
5.1
3.3
2.7
2.8
LEGEND
Cluster - Descriptor
1 – High TDS, saline
2 – Mixed, Intermediate TDS
3 – Mixed, Intermediate TDS
4 – Recent Recharge
5 – Evolved, Intermediate TDS
b15 fault
b70 fault
Northern Bowen Faults
Figure 15 Spatial Distribution of the k-means Clusters for the Permian Volcanics
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
4.2
Low er Permian Sandstones
4.2.1
Hydr ogeology and Hydraulics
Reg ion a l F lo w Pat t e r n s
Groundwater level measurements are available for seventy-nine bores completed within the Lower
Permian Sandstones; however, no data was available between the target period 1960 and 1965 and only
one monitoring location was available during 2006. The time period containing the most groundwater level
measurements was in 1972. Sixteen bore locations were monitored during this 12-month period; however,
all but three of the 16 bores were located within a clustered area approximately 50 km west of
Rockhampton. Due to the poor spatial distribution of data points during this, or any other, period across
the 1927 to 2006 record, potentiometric surface maps and an interpretation of regional groundwater flow
patterns could not be assessed.
4.2.2
Hydr ochemistr y
G en e ra l Ch ar a ct e r ist i cs
Based on the data compiled for this study, groundwater in this interval has a pH ranging from 6.7 to 8.6,
and TDS ranging from 246 mg/L to 9,831 mg/L (Appendix 3). Generally, the groundwater within this
aquifer can be considered fresh (median TDS = 836 mg/L) and slightly alkaline in character (median pH =
7.9).
+
The dominant cation present within the groundwater of the Lower Permian Sandstones is Na (range = 26
2+
2+
2+
to 2,030 mg/L; median = 49.5 mg/L), with Ca and Mg present in subordinate concentrations and Mg
2+
generally dominating Ca . Bicarbonate is present as the dominant anion in the majority of cases (range =
37 to 923 mg/L; median = 512 mg/L), with Cl becoming more significant in the more saline portions of the
aquifer interval. In some cases, chloride concentrations exceed those of HCO3 . Sulfate also occurs in
significant concentrations (up to 1,400 mg/L; median = 85 mg/L). In terms of total ionic mix, groundwater
+
+
in the Lower Permian Sandstones interval ranges from Na -HCO3 to Na -HCO3 -Cl hydrochemical types.
Similar to the Permian Volcanics, the Lower Permian Sandstones exhibits elevated silica concentrations,
th
with a range from 6 to 60 mg/L, a median of 20 mg/L, and a 90 percentile of 56 mg/L. This again
suggests the presence of more easily weathered silicate minerals in the host rocks.
Co rr el at io n a n d St at i st i c al An a l ysi s
+
Based on Spearman correlation coefficients (Appendix 3), a strong negative relationship is noted for K ,
2+
2+
HCO3 , Ca and Mg . In contrast, positive correlations are indicated for:
-
•
Cl with TDS;
•
Na and SO4 ;
•
TDS with Na ; and
•
SO4 and Ca ,
2-
+
+
2-
2+
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
+
2-
-
Influence from sediments deposited in a marine environment is suspected based on the Na , Cl and SO4
relationships.
Results of PCA analysis are summarised in Table 9. The parameters associated with the first component,
2and the order in which they appear, is unique to the Lower Permian Sandstone in that SO4 represents
2the strongest loading and F is included. Typically, F does not load with SO4 . Further, the second
2+
2+
component has strong loadings for SiO2 , Mg and Ca . This is also an unusual component loading,
2+
2+
possibly suggesting the weathering of Ca and Mg silicates.
Table 9 Principal Component Analysis Results for the Lower Permian Sandstones
Principle Component (% of Lower Permian
total variance explained by
1 (31%)
2 (23%)
rotated components)
SiO2
SO4
Parameters loading onto Na
each PCA component (in TDS
order of descending loading Cl
value)
F
Mg
Ca
The results for k-means cluster analysis are shown in Table 10 and Figure 16. Cluster 2 is interpreted as a
shallow marine depositional environment or coal-forming environment. Nitrate concentrations are typically
higher in association with these marine/coal settings, which may reflect ongoing degradation of organic
matter. Cluster 1 has a unique composition, in that it shows an intermediate evolved signature
22+
(intermediate Na /Cl ratio, intermediate SAR, intermediate (CO3 +HCO3 )/Cl ratio). However, the SO4
/Cl ratio is higher than that observed in any other hydrostratigraphic unit in the study area. Sulfate
2concentrations are higher in Cluster 2 than in Cluster 3, suggesting a localised source of SO4 , possibly
derived from oxidation and weathering of sulfides (i.e. pyrite). The presence of trace pyrite has been
confirmed in sediments analysed from the Back Creek Group (Grigorescu, 2011a). Cluster 3 represents
recently recharged water, and sampling locations with this hydrochemistry appear to be located in
recharge areas along the western section of the aquifer.
Table 10 k-means Cluster Results for Lower Permian Sandstones
Lower Permian Sandstone
Cluster
1
2
3
All
N
Stats
Mean
1σ
Mean
8
1σ
Mean
12
1σ
Mean
31
1σ
11
TDS
1087
500
3944
2831
561
193
1658
2050
301001-01210-CSG Water Chemistry
pH
8.1
0.3
7.6
0.5
7.9
0.4
7.9
0.5
Na/Cl
2.77
1.28
0.94
0.24
2.12
1.68
2.05
1.46
SAR
7
4
11
6
2
1
7
6
(CO3+
HCO3)/Cl SO4/Cl F/Cl
2.4
1.3
0.3
0.3
7.4
9.5
3.8
6.6
0.56
0.38
0.11
0.14
0.18
0.22
0.30
0.33
6.8E-03
4.8E-03
1.0E-03
9.1E-04
1.0E-02
7.5E-03
6.6E-03
6.5E-03
NO3
SiO2
K
(mg/L) (mg/L) (mg/L)
2.0
1.0
5.9
7.4
1.2
0.9
2.6
4.0
15
6
44
18
29
15
27
16
13.4
8.7
5.4
7.4
4.9
3.8
7.6
7.4
Page 35
SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
LEGEND
Cluster - Descriptor
1 – Mixed, somewhat evolved
2 – Shallow Marine/coal
3 – Recent recharge
b15 fault
b70 fault
Northern Bowen Faults
Figure 16 Spatial distribution of the k-means Clusters for the Lower Permian Sandstones
Iso t op es
Isotope data for groundwater from the Lower Permian Sandstone is limited to one sample. However,
14
13
18
2
measurements for C, δ CDIC, and δ OH2O and δ HH2O are available (Appendix 5). The calculated age of
this sample (22,270 years BP), along with the assumed groundwater velocities of 1 to 3 m/a, suggests this
13
bore is located anywhere from 22 to 67 km from the recharge area. The δ CDIC value of -14.7‰ is also
consistent with the expected values obtained from mineralization of vegetation associated with arid
18
2
regions (Clark and Fritz, 1997), and the δ O and δ H values (-3.9‰ and -23.9‰, respectively) are
generally consistent with local meteoric water (established in BRS and NRM, 2003).
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
4.3
Upper Permian Coal
4.3.1
Hydr ogeology and Hydraulics
Reg ion a l F lo w Pat t e r n s
Groundwater level data was available for seventeen bore locations completed within the Upper Permian
Coal intervals. The available data set spans the period between 1965 and 2001, and in almost all cases
no two locations were monitored during the same year. In general, the expectation is that flow patterns will
be influenced by major topographic features at the basin margins, with flow occurring from higher
elevation areas to lower lying regions towards the centre of the basin.
Re si du al H ea d Co n d i t io n s
Residual head mapping could not be carried due to insufficient data.
4.3.2
Hydr ochemistr y
G en e ra l Ch ar a ct e r ist i cs
Based on the data compiled for this study (Appendix 3), groundwater in this interval yields a range of pH
values from 7.2 to 8.5, but is generally considered to be slightly alkaline (median pH = 7.8). The TDS
exhibits a considerable range of 298 to 27,527 mg/L, but is generally considered brackish in character
(median TDS = 4,944 mg/L).
Sodium is the dominant cation present (range = 66 to 8,947 mg/L; median = 1,357 mg/L), followed by the
2+
2+
less dominant Mg and Ca . Bicarbonate exists as the dominant anion, with values ranging from 268 to
2,987 mg/L (median = 957 mg/L); however, significant concentrations of chloride (up to 16,560 mg/L;
+
+
median = 1,475 mg/L) are noted. In general, the groundwater varies from a Na -HCO3 to Na -HCO3—Cl
hydrochemical type.
-
One distinguishing features of this interval is its NO3 concentrations. The range of values associated with
this analyte is from 0.5 to 23.2 mg/L, with a median of 3 mg/L, which is the highest for all the Bowen Basin
formations.
Co rr el at io n a n d St at i st i c al An a l ysi s
There is insufficient data for the Upper Permian Coal to allow for a rigorous statistical analysis beyond the
statistics, shown in Table 11. The results indicate that water from the Permian Coal, is, as expected from
the depositional environment, highly mineralised (mean = 9,487 mg/L). The groundwater chemistry, as
+
described above, differs from that of the shallower units of the Surat Basin–Na /Cl ratios are higher as are
22CO3 +HCO3 /Cl , whereas SAR values are lower and SO4 concentrations are an order of magnitude
lower than the identified ‘marine’ signature. The latter observation may be a result of reducing
groundwater conditions associated with the deeper hydrostratigraphic units.
301001-01210-CSG Water Chemistry
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
Table 11 Summary of Selected Parameters and Ion Ratios for the Upper Permian Coal
Upper Permian Coal
N
All
12
Stats
TDS
Mean
1σ
9487
11097
pH
7.8
0.4
Na/Cl
1.88
1.56
SAR
30
22
(CO3+
HCO3)/Cl SO4/Cl F/Cl
1.7
2.8
0.15
0.39
1.5E-03
2.2E-03
NO3
SiO2
K
(mg/L) (mg/L) (mg/L)
6.3
9.5
21
13
8.7
9.1
Iso t op es
13
The only isotope measurements taken from the Upper Permian Coal intervals were for δ Cof the
13
dissolved methane fraction (Appendix 5). Results indicate a range of δ CCH4 values from -51‰to -38.6‰,
with the majority of values being less than -45‰. Suspected processes leading to the formation of
methane in these coal deposits are methanogenesis (microbial-related) and thermogenesis (heat-related)
(Whiticar, 1999).
4.4
Upper Permian Sandstones
4.4.1
Hydr ogeology and Hydraulics
Reg ion a l F lo w Pat t e r n s
Data coverage for the Upper Permian Sandstones was similarly poor for the 1960 to 1965 period. A
potentiometric surface map was developed using data collected during 2006 (Appendix 7, Figure 1).
Water level data collected during 2006, while limited in quantity, did provide good spatial coverage over a
large area of the Bowen Basin. Water levels were generally highest in the north (~300 mAHD) and south
(330 mAHD) close to the topographically elevated basin margins. Groundwater flow directions converged
on the eastern central basin boundary, approximately 50 km north-east of Blackwater, where minimum
groundwater levels of approximately 98 mAHD were recorded. This topographic and potentiometric low
point corresponds with what are assumed to be a regional discharge area.
Horizontal hydraulic gradients were found to be relatively consistent across the region ranging from 0.001
to 0.002 (between bores 13040284 and 13040286; 13040291 and 3040294A, respectively).
4.4.2
Hydr ochemistr y
G en e ra l Ch ar a ct e r ist i cs
Based on the data compiled for this study (Appendix 6), pH of the groundwater within this interval ranges
from 6.9 to 8.9, but can be considered slightly alkaline based on a median pH of 7.9. The TDS also
exhibits a considerable range from 223 to 33,478 mg/L (median = 1,767 mg/L).
Sodium exists as the dominant cation, exhibiting a range from 31 to 10,001 mg/L (median = 460 mg/L);
2+
2+
while,Ca and Mg contribute less to the mix. Varying dominance of HCO3 and Cl anions is evident,
indicating the presence of a Cl source within the system. Bicarbonate tends to dominate, however,
indicating a median value of 691 mg/L compared to 551 mg/L for Cl . Nevertheless Cl indicates a higher
th
concentration in the 90 percentile (3,492 mg/L) versus HCO3 (1,265 mg/L). In terms of hydrochemical
+
+
facies, the groundwater in this interval varies from Cl -HCO3 to Na -HCO3 and Na -Cl types.
301001-01210-CSG Water Chemistry
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
One distinguishing feature of the Upper Permian Sandstones is its comparatively higher silica
th
concentrations, with a range from 1 to 85 mg/L, a median of 24 mg/L, and a 90 percentile of 51 mg/L.
This suggests the presence of more easily weathered silicate minerals associated with the host
rocks/sediments. Similar to the underlying Upper Permian Coal, NO3 concentrations tend to exhibit
th
comparatively elevated concentrations (range = 0.1 to 56.5 mg/L; median = 1.3 mg/L; 90 percentile =
17.2 mg/L).
Co rr el at io n a n d St at i st i c al An a l ysi s
Spatial coverage within the Upper Permian Sandstones in terms of sampling locations is relatively good
compared to the other units. Consistent with the generally ‘marine-influenced’ depositional environment,
Spearman correlation coefficients (Appendix 3) showed strong positive associations for:
-
•
Cl with TDS; and
•
Cl with Na , Mg , SO4 and Ca .
-
+
2-
2+
2+
Additionally, although there are large standard deviations associated with the measurements, there are
higher NO3 concentrations measured in this interval compared to shallower hydrostratigraphic units of the
Bowen Basin. Strong positive Spearman correlation coefficients were also yielded for:
-
-
•
F with HCO3 ;
•
TDS with Na , Mg
•
Na with SO4 ; and
•
Ca
+
+
2+
2+
2-
and SO4 ;
2-
2+
with Mg .
Principal component analysis indicates that four components are required to adequately explain the total
variance in the dataset. The parameters loading onto the first two components are shown in Table 12.
Similar to the Permian Volcanics, the first component reflects the concentrations of the major ions, and the
second, increased pH values as a result of groundwater evolution.
Table 12
Principal Component Analysis Results for the Upper Permian Sandstones
Principle Component (% of Upper Permian Sandstone
total variance explained by
1 (36%)
2 (16%)
rotated components)
Cl
pH
CO3
Parameters loading onto TDS
each PCA component (in Mg
order of descending loading Ca
value)
SO4
Na
K-means cluster analysis yielded the relationships shown below in Table 13 provides a summary of results
for the k-means cluster analysis, while the spatial distribution of water types is shown in Figure 17.
+
-
Cluster 1 has the signature of groundwater that suggests it has recently been recharged (Na /Cl ratio of
2approximately 1, low (CO3 +HCO3 )/Cl ratio). Cluster 3 has the lowest mean TDS value (714 mg/L), and
301001-01210-CSG Water Chemistry
Page 39
SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
spatially, sampling locations with this geochemistry appear to be either located in the recharge zone in the
western section of the aquifer, or, located near fault features. Cluster 4 has a strong marine signature,
while Cluster 2 seems to fall somewhere in-between.
Table 13 k-means Cluster Results for Upper Permian Sandstones
Upper Permian Sandstone
Cluster
N
1
25
2
69
3
35
4
31
All
160
Stats
Mean
1σ
Mean
1σ
Mean
1σ
Mean
1σ
Mean
1σ
TDS
1711
1371
1950
801
714
317
9493
6545
3149
4375
301001-01210-CSG Water Chemistry
pH
7.6
0.5
7.9
0.3
8.2
0.3
7.7
0.4
7.9
0.4
Na/Cl
0.98
0.31
1.32
0.61
3.98
3.83
0.83
0.22
1.76
2.18
SAR
9
8
10
10
8
7
21
15
13
13
(CO3+
HCO3)/Cl SO4/Cl F/Cl
0.6
0.6
1.1
0.9
6.5
6.6
0.2
0.1
2.0
3.9
0.08 1.4E-03
0.09 1.7E-03
0.10 3.5E-03
0.09 9.6E-03
0.13 1.1E-02
0.15 1.3E-02
0.08 3.0E-04
0.08 2.7E-04
0.10 4.2E-03
0.11 9.7E-03
NO3
SiO2
K
(mg/L) (mg/L) (mg/L)
1.4
1.9
6.0
10.6
6.8
14.9
10.4
14.7
6.3
12.1
21
16
37
19
24
13
25
12
29
17
11.5
8.1
3.4
3.3
3.7
3.7
12.9
11.5
6.5
7.5
Page 40
SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
LEGEND
Cluster - Descriptor
1 – Recent recharge, high TDS
2 – Mixed, Intermediate TDS
3 – Evolved, recharge
4 – Shallow Marine/coal
b15 fault
b70 fault
Northern Bowen Faults
Figure 17 Spatial Distribution of the k-means Clusters for the Upper Permian Sandstones
Iso t op es
Isotope results are available for four samples in the Upper Permian Sandstones. Data for one of the sites
14
13
18
2
13
exists for C, δ CDIC, and δ OH2O and δ HH2O, while the remaining four only have results for δ CDIC
(Appendix 5). The one groundwater age obtained (34,760 years BP) indicates a relatively mature
groundwater at the location of sampling, which assuming a flow velocity range of 1 to 3 m/a, would be
13
between 35 to 104 km from the recharge area. The range of ‰ CDIC values (-16.0 to -8.5‰) indicates
contributions from more than one carbon source (i.e. vegetation and/or carbonate minerals), but the
18
majority of values suggest mineralisation of vegetation as the source (Clark and Fritz, 1997). The δ OH2O
2
and δ HH2O values measured also indicate a meteoric water source given their close association with the
Local Meteoric Water Line (established in BRS and NRW, 2003).
301001-01210-CSG Water Chemistry
Page 41
SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
4.5
Rew an Group
4.5.1
Hydr ogeology and Hydraulics
Reg ion a l F lo w Pat t e r n s
Only three groundwater level measurements were available for the Rewan Group during the 1960 and
1965 period. A slightly more extensive data set comprising nine locations was available for the 2006
period. These locations were focussed in the area west of Moura and Theodore (Appendix 7, Figure 2).
The limited assessment of Rewan Group potentiometry indicates a north to north-westerly groundwater
flow direction away from the southern basin margin generally following the surface topography.
Groundwater levels ranged from 279 mAHD (bore 13050014) to 98 mAHD (bore 13030831). Hydraulic
gradients were estimated to be in the order of 0.0025; however, given the relative position in the
topographically elevated basin margin, this estimate could be at the upper end of the range.
4.5.2
Hydr ochemistr y
G en e ra l Ch ar a ct e r ist i cs
Based on the data compiled for this study, groundwater sampled from this interval indicates a pH in the
range of 7.1 to 8.6, and a TDS ranging from 144 mg/L to 20,182 mg/L (Appendix 3). Generally, the
groundwater can be considered brackish (median TDS = 1,490 mg/L), with a slightly alkaline character
(median pH = 7.8).
+
The dominant cation present within the groundwater is Na (range = 42 to 4,050 mg/L; median = 557
2+
2+
mg/L), with subordinate contributions from Ca and Mg . Chloride exists as the dominant anion with
values reaching as high as 12,000 mg/L (median = 592 mg/L). Bicarbonate is also present in significant
concentrations, but is generally subordinate to Cl . Based on the total ionic mix, groundwater in this
+
+
interval may be described as a Na -Cl to Na -Cl -HCO3 hydrochemical facies type.
Co rr el at io n a n d St at i st i c al An a l ysi s
Based on Spearman correlation factors (Appendix 3), strong associations were noted for:
-
•
Cl with TDS;
•
Cl with Na , Mg
•
TDS with Na , Mg
•
SO4 with Mg ; and
•
CO3 with NO3 .
-
+
2+
+
2-
2+
2-
-
2+
and Ca
2+
2+
and Ca ,
Additionally, strong positive correlations were noted amongst the various major cations, with the exception
+
of K .
-
+
2+
2+
2-
Total dissolved solids, Cl , Na , Ca , Mg and SO4 loaded strongly onto the first PCA factor which
2accounted for 40% of the total variance. In turn, CO3 and pH loaded onto the second component
301001-01210-CSG Water Chemistry
Page 42
SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
(accounting for 20% of the total variance; Table 14). These relationships are similar to those noted for the
Permian Volcanics and the Upper Permian Sandstone.
Table 14 Principal Component Analysis Results for the Rewan Group
Principle Component (% of Rewan Group
total variance explained by
1 (40%)
2 (20%)
rotated components)
CO
TDS
3
Parameters loading onto Cl
each PCA component (in Na
order of descending loading Ca
value)
Mg
SO4
pH
HCO3
K-means cluster analysis yielded the relationships shown below in Table 15. Spatial coverage of the
various clusters is shown in Figure 18. The mean TDS values for clusters 1 and 2 were relatively high
(7,229 mg/L and 6,890 mg/L, respectively). These high TDS groundwaters likely reflect pore water in
association with finer-grained sediments (e.g., mudstones) that characterise this hydrostratigraphic unit.
Sulphate concentrations in the high TDS-clusters of the Rewan Group are lower, while bicarbonate
concentrations appear higher than those observed with groundwater of similarly high TDS content of the
upper Surat Basin units. Such a pattern is suggestive of microbial sulfur-reducing conditions in the
presence of easily mineralised organic matter.
Cluster 3 of the Rewan Group has a low TDS value and a hydrochemistry that is very similar to that
observed in the Clematis Sandstone (Table 17), suggesting a similar groundwater origin. Cluster 4 of the
Rewan Group has a chemistry that could be derived by mixing clusters 1 and 2 with groundwater of
cluster 3. Although there is not very much data available for this unit, the spatial distribution suggests that
high TDS water is encountered across the Basin, whereas the low TDS water originates at the southern
basin margin. Relative depths of the various bores may play a role, as well as proximity to recharge areas.
Table 15 k-means Cluster Results for Rewan Group
Rewan Group
Cluster
N
1
16
2
12
3
19
4
16
All
63
Stats
Mean
1σ
Mean
1σ
Mean
1σ
Mean
1σ
Mean
1σ
TDS
7229
4896
6890
3770
358
118
1246
598
3571
4311
301001-01210-CSG Water Chemistry
pH
7.6
0.3
7.6
0.4
7.8
0.3
8.1
0.3
7.8
0.4
Na/Cl
0.77
0.15
0.86
0.16
2.14
1.52
1.78
0.85
1.45
1.10
SAR
17
5
25
10
4
2
11
8
15
12
(CO3+
HCO3)/Cl SO4/Cl F/Cl
0.1
0.1
0.1
0.0
3.0
2.1
1.9
1.8
1.4
1.9
0.02
0.01
0.00
0.00
0.06
0.05
0.06
0.06
0.04
0.05
1.1E-04
8.4E-05
4.3E-04
6.4E-04
8.5E-03
1.5E-02
4.5E-03
4.8E-03
3.5E-03
8.5E-03
NO3
SiO2
K
(mg/L) (mg/L) (mg/L)
0.9
1.0
7.1
9.6
1.2
1.8
0.8
0.5
2.2
4.9
26
5
10
3
12
2
24
14
17
10
11.5
7.2
10.2
5.6
12.3
4.5
3.4
1.4
9.4
6.1
Page 43
SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
LEGEND
Cluster - Descriptor
1 – High salinity, shale, evolved
2 – High salinity, shale
3 – Low TDS, intermediate evolved
4 – Mixed
b15 fault
b70 fault
Northern Bowen Faults
Figure 18 Spatial Distribution of the k-means Clusters for the Rewan Group
4.6
Clematis Sandstone
4.6.1
Hydr ogeology and Hydraulics
Reg ion a l F lo w Pat t e r n s
Groundwater level data was available for twenty-two monitoring locations completed in the Clematis
Sandstone, with between one and five measurements available at each location. Data was collected
periodically between the years of 1941 and 1992, but does not provide sufficient spatial coverage to
permit any meaningful assessment of regional groundwater flow directions or temporal trends.
Re si du al H ea d Co n d i t io n s
Modelled residual head conditions between the Permian coal measures and the Clematis Sandstone are
shown in Figure 19. Areas shown in red indicate neutral residual head conditions (possible hydraulic
301001-01210-CSG Water Chemistry
Page 44
SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
interaction), while orange to blue areas indicate differences between the hydraulic heads of the two
formations, suggesting the presence of an intervening barrier. Although neutral residual head conditions
predominate, there are some significant head differences in the northern part of the area shown,
suggesting the local presence of a hydraulic seal between these two units, most likely caused by the
Rewan Group Aquitard.
For more information on the generation of the residual head maps discussed in this report, refer to Section
2.2.3.
Figure 19 Modelled residual head condition for Clematis and Permian Coal Measures
4.6.2
Hydr ochemistr y
G en e ra l Ch ar a ct e r ist i cs
Based on the data compiled for this study, pH of groundwater in this interval varies from 6.1 to 9.0, but is
generally considered slightly alkaline based on a median value of 7.9. Groundwater TDS values range
from 81 mg/L to 1,193 mg/L, but the pore water is generally considered fresh given the relatively low
median value of 387 mg/L (Appendix 3).
+
The dominant cation present within the groundwater is Na , with a range of 12 to 402 mg/L (median = 85
2+
2+
+
mg/L). Both Ca and Mg are subordinate to Na , with the influence of each being about equal to the
other. The dominant anion is HCO3 , with values ranging from 23 to 1,112 mg/L (median = 343 mg/L). The
301001-01210-CSG Water Chemistry
Page 45
SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
-
next most dominant anion is Cl , with 90% of the values indicating less than 110 mg/L. In terms of total
+
+
ionic mix, the groundwater in this interval may be described as a Na -HCO3 to Na -HCO3 -Cl
hydrochemical type.
+
Compared to the other Bowen Basin formations, K stands out as a characteristic element, with a range of
th
1.6 to 32 mg/L, a median of 13 mg/L and a 90 percentile of 21 mg/L.
Co rr el at io n a n d St at i st i c al An a l ysi s
Results for the Clematis Sandstone were somewhat unique amongst the hydrostratigraphic units of the
Bowen and Surat basins in that none of the parameters indicated a strong Spearman correlation
coefficient with Cl . Supporting this result were the hierarchical cluster analysis (Appendix 3) and PCA
analysis, which indicated that Cl loaded onto the fourth component (accounting for the least amount of the
total variance). It also indicated some relationship between Cl and NO3 (Appendix 3). Strong positive
Spearman correlation factors were noted for:
-
2-
+
2+
•
TDS with HCO3 , CO3 , Na and Mg ;
•
SO4 with Ca
•
Ca with Mg .
2-
2+
2+
2+
and Mg ; and
2+
Principal Component Analysis results indicated that four components were required to adequately
describe the variance in the dataset (Appendix 3); the first three are summarised below in Table 16.
Parameters loading on the first rotated component (accounting for approximately 27% of the total
22+
2+
variance) included SO4 , HCO3 , Mg , Ca and TDS, while the second component (accounting for
2+
approximately 22% of the total variance) included Na , CO3 and pH. The third component identified
+
positive loadings of F and SiO2 and negative loadings for K . Together, the three components are
interpreted in the following way: dominant controls on water chemistry in the Clematis Sandstone, in the
south of the Basin where the limited data is available, appear to be driven by sulfate forming conditions,
and carbonate and clay mineral weathering reactions.
Table 16 Principal Component Analysis Results for the Clematis Sandstone
Principle Component (% of Clematis Sandstone
total variance explained by
1 (27%) 2 (22%) 3 (15%)
rotated components)
CO3
F
Ca
Na
(-) K
Parameters loading onto SO4
each PCA component (in Mg
SiO2
pH
order of descending loading HCO
3
value)
TDS
K-cluster analysis yielded the relationships summarized in Table 17, and shown spatially in Figure 20.
Groundwater associated with each of the clusters yielded relatively low TDS. The lowest mean pH value
across both the Bowen and Surat basins was yielded by Cluster 1. Although groundwater associated with
each cluster does vary in chemical quality, they all appear to represent freshly-recharged water that has
undergone very little hydrochemical evolution. The chemistry of Cluster 3 is consistent with mixing of
301001-01210-CSG Water Chemistry
Page 46
SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
groundwaters associated with Cluster 1 and 2, and evolution of the mixed water through ion exchange and
weathering reactions (higher SAR, higher F /Cl ratio). Spatial results and distribution of groundwater
temperatures support this interpretation. Clusters 1 and 3 occur at the southernmost margins of the basin,
with low associated groundwater temperatures (20ºC to 30ºC), whilst groundwater of the more evolved
type, represented by Cluster 3, is associated with higher groundwater temperatures and typically extends
further to the northeast.
The groundwater chemistry of the Clematis Sandstone appears to be unique amongst the
hydrostratigraphic units of the Bowen Basin, with the exception of the groundwater represented by Cluster
3 of the Rewan Group. Groundwater chemistries for Cluster 3 of the Rewan Group and the Clematis
Sandstone are very similar, which suggests a similar origin. Similar low-TDS groundwaters were also
identified for the Precipice and Evergreen Sandstone units, which suggests (Habermehl, 1980) that there
may be some connection between the upper units of the Bowen Basin and the overlying and adjoining
aquifers of the Surat Basin.
Table 17 k-means Cluster Results for Clematis Sandstone
Clematis
Cluster
1
2
3
All
N
Stats
Mean
1σ
Mean
50
1σ
Mean
96
1σ
Mean
266
1σ
120
TDS
495
105
158
55
361
135
383
165
301001-01210-CSG Water Chemistry
pH
8.0
0.4
7.1
0.5
7.9
0.3
7.8
0.5
Na/Cl
2.81
1.34
1.14
0.53
2.73
1.70
2.46
1.52
(CO3+
HCO3)/Cl SO4/Cl F/Cl
SAR
3
1
2
1
6
5
5
5
6.4
4.2
1.5
0.8
3.1
2.1
4.3
3.7
0.11
0.06
0.04
0.02
0.02
0.02
0.06
0.06
9.0E-03
1.0E-02
7.6E-03
5.7E-03
7.9E-03
1.0E-02
8.4E-03
9.4E-03
NO3
SiO2 K
(mg/L) (mg/L) (mg/L)
0.9
1.2
0.5
0.3
0.8
0.7
0.8
0.9
14
4
15
2
17
6
15
5
11.8
3.9
18.4
5.7
12.8
6.2
13.5
5.8
Page 47
SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
LEGEND
Cluster - Descriptor
1 – Low TDS, evolved
2 – Recent recharge
3 – Mixed, evolved
b15 fault
b70 fault
Northern Bowen Faults
Figure 20 Spatial Distribution of the k-means Clusters for the Clematis Sandstone
(brown and purple lines indicate the location of faults)
301001-01210-CSG Water Chemistry
Page 48
SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
5
RESULTS – SURAT BASIN
5.1
5.1.1
Precipice Sandstone
Hydr ogeology and Hydraulics
Reg ion a l F lo w Pat t e r n s
The spatial distribution of data points for the Precipice Sandstone is limited to an area east of Injune, with
a few additional points located close to the towns of Wandoan and Taroom. A total of twelve
measurements were included to develop the groundwater contour map for the 1960 and 1965 period
(Appendix 7, Figure 3). Groundwater elevations ranged from a maximum of 430 mAHD (016157) in the
north close to Injune to approximately 200 mAHD (bore 015888) close to Wandoan in the southeast.
Regional groundwater flow is interpreted to be predominantly towards the east.
This easterly flow supports the findings of previous studies which identified a component of regional
groundwater flow, and ultimately discharge, to the Clarence-Moreton Basin in the east as well as the
Bowen Basin in the north (Hodgkinson et al., 2010).
Hydraulic gradients are greatest in the topographically elevated areas east of Injune (i.e., 0,003).
Gradients then decrease on the south-eastern plains to approximately 0.001. The distribution of data may
also have an effect on the observed gradients with the majority of data points being located in the northern
high-gradient areas.
Re si du al H ea d Co n d i t io n s
Comparison of hydraulic head differences between the Precipice Sandstone and Hutton Sandstone
aquifers provides an indication of the effectiveness of the intervening Evergreen aquitard as a hydraulic
barrier between the two aquifer formations. Potential hydraulic connections between the Precipice
Sandstone and Hutton Sandstone were the subject of a previous study conducted by the Geological
Survey of Queensland (Hodgkinson et al., 2010). Observed differences in hydraulic heads across these
formations were interpreted as evidence of the integrity of the Evergreen Formation as an effective
hydraulic seal for the Precipice Sandstone. Hodgkinson et al (2010) also concluded that the Evergreen
Formation acted as a regionally-extensive seal for the Precipice Formation aquifer, except in the central
region of the basin, where hydraulic connection is assumed based on comparable formation pressures.
Figure 21 shows the modelled residual head conditions between the Precipice and Hutton Sandstones.
Areas in red indicate neutral residual head conditions, which are an indication of potential hydraulic
interaction, while orange to blue areas indicate differences between the hydraulic heads of the two
formations, suggesting the presence of an intervening barrier. While head differences between the two
formations are neutral across much of the area shown, there are also large areas where head differences
approach or exceed 20 m, supporting the suggestion that the Evergreen Formation aquitard acts as an
effective hydraulic barrier in these areas.
For more information on the generation of the residual head maps discussed in this report, refer to Section
2.2.3.
301001-01210-CSG Water Chemistry
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
Figure 21 Modelled residual head conditions for the Precipice and Hutton Sandstones
5.1.2
Hydr ochemistr y
G en e ra l Ch ar a ct e r ist i cs
A summary of water quality results for the Precipice Sandstone is provided below, while additional
information can be found in Appendix 6 and Grigorescu (2011b). Based on the data compiled for this
project, the groundwater in this interval exhibits a pH ranging from 6.0 to 8.9, and TDS values ranging
from 51 mg/L to 5,025 mg/L. The groundwater can generally be considered fresh (median TDS = 150
mg/L) with a circum-neutral pH (median pH of 7.4). Sodium exists as the dominant cation, with values
ranging from 11.3 to 1,238 mg/L (median = 46 mg/L). In turn, HCO3 is the dominant anion, ranging from 6
+
to 2,391 mg/L (median = 148 mg/L). As such, the groundwater in this interval may be described as Na HCO3 hydrochemical type, with some indications of Cl influence in certain locations (range = 5 to 2,190
mg/L; median = 22 mg/L).
Co rr el at io n a n d St at i st i c al An a l ysi s
Based on Spearman correlation coefficients (Appendix 4), strong associations were noted for:
-
2-
2+
2-
+
•
Cl with Ca , TDS, SO4 , CO3 and Na ;
•
F with HCO3 and Na ;
•
TDS with Ca , HCO3 , Na , and CO3 ;
-
-
+
2+
-
301001-01210-CSG Water Chemistry
+
2-
Page 50
SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
2-
2+
2-
•
SO4 with Ca
•
HCO3 with Ca ; and
•
Ca
-
2+
and CO3 ;
2+
+
+
with Na and K .
PCA results for the Precipice Sandstone pore waters indicated that three components were required to
adequately describe the dataset variance. The first component accounted for approximately 42% of the
2+
2+
total variance and Cl , TDS, Ca , and Mg identified as major contributors - consistent with the correlation
+
coefficient results. The second component had strong factor loadings for TDS, HCO3 , SiO2 , Na and F
and accounted for 35% of the overall variance (Table 18).
Table 18 Principal Component Analysis Results for the Precipice Sandstone
Principle Component (%
Total Variance Explained)
Precipice Sandstone
1 (42%)
2 (35%)
K
F
Mg
Parameters
Ca
SO4
Cl
NO3
TDS
Na
HCO3
SiO2
TDS
Components 1 and 2 of the PCA analysis both accounted for a large amount of the total variance in the
data. Groundwater chemistry of the Precipice Sandstone reflects recharge at the peripheries of the basin
followed by a hydrochemical evolution down-gradient towards the basin centre. This is exhibited by
increasing SAR values. In turn, the increases in F concentrations are considered to reflect low flow
conditions in the central portion of the aquifer (Radke et al., 2000), and an indication of weathering of
basement rocks.
Results for the k-means cluster analysis, shown in Table 19 and Figure 22 support findings of the
Spearman correlation and PCA analysis. Clusters 1 and 2 are associated spatially with the outcropping
areas of the Precipice Sandstone and recharge zones; however, their hydrochemistries do differ,
suggesting different processes. The hydrochemistry of groundwater associated with Cluster 2 appears to
be more “evolved” with respect to groundwater associated with Cluster 1 (based the higher ion ratio and
SAR values); however, the pH value of these two groundwater types is very similar. Groundwater pH
generally increases with further movement along a flow path.
-
The data for Cluster 3 represents a water type that is more mineralized and evolved, with high (HCO3
2+CO3 )/Cl ratios and very high mean SAR values overall. Further, the mean F /Cl ratio noted for this
water type is higher than the F /Cl ratios noted in other aquifers of the Surat Basin. Overall, the results of
the K-means cluster analysis, correlation coefficients and PCA analysis suggest that the groundwater in
the Precipice Sandstone is different from that in the overlying aquifers. However, there are only 16 data
points included in Cluster 3, and one, located along the north-eastern out crop yields relatively high NO3
2and SO4 values. There is insufficient data to examine this occurrence further, but it is noted as an
important data gap.
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
Table 19 k-means Cluster Results for the Precipice Sandstone
Precipice
Cluster
N
1
33
2
64
3
16
All
113
Statistics
Mean
1σ
Mean
1σ
Mean
1σ
Mean
1σ
TDS
301
171
123
32
1,824
1,290
418
760
pH Na/Cl SAR
7.4
1.8
2
0.6
1.5
2
7.3
6.2
5
0.5
3.0
2
8.1
4.4
41
0.7
5.4
47
7.5
4.6
13
0.6
3.7
28
(CO3+
HCO3)/
Cl
SO4/Cl
3.7 0.121
3.8 0.086
7.3 0.077
4.2 0.059
4.1 0.082
6.1 0.129
5.8 0.091
4.7 0.082
NO3
SiO2 K
F/Cl
(mg/L) (mg/L) (mg/L)
7.5E-03
0.8
12
6.2
6.9E-03
0.6
1
1.9
4.3E-02
0.6
13
2.1
2.7E-02
0.6
2
0.5
5.3E-02
2.7
24
6.4
8.9E-02
3.4
10
7.9
3.4E-02
0.8
15
3.7
4.3E-02
1.0
6
3.6
Iso t op es
Groundwater samples from three bores completed in the Precipice Sandstone were analysed for various
13
stable and radiogenic isotopes (Appendix 5). All samples were analysed for δ CDIC and two were
14
18
2
14
analysed for C, δ OH2O and δ HH2O. The two samples analysed for C yielded groundwater ages of
30,160 years BP and 5,215 years BP indicating relative distances from recharge areas of 30 to 90 km and
5 to 16 km, respectively (assuming a groundwater flow velocity of 1 to 3 m/a). These same two samples
13
also yielded δ CDIC values ranging from -17.4‰ to -15.8‰ (median = -17.0‰) consistent with a local
18
2
vegetation source (Clark and Fritz, 1997). δ O values of -5.9‰ and -6.8‰ and δ H values of -37.0‰ and
-45.9‰ were also noted, more or less consistent with local meteoric water (established in BRS and NRW,
2003).
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
LEGEND
Cluster - Descriptor
1 – Low TDS, evolved
2 – Low TDS, recent recharge
3 – Intermediate TDS, highly evolved
Precipice Sandstone Outcrop
b15 fault
b70 fault
base of Precipice faults
Figure 22 Spatial Distribution of the k-means Clusters for the Precipice Sandstone
(brown, blue, green and purple lines indicate the location of faults)
5.2
Evergreen Formation
5.2.1
Hydr ogeology and Hydraulics
Reg ion a l F lo w Pat t e r n s
Despite there being groundwater level measurements available for approximately 120 bore locations,
which screen the Evergreen Formation aquitard, there are generally no more than five to ten locations
represented during any one to two year period across the monitoring record. As well, the majority of
locations contain only one groundwater level measurement recorded at the time of drilling, and drilling
301001-01210-CSG Water Chemistry
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
activities span the years between 1920 and early 2002. This is a common occurrence for many of the
formations in the Surat Basin.
Regional flow patterns in the Evergreen Formation have been assessed by Hodgkinson et.al, (2010).
Similar regional flow patterns have been identified in the underlying Precipice Sandstone aquifer
(Section 5.1.1) and the overlying Hutton Sandstone aquifer (Section 5.3.1). Lateral groundwater flow is
generally towards the east and north within this interval.
5.2.2
Hydr ochemistr y
G en e ra l Ch ar a ct e r ist i cs
A summary of water quality results for the Evergreen Formation can be found in Appendix 6, while
additional information is provided by Grigorescu (2011b). Based on the data compiled for this study,
groundwater within this interval has a pH ranging from 6.1 to 9.0 and a TDS content ranging from 82 mg/L
to 12,107 mg/L. Generally speaking, the groundwater can be considered slightly alkaline (median pH
greater than 7.6) and fresh (median greater than 250 mg/L).
2+
2+
Sodium exists as the dominant cation, with Ca and Mg subordinate in presence, particularly in the lower
permeability intervals. Bicarbonate exists as the dominant anion, with Cl as the next abundant anion,
again particularly in the lower permeability intervals. In terms of total ionic mix, the groundwater may be
+
described as a Na -HCO3 hydrochemical type.
One distinguishing feature of this interval is its silica concentrations, which range from 5 to 87 mg/L, and
th
+
have a median of up to 15 mg/L and 90 percentile close to 50 mg/L. Similarly, K stands out showing the
th
greatest range (0.4 to 99.7 mg/L) and 90 percentile of 21 mg/L) of all the Jurassic formations.
Co rr el at io n a n d St at i st i c al An a l ysi s
Basic and multivariate statistics were applied to hydrochemical data associated with the Evergreen
Formation (Appendix 4). Strong positive correlations (using the Spearman rank correlation method) were
noted for:
+
•
TDS with Na ;
•
TDS with Na , SO4 , NO3 and K ;
•
Cl with TDS and Na ;
•
Cl and SO4;
•
SO4 with Ca
•
SiO2 with Mg ; and.
•
F and HCO3 .
2-
+
-
-
+
+
-
-
2-
2+
-
2+
+
and K ;
-
-
2-
Uniquely, the positive correlation between Cl and SO4 is not observed in any of the other
hydrostratigraphic units of the Surat Basin, except for the Rolling Downs Group and shallower Tertiary
sediments.
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
PCA results indicated that three and four components, respectively, were required to adequately describe
the variance in the datasets for both the aquifer and aquitard intervals. For the Evergreen Formation, the
first component accounted for 44% of the total variance and, and species showing strong loadings on that
+
+
component included Cl , TDS, HCO3 , NO3 , Na and K . Component 2 accounted for 29% of the total
2variance, with species showing strong positive loadings were F , CO3 and pH, and strong negative
22+
loadings were SO4 and Ca (Table 20). These results are interpreted below in the discussion of the kmeans cluster analysis. The first and second components for the Evergreen Sandstone accounted for
35% and 19% of the total variance, and indicated strong loadings on each with respect to Cl , TDS, and
+
2+
2+
+
Na , and Ca , Mg and K , respectively (Table 20).
Table 20 Principal Component Analysis Results for the Evergreen Aquitard and Sandstone
Principle Component (%
Total Variance Explained)
Parameters
Evergreen AQT
Evergreen SS
1 (44%)
2 (29%)
1 (35%)
2 (19%)
Cl
pH
Cl
Mg
TDS
F
Na
Ca
Na
(-) Ca
(-) SO4
TDS
NO3
K
K
NO3
CO3
2+
HCO3
SO4
HCO3
CO3
2-
The negative relationships of Ca and SO4 to the other parameters loading onto factor two point to the
occurrence of discrepant water types in the Evergreen Aquitard sediments. In the context of the k-cluster
results discussed below, these discrepant water types are best described by differences in SAR values,
and with differences in pH, F and SiO2 concentrations. PCA results suggest more subtle differences in
the groundwater types within this confining interval, and potentially greater mixing with the Precipice
aquifer along the basin margins.
Available data for the Evergreen Formation is clustered around the northern margins of the Surat Basin,
where the formation is exposed at surface (Figure 23). Groundwater chemistry appears consistent with a
marine influence; however, the chemical signature in the Evergreen Formation is different from that
observed in the overlying formations, which is typified by the chemistry of Cluster 2 of the Rolling Downs
Group (Table 35). The differences in chemistry between these two “marine” signatures may reflect their
respective depositional environments and relative ages. In comparison to the “marine” signature of Rolling
2Downs Aquifer, the high TDS values in the Evergreen Formation, higher (CO3 +HCO3 )/Cl ratios, lower
2SO4 /Cl ratios and NO3 concentrations are similar to the Rewan Group pore waters. The strong
relationship between Cl and HCO3 in the correlation analysis and PCA results is explained by this
different water chemistry, and also by the fact that even in the low TDS waters of the Evergreen Formation
2the (CO3 +HCO3 )/Cl ratio is typically quite high.
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
Table 21 k-means Cluster Results for Evergreen Formations
N Statistics
Cluster
Evergreen Aquitard
Mean
1
11
1σ
Mean
2
15
1σ
Mean
3
17
1σ
Mean
All
43
1σ
Evergreen SS
1
2
3
4
All
13
Mean
1σ
Mean
22
1σ
Mean
8
1σ
Mean
39
1σ
Mean
69
1σ
301001-01210-CSG Water Chemistry
TDS
4,265
1,988
278
228
888
630
1,842
2,237
pH
7.6
0.4
7.8
0.7
8.3
0.2
7.8
0.6
(CO3+
HCO3)/
Na/Cl SAR Cl
SO4/Cl F/Cl
1.1
0.7
1.3
0.9
3.3
2.8
1.6
1.7
22
18
3
5
32
28
19
25
0.4
0.8
2.0
1.6
2.9
3.4
1.7
2.1
0.014
0.017
0.113
0.091
0.017
0.014
0.060
0.078
1.7E-02
5.2E-02
9.0E-03
7.9E-03
1.6E-02
1.6E-02
1.4E-02
3.3E-02
3,632 7.8
1.1
13
0.8
0.100 1.5E-03
4,063
483
172
507
225
169
48
741
1,805
0.4
1.1
0.6
3.5
4.0
1.3
0.5
1.4
1.5
11
2
2
18
5
1
1
6
10
1.0
1.5
1.1
3.4
5.7
3.0
2.2
6.2
9.7
0.138
0.153
0.132
0.043
0.039
0.219
0.249
0.166
0.200
0.3
7.7
0.4
8.3
0.6
7.3
0.7
7.6
0.6
1.7E-03
3.6E-03
3.1E-03
8.0E-03
1.5E-02
1.1E-02
1.2E-02
7.2E-03
9.9E-03
NO3
SiO2 K
(mg/L) (mg/L) (mg/L)
3.6
1.9
0.5
0.3
1.1
0.8
1.7
1.7
38
24
12
4
29
33
24
23
29.1
35.3
4.5
2.2
1.6
1.4
10.7
21.4
9.2
25
8.0
13.3
2.3
4.3
0.8
0.4
0.4
0.2
2.1
5.7
30
13
3
12
7
12
2
14
10
7.9
11.1
3.7
1.5
1.3
6.7
2.9
7.4
4.4
Page 56
SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
LEGEND
Cluster - Descriptor
1 – Marine
2 – Recharge, intermediate evolved
3 – Intermediate TDS, evolved
Evergreen Aquitard Outcrop
b15 fault
b70 fault
Figure 23 Spatial Distribution of the k-means Clusters for the Evergreen Formation aquitard
In both the finer-grained and coarser-grained intervals of the Evergreen Formation, sampling locations
showing the (evolved) marine signature occur more frequently to the east of the generally east-west
trending outcrop area (Clusters 1 for both the Aquitard and Sandstone units; Figure 23 and Figure 24. On
the western side, low-TDS groundwater occurs (Cluster 3 for the Aquitard and Cluster 4 for the
Sandstone; Table 21) along the outcrop that has a chemistry very similar to that observed in the
underlying Precipice Sandstone. In the down-gradient direction from these two very different water
chemistries (i.e. the marine and the low-TDS recharge), there is a mixed groundwater type that occurs in
association with wells located close to the Evergreen outcrop (Cluster 2 for the Sandstone) and more
evolved groundwater down-gradient (Clusters 3 for both hydrostratigraphic units). There is insufficient data
to comment on evolution of groundwater away from the basin margins in either unit due to the lack of
bores.
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
LEGEND
Cluster - Descriptor
1 – Marine, intermediate TDS
2 –Recharge, intermediate evolved
3 – Low TDS, evolved
4 – Low TDS, recent recharge
Evergreen Sandstone Outcrop
b15 fault
b70 fault
Figure 24 Spatial Distribution of the k-means Clusters for the Evergreen Formation sandstones
Iso t op es
13
Information from four bores was available for this study. All samples were analysed for δ CDIC with one
14
18
2
sample being analysed for C, δ OH2O and δ HH2O (Appendix 5). A groundwater age of 980 years BP was
noted for the one sample analysed, indicating a relatively young age and likely close proximity to the
recharge area (approximately 1 to 3 km assuming a groundwater flow velocity of between 1 and 3 m/a).
13
δ CDIC values ranged from -16.9‰ to -8.1‰, with a median value of -12.7‰. Considering this somewhat
isotopically-heavier median value, sources and processes other than just mineralisation of local vegetation
18
are suspected (e.g., CO2 reduction and methane formation). The two samples analysed for δ OH2O (2
4.57‰ and -4.45‰)and δ HH2O (-28.8‰ and -30.35‰) yielded values close to that expected for local
meteoric water (established in BRS and NRW, 2003).
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
5.3
Hutton Sandstone
5.3.1
Hydr ogeology and Hydraulics
Reg ion a l F lo w Pat t e r n s
A total of forty-nine measurements were available to develop the potentiometric surface map for the 1960
to 1965 period (Appendix 7, Figure 4). Water level data was concentrated in the northern region between
Injune and Wandoan, with additional data points located to the south-east between Wandoan and
Jandowae. Towards the south of the basin, a cluster of bores are located around Inglewood. Due to
limited spatial distribution of these bores, groundwater flow direction could not be interpreted in this area
of the basin.
Groundwater elevations ranged from a maximum of 460 mAHD (bore 016679) close to Injune, to a
minimum of 200 mAHD near Taroom (Appendix 7, Figure 4). From Injune, groundwater flow is interpreted
to occur from the northern areas and move towards the east and south-east, consistent with movement
from higher topographic areas towards lower lying areas close to Taroom. A northerly groundwater flow
direction is also observed between Wandoan and Taroom. A potential recharge zone is located north of
Jandowae, with contours showing localised radial flow.
Horizontal hydraulic gradients generally follow the topographic gradients, decreasing from a maximum of
-4
approximately 0.002north of Injune to a minimum of 9 x 10 on the lower plains to the east (Appendix 7,
Figure 4).
Re si du al H ea d Co n d i t io n s
Modelled residual head conditions between the between the Hutton Sandstone and Walloon Subgroup are
shown in Figure 25. Neutral head differences (red areas) predominate, suggesting the potential for
hydraulic connection between the two formations across most of the area shown. However, there are
small areas where there are significant head differences (orange to blue areas), suggesting localised
areas of hydraulic isolation between these two units.
For more information on the generation of the residual head maps discussed in this report, refer to Section
2.2.3.
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
Figure 25 Modelled residual head condition for Hutton Sandstone and Walloon Subgroup
5.3.2
Hydr ochemistr y
G en e ra l Ch ar a ct e r ist i cs
A summary of water quality results for the Hutton Sandstone can be found in Appendix 6, while additional
information is provided by Grigorescu (2011b). Based on the data compiled for this study, groundwater in
this interval exhibits a range in pH values from 6.5 to 9.0, and TDS content from 119 mg/L to 12,666 mg/L.
Generally speaking, the groundwater can be considered fresh (median TDS = 752 mg/L) and slightly
alkaline in character (median pH = 8.1).
+
The dominant cation is Na , ranging from 16 to 5,320 mg/L (median = 321 mg/). The dominant anion
varies from HCO3 to Cl . The median concentration for HCO3 (426 mg/L) is approximately double that of
th
Cl (222 mg/L); however, there is a large majority of higher Cl . values noted based on respective 90
percentile values ( Cl = 2,812 mg/L; HCO3 = 848 mg/L). Given the distribution of major ions, the
+
+
groundwater may be described as a Na -HCO3 to Na -Cl -HCO3 hydrochemical type.
One distinguishing feature of this interval is its silica concentrations, which exhibit a range of 3 to 88 mg/L,
th
a median of 16 mg/L and 90 percentile of 40 mg/L.
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
Co rr el at io n a n d St at i st i c al An a l ysi s
With respect to Spearman correlation coefficients (Appendix 4), strong positive associations were noted
for:
-
+
•
Cl with TDS and Na ;
•
CO3 with pH;
•
Ca
•
F with HCO3 .
2-
2+
2+
with Mg ; and
-
-
Principal Component Analysis results (Table 22) indicate that four components were required to
adequately describe the variance in the dataset; however, similar to the Walloons Subgroup (discussed
later), none of the individual components were very dominant (components account for 27%, 21%, 20%
+
and 12% of the total variance, respectively). Loadings were positive and strong for Cl , TDS, Na and
+
2+
2+
negative for SiO2 on component 1;positive for K , Mg and Ca on component 2; and, positive for HCO3 ,
CO3 , F and pH on component 3 (Table 22). Not shown in Table 22 (but indicated in Appendix 2) is that
2SO4 and NO3 yielded strong positive loadings on component 4, whereas F loaded negatively on that
component. Again, as for the Evergreen Sandstone, the PCA results for the Hutton Sandstone suggest
that there is a reasonably heterogonous composition of groundwater in the Hutton, but that differences in
the compositions reflect evolutionary processes or mixing of more discrete water types.
Table 22 Principal Component Analysis results for the Hutton Sandstone
Principle Component (%
Total Variance Explained)
Hutton
1 (27%)
Cl
Na
Parameters
TDS
(-) SiO2
2 (21%)
3 (20%)
HCO3
K
CO3
Mg
Ca
F
pH
Table 23 k-means Cluster Results for Hutton Sandstone
Hutton SS
Cluster
N
1
66
2
81
3
87
All
234
Statistics
Mean
1σ
Mean
1σ
Mean
1σ
Mean
1σ
301001-01210-CSG Water Chemistry
TDS
405
437
2,879
2,455
966
489
1,252
1,600
pH Na/Cl SAR
7.8
2.3
6
0.5
1.9
6
7.8
1.0
21
0.4
0.2
15
8.4
2.7
29
0.3
1.3
16
8.0
2.0
25
0.5
1.5
21
(CO3+
HCO3)/
Cl
SO4/Cl
3.6
0.093
3.9
0.138
0.3
0.024
0.3
0.016
2.2
0.027
1.5
0.028
1.9
0.045
2.6
0.082
NO3
SiO2 K
(mg/L) (mg/L) (mg/L)
F/Cl
1.2E-02
4.5
27
3.9
1.6E-02
10.0
20
2.5
6.5E-04
4.3
21
6.3
6.1E-04
3.8
12
3.9
1.1E-02
1.5
17
2.4
1.3E-02
1.3
8
1.7
7.7E-03
3.2
22
3.8
1.3E-02
6.3
15
2.9
Page 61
SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
The k-means cluster analysis for the Hutton indicates groundwater of three different compositions (Table
23 and Figure 26). Records included in Cluster 1 tend to occur in areas where the Hutton is exposed at
surface and show a hydrochemistry consistent with evolution of recharge along a flow path. Weathering
processes discussed in Section 7.1 are suspected. Cluster 2 yields a hydrochemistry very similar to that
attributed to the coal seams of the Walloon Subgroup. Mixing of pore waters between the Hutton and the
Walloon intervals along the north-eastern and eastern margins is supported by the observation of residual
heads oscillating between neutral and non-neutral along the northern intake beds zone (Figure 26).
LEGEND
Cluster - Descriptor
1 – Recharge, intermediate evolved
2 – Marine
3 – Intermediate TDS, highly evolved
Hutton Sandstone Outcrop
b15 fault
b70 fault
base of Hutton faults
Figure 26 Spatial Distribution of the k-means Clusters for the Hutton Sandstone
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
Cluster 3 shows a further evolved signature. Although the coverage in the middle of the basin is quite
limited, the highly evolved signature of Cluster 3 suggests that there may be homogenisation of pore
waters in the Hutton Sandstone, within the central portion of the aquifer, to a single dominantly evolved
hydrochemical type. This result at least suggests that the Hutton Sandstone is isolated from overlying and
underlying hydrostratigraphic units in the centre part of the basin. Comment cannot be made regarding
upward leakage of water from the Hutton Sandstone into the Springbok or Gubberamunda aquifers to the
southwest of the Eulo-Nebine Ridge, as is suggested by Radke et al.(2000), but the possibility exists as
the intervening aquitard layers pinch out against this structural feature.
Iso t op es
Isotope results were available from eight bores completed in the Hutton Sandstone. All samples were
13
14
18
2
analysed for δ CDIC, while only six were analyzed for C, δ OH2O and δ HH2O (Appendix 5). Groundwater
13
ages exhibited a range from 7,965 years BP to 35,070 years BP, and δ CDIC values were found to vary
from -18.1‰ to -9.4‰ with a median value of -14.6‰ (i.e. mineralisation of local vegetation and influence
18
2
from variable sources and processes). Measurements for δ OH2O (-6.4‰to -3.5‰)and δ HH2O (-40.2‰to21.3‰) were found to plot along a line that deviates from the Local Meteoric Water Line (established in
BRS and NRW, 2003), and at a slope consistent with an evaporative trend (Clark and Fritz, 1997). This
would suggest recharge during warmer and drier conditions.
5.4
Eurombah Formation
5.4.1
Hydr ogeology and Hydraulics
Reg ion a l F lo w Pat t e r n s
Groundwater level measurements were recorded at fifty-five locations between the years of 1921 and
2006. At almost all of these locations, only a single groundwater level (i.e. one event) was recorded with
the measurement dates spanning across the eighty-year period. An analysis of regional flow patterns
could not be completed as the influence of long-term trends would likely result in significant errors.
5.4.2
Hydr ochemistr y
G en e ra l Ch ar a ct e r ist i cs
The chemical dataset associated with the Eurombah Aquitard is quite limited (Appendix 6).However, from
available records, the groundwater within this interval exhibits a range in pH values from 7.2 to 8.7, and a
TDS content ranging from 178 mg/L to 2,077 mg/L. Based on this information, the groundwater may be
characterised as fresh (median TDS = 760 mg/L) with a slightly alkaline character (median pH = 8.2).
Sodium exists as the dominant cation, ranging from 42 to 1,585 mg/L (median = 308 mg/L). Subordinate
2+
2+
contributions are made by Ca and Mg . Bicarbonate exists as the dominant anion, with values ranging
from 94 to 718 mg/L (median = 530 mg/L), followed by Cl (range = 38 to 2,990 mg/L; median = 131 mg/L).
+
Based on the distribution of major ions, the groundwater in this interval may be described as a Na -HCO3
+
to Na -HCO3 - Cl hydrochemical type.
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
Co rr el at io n a n d St at i st i c al An a l ysi s
Limited data for the Eurombah interval precludes any significant statistical or spatial analysis of pore water
in this interval. Table 24 provides a summary of some of the basic properties.
Table 24 Summary of Selected Parameters and Ion Ratios for the Eurombah Aquitard
Eurombah Aquitard
N
All
10
Statistics
Mean
1σ
(CO3+
HCO3)/
NO3
SiO2
K
TDS pH Na/Cl SAR Cl
SO4/Cl F/Cl
(mg/L) (mg/L) (mg/L)
906 8.1
2.1
36
1.5
0.019 9.4E-03
1.6
19
2.1
675 0.5
1.2
24
1.3
0.026 7.3E-03
1.8
8
0.8
5.5
Walloons Subgroup
5.5.1
Hydr ogeology and Hydraulics
Reg ion a l F lo w Pat t e r n s
Intervals comprising the Walloons Subgroup were considered as one hydrostratigraphic unit for the
purposes of this assessment. A total of sixty-seven groundwater level measurements taken between 1960
and 1965 were used in the development of the potentiometric surface map for this interval (Appendix 7 –
Figure 5). The majority of data points were concentrated in the area around Taroom and Wandoan, but
coverage extended further west towards Mount Hutton and south-east following the alignment between
Miles, Dalby, and Inglewood. Groundwater levels ranged from a maximum 418 mAHD recorded at bore
16338 on the eastern slopes of Mount Hutton, to 183 mAHD at bore 15783 located near Taroom.
The narrow alignment of data permitted only limited assessment of groundwater flow directions within the
Walloon Subgroup. Groundwater flow directions were inferred to follow the surface topography east from
Mount Hutton towards the topographic low near Taroom. Local groundwater flow directions indicate a
northerly flow direction from the topographic ridge between Mount Hutton and Miles, which despite a lack
of data to the south, is inferred as a groundwater flow divide. Groundwater flow directions between Miles
and Inglewood indicate local systems controlled by surface topography.
Re si du al H ea d Co n d i t io n s
Modelled residual head conditions between the Walloon Subgroup and Springbok Sandstone are shown
in Figure 27. Neutral head differences (red areas) predominate, suggesting the potential for hydraulic
connection between the two formations across most of the area shown. However, there are small areas
where there are significant head differences (orange to blue areas), suggesting localised areas of
hydraulic isolation between these two units, likely caused by the finer-grained units of the upper Walloon
interval.
For more information on the generation of the residual head maps discussed in this report, refer to Section
2.2.3.
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
Figure 27 Modelled residual head condition for Walloon Subgroup and Springbok Sandstone
5.5.2
Hydr ochemistr y
G en e ra l Ch ar a ct e r ist i cs
A summary of water quality results for the Walloon Subgroup can be found in Appendix 6, while additional
information is provided by Grigorescu (2011b). Based on the data compiled for this study, groundwater
sampled from this interval has a pH ranging from 6.1 to 9.0, and TDS content ranging from 99 mg/L to
21,794 mg/L. Generally speaking, the groundwater may be described as slightly alkaline (median pH
greater than 8) and slightly brackish (median TDS greater than 1,680 mg/L).
+
-
-
The dominant cation is Na , and the dominant anions include Cl and HCO3 . Based on the distribution of
+
+
major ions, groundwater in this interval may be characterized as a Na -HCO3 to Na -Cl -HCO3
hydrochemical type.
Similar to the Hutton Sandstone, the Walloon Subgroup exhibits characteristic silica concentrations, with a
th
range of 1 to 74 mg/L, a median of 14 mg/L and 90 percentile of 37 mg/L.
Co rr el at io n a n d St at i st i c al An a l ysi s
Basic and multivariate statistics were applied to the Walloons Subgroup (Appendix 4). Strong positive
correlations were noted for:
•
-
+
Cl with TDS and the major cations (with the exception of K in the coal intervals);
301001-01210-CSG Water Chemistry
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
-
-
•
F with HCO3 (in the coal intervals);
•
SO4 with Ca
•
CO3 and pH; and
•
Ca , Mg
2-
2+
2+
and Mg
(in the coal intervals);
2-
2+
2+
+
and K (in the sandstone intervals).
PCA results for the Walloon Subgroup indicated that up to three components were required to adequately
describe the total variance in the dataset. For the Walloon coal intervals, components 1 and 2 accounted
for approximately the same amount of the total variance (33% and 32%, respectively; Table 25).
Component 1 represents the major ions and TDS, and component 2 reflects a negative relationship
22+
between Ca and pH, CO3 , HCO3 and F . This second component suggests weathering reactions as the
2+
controlling factor and reflects a shift from high relative Ca concentrations to higher pH values and
increased F . Aluminosilicates and clay minerals are implicated. The third component (accounting for
approximately 14% of the total variance – not shown) indicates that NO3 and silica (as SiO2 ) behave in a
manner opposite to each other. Nitrate concentrations are typically higher in association with the “marine”
signature across the basin, and may be the result of anthropogenic impacts, but also may reflect
degradation of organic matter in association with the coal seams. Increased SiO2 and decreased NO3
concentrations again suggest weathering of aluminosilicate minerals (including changed redox conditions)
with extended residence time of the pore water in is in these less permeable sediments.
Table 25 Principal Component Analysis Results for the Walloons Subgroup
Principle Component (%
Total Variance Explained)
Walloons Coal
Walloons SS
1 (33%) 2 (32%) 1 (29%) 2 (21%) 3 (19%)
CO3
Cl
(-) F
TDS
SiO2
TDS
Parameters
Cl
Na
(-) pH
(-) HCO3
Na
pH
HCO3
Mg
(-) CO3
K
pH
Ca
SO4
Ca
Mg
Ca
Results for the Walloon Subgroup intervals indicate that four components were required to adequately
describe the variance in the dataset; however, none of the individual components were very dominant
(components account for 30%, 21%, 19% and 17% of the total variance, respectively). Loadings were
2+
+
2+
strong for Cl , TDS, Na and K on component 1, for pH, HCO3 and CO3 on component 2, for SiO2 , Ca
22+
and Mg on component 3 (Table 25), and for SO4 and NO3 with a negative loading for F on component
4 (not shown).
Thus, the PCA results for the coal and sandstone intervals suggest the occurrence of differing
hydrochemical conditions associated with each lithology type. The results imply a stronger marine
signature for the coal intervals (all major ions of component 1 have high concentrations); however,
2opposing that signature is another strong water type that has a higher pH value, higher (CO3 +HCO3 )/Cl
ratio and higher F values (k-means cluster 2; Table 26). This opposing groundwater type has a high TDS
and is more hydrochemically evolved. This is an unusual hydrochemistry in either the Surat or the Bowen
basins, but also observed in the Walloons Sandstone (k-means Cluster 4; Table 26) and the Hutton
301001-01210-CSG Water Chemistry
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
Sandstone (Cluster 1; Table 23). Similar hydrochemistry is also observed in the Springbok Sandstone (kmeans Cluster 1; Table 28), and somewhat for the overlying Westbourne Formation aquitard (Table 29).
Table 26 k-means Cluster Results for Walloon Subgroup
N
Cluster
Walloons Coal
Statistics
Mean
1σ
Mean
2
56
1σ
Mean
3
61
1σ
Mean
All
302
1σ
Walloons Sandstone
Mean
1
19
1σ
Mean
2
13
1σ
Mean
3
29
1σ
Mean
4
21
1σ
Mean
All
63
1σ
1
185
TDS
pH
(CO3+
HCO3)/
Na/Cl SAR Cl
SO4/Cl F/Cl
NO3
SiO2
K
(mg/L) (mg/L) (mg/L)
5,969
4,794
2,566
1,118
1,106
605
4,151
4,245
7.9
0.4
8.5
0.3
7.8
0.5
8.0
0.5
0.9
0.2
2.5
1.4
1.6
0.8
1.3
0.9
21
11
59
29
17
14
33
28
0.2
0.2
1.8
1.7
1.1
1.4
0.7
1.2
0.025 4.3E-04
0.022 4.0E-04
0.006 1.1E-02
0.010 1.2E-02
0.064 3.1E-03
0.106 4.3E-03
0.030 2.9E-03
0.054 6.8E-03
3.1
6.9
2.0
2.1
2.1
2.4
2.6
5.1
24
12
10
4
24
18
21
13
7.5
6.6
4.4
2.9
2.7
2.0
5.6
5.5
727
51
564
329
4,882
3,745
2,198
922
2,438
2,814
8.2
0.2
7.5
0.6
7.9
0.5
8.2
0.3
8.0
0.5
4.9
1.4
1.4
0.5
1.0
0.2
1.9
1.1
2.2
1.8
31
8
6
7
26
20
48
19
38
27
4.6
1.7
1.3
1.0
0.2
0.2
1.2
1.3
1.6
2.0
0.069 1.2E-02
0.051 3.5E-03
0.058 4.1E-03
0.053 4.8E-03
0.040 4.4E-04
0.069 4.0E-04
0.007 6.9E-03
0.011 9.6E-03
0.041 5.3E-03
0.056 7.0E-03
1.2
0.9
3.8
3.1
7.3
7.7
2.4
2.4
3.6
4.9
15
3
45
16
20
10
13
4
21
14
1.5
0.4
4.4
2.5
21.3
56.6
3.1
1.7
8.5
32.0
Available data for the Walloons Subgroup is clustered around the northern, eastern and south-eastern
margins in the area referred to as the Walloon fairway (Figure 28 and Figure 29). Groundwater with quality
2+
+
2+
+
conditions consistent with coal seams (relatively high TDS, high Ca , Na , Mg , K , and NO3 ) is noted as
Cluster 1 of the Walloon coals (Table 26) along the eastern margin. Groundwater of a similar type is
represented by Cluster 3 for the Walloons sandstones, and occurs along the eastern and north-eastern
margins.
Mixing between groundwaters of the Springbok Sandstone and the Walloons Subgroup is suggested by
the relatively evolved groundwater chemistry (k-means Cluster 1) of some data points associated with the
Walloons Subgroup close to where the Springbok Sandstone is exposed at surface. However, there is
insufficient data coverage in the Springbok Sandstone to comment further on this possibility.
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
LEGEND
Cluster - Descriptor
1 –Coal seams
2 –High TDS, mixed, evolved
3 – Low TDS, evolved
Walloons Subgroup Outcrop
b15 fault
b70 fault
base of Walloon faults
Figure 28 Spatial Distribution of the k-means Clusters for the Walloon Subgroup coals
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
LEGEND
Cluster - Descriptor
1 – Recharge, evolved
2 – Recharge, intermediate evolved
3 – Coal seams
4 – High TDS, mixed, evolved
Walloons Subgroup Outcrop
b15 fault
b70 fault
base of Walloon faults
Figure 29 Spatial Distribution of the k-means Clusters for the Walloon Subgroup sandstones
Iso t op es
13
Isotope data that was available at the time of writing was limited to δ C in the dissolved methane fraction
13
(Appendix 5). δ CCH4 values were found to range from -56.8‰ to -54.2‰, with a median value of -56.1‰.
When assessed against values anticipated for various processes of natural gas formation (Whiticar, 1999)
methane in the Walloon Subgroup appears to be consistent with bacterial oxidation, bacterial methylation,
and mixed or transitional phases.
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
5.6
Springbok Sandstone
5.6.1
Hydr ogeology and Hydraulics
Reg ion a l F lo w Pat t e r n s
Data coverage for the Springbok Sandstone aquifer is relatively limited when compared to the shallower
formations. A total of seventeen measurements were included in the development of the potentiometric
surface map for the 1960 to 1965 period (Appendix 7, Figure 6). Water level data is concentrated in the
northern region between Mitchell and Wandoan, with some additional data available for the southern
region near Dirranbandi and St. George.
Groundwater elevations range from a maximum of 478 mAHD (bore 26263) at Mount Hutton, to
225 mAHD (Bore 64) near Dirranbandi. The limited data coverage indicates a radial and southerly
direction of groundwater flow originating from Mount Hutton, between Mitchell and Injune. Regionally,
groundwater is expected to flow south to south west towards Dirranbandi and Cunnamulla. A groundwater
divide is inferred along the topographic ridge between Mount Hutton and the town of Miles with a welldefined northerly groundwater flow direction observed towards the Bowen Basin and the town of Taroom.
The close positioning of data points along the northern edge of the ridge captures the high hydraulic
-4
gradient 0.015(between bore 16940 and bore 16939). Gradients then flatten to 7.5 x 10 on the southern
plains (between bore 15523 and Bore 64).
Flowing artesian conditions are noted towards the centre of the Surat Basin around bores 64149 and
15523. Similar conditions are likely to extend further beyond the data coverage.
Re si du al H ea d Co n d i t io n s
Modelled residual head conditions between the Springbok Sandstone and the Gubberamunda Sandstone
aquifers are shown in Figure 30. Areas of neutral head differences (red areas) are predominant,
suggesting a possible interaction between these two intervals. The small areas indicating significant head
differences (orange to blue areas) suggests localised hydraulic isolation, likely caused by the finer-grained
units of the Westbourne aquitard. Groundwater flow across the Westbourne Formation is possible where
the unit is thin, fractured or absent (Radke et al., 2000). While it is difficult to assess, with confidence, that
any preferential pathways exists across the Westbourne Formation using the available data, localised
thinning is the most likely mechanism for any connection between the Springbok Sandstone and the
Gubberamunda Sandstone aquifers.
For more information on the generation of the residual head maps discussed in this report, refer to Section
2.2.3.
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
Figure 30 Modelled residual head condition for
Springbok Sandstone and Gubberamunda Sandstone
5.6.2
Hydr ochemistr y
G en e ra l Ch ar a ct e r ist i cs
A summary of water quality results for the Springbok Sandstone can be found in Appendix 6. Based on
the data compiled for this study, groundwater within this interval has a pH ranging from 6.4 to 9.0, and
TDS content ranging from 150 mg/L to 14,045 mg/L. The groundwater is considered to be fresh to slightly
brackish (median TDS = 1,211 mg/L) with a circum-neutral to slightly alkaline in character (median pH =
7.7).
The occurrence of lower salinity waters is generally noted along the south to southwest flow path within
the aquifer and may imply the occurrence of highly transmissive intervals and limited weathering of the
matrix minerals. Recharge sources to the Springbok Sandstone are likely to be present northwest of
Injune where the unit exists at surface and rests directly beneath surficial deposits.
+
The dominant cation is Na , exhibiting a range of 37 to 4,780 mg/L (median = 560 mg/L), with subordinate
2+
2+
contributions from Ca followed by Mg . The major anions comprise both HCO3 and Cl . The median Cl
concentration (730 mg/L) is significantly greater than the median HCO3 concentration (432 mg/L), as well
th
as the 90 percentile values (4,472 mg/L versus 801 mg/L, respectively). Given this distribution of major
+
+
ions, the groundwater in this interval may be described as a Na -HCO3 -Cl to Na -Cl -HCO3 hydrochemical
type.
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
-
Distinguishing features for the Springbok Sandstone include comparatively elevated silica (as SiO2 )
th
concentrations (range = 10 to 62 mg/L; median = 22 mg/L; 90 percentile = 43.6 mg/L) and NO3
th
concentrations (range = 0.2 to 27 mg/L; median = 1.8 mg/L; 90 percentile = 10 mg/L).
Co rr el at io n a n d St at i st i c al An a l ysi s
Based on Spearman rank correlation coefficients (Appendix 4), strong positive relationships were noted
for
-
2+
+
2+
•
Cl with TDS, Ca , Na and Mg ; and
•
F with HCO3 .
-
-
A unique feature is the strong negative correlation between:
•
2+
2-
Ca , pH, and CO3 .
PCA and k-means cluster analyses for the Springbok Sandstone aquifer suggest that there may be
divergent sources of groundwater to this aquifer (Table 27 and Table 28; Figure 31).The composition of
Clusters 1 and 4 appear fairly similar in character with Cluster 4 exhibiting lower TDS values. Based on
SAR and the various ion ratio values, these two waters appear to represent recently recharged (Cluster 4)
to somewhat evolved (Cluster 1) water types. With respect to Clusters2 and 3, these appear to more
highly evolved waters with a possible influence from the underlying Walloon Subgroup (i.e., marine
signatures).
Table 27 Principal Component Analysis Results for the Springbok Sandstone
Principle Component (%
Total Variance Explained)
Parameters
Springbok
1 (39%)
2 (30%)
(-) CO3
Cl
(-) pH
(-) HCO3
Na
K
Ca
TDS
Ca
(-) F
Mg
As expected, groundwater of lower TDS content typically occurs in association with the Springbok
Sandstone outcrop (Cluster 4; Table 28), and more evolved groundwater represented by Cluster 3 occurs
towards the center of the basin (Figure 31).
Any further comment on the Springbok Sandstone on the basis of the available data is too speculative.
The lack of data records associated with the Springbok Sandstone represents an important gap with
respect to evaluating the effectiveness of the seal provided by the upper Walloons Subgroup (which is
known to be absent in some areas) and any potential interaction with the underlying coal intervals.
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
Table 28 k-means Cluster Results for the Springbok Sandstone
Springbok
Cluster
N
1
21
2
34
3
12
4
12
All
79
Statistics
Mean
1σ
Mean
1σ
Mean
1σ
Mean
1σ
Mean
TDS
854
192
6,536
1,879
2,058
1,116
549
220
2,906
2,901
pH Na/Cl SAR
8.4
4.9
33
0.4
2.0
33
7.1
0.6
12
0.3
0.1
8
7.9
1.2
19
0.4
0.4
15
8.1
2.5
15
0.5
0.8
9
7.7
2.0
24
0.7
1.9
17
(CO3+
HCO3)/
Cl
SO4/Cl
4.6 0.083
2.6 0.079
0.1 0.009
0.0 0.005
0.5 0.023
0.3 0.056
2.3 0.087
1.6 0.075
1.6 0.041
2.1 0.054
NO3
SiO2 K
F/Cl
(mg/L) (mg/L) (mg/L)
2.9E-02
1.2
21
1.7
3.5E-02
0.9
6
1.0
1.8E-04
6.8
20
9.0
8.0E-05
4.6
1.3E-03
1.7
20
6.5
1.5E-03
1.2
9
3.5
7.4E-03
10.7
31
2.9
7.9E-03
12.7
15
2.1
9.0E-03
4.0
25
3.6
6.8E-03
13.4
136
5.5
LEGEND
Cluster - Descriptor
1 – Intermediate TDS, highly evolved
2 – Mixed, marine or coal
3 – High TDS, mixed
4 – Recharge, evolved
Springbok Sandstone Outcrop
b15 fault
b70 fault
base of Springbok faults
Figure 31 Spatial Distribution of the k-means Clusters for the Springbok Sandstone
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
5.7
Westbourne Formation
5.7.1
Hydr ogeology and Hydraulics
Reg ion a l F lo w Pat t e r n s
A total of seventy one bore locations were found to have at least one groundwater level measurement. Of
this data set, only eight locations were represented during the 1960 to 1965 period, with no other time
period providing any more significant temporal coverage. Regional groundwater flow directions within the
Westbourne Formation aquitard could not be interpreted from this limited data set. However, it is likely that
lateral hydraulic gradients within this hydrostratigraphic interval follow those observed in the adjacent
Springbok and Gubberamunda Sandstone aquifers (Appendix 7, Figures 6 and 7).
5.7.2
Hydr ochemistr y
G en e ra l Ch ar a ct e r ist i cs
A summary of water quality results for the Westbourne aquitard can be found in Appendix 6. Based on
the data compiled for this study, groundwater in this comparatively extensive and thick interval has a pH
values ranging from 6.7 to 8.7, and a TDS content ranging 500 to 11,009 mg/L. In general, groundwater
from this interval can be considered fresh to slightly brackish (median TDS = 1,195 mg/L) with a slightly
alkaline character (median pH = 8.2).
+
The dominant cation within this unit is Na , with a range of 200 to 2,287 mg/L (median = 335 mg/L), while
the dominant anion varies between HCO3 and Cl . The median Cl concentration is larger than that of
th
HCO3 (524 mg/L compared to 499 mg/L, respectively) and also exhibits a greater 90 percentile (3,560
versus 622 mg/L, respectively). Based on the major ion distribution, groundwater in this interval may be
+
characterised as a Na -Cl -HCO3 hydrochemical type.
Co rr el at io n a n d St at i st i c al An a l ysi s
Limited data for the Westbourne aquitard interval precludes any significant statistical or spatial analysis.
Table 29 provides a summary of some of the basic properties and ion ratios.
Table 29 Summary of Selected Parameters and Ion Ratios for the Westbourne Formation
Westbourne
N
All
11
Statistics
Mean
1σ
301001-01210-CSG Water Chemistry
(CO3+
HCO3)/
SiO2 K
NO3
TDS pH Na/Cl SAR Cl
(mg/L) (mg/L) (mg/L)
SO4/Cl F/Cl
3,379 8.0
2.1
34
1.5 0.067 3.5E-03
1.6
23
3.8
3,549 0.7
1.4
20
1.6 0.077 3.4E-03
1.0
9
3.5
Page 74
SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
5.8
Gubberamunda Sandstone
5.8.1
Hydr ogeology and Hydraulics
The distribution of data for the Gubberamunda aquifer between 1960 and 1965 is concentrated around the
northern and north-eastern areas where the formation is exposed at surface, with less data available for
the southern plains (Appendix 7, Figure 7). Data coverage extends from Miles and Inglewood in the east
to Mitchell and Cunnamulla to the west.
Groundwater potentiometric elevations decrease southwards from maximum values of around 700 mAHD
in the north (between Mitchell and Injune) to 150 to 200 mAHD at the latitude of St George (-28º latitude).
Regional groundwater flow is predominantly southerly from the elevated outcrop areas in the north
towards the lower lying plains. Flow directions are relatively consistent; however, hydraulic gradients to
the southwest of Wandoan suggest the presence of a groundwater divide following the ridge between
Roma and Miles. Groundwater north of this ridge may flow towards the Bowen Basin and Taroom. This is
also observed in the overlying BMO Sandstone aquifer, which is described later. The absence of data to
the north of the ridge prevents further clarification of local flow directions in this area of the basin.
Sub-artesian conditions prevail along the northern and eastern basin margins, and changes to artesian
conditions towards the centre of the basin around St. George and Dirranbandi. Artesian conditions were,
however, noted in the past between Goondiwindi and Inglewood and in close proximity to the mapped
outcrop of the Gubberamunda Sandstone along the eastern margin of the basin.
Hydraulic gradients were greatest (0.01, between bores 50069 and 14449) (Appendix 7, Figure 7) in the
topographically elevated areas between Mitchell and Injune, which also loosely corresponds to the
mapped areas of the Gubberamunda outcrop. Gradients decrease on the southern plains to approximately
-4
9.6 x 10 (between bores 13414 and 14049) (Appendix 7, Figure 7). The distribution of data also is
suspected to have an effect on the noted gradients, with the majority of data falling in the northern areas.
Re si du al H ea d Co n d i t io n s
Modelled residual head conditions between the Gubberamunda and BMO Group aquifers are shown in
Figure 32. Neutral head differences (red areas) are present across the majority of the coloured area,
indicating the potential for hydraulic connectivity between the two formations. There are some areas
where moderate head differences (orange to blue areas) occur, suggesting the possible presence of
intervening low permeability layers.
For information on the generation of the residual head maps discussed in this report, refer to Section
2.2.3.
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
Figure 32 Modelled residual head conditions between Gubberamunda Sandstones and BMO Group
5.8.2
Hydr ochemistr y
G en e ra l Ch ar a ct e r ist i cs
A summary of water quality results for the Gubberamunda Sandstone can be found in Appendix 6. Based
on the data compiled for this study, groundwater in this interval has a pH ranging from 6.2 to 9.0, and TDS
content ranging from 62 mg/L to 27,283 mg/L. In general, the groundwater can be described as fresh
(median TDS = 689 mg/L) and slightly alkaline in character (median pH = 8.3).
+
The chemistry of the Gubberamunda Sandstone groundwater is dominated by the cation Na (12 to 7,805
2+
2+
mg/L; median = 273 mg/L) with subordinate contributions from Ca and Mg . Bicarbonate and Cl are the
dominant anions, with Cl exhibiting a much larger range (18 to 15,740 mg/L) than HCO3 (12 to 1,524
th
mg/L). Similarly, the 90 percentile value for Cl is greater than that for HCO3 (1,309 versus 854 mg/L).
+
Given relative distribution of ions, the groundwater in this interval may be characterised as a Na -HCO3 to
+
Na -Cl -HCO3 hydrochemical type.
Co rr el at io n a n d St at i st i c al An a l ysi s
The Gubberamunda Sandstone had very few species pairs showing strong correlations in comparison to
the other formations of the Surat Basin. Strong positive correlations were identified for:
•
-
+
2+
Cl with TDS, Na and Ca ;
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
2-
•
CO3 with pH; and
•
Ca
2+
2+
with Mg
+
and K .
The PCA results (Appendix 4), showed a strong component 1 (explaining 42% of the variance), and much
weaker components 2 and 3 (Table 30). Species showing strong loadings on the first component included
22+
2+
+
+
Cl , TDS, Ca , Mg , Na and K . SO4 did not show strong loadings on any of the components identified.
Table 30 Principal Component Analysis Results for the Gubberamunda Sandstone
Principle Component (%
Total Variance Explained)
Gubberamunda
1 (42%)
2 (21%)
Cl
(-) F
(-) HCO3
TDS
Na
Parameters
(-) CO3
Ca
K
Mg
The k-means cluster analysis indicated that the Gubberamunda Sandstone, unlike the BMO Group,
appears to show a dominant composition over the majority of the aquifer (Figure 33). The mineralogy of
the Gubberamunda Sandstone is dominated by quartz, and the residual head mapping suggests that the
Gubberamunda Sandstone and the BMO Group aquifers may interact, but the potential for interaction with
the underlying Springbok Sandstone aquifer is remote; except along the basin margins where the
Westbourne aquitard is known to thin or be absent. The dominant cluster (Cluster 2) has a relatively
+
2evolved signature, with high SAR values, low relative K concentrations and high (CO3 +HCO3 )/Cl ratios.
Groundwater chemistry associated with Cluster 1 tends to display a marine signature, and with Cluster 3
has a freshly-recharged signature (Table 31).
Table 31 k-means Cluster Results for the Gubberamunda Sandstone
Gubberamunda
Cluster
1
2
3
All
N
Statistics
Mean
83
1σ
Mean
491
1σ
Mean
95
1σ
Mean
669
1σ
301001-01210-CSG Water Chemistry
TDS
6,856
5,522
746
358
654
429
1,475
2,791
pH Na/Cl SAR
7.6
0.9
21
0.5
0.3
10
8.4
4.1
29
0.3
1.8
12
7.7
2.1
10
0.4
1.1
8
8.2
3.4
33
0.5
2.0
18
(CO3+
HCO3)/
Cl
SO4/Cl
0.1
0.053
0.2
0.125
3.7
0.038
2.1
0.050
1.7
0.153
1.5
0.155
3.0
0.056
2.3
0.093
NO3
SiO2 K
F/Cl
(mg/L) (mg/L) (mg/L)
5.3E-04
7.7
19
17.0
1.3E-03
15.4
10
12.5
1.3E-02
1.5
21
1.8
1.5E-02
6.6
6
0.9
5.4E-03
3.2
32
4.8
7.7E-03
5.2
19
3.5
1.0E-02
2.4
22
3.7
1.4E-02
8.3
10
6.3
Page 77
SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
LEGEND
Cluster - Descriptor
1 –Marine
2 –Intermediate TDS and evolved
3 – Recharge, intermediate evolved
Gubberamunda Sandstone Outcrop
b15 fault
b70 fault
Figure 33 Spatial Distribution of the k-means Clusters for the Gubberamunda Sandstone
These groundwater types occur almost exclusively in association with the outcrop of the Gubberamunda
Sandstone; however, there are locations with a marine signature along the western margin of the aquifer
as opposed to a freshly-recharged signature on the northern margin. As well, the presence of discrepant
water types in the southwest corner of the basin near the Eulo-Nebine ridge area suggests potential
mixing with overlying (BMO Group) and possible the underlying Springbok Sandstone aquifer.
Iso t op es
13
18
2
13
Measurements from four bores were available for δ CDIC, δ OH2O and δ HH2O (Appendix 5). δ CDIC values
were found to range from -8.4‰ to -2.4‰, with a median value of -8.0‰, indicating an isotopically-heavier
source of carbon (possibly carbonate minerals), or the occurrence of fractionating processes (acetate
18
2
fermentation or reduction of CO2). δ OH2O (-7.3‰ to -6.6‰) and δ HH2O (-42.9‰ to -39.6‰) values plot
along a line that deviates from the Local Meteoric Water Line (established in BRW and NRW, 2003) in a
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
way that suggests various reactions like exchange with CO2, silicate mineral hydration, or exchange with
H2S (Clark and Fritz, 1997). Additional discussion is provided in Appendix 8.
5.9
BMO Group
5.9.1
Hydr ogeology and Hydraulics
Reg ion a l F lo w Pat t e r n s
The groundwater level data coverage for the BMO Sandstone extends from Chinchilla in the east to
Mitchell in the west and generally from Wandoan and Injune in the north, to Goondiwindi and Dirranbandi
in the South (Appendix 7, Figure 8).
Groundwater potentiometric surface elevations within the BMO Sandstones broadly decrease from
620 mAHD in the northwest to 160 mAHD in the southwest representing a prevailing southerly regional
groundwater flow direction. The topographic high-point running between Mitchell and Miles is reflected by
a groundwater divide between the predominantly southerly groundwater flow and what is suspected to be
a northerly flow component from the outcropping BMO Sandstone to towards Taroom. The spatial data
coverage limits understanding of this northerly component of flow but represents the potential for
inter-basin flow from the Surat to the Bowen Basin. Variations in the local groundwater flow directions are
observed particularly where they converge around surface drainage valleys to the south of the Warrego
Highway. The BMO Sandstone aquifer is sub-artesian throughout the northern basin margin where the
formation is exposed at surface. Artesian conditions develop within the low lying drainage network to the
south of the basin margin, and become more widespread towards the centre of the basin.
Horizontal hydraulic gradients generally follow the topographic gradients, decreasing from a maximum of
-4
approximately 0.01 (between bores 15446 and 15571) around Mitchell and a minimum of 8.3 x 10
(between bores 16291 and 15764) on the lower plains to the south.
Re si du al H ea d Co n d i t io n s
Residual head mapping was not carried out specifically for the BMO Group due to lack of sufficient
information for the overlying Rolling Downs Group.
5.9.2
Hydr ochemistr y
G en e ra l Ch ar a ct e r ist i cs
A summary of water quality results for the BMO Group can be found in Appendix 6. Based on the data
compiled for this study, groundwater in this interval has pH values ranging from 6.7 to 9.0, and TDS
content ranging from 73 mg/L to 28,594 mg/L. Generally, the groundwater within this unit can be
considered fresh to slightly brackish (median TDS = 1,080 mg/L) and slightly alkaline in character (median
pH = 8.3).
+
The dominant cation within this hydrostratigraphic interval is Na (range = 12 to 7,320 mg/L; median = 430
mg/L), with HCO3 as the dominant anion (range = 4 to 1,629 mg/L; median = 652 mg/L). Chloride also
exists in notable concentration ranging from 11 to 17,673 mg/L (median = 210 mg/L) and can dominate
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
the anion mix in some locations. Based on the relative distribution of ions, the groundwater in this interval
+
can be characterised as a Na -HCO3 to Na -HCO3 -Cl hydrochemical type.
-
Of particular note is the F content, which indicates comparatively higher statistical values than all the
th
Jurassic and Cretaceous formations (range = <0.1 to 56 mg/L; median = 0.7 mg/L; 90 percentile = 2.7
mg/L). Weathering of F -bearing minerals is the suspected source.
Co rr el at io n a n d St at i st i c al An a l ysi s
Based on Spearman correlation coefficients (Appendix 4), strong positive correlations were noted for:
-
2+
•
Cl with TDS and Ca ;
•
F with HCO3 and SO4 ;
•
Ca
•
Mg
-
-
2+
with Mg
2+
2+
with K .
2-
+
and K ; and
+
In contrast, there were strong negative correlations noted for:
•
F- and SO42-; and
•
Ca2+ and CO32-.
PCA results indicated that three components were required to adequately describe the variance in the
data set, with the components 1 and 2 accounting for near-equal (34% and 32%, respectively)
2percentages of the total variance (Table 32). The first component had a positive loading for SO4 and
22+
Ca , but negative loadings of F , pH CO3 and HCO3 . The second component had positive loadings for
+
2+
2+
+
Cl , TDS, Na , Ca , Mg and K . The third component (not shown) yielded a negative loading of SiO2 .
Table 32 Principal Component Analysis Results for the BMO Group
Principle Component (%
Total Variance Explained)
Parameters
BMO
1 (34%)
2 (32%)
(-) HCO3 Cl
(-) CO3
TDS
(-) F
Na
(-) pH
Ca
SO4
K
Ca
Mg
Results provided by the k-means cluster analysis suggest that there are four groundwater types
associated with the BMO group (Table 33 and Figure 34). Mixing of groundwater between the BMO Group
and the Gubberamunda Sandstone, at least in the eastern portion of these aquifers, is suggested by the
consistency in composition of Cluster 1 of the BMO and Cluster 2 of the Gubberamunda, as both have
relatively highly evolved chemical signatures with intermediate TDS values. Spatially, Cluster 2 dominates
the hydrochemistry of groundwater in the Gubberamunda (see section below); however, in the BMO, it
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
dominates only in areas where the Gubberamunda outcrop occurs at the north-eastern and eastern edges
of the basin.
Table 33 k-means Cluster Results for the BMO Group
BMO
Cluster
N
1
259
2
111
3
47
4
139
All
556
Statistics
Mean
1σ
Mean
1σ
Mean
1σ
Mean
1σ
Mean
1σ
301001-01210-CSG Water Chemistry
TDS
1,094
280
4,892
4,429
757
451
858
278
1,661
2,374
pH Na/Cl SAR
8.4
5.4
46
0.3
5.1
13
7.8
1.1
24
0.4
0.4
13
7.6
1.7
8
0.4
1.0
6
8.4
2.8
4
0.3
1.2
3
8.2
3.6
44
0.4
4.0
44
(CO3+
HCO3)/
Cl
SO4/Cl
5.4 0.013
6.1 0.018
0.2 0.134
0.2 0.224
1.4 0.187
1.6 0.287
1.9 0.114
1.4 0.106
3.1 0.077
4.8 0.154
NO3
SiO2 K
F/Cl
(mg/L) (mg/L) (mg/L)
6.7E-02
1.2
18
2.1
5.8E-01
1.4
5
2.2
6.2E-04
4.9
19
11.4
5.8E-04
5.7
22
11.2
5.5E-03
1.0
31
6.0
1.1E-02
1.2
18
5.8
5.5E-03
1.8
21
2.0
5.2E-03
2.3
14
1.2
3.4E-02
2.0
20
3.9
4.0E-01
3.1
14
6.2
Page 81
SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
LEGEND
Cluster - Descriptor
1 - Intermediate TDS, highly evolbed
2 - High TDS, marine
3 – Intermediate TDS, recharge
4 – Mixed, evolved
Bungil Outcrop
Mooga Outcrop
b15 fault
b70 fault
Figure 34 Spatial Distribution of the k-means Clusters for the BMO Group
2-
2+
The strong negative correlation amongst SO4 and Ca versus the carbonate species and pH is
22+
consistent with three distinct sources of water to the BMO Group. Some have higher relative SO4 , Ca ,
2+
and Mg concentrations with respect to the evolved groundwater of Cluster 1, and lower relative pH
values. Cluster 4 has a higher pH value and an intermediate composition to the other groundwater types
of the aquifer, suggesting some degree of mixing within the aquifer. However, although temperature
seems to homogenise to higher values in the center of the aquifer (Figure 34), the strong negative
correlations suggest that the groundwater is not homogenous even down towards the center of the basin.
Iso t op es
Results were available for eleven bores in the BMO Group (Appendix 5). These correspond to
13
18
2
13
measurements of δ CDIC, δ OH2O and δ HH2O (Appendix 5). δ CDIC values were found to range from 18
2
9.3‰ to +2.0‰with a median value of -2.2‰. δ OH2O (-7.6‰ to -6.4‰) and δ HH2O (-43.7‰ to -37.3‰)
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
values similarly deviated from the Local Meteoric Water Line (established in BRS and NRW, 2003) much
the same as the samples from the underlying Gubberamunda. This deviation is consistent with various
reactions like exchange with CO2 , silicate mineral hydration, or exchange with H2S (Clark and Fritz,
1997).
5.10
Rolling Dow ns Group
5.10.1 Hydr ogeology and Hydraulics
Reg ion a l F lo w Pat t e r n s
Groundwater level data between 1960 and 1965 for the Rolling Downs Group aquitard covers the area
bounded by Miles and Dirranbandi in the east, and Cunnamulla and Mitchell in the west (Appendix 7–
Figure 9). Groundwater elevations range from between 350 to 400 mAHD across the high ground
spanning between Mitchell and Miles (bores 15745, 16485, 16055, 15020A, and 15955)(Appendix 7 –
Figure 9), decreasing southward to less than 150 mAHD south of Cunnamulla and Dirranbandi (bores
13487, 16286, 15520, and 16287)(Appendix 7 – Figure 9). Groundwater pressures were found to be subartesian across the northern margin of the Surat Basin, becoming artesian in the southwest.
The regional groundwater flow direction is generally south to south-west with local variations in the flow
directions showing some correlation with areas of exposed Rolling Downs Group sediments. The influence
of the outcropping geology on groundwater flow directions can be attributed to both the increased
infiltration of rainfall likely to recharge the formation in these areas, as well as occurrence of outcrop with
-4
topographic highs. Horizontal hydraulic gradients are relatively uniform ranging from 7.24 x 10 (between
bores 16485 and 14694)(Appendix 7 – Figure 9) to 0.001 (between bores 16055 and 14682)(Appendix 7 –
Figure 9). Gradients generally decrease further to the south around the Balonne River near Dirranbandi
where the terrain is relatively flat.
Groundwater in the Rolling Downs Group to the south east of Cunnamulla becomes highly pressurised
with hydraulic heads ranging from 240 mAHD to 190 mAHD. This significant, but localised, variation in the
potentiometric surface may indicate increasingly effective confinement of the upper aquitard layers of the
Rolling Downs Group, or possibly the presence of an isolated flow system. This hydraulic pressure (and
strongly artesian zone) correlates with the occurrence of groundwater springs at surface.
Groundwater levels from 2006 (Appendix 7, Figure 9) draw on data sourced from different bore locations
from those available during the period of 1960-1965. This, in itself, is likely to cause variations in the local
flow directions; however, it can be seen that the regional groundwater flow direction remains in a south to
south-westerly direction and hydraulic gradients remain comparable. The data coverage does not permit
further analysis of the high pressure zone to the south-east of Cunnamulla.
Re si du al H ea d Co n d i t io n s
Residual head mapping was not carried out for the Rolling Downs Group due to lack of sufficient
information for both the overlying Alluvium/Tertiary Volcanics and the Rolling Down Group itself.
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
5.10.2 Hydr ochemistr y
G en e ra l Ch ar a ct e r ist i cs
A summary of water quality results for the Rolling Downs Group can be found in Appendix 6. Based on
the data compiled for this study, groundwater in this interval has pH values ranging from 6.0 to 9.0, and
TDS content ranging from 196 to 36,325 mg/L. Generally speaking, the groundwater can be described as
brackish (median TDS = 2,170 mg/L) and slightly alkaline in character (median pH = 8).
+
Major ion chemistry of the groundwater is dominated by Na (range = 20 to 12,172 mg/L; median = 710
2+
2+
mg/L), with subordinate contributions from Ca and Mg . Chloride exists as the dominant anion (range =
16 to 20,983 mg/L; median = 947 mg/) in the majority of samples assessed, followed by HCO3 (range = 5
to 1,889 mg/L; median = 410 mg/L). Sulfateis also present in notable concentrations, with a range of 0.1 to
3,790 mg/L, and a median of 130 mg/L. Based on the distribution of major ions, the groundwater may be
+
+
described as a Na -Cl to Na -Cl -HCO3 hydrochemical type.
-
Other features of the groundwater in the Rolling Downs Group include its silica (as SiO2 ) (median = 47
+
mg/L), NO3 (median = 2.5 mg/L) and K (median = 11 mg/L) concentrations.
Co rr el at io n a n d St at i st i c al An a l ysi s
Based on Spearman correlation coefficients, strong positive correlations were noted for:
-
+
2+
2-
•
Cl with TDS, Na , Ca , SO4 ;
•
Ca
•
TDS with SO4 , the major cations and NO3 ;
•
SO4 with the major cations; and
•
General correlation amongst the major cations.
2+
2+
2-
+
with Mg , SO4 and K ;
2-
-
2-
Some of these correlations are reflected in the strong first component of the PCA results (Appendix 4).
Further, the first PCA component for the Rolling Downs Group accounts for the almost 50% of the total
variance in the data set (Table 34). The second and third components account for 17% and 15% of the
2total variance, respectively, and have strong loadings for HCO3 , CO3 and pH and silica (as SiO2 ).
Fluoride loads onto both components 2 and 3, but not strongly (i.e. component loading was between 0.4
and 0.6).
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
Table 34 Principal Component Analysis Results for the Rolling Downs Group
Principle Component (%
Total Variance Explained)
Parameters
Rolling Downs Group
1 (47%)
2 (17%)
(-) CO3
Cl
TDS
(-) HCO3
Na
(-) pH
Ca
Mg
K
SO4
NO3
Interpretation of the PCA and correlation analysis is supported by the k-means cluster analysis. Results
yielded for the Rolling Downs Group suggest that there are four groundwater types associated with this
hydrostratigraphic interval (Table 35 and Figure 35). Cluster 2 has the highest mean TDS content (greater
than 18,000 mg/L) observed for both the Surat and Bowen basins, relatively low pH and SAR values, very
2+
low F /Cl and (CO3 +HCO3 )/Cl ratios and high mean concentrations of K . Groundwater of the 'marine'
type dominates in the eastern outcrop region, presumably related to the depositional environment in that
area of the basin, and down-gradient to the southwest until approximately Dirranbandi, where there
appears to be a divide in groundwater flow. In turn, a ‘marine’ signature is not typically seen to the west of
this divide.
Table 35 k-means Cluster Results for the Rolling Downs Group
Rolling Downs Group
Cluster
N
1
165
2
110
3
258
4
83
All
616
Statistics
Mean
1σ
Mean
1σ
Mean
1σ
Mean
1σ
Mean
1σ
TDS
874
396
18,680
8,290
3,262
1,742
629
373
949
7,476
pH
8.3
0.3
7.3
0.5
7.8
0.5
7.7
0.5
7.9
0.6
(CO3+
HCO3)/
Na/Cl SAR Cl
SO4/Cl F/Cl
3.9
2.4
0.8
0.1
1.1
0.3
2.2
1.2
2.0
1.8
23
9
28
8
17
6
7
5
8
12
3.6
2.9
0.0
0.0
0.3
0.3
2.2
2.1
1.4
2.3
0.060
0.087
0.058
0.026
0.071
0.069
0.136
0.149
0.075
0.088
2.0E-02
4.0E-02
5.1E-05
5.4E-05
5.6E-04
5.9E-04
5.0E-03
4.9E-03
6.2E-03
2.2E-02
NO3
SiO2 K
(mg/L) (mg/L) (mg/L)
1.9
2.5
16.1
8.2
5.4
6.4
2.8
6.4
5.0
7.1
25
12
53
22
48
43
72
46
46
38
2.6
1.9
57.1
48.2
14.0
7.0
6.2
5.1
18.2
28.9
Groundwater to the west of the divide is represented by Cluster 1, which shows intermediate
+
mineralisation and evolution towards a Na -HCO3 hydrochemical type. Associated with this groundwater
is a higher mean pH value relative to the other groundwater types of the Rolling Downs Group, and a
higher F /Cl ratio. To the north and in the center of the basin, groundwater has compositions of the type
2represented by Clusters 2 and 4. The high SAR and SO4 /Cl ratios and intermediate TDS content
associated with Cluster 4 suggests an influence from mineral weathering processes leading to the
2elevated (CO3 +HCO3 /Cl ratios. Influence from connate water (i.e. ancient seawater water trapped
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
2-
during deposition of the unit) would also account for the source of SO4 , considering the lack of detectable
sulfide minerals in the mineralogy.
LEGEND
Cluster - Descriptor
1 - Intermediate TDS, evolved
2 - High TDS, marine
3 – Mixed
4 – Intermediate TDS, recharge
Rolling Downs Grroup Outcrop
b15 fault
b70 fault
Figure 35 Spatial Distribution of the k-means Clusters for the Rolling Downs Group Aquitard
5.11
Cainozoic Cover (Alluvium and Tertiary Volcanics)
5.11.1 Hydr ogeology and Hydraulics
Reg ion a l F lo w Pat t e r n s
Only limited water level data was available for the surficial sediments during the 1960 to 1965 assessment
period, thus precluding development of potentiometric surface maps for this interval. In most cases, the
observation bores and stock and domestic bores which are completed within the alluvium were installed
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
during the late 1980s. Data from this period indicate that groundwater flow directions are predominantly
controlled by the surface topography.
5.11.2 Hydr ochemistr y
G en e ra l Ch ar a ct e r ist i cs
A summary of water quality results for the Alluvium and Tertiary Volcanics can be found in Appendix 6.
Groundwater quality for each is described below.
Based on the data compiled for this study, the groundwater in the Alluvium interval has a pH ranging from
6.1 to 9.0, and TDS content ranging from 50 to 86,959 mg/L. The excessive TDS value recorded is
th
somewhat suspect, considering that the 90 percentile for this interval is 5,670 mg/L. Constituents that
+
are attributed to the high TDS value include Cl (50,622 mg/L), followed by Na (21,593 mg/) and HCO3
(11,907 mg/L). With the exception of this apparently anomalous data record, the groundwater in this
interval can be described as fresh (median groundwater TDS = 714 mg/L) with a slightly alkaline character
(median pH = 7.6).
+
th
The groundwater chemistry in the alluvium is dominated by Na (median = 151 mg/L; 90 percentile =
2+
th
1,510 mg/L), with notable contributions from Ca (median = 54 mg/L; 90 percentile = 252 mg/L) followed
2+
th
by Mg (median = 38 mg/L; 90 percentile = 156 mg/L). The dominant anion is HCO3 (median = 454
th
th
mg/L; 90 percentile = 1,034 mg/L) followed by Cl (median = 195 mg/L; 90 percentile = 3,140 mg/L). In
+
terms of total ionic mix, the groundwater may be characterised as a Na -HCO3 hydrochemical type, with
variable influence from Cl ions. A distinguishing feature of this interval is its comparatively elevated silica
(as SiO2 ) concentrations (range = 1 to 140 mg/L; median = 44 mg/L), indicating the presence of more
easily weathered silicate minerals (micas and clays).
+
The groundwater chemistry in the Tertiary Volcanics is dominated by the Na ions (median = 151 mg/L;
th
2+
th
90 percentile = 560 mg/L), followed by Mg (median = 50 mg/L; 90 percentile = 156 mg/L) and
2+
th
Ca (median = 36 mg/L; 90 percentile = 94 mg/L). With respect to the anions, HCO3 is dominant
th
th
(median = 585 mg/L; 90 percentile = 985 mg/L) followed by Cl (median = 125 mg/L; 90 percentile = 744
mg/L). Based on the distribution of major ions, the groundwater in this interval may be characterised as a
+
Na -HCO3 hydrochemical type. Distinguishing features of this interval include its comparatively elevated
th
th
SiO2 (median = 49 mg/L; 90 percentile = 74 mg/L), and NO3 (median = 3.8 mg/L; 90 percentile = 29.7
mg/L).
Co rr el at io n a n d St at i st i c al An a l ysi s
Based on Spearman correlation coefficients, strong positive associations were noted for:
-
+
2-
2+
-
•
Cl with TDS, Na , Mg , SO4 , HCO3 ;
•
Ca , in the Mg
•
TDS with SO4 , HCO3 and the major cations (except K ); and
•
SO4 with Na and Mg .
2+
2-
2+
+
and Na ;
2-
-
+
2+
+
PCA results indicate that at least three components are required to adequately describe the variance in
2+
2+
2+
the datasets (Appendix 4). For the Alluvium, the first component comprises Mg , TDS, Cl , Ca , Mg ,
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
2-
-
2-
SO4 , HCO3 which accounts for 43% of the variance, followed by CO3 and pH in component 2 (21%).
As for the Tertiary Volcanics, component 1 in the PCA results accounts for 38% of the total variance in the
2+
2+
dataset, with TDS, Na , Cl and SO4 as dominant constituents. The second component, which Ca and
2NO3 are associated with, accounts for 16%, followed by CO3 and pH (15% of variance).
Table 36 Principal Component Analysis Results for the Alluvium and Tertiary volcanics
Principle Component (%
Total Variance Explained)
Parameters
Alluvium
1 (43%)
Cl
TDS
SO4
HCO3
Ca
Mg
Na
2 (21%)
pH
CO3
Tertiary Volcanics
1 (39%)
2 (16%)
TDS
NO3
Na
Ca
Cl
Mg
SO4
K
CO3
Mg
PCA results (Table 36) indicated that three components were required to adequately describe the
variance in the dataset available for the Tertiary Volcanics. Similar to the alluvium, the first component
2+
+
(accounting for 39% of the total variance) yielded strong loadings for Cl , TDS, SO4 , HCO3 , Na and K .
The two other components accounted for a very similar amount of the total variance (16% and 15%) and
22+
had strong loadings of Ca and NO3 and CO3 and pH, respectively.
In the Alluvium, four clusters were identified by the k-cluster analysis (Appendix 4). The distribution of
various cluster types is quite variable, and a most likely related to local influences such as:
•
Elevation and terrain;
•
Geology of the catchment ;
•
Mineralogy of the sediments;
•
Local hydraulic properties; and
•
Interactions with surface water bodies, etc.
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LEGEND
Cluster - Descriptor
1 –High TDS, intermediate evolved
2 – Basalt
3 – low TDS, somewhat evolved
4 – Recently recharged
b15 fault
b70 fault
Northern Bowen Faults
Figure 36 Spatial Distribution of the k-means Clusters for the Alluvium
High TDS groundwater associated with Cluster 2 do appear to occur more frequently along the eastern
margins of the Surat Basin, and northward along the eastern margins of the Bowen Basin, which may
align with accumulations of basalt in these areas. Mean TDS values for the four clusters range from 347
mg/L to 7,507 mg/L. The lowest mean pH value (6.9) encountered in all the hydrostratigraphic was
associated with Cluster 3 of the alluvium. Groundwater in the alluvium varies in hydrochemical type but is
overall consistent with recharge water that has percolated through overlying soils and reflects various
degrees of carbonate, sulfate, and silicate mineral weathering.
Similarly, the hydrochemistry of bores associated with the Tertiary Volcanics (Appendix 6) is considered to
reflect localised conditions, with no apparent spatial patterns with respect to the different hydrochemical
clusters.
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Table 37 k-means Cluster Results for the Alluvium and Tertiary Volcanics
Alluvium
Cluster
N
1
437
2
3
4
Statistics
Mean
Stdev
Mean
307
Stdev
Mean
288
Stdev
Mean
265
Stdev
Mean
All
TDS pH
1178
690
7507
9130
347
207
512
176
2320
5264
8.0
0.4
7.7
0.5
7.5
0.4
6.9
0.3
7.6
0.6
(CO3+
Na/Cl SAR HCO3)/Cl SO4/Cl F/Cl
1.6
1.6
0.8
0.3
1.7
1.0
1.2
0.3
1.3
1.1
8
13
16
12
3
3
2
1
9
13
1.8
2.3
0.3
0.3
2.6
2.8
2.7
1.2
1.8
2.2
0.075
0.089
0.143
0.173
0.076
0.118
0.034
0.023
0.06
0.09
5.7E-03
1.4E-02
3.6E-04
1.2E-03
7.9E-03
9.5E-03
4.6E-03
3.3E-03
4.7E-03
1.0E-02
NO3
SiO2
(mg/L) (mg/L)
4.6
9.3
8.6
14.4
3.9
6.4
0.6
0.8
4.3
9.4
46
21
35
14
39
17
54
9
44
18
K
(mg/L)
3.6
4.6
8.2
9.4
4.1
3.8
0.8
0.5
4.3
6.2
Tertiary Volcanics
Cluster
N
1
186
2
3
4
All
Stats
Mean
Stdev
Mean
98
Stdev
Mean
92
Stdev
Mean
43
Stdev
Mean
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TDS pH
824
235
352
109
2793
1820
505
199
1108
1256
8.1
0.3
7.8
0.4
8.0
0.4
8.4
0.2
8.0
0.4
(CO3+
Na/Cl SAR HCO3)/Cl SO4/Cl F/Cl
2.3
1.8
3.7
2.6
1.0
0.4
4.9
4.3
2.6
2.5
4
3
2
2
7
5
13
6
6
6
4.1
3.7
9.1
5.9
0.8
0.7
5.0
5.3
4.6
5.0
0.114
0.135
0.113
0.136
0.158
0.173
0.110
0.134
0.12
0.15
9.3E-03
1.8E-02
3.2E-02
2.6E-02
1.1E-03
1.3E-03
3.6E-02
8.1E-02
1.5E-02
3.3E-02
Nitrate Silicate K
mg/L) (mg/L) (mg/L)
13.1
16.0
3.5
4.9
13.1
21.1
2.1
2.8
9.9
15.6
52
15
52
23
54
18
22
11
50
20
2.8
2.7
2.0
3.3
7.7
5.1
2.0
2.0
3.6
4.0
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
LEGEND
Cluster - Descriptor
1 – Intermediate TDS, evolved
2 – Recent recharge
3 – Intermediate to high TDS, volcanics
4 – Low TDS, evolved
b15 fault
b70 fault
Northern Bowen Faults
Figure 37 Spatial Distribution of the k-means Clusters for the Tertiary Volcanics
Iso t op es
14
13
Isotope data for the Cainozoic units is limited to one sample in the Tertiary Volcanics for C, δ CDIC,
18
2
δ OH2O and δ HH2O (Appendix 5). The groundwater age identified (27,970 years BP) suggests that the
bore is located some distance from a recharge area, and assuming a groundwater flow velocity between 1
13
and 3 m/a, would be of the order of 28 to 84 km. The δ CDIC value of -18.1‰ is consistent with
mineralization of local vegetation, but is slightly lighter isotopically than expected suggesting some other
18
2
fractionating processes at play. With respect to the δ OH2O and δ HH2O values (-6.4‰ and -40.6‰,
respectively), these lie quite close to the Local Meteoric Water Line (established in BRS and NRW, 2003)
and are consistent with local recharge.
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6
6.1
GROUNDW ATER FLOW REGIMES
Recharge and Discharge Phenomena
Nat ur al P ro ce s se s an d F eat u r e s
Research conducted on the recharge phenomenon in the GAB has indicated that replenishment to the
groundwater systems primarily occurs in well-defined areas. Infiltration of rainfall to intake beds exposed
at outcroppings of permeable material generally occurs within the elevated northern and eastern margins
of the area (BRS and NRM, 2003). Recharge can also occur via leakage of water from creeks and rivers
that have eroded into permeable formations (BRS and NRM 2003, Habermehl 2002). Percolation through
unconsolidated sediments which overlie regional aquifers can also facilitate some recharge to the
groundwater systems, but this is considered a minimal contribution.
A more complete review of recharge phenomena is summarised in the literature review associated with
Task 1 of this study (WorleyParsons, 2010).
Natural discharge from the confined aquifers in the basin occurs by the following means (Habermehl, 2002
and BRS and NRM, 2003):
•
Baseflow to rivers;
•
Vertical upward leakage from the Lower Cretaceous-Jurassic age aquifers towards the Cretaceous
age aquifers and regional water table; and
•
Subsurface outflow into neighbouring basins.
Concentrated outflow from springs is also another important discharge mechanism. Most springs in the
GAB are concentrated in groups, and are therefore distributed over relatively small areas (Figure 38).
These discharge features tend to occur at the contact between a formation of higher permeability and an
underlying formation of lower permeability (a contact spring),as a result of structural features or resulting
pathways, such as faults, folds, monoclines and intersecting lineaments. The latter type tends to occur at
the abutment of aquifers against lower permeability bedrock or where confining beds thin near discharge
margins. Diffuse discharge from the artesian aquifers through the confining beds towards the ground
surface is known to occur in the down-gradient, southwest margin of the Surat Basin near the Eulo-Nebine
Ridge (Figure 38). In this area, overlying and underlying confining beds are relatively thin (Rolling Downs
Group) or non-existent (Westbourne Formation) and vertical hydraulic gradients between the aquifer
formations are comparatively high. Therefore groundwater surface elevations tend to be quite shallow or
in excess of the land surface resulting in mound spring complexes (Woods et al., 1990).
Groundwater springs can be classified as either recharge or discharge springs. Recharge springs are
common in recharge areas along the northern margin of the Surat Basin and further north into the Bowen
Basin. These springs generally fall within the Springsure Supergroup and represent discharge features for
local flow systems within defined recharge areas (i.e., short flow-path types).
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
Figure 38 Location of Major Spring Groups and Potential River Baseflow Interaction (Dept. of
Natural Resources and Mines, 2005) (red box indicates the location of the Bowen and Surat basins and
grey areas indicate the location of major intake beds)
Springs nearer the centre of the Surat Basin and to the southwest Supergroup typically represent
discharge springs (i.e., longer flow-path types). These springs commonly have a deeply buried structural
control such as faulting, folding, monoclines and intersecting lineaments (Habermehl, 2002). Groundwater
is transmitted along open faults, or where the hydrostatic pressure has broken through thin confining
formations (Habermehl, 1980). Other natural discharge features of the Bowen and Surat basins include
upward diffusion of groundwater between aquifers and subsequent loss through evapotranspiration from
the water table and discharge to streams as baseflow.
Ant h rop o g en i c Ef f e ct s
Free or controlled artesian flow and pumped abstraction from the various aquifers currently form the major
anthropogenic influence on groundwater conditions in the Bowen and Surat basins. Recent depressuring
of deeply buried coal seams in the Bowen and Surat basins for the production of coal seam gas has
lowered the potentiometric surface within those intervals in local areas. Further development will increase
this effect and will eventually influence adjacent aquifers (i.e., Springbok Sandstone). Drawdown in the
various coal measures can reach several hundreds of meters in areas where they are deeply buried.
Because the coals seams tend to be relatively thin (<3 m) but numerous, production intervals span large
vertical distances – possibly several hundred meters. The net effect is an extended zone of lower
hydraulic pressure, which will act as a sink for water originating from adjacent units. Pathways for fluid
transfer may be via direct hydraulic connection between the CSG zone and an overlying or underlying
aquifer interval, or more diffuse through an intervening aquitard.
The competency and thickness of the aquitard will play a major role in the magnitude of effect. Thicker,
more competent aquitard intervals, like the Westbourne Formation in the Surat Basin, serve to diminish
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
drawdown effects and resulting fluid transfer that may be occur between adjacent formations due to large
groundwater diversions from CSG production or other large entitlements.
An important fact to remember is that water levels can fluctuate in otherwise discrete aquifer intervals
without fluid transfer occurring. The associated change in the subsurface stress field accompanied by a
reduction (pumping) or increase (injection) in pore pressure in a confined interval may still cause water
levels to move in adjacent formations based on changes to the balancing forces of effective stress (grain
to grain contact) and fluid pore pressure that support total stress applied by the overlying bedrock and
sediments.
6.2
General Groundwater Flow Patterns
General groundwater flow patterns in the Bowen and Surat basins have been identified to be controlled, to
a large degree, by regional topography (i.e., Great Dividing Range) and buried structural features defining
the limits of the basins (e.g. Eulo-Nebine Ridge, Kumbarilla Ridge). Figure 39 provides an example,
showing the patterns of flow in the Surat Basin, south down to the Coonamble Embayment in New South
Wales and across the Eulo-Nebine Ridge to the west.
Figure 39 General Groundwater Flow Patterns in the Upper Jurassic and Cretaceous Intervals of
the Surat Basin and Adjacent basins (Radke et. al.,2000)
Much of the groundwater flow originates from the northern and eastern margins of the Surat Basin, in and
around the major recharge zones for the system. Age-dating in previous studies, using radiogenic
isotopes such as chlorine-36 (half-life = 301,000 ± 4,000 years) and carbon-14 (half-life = 5,730 ± 40
years), has indicated groundwater flow velocities of the order of less than 1 m/a up to 3 m/a east of the
Bowen and Surat basins(Radke et al., 2000). These results imply an extended residence time of the pore
water in the various water-bearing intervals, which of course will influence attendant pore water chemistry.
Areas of very low flow velocities and potentially areas of flow stagnation or flow isolation have also been
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
cited as reasons for the variable groundwater quality conditions noted in a number of the major aquifer
intervals beneath both basins.
A general flow divide is noted in the vicinity of the Eulo-Nebine Ridge area, where groundwater movement
occurs parallel to sub-parallel along the ridge from northeast to southwest. Similarly, the Great Dividing
Range acts as a regional flow divide between areas to the north and east.
Natural discharge from the Surat Basin occurs as concentrated outflow from springs ranging from 1 to 150
L/sec (Habermehl, 2002), as well as vertical leakage from the Lower Cretaceous–Jurassic aquifers
towards the Cretaceous aquifers and upwards to the regional water table (Habermehl, 2002). Sub-surface
discharge to neighbouring basins is also a major component of natural groundwater movement from the
Surat Basin to adjoining areas.
Water quality results discussed in Section 5 are consistent with the groundwater flow patterns derived by
others, and indicate some notable spatial patterns with respect to the lateral distribution of the various
aquifers and confining aquitard layers. As well, variability within discrete aquifer intervals are noted, which
aligns with the notion of:
•
Recharge along the northern and eastern margins of the basin;
•
Down-gradient evolution of the pore water as a result of physical and chemical processes;
•
Interaction between certain units leading to somewhat unique chemistry in the receiving interval;
and
•
Discharging conditions to the southwest near the Eulo-Nebine Ridge area.
Major recharge areas for the Late Jurassic and Cretaceous formation in the Surat Basin are shown in
Figure 40. Unfortunately, major recharge areas have not been mapped for the Bowen Basin, but are
inferred to exist along the western edges of that basin. Major recharge areas associated with both basins
are generally situated in upland areas, where formations are exposed at surface and receive direct or
diffuse infiltration of precipitation. Conversely, recharge areas can occur where permeable formations (i.e.,
Precipice, Hutton and Kumbarilla beds), exist in direct contact with overlying aquifers comprising
unconsolidated and permeable materials (i.e. Alluvium). Recharge is also suspected to occur from
perennially flowing rivers cutting across these permeable formations and delivering water to them via
leakage through the streambeds (Figure 41 and Figure 42).
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Figure 40 Location of Major Recharge Areas for Surat Basin (Radke et. al., 2000)
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
Area 1
Figure 41 Locations along the eastern edge of the Surat Basin where
recharge via riverbed losses is suspected (AGE, 2005)
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
Area 2
Area 3
Figure 42 Locations along the northern edge of the Surat Basin where recharge via riverbed losses
is suspected (AGE, 2005)
As discussed in Sections 2.2.2 and 2.2.3, groundwater contouring and residual head mapping was used to
help interpret the potential directions of water movement within and between major formations.
Groundwater flow directions generated from water levels present in the unified database indicate similar
results as others (Radke et al., 2000, Hodgkinson et al., 2009). For the Bowen Basin, a general flow
direction from the uplands flanking the west side of the Great Dividing Range towards the low-lying areas
in the eastern portions of the basins is indicated for the Upper Permian Sandstones (Appendix 7, Figure
1), Rewan Group (Appendix 7, Figure 2) and likely for the Tertiary formations. Groundwater flow was
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
generally from the western and southern outcrop areas flowing towards the centre-east of the basin,
where the Funnel River and Lake Mary have been identified as potential regional discharge features.
Local groundwater flow systems have been identified along the basin margin, where sub-artesian
groundwater flow direction is predominantly controlled by the surface topography.
In the Surat Basin, groundwater flow originates in the north to northwest portion of the basin and generally
moves towards the south and southeast in the Jurassic formations (Precipice Sandstone, Hutton
Sandstone and Walloon Subgroup and Springbok Sandstone); however, data was restricted to the
northern portion only (Appendix 7, Figures 3, 4, 5 and 6, respectively). With respect to groundwater flow in
the shallower and overlying Late Jurassic to Cretaceous, direction off low tends to be more consistently
from the north towards the south for the Gubberamunda Sandstone, BMO Group and Rolling Downs
Group (Appendix 7, Figures 7, 8 and 9, respectively). As for the uppermost Cainozoic Units, a general
east to west direction is anticipated, but local patterns are expected to be highly variable due to natural
land type, interaction with drainage features, and anthropogenic influences (i.e. long-term water
withdrawal).
General vertical flow conditions prior to human major human development are shown in Figure 43 for the
Cretaceous formations of the Surat Basin; indicating the occurrence of near-neutral or slightly downward
vertical hydraulic gradients in the recharge area along the basin margins where the various formations
outcrop. This area has been described by others as the sub-artesian zone of the GAB. Within 100 km or
less from the recharge areas, conditions shift to artesian and remain so across the rest of the basin.
Figure 43 General Vertical Flow Conditions in the Cretaceous Formations of the Surat Basin before
1880 (Radke et al., 2000)
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This is in line with the existing conceptual hydrogeological model which assumes a degree of mixing in the
heterogeneous recharge zone, which then develops a homogenous water type along the flow path before
becoming confined and isolated at depth, as it moves towards the basin centre (Radke et al., 2000).
The regional scale groundwater flow regimes identified by previous studies in Surat Basin (Audibert, 1976
and Habermehl, 1980) were confirmed in the current investigation by incorporating data points previously
omitted due to the absence of surveyed bore locations. This has resulted in increased resolution of
potential local flow regimes in the sub-artesian region of the basin margins, particularly in the shallow
aquifers (i.e. BMO aquifer). Potentiometric surfaces of the Surat Basin aquifers indicate a south to southwesterly regional groundwater flow direction towards the centre of the Surat Basin. Meanwhile, work
conducted by Hodgkinson et al (2010) for the Precipice, Evergreen and Hutton intervals indicates a more
complex flow pattern of a semi-radial fashion outward from two main areas of the basin – the northwest
portion and the southeast corner. Residual head mapping between the Precipice and Hutton intervals
(Figure 21) indicates the potential for upward flow from the Precipice Sandstone to the Hutton Sandstone,
in areas along in the northern margin of the basin and an area near the centre of the basin.
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
7
SP ATI AL V ARI ABILITY OF GROUNDWATER CHEMISTRY
The chemical composition of groundwater is controlled by the interaction between recharging precipitation
and various minerals present in soils and rock formations. Recharging precipitation tends to be slightly
acidic by nature. When in equilibrium with atmospheric CO2 , pH of rainfall is in the order of 5.6. If other
significant acid-forming gases are present (nitrogen-oxides (NOx ) or sulfur-oxides (SOx ), the pH can
become even lower and thus more aggressive as a weathering agent.
Processes of microbial degradation of organic matter residing in soil horizons or aquifer formations also
influence the weathering potential of groundwater as it moves through the subsurface. In short, the
evolution of groundwater represents a continuum of interactions and processes resulting in the chemical
quality of the water that is produced from a well, discharged from a spring or contributed to a water body
as baseflow or seepage.
In Section 3.1of this document, background information was presented on the geology of each major
formation, or grouping of formations, located within the Bowen and Surat basins. As previously mentioned,
the predominant minerals identified in the Surat Basin formations are, in approximate order of abundance:
quartz, feldspars and kaolinite (Grigorescu, 2011). Other secondary minerals such as mixed-layer
silicates, carbonates (e.g. calcite, siderite) and iron oxides (e.g. goethite, hematite) have a localised
occurrence. To date, the mineralogical study of the various formations in the Bowen Basin has been
limited to a few samples; however, a mineral composition similar to the Surat Basin has been observed.
General Weathering Reactions
7.1
A number of weathering reactions have been identified as dominant influence to major pore water
chemistry (Radke et al. 2000). Additional information pertaining to these weathering reactions is provided
in Appendix 8. In general, the following statements can made which apply to both the Bowen and Surat
basins:
•
Mineral weathering is the dominant process responsible for solution chemistry, with various ions
being released and pH conditions becoming more alkaline as the pore waters react with minerals
along established flow paths;
•
Na , Ca , K are sourced from the feldspar minerals plagioclase (Ca
+
present, and orthoclase (K ), referred to herein as K-feldspar;
•
Mg
•
HCO3 is predominantly sourced from microbial degradation of organic matter (that forms CO2 ) and
2the dissolution of carbonate minerals (that forms CO3 ). Some is likely associated with weathering
minerals, like feldspars and clays. Microbial degradation is also responsible for the formation of
methane in highly reducing conditions;
•
SO4 is suspected to be linked to a marine source (i.e., leaching of saline pore water trapped in
sediments during deposition) and oxidation metal-sulfides, if present;
•
Cl is a highly conservative (recycled in the system) ion that is sourced from marine aerosols
incorporated in recharged precipitation, and leaching of saline pore waters trapped in sediments
during deposition;
+
2+
2+
+
2+
+
+
and Na ), albite (Na ) if
is sourced from weathering of chlorite and other silicate minerals including micas;
-
-
2-
-
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
•
Overall mineralization of the various groundwaters is a function of residence time and contact
between the pore water and minerals. The type of minerals present and their weathering potential
is a controlling factor;
•
Removal of Ca and Mg ions from solution following exchange with of Na ions present on clay
2+
2+
mineral surface is considered a dominant control on Ca and Mg concentrations. The result is
increasing SAR values as the water evolves. Some carbonate mineral precipitation may also occur,
2+
2+
removing Ca and Mg ions from solution; and
•
Marine influences have been minor and episodic during the course of sedimentation of the Bowen
and Surat basins. The predominant depositional settings have been fluvial-continental to marginalmarine, with extensive coal-forming swamps and estuaries.
2+
2+
+
The following review of groundwater quality conditions and potential interactions between formations in
both the Bowen and Surat basins is predicated on variations in mineralogy, weathering and mineral
exchange reactions, mineral precipitation reactions (under the appropriate conditions) and other minor
reactions in response to variable solution chemistries, pH conditions, redox states and subsurface
temperature conditions.
7.2
Spatial Anal ysis of Hydrochemical Parameters
The spatial variability of certain parameters pertaining to hydrochemical evolution of groundwater in the
Bowen and Surat basins was assessed to determine particular areas and patterns in groundwater quality
that might help clarify origins and interactions. For ease of viewing, major aquifer formations within the
Permian time sequence (i.e., Bowen Basin) were presented with the Jurassic formations of the Surat
Basin (Precipice to Springbok formations). Similarly, data associated with the Late Jurassic to Cretaceousaged formations above the Westbourne Formation (i.e. Gubberamunda Sandstone to BMO Group) were
presented with the data from the more recent Alluvium and Tertiary Volcanics.
The decision to present the data as groupings is based on the general dominance of more marginalmarine type depositional settings in the shallower formations(Gubberamunda Sandstone and shallower)
as opposed to more continental-fluvial type settings (including coal-forming swamps) prevailing in the
deeper formations (Springbok to Precipice Sandstones).
To support assessment of water-type origins, and potential weathering reactions leading to the associated
pore water chemistry, mineral saturation indices (SI) were calculated for a select set of minerals generally
present in the various aquifers based on mineralogy (primarily silicates and smaller amounts of
carbonates). A summary of results for calcite (an analogue for carbonates), quartz and albite (an analogue
for feldspars, or the more crystallised aluminosilicates) is provided in Table 38.
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Table 38 Summary of mineral saturation conditions (SI values) in Bowen and Surat formations for
calcite, quartz and albite.
Formation or
(Sub)Group
Mineral SI values calcite
Count
% below
saturation
Mineral SI values quartz
Count
% below
saturation
Mineral SI values albite
Count
% below
saturation
Alluvium
1491
43%
1073
1%
131
17%
BMO Group
1417
49%
654
1%
48
63%
Gubberamunda SST
1430
58%
868
1%
134
100%
Springbok SST
123
36%
46
0%
--
--
Walloon Subgroup
497
26%
143
4%
12
58%
Hutton SST
397
52%
226
4%
12
75%
Precipice SST
340
83%
207
2%
22
95%
Clematis SST
288
99%
293
1%
8
100%
37
22%
24
0%
24
0%
Lower Permian SST
Note: SST = sandstone
Results for calcite indicate a higher percentage of negative (-) SI values for the Clematis and Precipice
Sandstones indicating conditions favourable for dissolution of this mineral, as opposed to mixed conditions
for the other intervals. This result is consistent with the mineralogy data presented in Section 3.1, where a
lack of carbonate minerals is noted for the Precipice Sandstone, as opposed to overlying formations. With
respect to quartz, all formations indicate under-saturated conditions with respect to this silicate mineral
and a high potential for weathering. As such, the presence of silica (as SiO2 ) concentrations in the
various formation waters may be attributed to dissolution of this mineral, considering its dominance of the
mineral mix of most formations (Section 3.1). As for the feldspars (i.e. albite), with the exception of the
Lower Permian sediments and the Alluvium, the remaining formations indicate conditions favourable for
mineral dissolution. Higher percentages of negative SI values are noted for the Clematis, Precipice,
Hutton and Gubberamunda Sandstones. Weathering of plagioclase (as well as micas) would account for
2+
+
+
some of the Ca , Na and K ions present in the associated pore waters of these, and the other,
formations.
7.2.1
Temperat ure
Assessment of groundwater temperatures within a regional setting can provide very useful information
regarding groundwater flow phenomenon (e.g. recharge versus discharge) and the potential for various
reactions that may influence groundwater chemistry and suspected origins. For example, diffusion rates
typically increase at higher temperatures, as does the solubility of certain minerals. The reverse is true for
most carbonate minerals given their retrograde solubility. As a general rule, the rate at which a chemical
reaction proceeds doubles for every 10°C increase in temperature (Langmuir, 1997). Diagenetic
processes resulting in the formation of authigenic mineral overgrowths can result causing considerable
reduction in primary porosity and associated permeability. Therefore, deeper sections of basins exhibiting
elevated temperature conditions are likely to have significantly reduced hydraulic conductivity due to
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mineral in-filling of pore and fractures spaces, and hence lower groundwater flow rates. Evidence of this
effect in the GAB aquifers has been documented by other researchers (Radke et al., 2000).
With respect to the Bowen Basin, the density of data points is somewhat sparse or clustered (Figure 44).
The lack of significantly elevated groundwater temperatures in the bores assessed suggests that most are
located in or near recharge areas or at shallower depths. The cluster of bores located in the southern part
of the basin displaying comparatively higher temperatures than adjacent wells is of note, and possibly an
indication of cross-formational flow from deeper, warmer intervals. The unit represented by this cluster of
values is the Clematis Sandstone.
P to J
Late J to T
Figure 44 Groundwater Temperatures (in ºC) for the Permian (P) to Jurassic (J) formations (left
panel) and Late Jurassic (Late J) to Tertiary (T) formations (right panel): brown and purple lines
indicate locations of identified faults; major recharge areas have been provided for reference.
With respect to the Surat Basin, the general trend is one of increasing groundwater temperatures towards
the basin centre, with natural geothermal gradients of about 20°C/km to 30°C/km (Grigorescu, 2011a). At
the elevated temperatures noted in the basin centre, rates for chemical reactions could be expected to be
10times that of reactions occurring in the near surface. As such increased mineralisation of the water
(TDS content) is to be expected due to increased solubility and weathering reaction. Variable values for
mineral SI were noted for the mineral calcite (Table 38) with most of the formations indicating dominantly
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saturated conditions. Notable exceptions include the Clematis and Precipice Sandstones which are
dominated by geochemical conditions consistent with mineral dissolution
Areas of lower groundwater temperatures exist where active recharge is anticipated to be occurring. This
is primarily noted along the north and east margins of the Surat Basin. The occurrence of comparatively
lower groundwater temperatures along the western edge, near the Eulo-Nebine Ridge area, supports
previous conclusions for the potential downward movement of cooler groundwater water from shallower
formations and mixing with the comparatively deeper pore waters (Radke et al.,2000).
7.2.2
Total Dissol ved Sol ids
Mineral weathering and dissolution reactions control the concentration of dissolved species in natural
waters. This results in mineralisation of the pore waters, which is often measured as total dissolved solids
or TDS content. Infiltrating water sourced from precipitation starts as very low TDS water (e.g. TDS less
than 50 mg/L). As the water enters the soil, reactions occur that add dissolved ions and increase the TDS
content. Typical reactions include the formation of low molecular weight organic (LMWO) acids, ion
exchange, dissolution of soluble salts precipitated in the soil horizon, acid weathering of soil minerals and
so on. The types of minerals present in the soil and rock matrix will dictate the degree to which weathering
occurs, and hence the resulting pore water chemistry. For instance, silicate minerals tend to be more
resistant to weathering reactions than carbonates or sulfates.
Figure 45 shows the distribution of TDS values of groundwater for the various formations in the Bowen
and Surat basins. Although the greater majority of data points identified in the figures indicate
comparatively low TDS values (less than 2,000 mg/L), there are some that exceed 5,000 mg/L which
implicates mixing of fresher pore water with more saline pore water. Further review of the spatial
distribution of TDS values indicates some interesting patterns.
For example, the higher TDS values in the aquifers associated with the Jurassic formations of the Surat
Basin (left panel of Figure 45) tend to be located in the area consistent with the Walloon Subgroup.
Outside of this area, TDS values appear lower in magnitude, with a slight increasing trend towards the
basin centre (i.e., the direction of groundwater flow). The occurrence of higher TDS values in the Walloon
Subgroup is suspected to be linked to geochemical and weathering reactions occurring in this interval.
The mineralisation of the organic matter (i.e., coal) and enhanced mineral weathering reactions is
suspected. The occurrence of a greater proportion of more easily weathered minerals (refer Section 3.1)
compared to other formations is apparent. Similarly, the likely occurrence of saline connate waters
trapped during deposition of the mudstones, siltstones and sandstones followed by leaching of these salts
by the resident pore waters subsequent to burial is also suspected. The considerable variation of TDS
values within the various formations is a testament to their variable hydraulic properties, the degree of
water-rock interaction, differing mineralogy and the settings within which they were deposited.
Conversely, the lack of similarity in pore waters between discrete aquifer intervals provides some
evidence for hydraulic isolation and lack of cross-formational flow.
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P to J
Late J to T
Figure 45 Total Dissolved Solids (TDS) (in mg/L) for the Permian (P) to Jurassic (J) formations (left
panel) and Late Jurassic (Late J) to Tertiary (T) formations (right panel): brown and purple lines
indicate locations of identified faults; major recharge areas have been provided for reference.
With respect to the late Jurassic and Tertiary formations, higher TDS values are located near areas where
the formations are exposed at or near surface, or are in contact with the overlying Rolling Downs Group.
Leaching of soluble salts from these marginally marine dominated formations, or direct mixing of pore
waters, would appear to be a contributing factor to the chemistries of the BMO Group and to some degree
the Gubberamunda Sandstone.
The apparent freshening of pore waters with proximity to the Surat Basin centre is interesting considering
that groundwater temperatures are increasing. As such, one would expect to see increased salinity of
groundwater due to increased residence time and associated mineral dissolution and weathering
reactions; likely due to elevated groundwater temperatures. As noted in Section 3.1.2, mineralogy of the
Precipice Sandstone is dominated by quartz, while mineralogy of the Hutton Sandstone is dominated by
quartz and feldspars. Both of these minerals possess a low susceptibility to weathering and the kinetics of
the weathering reactions tend to be slow. Mineralogy for the Hooray Sandstone (equivalent of the
Gubberamunda and Mooga Sandstones) is also quartz-dominated. Therefore, lower TDS values would
reflect the lack of easily weathered minerals.
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7.2.3
Al kalinit y
Alkalinity is a useful indicator of mineral weathering and hydrochemical evolution (Radke et al., 2000).
This parameter represents the acid neutralising capacity of a water sample and consists of the sum of
titratable carbonate and non-carbonate chemical species in a filtered sample. Carbonate alkalinity is
2typically associated with the ions HCO3 and CO3 , while non-carbonate alkalinity is associated with any
+
other constituent with the ability to neutralise hydrogen (H ) ions (i.e., inorganic ligand or organic
molecule). Alkalinity is formed during reactions resulting between various minerals and carbonic acid.
Similarly, dissolution of carbonate minerals (e.g. calcite) will produce carbonate anions. Examples of these
reactions are presented in Appendix 8.
Figure 46 shows total alkalinity for the Permian to Jurassic formations and Late Jurassic to Tertiary
formations. The pattern of data distribution in the Bowen Basin is quite variable with elevated values
occurring along the flanking areas as well as towards the basin centre. Of note again is the cluster of
values in the southern part of the basin, which coincides with the Clematis Sandstone. The grouping of
comparatively elevated values to the southern end of the cluster indicates higher values than the northern
2end. Considering the presence of elevated SO4 in the upper recharge areas, this suggests possible
mixing with pore waters from shallower intervals, and hence an area of interaction. Limited potentiometry
and residual head contours also indicate that this area is close to a zone where a neutral gradient exists
between the Rewan Group and underlying Upper Permian coals, and where the presence of identified
faulting exists. As such, the existence of a possible hydraulic communication exists between those
intervals, and possibly the Clematis given its rather distinct pore water chemistry.
In contrast to the Bowen Basin, the spatial distribution of alkalinity in Jurassic formations of the Surat
Basin follows a somewhat predictable pattern from lower values near the recharge areas (<250 mg/L), to
higher values in the Walloon Subgroup situated in along the eastern edge (>500 mg/L). This pattern is
followed by progression of more elevated values in the Hutton and Precipice Sandstones towards the
centre of the basin (i.e.>250 mg/L), and is indicative of mineral reactions and natural hydrochemical
evolution.
Based on the mineralogy of the various formations comprising the Jurassic, much of this alkalinity would
appear to be associated with dissolution of carbonate minerals, such as calcite and siderite, as well as a
2+
contribution from the weathering of feldspars (primarily Ca -based given their higher weathering potential
+
+
as opposed to Na or K -based forms. Except for the Precipice Sandstone, mineral saturation conditions
for calcite are generally under-saturated in the remaining intervals (Table 38).
The occurrence of alkalinity in the overlying Late Jurassic and Tertiary formations of the Surat Basin
follows a similar pattern with a propensity for higher values in more down-gradient areas (i.e., evidence of
hydrochemical evolution). Of note is the general clustering of lower alkalinity values in the north-western
as opposed to the north-central and eastern areas of the basin. This would suggest the occurrence of
different mineralogy, different geochemical conditions, or a combination of both.
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P to J
Late J to T
-
Figure 46 Total Alkalinity (in mg/L as CaCO3 ) for the Permian (P) to Jurassic (J) formations (left
panel) and for Late Jurassic (Late J) to Tertiary (T) formations (right panel): brown and purple lines
indicate locations of identified faults, major recharge areas have been provided for reference.
7.2.4
Sulfate
2-
The distribution of sulfate (SO4 ) concentrations in the Bowen and Surat basins shows an almost inverse
relationship to that of alkalinity, with higher concentrations tending to occur near the recharge areas and
then remaining relatively consistent across each basin (Figure 47). The rather anomalous configuration of
values in the cluster associated with the Clematis Sandstone again stands out, and suggests the presence
of anomalous groundwater conditions.
2-
In the Surat Basin, comparatively elevated concentrations (>100 mg/L) of SO4 are associated with the
2Walloon Subgroup along the east side of the basin. Other notable features include a cluster of lower SO4
values (<5 mg/L) near the northern end of the Burunga-Leichardt fault zone, and where the Precipice
Sandstone is close to, or exposed at, surface.
In the Late Jurassic formations, a swath of elevated concentrations (>500 mg/L) is noted in the northern
recharge area, with lower concentrations (<10 mg/L) occurring in the eastern recharge areas, and at
locations hydraulically down-gradient. In turn, sulfate values on the eastern side of the basin are
noticeably lower. This pattern roughly coincides with the lateral distribution of the overlying Rolling Downs
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Group. Leaching or direct interaction of sulfate-laden pore water from that marginal-marine dominated
unit into the BMO Group and possibly the Gubberamunda Sandstone would explain this configuration.
P to J
Late J to T
Figure 47 Sulfate Concentrations (in mg/L) for the Permian (P) to Jurassic (J) formations (left
panel) and Late Jurassic (Late J) to Tertiary (T) formations (right panel): brown and purple lines
indicate locations of identified faults; major recharge areas have been provided for reference.
2-
The transition from high SO4 values in recharge areas to lower values in down-gradient areas suggests
the possible occurrence of sulfate-reducing conditions or precipitation (removal of sulfate from solution) of
sulfate minerals. However, to date, no major sulfide or sulfate minerals have been detected in these
formations (Grigorescu, 2011a). Given the dominance of under-saturated conditions for the mineral
gypsum, sulfate reduction would appear to be the reason.
2-
An area of generally low SO4 concentrations (10 mg/L or less) is also noted to occur with the south-west
corner of the Surat Basin near the Eulo-Nebine Ridge. This occurs in the area where the Hutton,
Springbok and Gubberamunda Sandstones are believed to have hydraulic interaction due to thinning of
the Eurombah, Walloon Subgroup and Westbourne Formation aquitard layers.
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7.2.5
Silica
In groundwater, dissolved silica (as SiO2) will be sourced from the weathering of aluminosilicate minerals
and quartz. The primary silicate-based minerals in the study area include (in order of least to most
weathering potential):
•
Quartz;
•
Plagioclase feldspars; and
•
Various clay minerals (kaolinite, illite and mixed-layers silicates (i.e. smectites).
The solubility of silicate minerals is greatly affected by temperature, redox conditions and the
concentration of other species in solution; where results indicate that higher groundwater temperatures
tend to yield higher concentrations of dissolved silica (as SiO2 ).
The distribution of dissolved silica concentratons for both the Bowen and Surat basins is shown in Figure
48. For the most part, dissolved silica concentrations in the Surat Basin are low in proximity to major
recharge areas, with a general increase in areas where higher groundwater temperatures are noted,
particularly in the Surat Basin. This pattern is consistent with normal mineral weathering and enhanced
solubility of silcates in relation to increased groundwater temperatures.
P to J
Late J to T
Figure 48 Dissolved Silica (in mg/L) for the Permian (P) to Jurassic (J) formations (left panel) and
Late Jurassic (Late J) to Tertiary (T) formations (right panel): brown and purple lines
indicate locations of identified faults; major recharge areas have been provided for reference.
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The propensity for higher magnitude values in the Bowen Basin is also noted, and is particularly
associated with the Tertiary-age sediments along the basin edges. Given the occurrence of comparatively
lower groundwater temperatures, it would appear that mineralogy plays a role with the occurrence of a
mineral species more susceptible to weathering (i.e. igneous minerals like micas and feldspars versus
quartz). Complex flowpath phenomena is also suspected as a factor, with extended residence time
occurring in less permeable intervals versus more well-connected and laterally continuous aquifers.
7.2.6
Fluoride
-
Fluoride (F ) is an anion derived from the element fluorine, which is one of the lightest and most reactive
halides in the periodic table of elements. Its presence in groundwater is typically associated with the
weathering of igneous or sedimentary rocks, or F -based minerals such as fluorite (CaF2). Fluorine can
substitute for hydroxyl (OH ) groups in the lattices of many minerals and therefore tends to accumulate in
granites and pegmatite dykes.
The propensity for higher magnitude values in the Bowen Basin is noted, and is particularly associated
with the Tertiary-age sediments along the basin edges. Given the occurrence of comparatively lower
groundwater temperatures in that basin, it would appear that the mineralogy plays a role with the
occurrence of a mineral mix more susceptible to weathering (i.e. more highly weatherable igneous
minerals like micas and feldspars versus quartz). Complex flowpath phenomena is also suspected as a
controlling factor, with extended residence time occurring in less permeable and continuous aquifer
intervals.
-
Figure 49 shows the distribution of F concentrations in both the Permian to Jurassic formations and Late
Jurassic to Tertiary formations of the Bowen and Surat basins. This distribution shows a remarkable
similarity to the distribution of total alkalinity, implicating weathering of feldspars and micas (with fluorine
present in the mineral lattice) as the cause, in turn generating HCO3 ions (see Appendix 2).
Concentrations exceeding 1.5 mg/L tend to cluster slightly down-gradient of the main recharge areas in
the Surat Basin, and remain elevated towards the basin centre. The highest values tend to be noted in the
BMO Group, followed by the Walloons Subgroup, Springbok Sandstone and then Hutton Sandstone. The
presence of the relatively thick Westbourne aquitard precludes any interaction between the Walloon
Subgroup and the BMO; however, the similarity of values with the Hutton Sandstone does suggest some
mixing of these waters.
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P to J
Late J to T
Figure 49 Fluoride Concentrations (in mg/L) for the Permian (P) to Jurassic (J) formations (left
panel) and Late Jurassic (Late J) to Tertiary (T) formations (right panel): brown and purple lines
indicate locations of identified faults; major recharge areas have been provided for reference.
7.2.7
Boron
Boron is one of the less abundant elements in nature and shares similar properties to its neighbouring
elements in the periodic table, carbon and silicon. In igneous rocks, boron tends to associate with such
minerals as tourmaline, biotite and amphiboles. In the sedimentary environment, boron is typically
associated with marine shales and deep-sea clays. Boron possesses similar physical and chemical
properties as silicon and is a temperature-sensitive element.
The distribution of boron across the Bowen and Surat basins is shown in Figure 50. For the most part,
concentrations of this dissolved constituent are <1 mg/L. Exceptions are particularly noted for areas near
the middle of the Bowen Basin (although data is sparse), and within the shallower Cretaceous to Tertiary
interval beneath the Surat Basin. The occurrence of elevated boron values generally occurs nearer the
deeper parts of the Bowen and Surat basins, or in the areas down-gradient from outcroppings of the BMO
Group and Gubberamunda Sandstone (along the northern and eastern edge of the Surat Basin). An area
of generally elevated boron concentrations also occurs along the south-western portion of the Eulo-Nebine
Ridge, along the west side of the Surat Basin. Because boron is a common constituent in seawater
(approximately 27 mg/L) the suspected cause of these elevated values is interaction with the overlying
Rolling Down Group formations, which were deposited in a predominantly marine environment. The
2occurrence of occasional elevated SO4 values (as noted in Figure 47) supports this conclusion.
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In contrast, the occurrence of elevated boron concentrations near the deeper portions of the two basins
implies weathering of boron-containing minerals like micas, or the possible liberation of this element from
clay mineral sites in response to elevated groundwater conditions.
P to J
Late J to T
Figure 50 Boron Concentrations (in mg/L) for the Permian (P) to Jurassic formations (left panel)
and in Later Jurassic (Late J) to Tertiary (T) formations (right panel): brown and purple lines indicate
locations of identified faults; major recharge areas have been provided for reference.
7.2.8
HCO 3 - to Cl - ratios
-
-
Figure 51 provides a review of the spatial variability of HCO3 to Cl ratios. Variability in the ratio values
and their relative distribution in a given area can provide insight into mixing phenomena between
formations of differing provenance (i.e., marine versus continental) and can identify areas where mineral
weathering is leading to more evolved pore water conditions.
-
-
From a review of Figure 51 it is apparent that the distribution of HCO3 to Cl ratios is complex, indicating
the occurrence of variable geochemical conditions throughout the Bowen and Surat basins. In the Bowen
Basin, the Permian Coal exhibits consistently elevated ratio values in excess of 4, indicating a higher
proportion of HCO3 as opposed to Cl . This is quite different from the Walloon Subgroup of the Surat
Basin where ratios were quite low, suggesting a different set of geochemical conditions. The return to
lower values in the Lower Permian Sandstone suggests an influence from recently recharged water in
areas where the associated sandstones are exposed at surface.
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A general pattern is noted for the Surat Basin, where lower values tend to occur along down-dip flanks of
exposed formations (i.e., BMO Group and Gubberamunda), followed by an increase in ratio values downgradient. This suggests carbonate mineral dissolution, biological mineralization of residual organic matter
in the sediments, and weathering of feldspars (see Appendix 8). The occurrence of lower ratio values in
the central parts of each basin reflects either the addition of Cl or the removal of HCO3 from the host pore
waters. Given the variability in calcite mineral SI values in the central Surat Basin, and occurrence of
carbonate mineral precipitation in response to the higher groundwater temperatures, release of Cl ions
from overlying marine units is suspected.
With respect to the Jurassic formations, a propensity for elevated values is evident particularly in the
northern section of the Surat Basin. Formation of HCO3 ions following the weathering of minerals
present, and the oxidation of organic matter to produce CO2 , is a likely cause. With respect to the
Walloon Subgroup and Springbok Sandstone, lower ratios are generally noted, which supports the notion
of connate water leaching, contributions of Cl to the associated pore waters, and mixing of pore waters
between those two formations. Lower ratio values may also result from CO2 reduction and the formation
2of methane gas, thus reducing the potential for CO3 or HCO3 ions to form.
Ratio values noted in the Hutton Sandstone again are quite variable, but tend to exhibit a higher density of
lower values compared to overlying formations. Conversely, in the underlying Evergreen Formation ratios
tend to exhibit comparatively higher values, which may be explained by increased weathering reactions
involving carbonate minerals and other less resistant igneous minerals (i.e., feldspars and micas).
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P to J
Late J to T
Figure 51 HCO3/Cl ratios for Permian (P) to Jurassic (J) formations (left panel) and Late Jurassic
(Late J) to Tertiary (T) formations (right panel): brown and purple lines indicate locations of
identified faults; major recharge areas have been provided for reference.Sodium Adsorption
Ratios (SAR)
SAR values provide an assessment of the dominance of the sodium ion over its counter-part cations,
2+
2+
Ca and Mg . Elevated SAR values tends to reflect waters having been in contact with the soil and rock
+
matrix for an extended period of time, either resulting from the release of Na ions due to ion exchange
reactions, mineral weathering, or interaction with marine-type waters (see Appendix 8). Such elevated
values are typically considered an indication of, hydrochemically-evolved, pore water.
Figure 52 provides a review of the spatial variability associated with SAR. In the Bowen Basin, SAR
values tend to be quite similar in magnitude and variability for both the Clematis Sandstone and the Upper
Permian Sandstones. The occurrence of consistently elevated values near the centre of the basin in both
the Upper Permian Sandstones and the Permian Coal suggests a possible interaction between these two
intervals; however, the absence of residual head measurements makes it difficult to substantiate this
hypothesis. Again, the values in the Lower Permian Sandstones are quite different from the overlying units
in that they tend to reflect comparatively lower values, suggesting hydraulic separation between this unit
and its overlying counterparts.
In the alluvial deposits, SAR values exhibit a considerable range from 1 to greater than 10. The majority
of elevated values reside in areas downgradient from the flanking recharge areas suggesting
hydrochemical evolution as the reason. The BMO Group and Gubberamunda Sandstone display this
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
pattern in the right panel of Figure 52. Clustering of elevated values is relatively consistent within the
eastern half of the Surat Basin.
Although data are sparse, SAR values in the Springbok Sandstone show similar magnitude values in both
the overlying Gubberamunda and underlying Walloon Subgroup. However, considering the expansive
thickness of the Westbourne Formation aquitard, interaction between the Walloon Subgroup and
Springbok Sandstone with shallower aquifer intervals is not considered the reason. Exceptions do exist in
areas where the aquitard thins (along the basin margins) and where surficial sediments rest in contact with
the exposed portions, along the northern and eastern margins of the basin. Interaction is, however, more
readily indicated between the Walloon Subgroup and Springbok Sandstone where the latter unit has
eroded through the top part of the Walloon interval and is in direct contact with the associated coal
intervals (Australia Pacific LNG, 2010).
Similarity between the clustering of values within the Hutton Sandstone and the Walloon interval,
particularly in the northern area of the Surat Basin and along identified fault features, is highly suggestive
of interaction between these formations. Assessment of the residual head map of the Hutton Sandstone
and Walloon Subgroup intervals (Figure 25) indicates a fairly neutral condition between the Hutton and
Walloon intervals, which suggests the potential for cross-formational flow via any open pathways through
the comparatively thin Eurombah aquitard.
There is an overall dissimilarity between SAR values for the Hutton and Precipice, with the exception of a
few consistently elevated values near the recharge areas of the Surat Basin. This finding is consistent with
the general understanding of the effectiveness of the Evergreen Formation aquitard in maintaining
hydraulic separation between these two formations.
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P to J
Late J to T
Figure 52 SAR Values for Permian (P) to Jurassic (J) formations (left panel) and Late Jurassic (Late
J) to Tertiary (T) formations (right panel): brown and purple lines indicate locations of identified faults;
major recharge areas have been provided for reference.
7.2.9
Isotopes
O x yge n an d D eu t e ri u m
All pairs of oxygen and deuterium isotope were plotted against the Local Meteoric Water Line (LMWL) for
2
18
the intake beds associated with these aquifers (equation: δ H = 7.618δ O + 10.883 in Figure
53;established in BRS and NRW, 2003). The LNWL represents where, on a plot of stable stable oxygen
and hydrogen, water samples are expected to fall when derived from precipitation. Deviations of data points
from this line can reveal certain information regarding processes and water-rock reactions.
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
-20
Exchange with
H2S
Silicate
hydration
Evaporative
trend
-25
Exchange
with CO2
-30
High temperature exchange
with minerals
Condensation
δ2H (‰)
-35
Tertiary Volcanics
BMO Group
-40
Gubberamunda Sandstone
Hutton Sandstone
Evergreen Sandstone and Aquitard
-45
Precipice Sandstone
Upper Permian Sandstone
Lower Permian Sandstone
-50
-8
-7
-6
-5
-4
-3
δ18O (‰)
Figure 53 Oxygen and Deuterium Isotopes for Groundwater Samples in the Study Areas
(arrows represent expected trajectory of data points in response to various fractionation
processes (as described in Clark and Fritz, 1997).
Most isotopic pairs plotted near the LMWL, which indicates that the groundwater within these basins is
meteoric in origin. There are two distinct groupings of groundwater based on their composition of oxygen
and deuterium isotopes:
•
A relatively isotopically-lighter groundwater present in the Precipice Sandstone, Gubberamunda
Sandstone, BMO Group and Tertiary Volcanics (and some groundwater within the Hutton
Sandstone).
•
A relatively isotopically-heavier groundwater present in the Upper Permian Sandstone, Lower
Permian Sandstone, Evergreen Formation (Sandstone and Aquitard intervals), and some
groundwater within the Hutton Sandstone.
Based on the data available, isotope trends that deviate from the LMWL exist for samples collected from
the BMO Group, Gubberamunda Sandstone, and Hutton Sandstone. Isotope trends for the other aquifers
could not be meaningfully determined due to the limited data available. These deviations from the LMWL
indicate the occurrence of isotope fractionation processes either as water infiltrates into the aquifers or as
the water moves along established groundwater flow paths.
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The relatively isotopically-heavier groundwater present within the Upper Permian Sandstone, Lower
Permian Sandstone, Evergreen Formation (Sandstone and Aquitard intervals), and some groundwater
within the Hutton Sandstone suggests the occurrence of an evaporative effect. As rainfall infiltrates the
soil, it can undergo evaporation in the unsaturated zone and may eventually precipitate salts. As the water
shifts its phase from a liquid to vapour, the lighter isotopes of oxygen and hydrogen preferentially enter the
vapour phase leaving the heavier isotopes behind in the remaining liquid. Water that has experienced
evaporation will tend to fall along a trend line situated below the meteoric water line and possessing a
slope of around 5 (as opposed to the general slope of 8 for the meteoric water line). This would appear to
describe the process that samples collected from the Hutton Sandstone, Lower Permian Sandstones and
Evergreen Sandstone have experienced. The occurrence of palaeo-drought conditions (Radke et al.,
2000) extending for long periods of time are known to have occurred in the study area, and therefore the
occurrence of an evaporative trend is not surprising.
What is surprising are results obtained for the BMO Group and Gubberamunda Sandstone, which are
quite anomalous in comparison to the other samples, and fall above the Local Meteoric Water Line
(established in BRS and NRW, 2003). This occurrence can be explained by a number of fractionating
processes, such as exchange of oxygen with CO2, the exchange of hydrogen with H2S, or the hydration of
silicate minerals. Given that the groundwater has been isolated in the subsurface for many years, the most
likely mechanisms would be either the exchange with gases, like CO2 and/or H2S, present in the
subsurface and/or silicate mineral hydration (Coplen and Hanshaw, 1973).
Ca rb o n- 1 3 in D i sso lv ed In o rg an i c Ca rbo n ( δ
13
CDIC)
13
The values of δ CDIC present within the dissolved inorganic carbon components of groundwater vary
between the different aquifers. This variation is highly suggestive of different sources of carbon in the
dissolved inorganic fraction, or more commonly referred to as carbonate alkalinity. Table 39 summarises
the ranges of values for the various formations (where data is available), while Figure 54 shows the
distribution of values across the Bowen and Surat basins. Although the number of formations with related
measurements is quite low, there are some notable differences between the Cretaceous formations and
the underlying Jurassic formations.
Differences in the range of values noted for the two intervals suggest a contribution from different sources
of carbon. The isotopically lighter values tend to lie close to values expected from the mineralisation of
plant material originating in a semi-arid region (between -25‰ and -20‰) (Clark and Fritz, 1997).
13
Vegetation in such areas can be expected to have δ C of the order of -23‰ to -25‰. Weathering of this
organic carbon, and subsequent conversion to CO2, is accompanied by a shift towards heavier values of
approximately 10‰ in the resulting bicarbonate ions produced (Clark and Fritz, 1997). The values
obtained for the Permian and Jurassic intervals are generally consistent with this mechanism as
13
δ CDICvalues would be expected to fall around -15‰.
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Table 39 δ CDIC Values for Cretaceous, Jurassic and Permian Formations
13
Bowen
Surat
Basin
Age
Formation or Interval
Cretaceous
BMO Group
-9.3 to +2.0
-2.2
11
Late Jurassic
Gubberamunda Sandstone
-8.0 to -2.4
-8.2
6
Jurassic
Hutton Sandstone
-18.1 to -9.4
-14.6
8
Evergreen Formation
-16.9 to -8.1
-12.7
4
Precipice Sandstone
-17.8 to -15.8
-17.4
3
Upper Permian Sandstone
-16.0 to -8.5
-14.0
4
Lower Permian Sandstone
-14.7
--
1
Permian Volcanics
-17.7
--
1
Permian
Range
(‰ PDB)
Median
(‰ PDB)
No. of
readings
13
It is evident from a review of Table 39 that there are two distinct groupings of δ CDIC values:
•
Isotopically heavier values within the Cretaceous formations (ranging from -9.3‰ to +2.0‰); and
•
Isotopically lighter values within the Permian and Jurassic formations (ranging from -18.1‰
to -9.4‰)
Conversely, the isotopically heavier values associated with the Cretaceous formations are consistent with
13
values related to carbonate minerals present in the GAB sediments, and that yield δ C values around6.5‰ (Schulz-Rojahn, 1993). Therefore, carbonate dissolution would explain the occurrence of the
isotopically heavier values. The much heavier (and positive) values further suggest fractionating
processes such as anaerobic fermentation of dissolved organic matter or reduction of CO2as possible
causes. Examples of these reactions are provided below:
CH3OH + H2→ CH4 + H2O
CO2 + 4H2→ CH4 + 2H2O
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Lower and
Upper Permian
Precipice
Evergreen
Hutton
Gubberamunda
BMO Group
Figure 54 Distribution of δ CDIC values (as ‰ PDB) in the Jurassic and Cretaceous formations
13
7.2.10 Dissol ved Methane
Dissolved methane concentrations for various aquifers within the Bowen and Surat basins are provided in
Appendix 5. Overall, dissolved methane concentrations are higher within the Walloon Coal Measures,
Bandanna Coal Measures, and Baralaba Coal Measures (up to 20,000 ppm). Notable concentrations of
dissolved methane were also detected in the Gubberamunda Sandstone and BMO Group (up to 75 ppm),
with minor concentrations (generally <1 ppm) detected in the Upper Permian Sandstone, Hutton
Sandstone, Precipice Sandstone and Evergreen Formation hydrostratigraphic units.
The occurrence of methane across hydrostratigraphic units within the Bowen and Surat basins suggests
that two possible production processes are occurring in isolation, or in tandem:
•
Methane gas has been produced in-situ in most/all aquifers within these basins through chemical
reducing processes; and
•
Methane gas has been produced insitu within the coal seams present, then seeping into overlying
and underlying aquifers and aquitards through geological structures (e.g. faults) and other hydraulic
connections.
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Unfortunately, determining which of the above processes may be leading to the presence of dissolved
methane in the aquifers outside of the Bowen and Surat coal intervals cannot be achieved with the
information currently available.
Ca rb o n- 1 3 o f Di s so lv ed M et h an e (δ
13
CCH4)
13
Methane is comprised of one carbon atom coordinated with four hydrogen atoms. As such, the δ C
values of the methane fraction can provide some indication as to how it was formed. Methane gas can be
produced by thermogenic processes, which tend to occur deep in the earth and over long periods of time,
as well as by methanogenic processes facilitated by bacteria. Each of these processes will yield a
13
characteristic δ C value for the resulting methane.
1000000
Walloon Coal Measures
predominantly
CO2 reduction
10000
C1/(C2 + C3)
Bandanna Coal Measures
Migration
100000
predominantly methyltype fermentation
Baralaba Coal Measures
Oxidation
BACTERIAL
PROCESSES
Migration
1000
Kerogen
Type II
100
MIXING
10
THERMOGENIC
PROCESSES
1
-80
-70
-60
-50
-40
-30
-20
-10
δ13CCH4 (‰ PDB)
Figure 55 δ C Isotopes Values for Dissolved Methane of the Various Coal Measures (shaded
areas indicate dominant processes leading to methane productions (after Whiticar, 1999)
13
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
13
A number of δ C CH4 values were identified in the unified database, which were plotted in relation to
reference fields describing various processes responsible for the formation of methane to determine
13
potential sources for the dissolved gas. Whiticar (1999) compared δ C to the ratio between the C1
(methane) hydrocarbon fraction and the sum of the C2 (ethane) and C3 (propane) hydrocarbon fractions,
as a way of deciphering the source. Data available for this study was similarly plotted to determine where
they would fall in relation to these established fields (Figure 55). The results of this processes indicated
the following potential sources for methane in the various coal measures:
•
Walloon Subgroup – bacterial and/or bacterial methyl-type fermentation, with one sample indicating
possible mixing or migration.
•
Bandanna Coal Measures – primarily bacterial processes.
•
Baralaba Coal Measures – one sample indicating formation consistent with thermogenic formation
and the other possibly the result of migration or oxidation of a bacterial source.
Regardless of the actual processes responsible, it would appear that the methane present in the Walloon
Subgroup is more associated with bacterial processes, as opposed to a thermogenic source.
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8
INDICATORS OF POTENTI AL HYDRAULIC CONNECTIVITY
8.1
Inter-Aquifer
Assessment of potential hydraulic connectivity between the major formations of the Bowen and Surat
basins was accomplished by a comparison of residual head values between the major formations, a
comparison of spatial distribution of water quality and an assessment of potential fault pathways and
areas of aquitard thinning between major aquifer formations. Results for each of the basins are
summarised below.
8.1.1
Bow en Basin
There is very little temporal and spatial overlap in the potentiometric datasets for aquifers in the Bowen
Basin. Comparison of aquifer formation water pressures could not be accurately assessed and, as a
result, a heavy reliance was placed on the work of previous studies conducted in the basin. Although
groundwater quality conditions on their own are not sufficient to clarify potential interactions between
otherwise discrete formations, they can provide some indications of interaction.
When attempting to manage the potential impacts of CSG extraction on aquifers of the Bowen Basin,
arguably the most important interactions are those which involve the coal-bearing formations (the Upper
Permian Coal). Previous work by the Centre for Water in the Minerals Industry(CWiMI, 2008) offers
analysis of the potential for hydraulic connection between the Upper Permian Coal and the overlying
Clematis Sandstone aquifer. This is based primarily on the thickness and competency of the intervening
Rewan Group aquitard. This study found that the potential for hydraulic connection across the Rewan
Group aquitard was low, particularly when compared with the aquifers of the Surat Basin coal seams
(Condamine Alluvium, Springbok Sandstone and Hutton Sandstone).
The different groundwater quality present in the Clematis Sandstone (median TDS = 387 mg/L, dominant
2+
Na , HCO3 ; median SO4 and SiO2 = 5.5 and 15 mg/L, respectively – Appendix 6) compared to the
+
underlying Upper Permian Sandstones (mean TDS = 1,767 mg/L, dominant Na , HCO3 and Cl , median
2SO4 and SiO2 = 80 and 24 mg/L, respectively – Appendix 6) provides some evidence that the Rewan
Group is acting as an effective barrier at least in the middle portion of the basin. However, the lack of
residual head measurement limits the ability to confirm this suspicion.
Certain assumptions regarding the potential for interactions between the Upper Permian Sandstone and
the Upper Permian Coal can be drawn from this work. The similarity in water quality between these two
intervals provides some evidence of interactions (i.e. the results determined for the Upper Permian Coal
[Table 11] and Cluster 4 determined for the Upper Permian Sandstones [Table 13]). Again, the lack of
residual head measurements makes confirmation difficult.
The presence of numerous faults throughout the Bowen Basin also presents the potential for pathways to
exist between any intervening aquitards. However, infilling of these pathways is likely due to secondary
mineral precipitation. In general, a moderate to high potential for aquifer interaction is inferred; however,
development of a more robust groundwater level dataset in strategic locations across the basin would be
helpful in elucidating interactions between the CSG production intervals and adjacent aquifer formations.
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
8.1.2
Surat Basin
The general understanding of inter-aquifer connectivity in the Surat Basin has been based on published
literature and the potentiometric surface analysis completed by Radke et al. (2000) and Hodgkinson et al.
(2009). This understanding has been supplemented with residual head mapping and geochemical
analysis completed in this study.
At the base of the stratigraphic sequence, the Moolayember Formation aquitard represents the basal
confining layer for the Precipice Sandstone. Based on potentiometry work conducted for this study, and by
others (Hodgkinson et al., 2010), the reasonable separation of iso-contours of groundwater levels in the
Precipice Sandstone and the Hutton Sandstone (Figure 21) supports the effectiveness of the Evergreen
Formation as a regional aquitard. This sealing effect may be compromised at the basin margins where a
thinning of the Evergreen is suspected. Further evidence of effectiveness of the Evergreen aquitard is
provided by the relatively distinct water qualities of the Precipice Sandstone (median TDS= 150 mg/L;
+
dominant Na and HCO3 – Appendix 6) compared with the overlying Hutton Sandstone (median TDS =
+
752 mg/L; dominant Na , HCO3 and Cl – Appendix 6).
Hydraulic connectivity between the Hutton Sandstone and Walloon Subgroup is suspected in certain
areas, due to localised thinning of the Eurombah Aquitard. These areas are shown in Figure 25. The
concurrence of data points with a k-means cluster signature consistent with a marine signature in the
Hutton (Cluster 2 in Table 23) and the coal intervals in the Walloon Subgroup (Cluster 1 in Table 26)
provides some evidence to support this conclusion.
The occurrence of neutral residual head values between the Walloon Subgroup and the overlying
Springbok Sandstone in the middle portions of the basin provide some evidence of potential interaction
between the two formations (Figure 27). Evidence based on an assessment of isopach thickness maps
suggests that erosional down cutting of the upper Walloon Subgroup mudstones has led to the Springbok
being in direct connection with the uppermost Macalister Coal Seam near the middle of the Jurassic coal
measures (APLNG, 2010). Although the data is limited for the Springbok Sandstone, the concurrence of a
similar water type (Cluster 2 – Table 28) with that of the Walloon Subgroup (Cluster 1 – Table 26) along
the eastern margin of the basins supports this conclusion, indicating a potentially high risk from drawdown
effects associated with CSG development.
The Westbourne Formation aquitard represents a fairly extensive confining layer between the Springbok
Sandstone and the Gubberamunda Sandstone.
Figure 56 shows the mapped thickness of this aquitard interval. Although this confining layer extends
across the majority of the basin, thinning is shown to occur in certain areas along the basins margins, and
along the Eulo-Nebine Ridge thus allowing for potential interaction. This is demonstrated in Figure 30 for
the northern margin of the basin, but potentiometry information is not available for the Eulo-Nebine Ridge
area. Nevertheless, the general occurrence of artesian conditions in that area, combined with the thinning
of the Westbourne, suggests a high potential for upward movement from deeper formations towards the
Gubberamunda Sandstone.
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Figure 56 Thickness and extent of Westbourne Formation Aquitard
showing thinning towards Eulo-Nebine Ridge (from Radke et al., 2000)
Localised hydraulic connection across the Westbourne Aquitard and interaction between the
Gubberamunda, BMO Group and the Rolling Downs Group aquitard may exist in the western Surat Basin,
where near-neutral residual heads correlate with structural anomalies and/or mapped zones where the
aquitard thins.
Given that the Rolling Downs Group was deposited in a marginal-marine setting, pore water from that
interval tends to be distinct compared with the underlying formations of more continental origin. Similarities
in pore water chemistry in the Gubberamunda (Cluster 1 – Table 31) and BMO (Cluster 2 –Table 33)
along the eastern margin of the basins provides additional evidence of potential interaction between these
sandstone intervals. A similar situation is noted between the BMO (Cluster 2 – Table 33) and the Rolling
Downs Group (Cluster 3 – Table 35) along the northern margin of the basin. Other evidence of crossformational flow is provided by the occurrence of springs at the surface, which are believed to originate
from deeper confined intervals from at least the Gubberamunda Sandstone upward (Radke et al., 2000).
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Figure 57 Thickness of Rolling Downs Group sediments showing thinning over the Eulo-Nebine
Ridge and near the (stippled) recharge area (from Radke et al., 2000)
Interaction between the Cainozoic units and the underlying aquifers is generally restricted to areas where
the underlying formations approach the surface or are exposed. This primarily occurs along the basin
margins in proximity to the recharge beds, and where the Rolling Downs Group thins, or is absent. Figure
57 shows the mapped thickness of the Rolling Downs Group across the Surat Basin, and demonstrates
the thinning that occurs at the basin margins and along the Eulo-Nebine Ridge.
Additionally, interaction between the surficial deposits and the underlying formations will occur in areas
where rivers have eroded through confining layers and streambeds are in hydraulic connection with the
underlying permeable formations (Figure 41 and Figure 42). At these locations, fresher water will enter the
groundwater system, if conditions are appropriate. Unfortunately, the lack of data point information
associated with the Alluvium makes confirmation of this interaction difficult.
The role of major faults in the Surat Basin is believed to be minor in the formations extending from the
Walloon Subgroup to surface given the limited amount of displacement (< 50 m) that has been identified in
the area. The potential is greater for the deeper formations, but the significant differences in hydraulic
head values between the Precipice and Hutton Sandstones along the various major fault zones and the
general dissimilarity of hydrochemistry between the two formations (based on correlation coefficients, PCA
and k-means cluster analysis) suggests integrity of the Evergreen seal across the majority of the basin.
Some possibilities for connection exist near the northern-most extent of the Burunga-Leichardt and
Hutton-Wallumbilla fault systems along the eastern and northern margins of the basins, respectively,
where evidence is provided by similarities in hydraulic heads (Figure 21).
A summary of potential interactions and hydraulic connections between the various formations within the
Surat and Bowen basins is provided in Figure 58.
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Hydrostratigraphic Units
Comments on Inter-aquifer Connectivity
Rolling Downs Group
Effective isolation from surface; diffusional or contribution
from Rolling Downs Group to underlying units
BMO Group sandstones
General hydraulic connection (direct to weak depending on
Gubberamunda Sandstone
presence and extent of intervening low permeability layers)
Westbourne Aquitard
Effective isolation; zone of potential interaction at basin
edges and along Eulo-Nebine Ridge where unit thins
Springbok Sandstone
Hydraulic connection through erosion of upper
Walloons Subgroup
Walloon mudstone or thinning of aquitard layer.
Eurombah Aquitard
Effective isolation with some possible interaction where
aquitard layers are thin (limited data coverage).
Hutton Sandstone
Evergreen Formation
Effective isolation with likely interaction at basin edges and
along Eulo-Nebine Ridge where unit thins
Precipice Sandstone
Possible localized interaction with Permian coals
Moolayember Formation
Clematis Sandstone
Effective isolation; possible local interaction near northern
margin of Surat Basin
Possible localized interaction with Permian coals
Rewan Group
Effective isolation with possible fault pathways
Upper Permian Sandstones
Localised hydraulic connection likely via thin or
Upper Permian Coals
absent aquitard layers and fault pathways
Lower Permian Sandstones
Basement
Barrier to further downward movement of groundwater
Figure 58 Summary of Potential Inter-aquifer Connectivity for the Study Area
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8.2
Inter-Basin
A component of groundwater flow has been identified from the northern margin of the Surat Basin
discharging into aquifers of the Bowen Basin. This flow system, identified in the BMO Sandstone,
Gubberamunda Sandstone, Springbok Sandstone, Walloon Subgroup and Hutton Sandstone aquifers,
indicates the presence of a regional groundwater divide following the topographic ridge between Mount
Hutton and Miles. Groundwater north of this divide is expected to flow northward before discharging to
aquifers of the Bowen Basin. Groundwater south of the divide moves south to south-westerly towards the
centre of the Surat Basin.
While the data coverage did not permit a detailed assessment of interaction between the Surat Basin and
the Clarence-Moreton Basin to the east, the groundwater flow direction interpreted for the Precipice
Sandstone supports the findings of Hodgkinson et al.,(2010) who identified a component of easterly
regional groundwater flow, and ultimately discharge to the Clarence-Moreton Basin. The relationship
between the basins is suggested to reverse for the shallower Middle to Late Cretaceous and Jurassic
formations, which find the Surat Basin receiving subsurface recharge from the Clarence-Moreton Basin
(Hodgkinson et al., 2010).
The prevailing regional groundwater flow direction in the Surat Basin is towards the south-western corner
of the basin where groundwater discharges to the Bogan River, Bourke and Eulo spring systems. A
component of interaction (primarily discharge from the Surat Basin) between the Coonamble Embayment
to the south and the Eulo-Nebine Ridge Area to the west is expected (Radke et al., 2000). These
interactions are beyond the data coverage of this study.
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9
IMPROVING KNOWLEDGE AND UNDERSTANDING
The present study has demonstrated the need for additional data to better understand the spatial pattern
and degree of groundwater interaction (i.e. connectivity) between coal formations and surrounding
aquifers, in the Bowen and Surat basins. For example, it is important that future data gathering be more
uniformly distributed across the basins, both across the surface areas and throughout the stratigraphic
profile and gather data encompassing more comprehensive chemical elements (i.e. isotopes). Table 40
indicates the type of data that is required to better understand the physical and hydrochemical
characteristics of groundwater in the Surat and Bowen basins, and how the additional information gained
may be used.
Data from CSG companies, and the regional water quality monitoring strategy designed by the
Queensland Water Commission (QWC), could be used to build on the data collated and analysed in the
course of this investigation.
Table 40 Data acquisition recommendations
Recommendations
Advantages of additional data
Gather more information spatially and down
Improve definition of pore water chemistry, in particular to
stratigraphic profile, regarding the mineralogy and
resolve some of the variability noted in chemical composition
geochemical conditions of the host aquifer and the
of water samples within, and between, aquifer intervals.
hydrochemistry of the groundwater (formation by
formation).
Gather additional information pertaining to the
Resolve highly variable water quality conditions and refine
distribution of hydraulic properties within discrete
knowledge of flow path phenomena.
intervals.
Design future monitoring and sampling to ensure
Improve the identification and interpretation of the origin of
there is a more uniform distribution of data across
pore waters, regional flow paths and potential groundwater
the basins.
mixing relationships.
Collect information regarding the variable
Improve understanding of potential interactions between CSG
thickness, hydraulic properties and spatial
intervals and adjacent formations along with the hydraulic
distribution of hydraulic properties for the major
integrity of confining layers.
aquitard layers (i.e. Evergreen, Eurombah, Upper
Walloon Subgroup, Westbourne, BMO Group,
Rolling Downs Groups).
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Recommendations
Advantages of additional data
Gather measurements for a more comprehensive
Refine interpretations regarding key process and interactions
suite of water quality information, including trace
within, and between, CSG intervals and adjacent aquifers.
elements (e.g., As, Ba, Br, Li, Sr) and dissolved
organics.
Collect dissolved gas and isotope data (stable and
radiogenic-
18
2
O, H,
13
C,
34
S,
14
C,
36
Cl,
87
Sr).
Improve understanding of the origin of groundwater in
individual units, geochemical processes and flow path
phenomena (i.e. groundwater ages and flow velocities).
Improve understanding of spatial distribution of gases in
aquifer pore waters and whether the gases are the result of
migration from CSG production zones or the result of natural
in-situ processes.
Collect additional water chemistry data for:
a)
Surficial deposits and all formations below
the Gubberamunda Sandstone of the
Surat Basin;
b)
All formations within the Bowen Basin.
Particular attention is required for the Precipice,
Improve the ability to resolve spatial patterns within discrete
formations and potential mixing relationships with adjacent
formations.
With particular focus on the Precipice, Hutton and Springbok
Sandstone aquifers, additional data will greatly improve
understanding the risks to groundwater quantity and quality
from CSG development.
Hutton and Springbok Sandstones in the more
southern and western parts of the Surat Basin.
Obtain refined data from CSG operators regarding
Enhance the current understanding of potential pore water
chemical information and hydraulic properties
interactions within and between these formations, their
pertaining to the Walloon Subgroup and adjacent
unique chemical signature and the distribution and source of
formations.
dissolved gas present in the groundwater (i.e. gas migration
from coal measures to shallower aquifers, or natural
formation by in-situ processes).
Collect additional data of groundwater levels,
Resolve potential hydraulic interactions between the various
including information from strategically positioned
formations, particularly in relation to potential interaction
nested bores.
between the Walloon Subgroup and the Springbok and
Hutton Sandstones. This is essential to improve
understanding of residual head conditions, which; likely, overestimate the extent of potential interaction areas.
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10 SUMMARY OF FINDINGS
This section summarises the key findings from the analyses presented in previous sections, and highlights
the distinguishing water quality features of various formations in the Bowen and Surat basins that could be
used in future investigations involving groundwater flow regimes and aquifer connectivity. Findings are
presented separately for the Bowen and Surat basins.
10.1 Bow en Basin
The major formations of the Bowen Basin comprise a complex arrangement of sandstones of fluvial,
lacustrine, aeolian and marine origin, along with substantial accumulations of coal. The sandstone
formations associated with the Bowen Basin include the Lower and Upper Permian Sandstones of the
Back Creek Group, Rewan Group and the overlying Clematis Sandstone. Intervening layers of lower
permeability sediments also reside in the Back Creek Group and Rewan Group. The Bandana and
Baralaba Coal Measures are the target formations for CSG production in the southern portion of the basin.
These coal measures occur between the overlying Rewan Group and other Upper Permian sandstones.
Extensive coal seams also occur in the northern and central areas of the Basin (Moranbah Coal Measures
and German Creek Coal Measures).
Mineralogical data from sediment cores collected from the south-eastern portion of the basin (Back Creek
Group, Rewan Group, Clematis Sandstone and Moolayember Formation) indicates a predominance of
quartz, plagioclase and kaolinite. These cores also indicate minor occurrences of carbonates (calcite and
siderite), iron-oxides (hematite, restricted to the Rewan Group) and metal-sulfides (pyrite and marcasite,
restricted to the Back Creek Group).The Back Creek Group (Figure 1) possesses a higher plagioclase and
lower kaolinite content than the Rewan Group (Figure 2). Also of note is the comparatively lower chlorite
content and lack of measureable pyrite in the Rewan Group.
10.1.1 Hydr ochemistr y
+
Groundwater chemistry in the Bowen Basin generally is dominated by sodium (Na ) and bicarbonate
(HCO3 ), while chloride (Cl ) concentrations varies from a dominant to subordinate constituent. Salinity
values can be quite variable ranging from the low 100s of mg/L up to several 1,000s of mg/L. Mineral
weathering and ionic exchange reactions appear to be the dominant controls on pore water composition,
with feldspars and aluminosilicates, controlling the major cation concentrations.
Groundwater within other formations exhibits considerable variation across the basin, and in the recharge
areas. Variable recharge conditions, changes in lithology and complex groundwater flow conditions are
believed to be the controlling factors.
More specific points are made below:
•
Groundwater salinity is variable by depth and location, with the highest TDS values occurring in the
Lower and Upper Permian Sandstones and coal intervals beneath the northern half of the basin
(Figure 45);
•
Silica (Figure 48), fluoride (F ) (Figure 49), and Sodium Absorption Ratio (SAR) values (Figure 52)
exhibit a similar distribution across the entire basin, suggesting that there are similar mineral
assemblages throughout the major formations of the basin, and that the water-rock interactions in
these locations generate pore water of comparable composition;
301001-01210-CSG Water Chemistry
-
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
+
•
Increased Na in the pore waters to the north (as defined by the SAR values) is believed to be
22+
2+
linked to ionic exchange reactions involving Ca and Mg liberated from carbonate (CO3 )
dissolution and/or silicate-based mineral weathering (feldspars and micas); and
•
Comparatively higher SO4 values also exist in the northern part of the basin as well as along the
western and eastern margins (Figure 47). A zone of notably lower values is also evident near the
cluster of wells associated with the Clematis Sandstone. This occurrence is truly anomalous and
2contributes to the distinctive water quality of this formation. Suspected sources of SO4 include:
2-
o
o
Contributions of marine-influenced pore waters from adjacent formations; and/or
Oxidation of sulfide minerals(i.e. pyrite (FeS2) (where present).
2-
Stable isotopes of sulfur in the SO4 ions would help substantiate this conclusion. Bicarbonate:chloride
ratios (Figure 51) appear lower in magnitude in the northern part of the basin compared to the south,
indicating a source of Cl (i.e., from a marine influence).
Spearman correlation coefficients and PCA results (Table 9 and Table 12, respectively) for the Lower and
Upper Permian Sandstones indicate a similarity between the pore water of these two intervals. Notable
2+
influences from Na , Cl and SO4 suggest a marine influence likely derived from waters trapped during
2+
2+
deposition in a marginal-marine setting. Secondary to that, is a correlation between Ca , Mg , SiO2 ,
CO2 , and pH consistent with the weathering of aluminosilicate minerals.
K-means cluster analysis for the Lower and Upper Permian Sandstones (Table 10 and Table 12,
respectively) indicates a highly variable and somewhat random distribution of major water groupings
across the basin. Much of this variability is situated near known faults. However, the potential importance
of faults in pore water interactions cannot be resolved due to the lack of supporting hydraulic data.
Table 41 summarises some of the distinguishing water quality features for the various formations in the
Bowen Basin. These features should be included in any investigation into connectivity with surrounding
formations.
Recharge and groundwater flow phenomena
Recharge to the Bowen Basin occurs in the flanking uplands along the western and eastern edges where
the various formations are exposed at the surface. Some contribution might occur from rivers that are in
hydraulic connection with the same bedrock intervals; however, this was not confirmed as part of this
study. Groundwater surface elevations are limited but, where available, indicate a general regional flow
pattern from the upland areas on the western side of the basin (near the Nebine Ridge) towards the
adjacent low-lying areas to the east and south. This flow pattern is consistent with the local terrain,
indicating topographic control on flow patterns.
Previous estimates of groundwater flow velocities in the GAB have indicated rates of the order of 1 to 3
metres per annum (based on groundwater age-dating).
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Table 41 Distinguishing water quality features of the Bowen Basin formations
Formation or
Interval
Clematis Sandstone
Distinguishing
Constituent(s)
+
Spearman Correlation(s)
-
K
No correlation between Cl and
major cations; no correlation
+
2between Na and SO4 ; (-) F
+
with K (approaching 0.6)
Rewan Group
--
No correlation of Na and
+
2SO4 ; (-) F with K
(approaching 0.6)
Upper Permian
Sandstones
SiO2 and NO3
Upper Permian Coal
NO3
+
-
-
-
Lower Permian
Sandstones
--
Permian volcanics
SiO2
+
2-
(+) Ca
2+
and SO4
+
(+) Na and Cl
2+
2-
+
(+) Mg
(Insufficient data)
(Insufficient data)
+
2+
and Cl
-
(+) Na with SO4 ; (+) F with
HCO3
-
-
-
Dominant ions in
Factor 1 of PCA
+
2-
(-) HCO3 with K ; (-) Ca and
2+
+
+
Mg with K ; (+) F with Na
(approaching 0.6)
(+) Na and SO4
--
(+) Na and Cl
+
-
Note: (+) = positive relationship; (-) = negative relationship
Potential connectivity between coal seams and other aquifers
Modelled results of residual hydraulic head conditions between the Clematis Sandstone and Permian Coal
Measures (Figure 19) suggest that a hydraulic connection, or neutral residual head conditions, between
these two formations exists. Although neutral residual head conditions dominate the modelled area, a
notable head difference (>20m) is present in some areas. This head difference indicates a hydraulic
separation between these units and is likely caused by Rewan Group Aquitard.
The Rewan Group aquitard represents an extensive low permeability layer separating the Clematis
Sandstone from the underlying Upper Permian sandstones. The Clematis Sandstone is confined from
above by the Moolayember Formation aquitard and separated from the neighbouring aquifers of the Surat
Basin, except on the western edge near the Nebine Ridge where the aquitard thins. Hydraulic separation
of the Clematis Sandstone from the underlying Upper Permian sandstones is also supported by the
notable difference in water qualities between the two intervals.
Some evidence exists to suggest mixing of pore waters between the Clematis Sandstone and the Rewan
Group, and possibly the Upper Permian Coal and Sandstones in the southern part of the basin. This
evidence is provided by the occurrence of anomalous water quality conditions (expressed by various
constituents and ionic ratios) at the southern end of a data cluster associated with the Clematis
Sandstone. Numerous faults have been identified in the formations (SRK, 2008) that may provide the
potential for groundwater mixing between certain formations. The similarity of water qualities for the Lower
and Upper Permian sandstones and the adjacent coal members supports interaction between these
intervals. However, the vertical extent of this faulting is unknown. This represents an area for further
investigation and possible additional monitoring.
Lack of refined water level and water quality data through the majority of the Bowen Basin limits the ability
to resolve potential interactions between the Bandanna and Baralaba Coal Measures and overlying
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
formations – particularly the Clematis Sandstone. Additional information will be needed to further refine
knowledge and understanding of the hydrogeological conditions, and properly frame the risks to these
formations as a result of CSG development (i.e., water level drawdown and potential gas migration).
10.2 Surat Basin
The formations of the Surat Basin comprise a series of continental fluvial and marginal-marine sandstones
deposited by braided and meandering stream systems. Coarser-grained formations include (from the
base of the section upward) the Precipice Sandstone, Hutton Sandstone, Springbok Sandstone,
Gubberamunda Sandstone, Bungil Formation, Mooga Sandstone and Orallo Formation (BMO Group).
Permeable sediments of Cainozoic age comprise the alluvium blanketing the area, which includes the
Condamine Alluvium on the eastern side of the basin. Extrusive volcanic rocks (olivine-rich basalt) of
Tertiary-age are also present.
The more permeable formations are confined by lower permeability layers of mudstones and siltstones
associated with (from the base up) the Evergreen Formation, Eurombah Formation, upper Walloon
Subgroup, Westbourne Formation and Rolling Downs Group.
Three major coal seam intervals in the Walloon Subgroup represent the target for CSG development.
Thickness of the coal seams is generally limited to less than 1m, but can extend up to approximately 3m.
The lateral continuity of the coal seams is limited.
Mineralogical data from the north-eastern Surat Basin indicates a dominance of quartz, plagioclase and
kaolinite with minor occurrences of carbonates (calcite, siderite, and magnesite) and iron-oxides (hematite
and goethite), and a lack of detectable metal-sulfides.
10.2.1 Hydr ochemistr y
Notable differences exist between the pore waters of the Jurassic formations and the Cretaceous
formations, with higher salinity values tending to occur in the overlying Cretaceous formations. This is due
to a change from a continental-fluvial depositional setting during the Jurassic Era to a more marginalmarine setting during the Cretaceous Era.
Salinity values vary considerably within and between formations, ranging from several 100s of mg/L to
several 1,000s of mg/L. Lower salinity values tend to occur in areas near upland recharge areas, with
concentrations increasing in a down-gradient direction towards the centre of the basin. Mineral
weathering and ionic-exchange reactions appear to be the dominant processes influencing pore water
chemistry. Notable difference in water quality from bores located in recharge areas suggest a link to
variable recharge processes, groundwater flow rates and resulting geochemical reactions. Water types
+
are generally dominated by Na and HCO3 ions, with variable concentrations of Cl .
Elevated TDS values in the deeper Jurassic formations are generally restricted to the areas near the
northern and eastern margins of the basins. This distribution is consistent with the location of the
comparatively higher mineralised Walloon Subgroup (Figure 45). In the shallower Late Jurassic to Tertiary
sediments, the area of higher TDS is generally restricted to the northern part of the basin near the intake
beds (Figure 45). The absence of elevated TDS values on the eastern margin of the basin is consistent
with a lack of Rolling Down Group sediments covering the area. Therefore, influence of the more
mineralised waters of the Rolling Downs Group on the TDS of the underlying formations in the northern
part of the basin is apparent.
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A general shift from slightly lower TDS values near the intake beds of the Late Jurassic to Tertiary
formations to slightly higher TDS values towards the basin centre (Figure 45) is consistent with the natural
evolution of groundwater and acquisition of dissolved mineral content as it moves through the formations
and gains constituents from weathering and ion-exchange reactions. The occasional elevated TDS value
also occurs near the Eulo-Nebine Ridge (Figure 45), which is again linked to contribution of higher
mineralised water from the overlying Rolling Downs Group down into the underlying BMO Group and
Gubberamunda Sandstone.
Other points of interest include:
•
Alkalinity displays a similar pattern of spatial distribution as TDS, with the highest values occurring
within the Walloons fairway (Figure 46). The dissolution of carbonates, and to a lesser degree,
silicate minerals is the suspected cause. In-situ production of CO2 from microbial oxidation of
organic matter in organic-rich formations, like the Walloon Subgroup, will also add to the overall
alkalinity of associated pore waters. The noted reduction in alkalinity values towards the basin
centre (Figure 46) is consistent with the precipitation of carbonate minerals in response to higher
groundwater temperatures (i.e., retrograde solubility characteristics).
•
Sulfate concentrations exhibit the greatest concentration of elevated values immediately downgradient of the northern recharge beds and in the formations above the Westbourne Formation
aquitard (Figure 47). Contribution of marine-influenced pore waters from the overlying Rolling
Downs Group into the BMO Group and deeper Gubberamunda Sandstone is implied; which is
further supported by the occurrence of low HCO3 /Cl ratios (Figure 51). Lack of Rolling Downs
Group sediments on the east side of the basin supports this conclusion based on the presence of
lower sulfate values and higher HCO3 /Cl ratios.
•
Silica concentrations are variable near the recharge areas, but show a general increase in
magnitude towards the middle part of the basin (Figure 48). This is likely to be due to elevated
groundwater temperatures (Figure 44) resulting in increased solubility of silicate-based minerals;
•
The distribution of F concentrations across the basin (Figure 49) is consistent with the spatial
pattern displayed for alkalinity(refer Section 7.2.3)(Figure 46). That is, the highest values occur
down-gradient of the eastern and northern margins of the basin. This pattern implies weathering of
feldspars as the source.
•
Boron values in the Late Jurassic to Tertiary formations show a general clustering of elevated
values nearer the basin centre and in the south-western part of the basin near the Eulo-Nebine
Ridge (Figure 50). This finding is consistent with the contribution of marine-influenced pore water
from the Rolling Downs Group down to the BMO Group and Gubberamunda Sandstone. The
concurrence of some elevated concentrations, with higher groundwater temperatures (Figure 44),
also suggests the release of boron from clay minerals contained in these shallower formations.
•
SAR values (Figure 52) show a very similar spatial distribution as alkalinity (Figure 46) and imply an
+
2+
2+
increase in Na as a result of ion exchange (i.e., substitution of Ca and Mg on the surfaces of
clay minerals).
-
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Table 42 summaries features for the various formations in the Surat Basin.These features should be
included in any investigation into connectivity with surrounding formations.
Table 42 Distinguishing water quality features of the Surat Basin formations
Formation or
Interval
Distinguishing
Constituent(s)
Alluvium
SiO2
Tertiary volcanics
SiO2 , NO3
Rolling Downs Group
K , SiO2 , NO3
BMO Group
F
Gubberamunda
--
Springbok
SiO2 , NO3
Walloon Coals
--
Walloon Sandstones
SiO2
-
(-) F with SO4 and Ca
Hutton Sandstone
SiO2
-
(+) F with HCO3 ; lack of correlation
2+
2+
between Cl , Ca and Mg
Evergreen Aquitard
SiO2 , K
(+) F with HCO3 ; (+) SiO2 with Ca ;
+
+
(+) NO3 with Na and K ; (+) HCO3
+
with Na
Evergreen Sandstone
--
lack of correlation between Cl , Ca
2+
and Mg
Precipice Sandstone
--
(+) F with Na and HCO3 ; (+) HCO3
+
2+
with Na and Ca
-
Spearman Correlation(s)
-
+
2+
(+) Na and Cl
-
+
2+
(+) Mg and Cl
(+) HCO3 with Na and Mg
-
+
-
-
(+) HCO3 with Na and Mg
-
-
+
2-
2-
2-
(-) F with SO4 ; (-) CO3 with Ca
-
2+
+
-
-
(-) HCO3 and F
+
-
-
-
-
-
-
+
-
(+) F with HCO3 ; (-) HCO3 and CO3
2+
2+
with Ca ; (-) Ca with pH
(+) SiO2 with Ca
-
-
-
-
+
-
(-) HCO3 and F
-
-
2+
-
(+) Na and Cl
+
-
+
-
+
-
+
-
-
(+) Na and Cl , (-)
SiO2
2+
-
-
-
(-) HCO3 and F
-
-
2-
2+
2-
-
(+) Na and Cl ;
significant NO3
(+) Na and Cl
-
-
2+
(+) TDS with NO3 and SO4 ; (+) NO3
+
with Na
-
-
Dominant ions in
Factor 1 of PCA
2+
(+) Na and Cl ;
significant NO3
(+) Na and Cl ;
significant NO3
-
+
2-
(+) K and SO4 ;
significant NO3
Note: (+) = positive relationship; (-) = negative relationship
Recharge and groundwater flow phenomena
Recharge to the formations generally occurs along the northern and eastern edges of the basin. This
recharge occurs into formations or sediments that are exposed at surface and are in hydraulic connection
with major streams and rivers flowing through the area.
Groundwater movement is generally from topographically elevated areas in the north and east along the
flanking areas of the Great Dividing Range towards adjacent low-lying areas to the south and west. Flow
on the western side of the basin occurs in a parallel to sub-parallel direction along the Eulo-Nebine Ridge,
and represents a hydraulic divide between the Eromanga and Surat basins.
Estimates of groundwater flow velocities in the Surat Basin are similar to those for the Bowen Basin (i.e.,
of the order of 1 to 3 metres per annum).
Pot ent ia l co n n ect iv it y b et w e en co a l se am s and ot h e r aq uif e rs
Residual hydraulic heads between the various formations suggest possible interaction between otherwise
discrete aquifers in areas where differences are neutral to near-neutral, and where intervening aquitards
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
thin out. The transfer of associated pore waters may be direct in some cases, or more diffuse across
thinner layers of intervening mudstone and siltstone.
Based on modelled residual head conditions, areas of potential hydraulic connection are noted for the
Walloon Subgroup and Springbok Sandstone (Figure 27), where the upper mudstone intervals of the
Walloon Subgroup are absent or thin out (i.e., in the middle of the Walloons fairway). This particular
situation represents the highest risk to any Surat Basin aquifer from CSG development (i.e., drawdown
and potential gas migration).
Some similarities exist between pore water compositions for the Hutton Sandstone and Walloon
Subgroup, as shown in the statistical correlations (Table 22 and Table 25, respectively) and k-means
cluster analysis (Cluster 2 – Table 23 and Cluster 1 – Table 26). Results from residual head mapping also
indicate that neutral head conditions (suggesting hydraulic connectivity) dominate throughout the modelled
area (Figure 25) where the formations are closer to surface and thinning of the Eurombah Aquitard occurs.
The difference in water quality and hydraulic head conditions between the Precipice and Hutton
Sandstones (Figure 21) indicates that the Evergreen Formation aquitard is serving as an effective barrier.
With reference to Figure 21, the hydraulic integrity of this aquitard diminishes where the formation is thin
or absent and potential for hydraulic interaction of pore waters in permeable intervals becomes greater.
Given the general extent and thickness of the Westbourne aquitard (Figure 56), this low-permeability
formation is believed to form an effective barrier between the Walloon Subgroup and the overlying
Gubberamunda Sandstone and BMO Group.
The Rolling Downs Group aquitard is also believed to provide an effective isolation between the surficial
formations, the BMO Group and Gubberamunda Sandstone, except at the basin margins (where it is thin
or absent), or in areas where local rivers have eroded through this confining layer, and potentially are in
hydraulic connection with these intervals (i.e. eastern and northern margins of the basins – Figure 41 and
Figure 42).
The presence of Walloon Subgroup sediments beneath, and in direct hydraulic connection with Alluvial
deposits (namely the Condamine Alluvium on the eastern margin of the Surat Basin), represents a risk to
that surficial aquifer and any other features connected to it (i.e. groundwater dependent ecosystems).
This risk is highest near CSG tenements on the eastern side of the basin, and more data needs to be
acquired to more fully assess potential connectivity between these two intervals.
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
11
CONCLUSION
This study, the Spatial Analysis of CSG Water Chemistry, is a consolidation of information regarding the
mineralogical, hydrogeological and geochemical properties of all coal units and adjacent aquifer systems
making up the Bowen and Surat basins. This study has expanded on previous scientific work from the
perspective of better understanding the origins, recharge and flow regimes of groundwater from CSG units
and other water-bearing formations. However, the scopes of the previous investigations were limited by
focussing on the aspects of particular formations in various sub-basins. This study is the first-ever attempt
to encompass and understand the entire stratigraphic sequence of the Bowen Basin and the overlying
Surat Basin.
The objective of this study was to collate and interpret water chemistry data from the Bowen and Surat
basins in an effort to:
•
characterise and determine the origin, recharge and flow regimes of coal seam groundwaters; and
•
identify potential indicators of groundwater connectivity between coal seams and other aquifers.
This study assembled and analysed a large volume of hydrogeological data and utilised some innovative
approaches to assess hydraulic conditions between aquifers and geochemical attributes that may be
useful in identifying a particular water-type for each formation. Despite the effort expended, data
availability, distribution and variability have been the primary constraints on the extent to which the study’s
objectives could be achieved.
The assessment of the hydrogeological conditions in the Bowen Basin was limited by the poor spatial
extent, quality and magnitude of the data available, including information on physical and chemical
properties of key aquitards (a major data gap). In order to address these data gaps additional regional
groundwater monitoring and sampling is needed. However, this investigation did identify distinctive water
chemistry for the Clematis Sandstone, and the data indicates that this formation is hydraulically separated
from the underlying Upper Permian Sandstones.
When compared to the Bowen Basin, a much greater body of information was available for assessment in
the Surat Basin, albeit generally restricted to the shallower formations (surface down to the
Gubberamunda Sandstone). Data associated with the deeper formations was either sparse (Springbok
Sandstone) or clustered near basin margins (Hutton and Precipice Sandstones). However, in the case of
the Surat Basin, information regarding the physical and chemical properties of the major aquitard layers
was lacking. This is a significant data gap and one that should be addressed through future investigation
and monitoring efforts. Regardless, the assessment indicated that the formations with the most significant
potential connectivity to the Walloon Subgroup include the Springbok Sandstone (highest), followed by the
Hutton Sandstone. This degree of potential connectivity is due to the variable thickness, or absence, of
low permeability aquitard sediments between these sandstone intervals and the Walloon Subgroup.
Additionally, the presence of neutral modelled residual head conditions and similar water qualities
supports the likelihood for connectivity.
Additionally, the Condamine Alluvium, located on the east side of the Surat Basin extending from
Toowoomba through Dalby and Chinchilla, is in direct hydraulic connection with the Walloon Subgroup.
Considering the proximity of certain CSG tenements to this sensitive and already over-used aquifer,
regional monitoring efforts should focus on this aquifer system.
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This report has proposed that the above knowledge gaps be addressed by collecting data regarding a
variety of attributes. These knowledge gaps include the following and were discussed in greater detail in
(Section 9):
•
Mineralogical characteristics (to assist in establishing mineral dissolution kinetics and water
chemistry etc.).
•
Hydraulic properties (e.g. lateral and vertical hydraulic conductivity (k)).
•
Trace elements (e.g., arsenic (As), barium (Ba), bromide (Br), lithium (Li), strontium (Sr)) and
dissolved organics.
•
Dissolved gases (e.g. methane).
•
Stable and radiogenic isotopes (e.g.
•
Groundwater elevation measurements (e.g. resolve potential hydraulic interactions).
18
2
O, H,
13
C,
34
S,
14
36
C, Cl and
87
Sr).
It is essential that any enhanced data collection strategies ensure that all information is acquired more
uniformly across the basins and throughout the depth of the stratigraphic profile. The collection of such
additional information will augment the understanding of key areas such as the origin of the various pore
waters, chemistry, potential groundwater mixing relationships, and the interconnection between otherwise
discrete formations and surface waters.
The database and analyses generated by the present study provides a foundation that can be further
developed as additional data becomes available, including data held by the CSG industry and that
produced by proposed regional monitoring activities. Further expansion of the database will also be
invaluable for refining the design of groundwater monitoring activities and interpretation of results obtained
from monitoring infrastructure.
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12
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and Mineralogy of Fresh Groundwater Systems, Queensland, Australia: Part I – Aquifers.
International Congress of Chemistry and Environment, Thailand.
Preda, M. and Hodgkinson, J. 2009. The Potential Influence of Carbon Geostorage on the Hydrochemistry
and Mineralogy of Fresh Groundwater Systems, Queensland, Australia: Part II – Regional Seal.
International Congress of Chemistry and Environment, Thailand.
Hodgkinson, J., Hortle, A. and McKillop, M. 2010. The Application of Hydrodynamic Analysis in the
Assessment of Regional Aquifers for Carbon Geostorage: Preliminary Results for the Surat Basin,
Queensland. APPEA Journal 2010.
Klohn Crippen Berger (KCB) 2010. Central Condamine Alluvium: Stage 2 – Conceptual Hydrogeological
Summary. Report prepared for the Department of Environment and Resource Management,
Queensland Government, File No. M09631 A01, July 2010.
Norušis, MJ, SPSS 13.0 Guide to Data Analysis. Englewood Cliffs: Prentice Hall, 2005.
Queensland Water Commission (QWC) 2011. Report for QWC17-10 Stage 2: Surat Management Area
Groundwater Model Report. Prepared by GHD Pty Ltd for The State of Queensland as
represented by the Queensland Water Commission (currently in draft), December 2011.
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
Radke, B.M., Ferguson, J., Cresswell, R.G., Ransley, T.R. and Habermehl, M.A. 2000. Hydrochemistry
and Implied Hydrodynamics of the Cadna-owie – Hooray Aquifer, Great Artesian Basin, Australia
Bureau of Rural Science, Canberra.
Schulz-Rojahn 1993. Calcite-cemented zones in the Eromanga Basin: clues to petroleum migration and
entrapment? APEA Journal, 33 (1), p. 63-76.
SRK Consulting (SRK) 2008.Bowen and Surat Basins Regional Structural Framework Study.
Scott, S., Anderson, B., Crosdale, P., Dingwall, J. and Leblang, G. 2004.Revised Geology and Coal Seam
Gas Characteristics of the Walloon Sub-group.PESA Eastern Australasian Basins Symposium II,
Adelaide.
Scott, S., Anderson, B., Crosdale, P., Dingwall, J. and Leblang, G. 2007. Coal petrology and coal seam
gas contents of the Walloon Subgroup – Surat Basin, Queensland, Australia. International Journal
of Coal Geology, 70 (2007), 209-222.
Stumm W. and J.J. Morgan, 1981.Aquatic Chemistry: An Introduction Emphasizing Chemical Equilibria in
Natural Waters (2nd Edition). John Wiley & Sons, ISBN 0-471-09173-1, 780 pp.
Thomas, B.M. and Reiser, R.F. 1971, Surat, Queensland, 1:250 000 geological series map.Sheet SG/5516, 1st edition, Bureau of Mineral Resources, Australia.
Whiticar, M.J. 1999. Carbon and hydrogen isotope systematic of bacterial formation and oxidation of
Methane. Chemical Geology, Vol. 161, pp. 291-314.
Woods, P.H., Walker, G.R. and Allison, G.B. 1990. Estimating groundwater discharge at the southern
margin of the Great Artesian Basin near Lake Eyre, South Australia. In: Proceedings of the
International Conference on Groundwater in Large Sedimentary Basins, Perth, 9-13 July 1990.
Australian Water Resources Counsel Conference Series No. 20, p.298-309.
WorleyParsons 2010. Spatial Analysis of Coal Seam Water Chemistry; Task 1: Literature Review.
Prepared for the Department of Environment and Resource Management, December 2010.
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
13 ABBREVI ATIONS
Abbreviation
Meaning
BMO
Bungil Formation, Mooga Sandstone and Orallo Formation
BRS
Bureau of Rural Sciences
2+
Ca
Calcium
CSG
Coal Seam Gas
-
Cl
Chloride
-
Carbon Dioxide
CO3
2-
Carbonate
DERM
Department of Environment and Resource Management
EPA
Environmental Protection Agency
GAB
Great Artesian Basin
GABCC
Great Artesian Basin Coordinating Committee
GDE
Groundwater Dependent Ecosystem
GL
Gigalitres
CO2
-
HCO3
+
Bicarbonate
K
Potassium
Km
Kilometre
LNG
Liquefied Natural Gas
m
Metres
Ma
Million Years Before Present
m AHD
Metres Above Australian Height Datum
m bgl
Metres Below Ground Level
mD
Millidarcy
mg
Milligrams
m/a
Metres Per Annum (year)
m/day
Metres Per Day
2
m /day
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Metres Squared Per Day
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
Abbreviation
Meaning
mg/L
Milligrams Per Litre
2+
Mg
Magnesium
ML
Megalitres
ML/day
Megalitres Per Day
mm/a
Millimetres Per Annum (year)
+
Na
Sodium
NRM
Natural Resources Management
PDB
Pee Dee Formation Belemnite (an isotope-correction
standard)
PJ
Petajoules
ppm
Parts per million
TDS
Total Dissolved Solids
RWL
Reduced Water Level
2-
SO4
Sulfate
mS/cm
MilliSiemens Per Centimetre (1 mS/cm = 1000µS/cm)
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14 GLOSS ARY
Term
Description
Aeolian
The erosion, transport, and deposition of material due to the action of the wind
at or near the Earth’s surface.
Alluvial
Applying to the environments, actions, and products of rivers or streams.
Alluvium
Clay or silt or gravel (sediment) carried by rivers or streams and deposited
where the stream slows down.
Anion
A negatively-charged ion (i.e. an atom or complex of atoms, that has lost one
or more electrons and is left with an overall negative charge).
Aquifer
A water-saturated geologic unit that is capable of transmitting significant or
usable quantities of groundwater under ordinary hydraulic gradients.
Aquitard
A water-saturated sediment or rock whose permeability is so low it cannot
transmit any useful amount of water. An aquitard allows some measure of
leakage between the aquifer intervals it separates.
Artesian
A condition which applies to aquifers which are confined by layers of low
permeability, and where the hydraulic head in the aquifer is higher than the
overlying ground surface. Wells penetrating such aquifers may result in
groundwater flowing at the surface without pumping.
Authigenic
Minerals or materials that are formed in place rather than having been
transported and deposited.
Basalt
A dark-coloured, fine-grained, extrusive igneous rock composed of plagioclase
feldspar, pyroxene, and magnetite, with or without olivine, and containing not
more than 53 wt.% SiO2. Many basalts contain phenocrysts of olivine,
plagioclase feldspar and pyroxene.
Baseflow
Amount of groundwater flowing into a river.
Basin
A topographic depression containing, or capable of containing, sediment.
Bedrock
The solid rock that underlies unconsolidated surficial sediments.
Bore/borehole
A hole drilled into the ground for exploratory purposes. See “wellbore”.
Brackish Water
Water that contains relatively low concentrations of soluble salts. Brackish
water is saltier than fresh water, but not as salty as salt water.
Cainozoic
The period in geologic time between 65 million years ago and the present
Calcareous
Composed of, or containing, or resembling calcium carbonate, or calcite, or
chalk.
Catchment
The area of land drained by a creek or river system, or a place set aside for
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
Term
Description
collecting water which runs off the surface of the land. Catchments provide the
source of water for the dams and reservoirs in which our drinking water is
collected.
Cation
A positively-charged ion (i.e. an atom or complex of atoms, that has lost one or
more electrons and is left with an overall positive charge).
Clastic
(1) Applied to the texture of fragmental sedimentary rocks. (2) Sediment
composed of pre-existing rocks.
Climatic
Of or relating to the climate.
Coal Seam
A layer, vein, or deposit of coal.
Confined Aquifer
Exists where the groundwater is bounded between layers of impermeable
substances like clay or dense rock. When tapped by a well, water in confined
aquifers is forced up, sometimes above the soil surface. This is how a flowing
artesian well is formed.
Confining Layer / Unit
Geologic material with little permeability or hydraulic conductivity. Water does
not pass through this layer or the rate of movement is extremely slow.
Conformable /
Conformably
Applied to a sequence of strata deposited in an apparently continuous
succession.
Conglomerate
Coarse grained sedimentary rock with rounded clasts greater than 2 mm in
size.
Consolidated Rock
Tightly bound geologic formation composed of sandstone, limestone, granite, or
other rock.
Cretaceous
The period in geologic time between 140 and 65 million years ago; also, the
corresponding system of rocks deposited during that time range.
Darcy
A unit of intrinsic permeability. One Darcy is equal to 0.987 × 10
-6
10 m/s.
Darcy’s law
A groundwater movement equation formulated by Henry Darcy during the
mid-1800s based on experiments on the flow of water through beds of sand.
Darcy's Law forms the scientific basis of fluid permeability used in earth
science.
Depressurisation
The lowering of the groundwater potentiometric surface over the desired area.
Devonian
The period in geologic time between 408 and 362 million years ago; also, the
corresponding system of rocks deposited during that time range.
Diagenetic
The physical, chemical or biological alteration of sediments into sedimentary
rock at relatively low temperatures and pressures that can result in changes to
the rock's original mineralogy and texture.
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2
m or 9.66 x
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
Term
Description
Discharge
An outflow of water from a stream, pipe, groundwater aquifer, or watershed; the
opposite of recharge.
Discharge Area
The area or zone where groundwater emerges from the aquifer. The outflow
maybe into a stream, lake, spring, wetland, etc.
Dissolved Solids
Minerals and organic matter dissolved in water.
Drawdown
A lowering of the groundwater level caused by pumping.
EC
Electrical Conductivity. Usually measure in Siemens per unit length
(e.g. milliSiemens per centimetre (mS.cm).
Erosion
The process by which material, such as rock or soil, is worn away or removed
by wind or water.
Evapotranspiration
The process by which water is discharged to the atmosphere as a result of
evaporation from the soil and surface-water bodies and transpiration by plants.
Transpiration is the process by which water passes through living organisms,
primarily plants, into the atmosphere.
Facies
Features of a rock type or water type from which its origins can be inferred.
Fault
A crack in the earth's crust resulting from the displacement of one side with
respect to the other.
Flow Rate
The time required for a volume of groundwater to move between points.
Typically groundwater moves very slowly – sometimes as little as millimetres
per year.
Fluvial
Material deposited by moving water.
Fluvial Deposits
(1) Particles of minerals or rocks which are transported by a river and deposited
along its valley. (2) All material, past and present, deposited by flowing water.
Formation
A geologic unit of distinct rock types that is large enough in scale to be
mappable over a region.
Fractured
A general term applied to any break in a material, but commonly applied to
more or less clean breaks in rocks of minerals that are not due to cleavage or
foliation.
Fresh Water
Water that is not salty, especially when considered as a natural resource.
Typically less than 1,000 mg/L total dissolved solids.
Gaining Stream
A stream in which groundwater discharges contribute significantly to the stream
flow volume. The same stream could be both a gaining stream and a losing
stream, depending on the conditions.
Goethite
An iron-bearing oxide mineral found in soil and other low-temperature
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
Term
Description
environments; FeO(OH).
Grading
The process of levelling off to a smooth horizontal or sloping surface.
Granite
A light-coloured, coarse-grained intrusive igneous rock, consisting of essential
quartz (at least 20%), alkali feldspar, mica (biotite and/or muscovite), with or
more commonly without amphibole, and accessory apatite, magnetite, and
sphene.
Groundwater
All the water contained in the pores/voids within unconsolidated sediments or
consolidated rocks (i.e. bedrock).
Group
A grouping of geological or hydrogeological formations.
Heterogeneous
Consisting of elements that are not the same kind or nature.
Homogenisation
The act of making something homogeneous or uniform in composition.
Hydraulic Conductivity
The capacity of a porous medium to transmit water. Hydraulic conductivity (K) is
governed by the size and shape of pore spaces, and the physical properties of
the fluid moving through the pore spaces.
Hydraulic Gradient
The change in hydraulic head or water level over a distance. Usually expressed
in metres/metre. For example, a hydraulic gradient of 0.01 indicates a onemetre drop in water level over a distance of 100 m. The hydraulic gradient is the
driving force that causes groundwater to flow.
Hydraulic Head
A measure of the groundwater pressure in an aquifer. Hydraulic head is
determined from water level measurements in wells.
Hydrocarbons
An organic compound containing only carbon and hydrogen.
Hydrochemical Type
The definition of a chemical composition of groundwater based on the relative
percentages of major cation and anion concentrations.
Hydrogeology
The science that relates geology, fluid movement (i.e. water) and geochemistry
to understand water residing under the earth’s surface. Groundwater as used
here includes all water in the zone of saturation beneath the earth’s surface,
except water chemically combined in minerals.
Hydrochemistry
The study of the chemical processes and reactions that govern the composition
of water in relation to its interaction with rocks, other water bodies, and soils. It
also covers the role of water in the cycles of matter and energy that transport
the Earth’s chemical components in time and space, and their interaction with
the hydrosphere and atmosphere.
Hydrostatic Head
The force (pressure) exerted by a body of fluid at rest.
Hydrostratigraphic Unit
Geological units that are not solely based on lithological characteristics but also
include characteristics related to water movement, occurrence and storage.
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SPATIAL ANALYSIS OF CSG WATER CHEMISTRY
Term
Description
Impermeable Layer
A layer of material (such as clay) in an aquifer through which water does not
pass.
Ion-exchange
A chemical process in which cations of like charge are exchanged equally
between a sediment and pore water phases.
Indicators
Anything that is used to measure the condition of something of interest.
Indicators are often used as variables in the modelling of changes in complex
environmental systems.
Infiltration
Flow of water from the land surface into the subsurface. Infiltration is the main
factor in recharge of groundwater reserves.
Isopach
A contour that connects points of equal thickness.
Isotope
One of two or more atoms with the same atomic number but with different
numbers of neutrons. A radiogenic isotope decays with time while a stable
isotope does not.
Jurassic
The period in geologic time between 208 and 145.6 million years ago; also, the
corresponding system of rocks deposited during that time range.
Kaolinite
A layered silicate mineral (clay mineral) with one tetrahedral sheet linked
through oxygen atoms to one octahedral sheet of alumina octahedral.
Lacustrine Deposits
Sedimentary material deposited in a lake environment.
Lagoon
A body of water enclosed by a barrier, such as a water storage pond.
Limestone
A sedimentary rock rich in calcium carbonate.
Lithic
Formed of rock.
Lithology
The systematic description of sediment and rocks, in terms of composition,
texture and internal structure.
Losing stream
A stream that is losing water to (or recharging) the groundwater system. The
same stream could be both a gaining stream and a losing stream, depending
on the conditions.
Mafic
Applied to any igneous rock which has a high proportion of iron and magnesium
silicates (pyroxene and olivine) such that its colour index is between 50 and 90
(i.e. it is dark coloured). Also ultra-mafic.
Member
A unit used in lithostratigraphy. Many members are grouped into a formation,
which is the fundamental unit used in lithostratigraphy.
Meteoric Water
Water derived from the earth's atmosphere.
Methanogenic
The process by which certain bacteria convert CO2 or organic molecules to
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Term
Description
methane.
Micaceous
A rock that contains a notable amount of the mineral mica (a sheet-silicate).
Mineralisation
Conversion of organic tissues to an inorganic state as a result of decomposition
by soil micro-organisms.
Monitoring Well
A constructed controlled point of access to an aquifer which allows groundwater
observations. Small diameter observation wells are often called piezometers.
Mound Spring
Mound springs are geomorphic formations raised above the surrounding land
surface formed by a deposit of minerals and sediment brought up from artesian
aquifers or confining beds by water at certain natural discharge points in the
Great Artesian Basin. Other spring systems not raised above the surrounding
land surface also occur throughout the Basin.
Mudstone
A fine-grained sedimentary rock whose original constituents were clays and
muds. Grain size is up to 0.625 mm with individual grains too small to be
distinguished without a microscope.
Orthoclase
A variety of feldspar, composed of potassium aluminum silicate, characterised
by a monoclinic crystalline structure and found in igneous or granitic rock. Also
called potassium feldspar or alkali feldspar.
Outcrop
The part of a rock formation which is exposed at the Earth’s surface.
Oxidation
A chemical reaction in which ions are transferring electrons, to increase positive
valence.
Pegmatite
A very crystalline, intrusive igneous rock composed of interlocking crystals
usually larger than 2.5 cm in size.
Percolation
The movement of water through the openings in rock or soil.
Permeability
A measure of the ability of a porous medium to transmit a fluid (any fluid) to
diffuse through it to another medium. Similar to hydraulic conductivity that
describes the ability of a porous medium to transmit water specifically.
Permian
The final period in the Palaeozoic Era that occurred between 299-251 Ma ago.
Per mil
Parts per thousand (‰)
pH
The logarithm of the reciprocal of hydrogen-ion concentration in gram atoms
per litre; provides a measure on a scale from 0 to 14 of the acidity or alkalinity
of a solution (where 7 is neutral and greater than 7 is more basic and less than
7 is more acidic).
Plagioclase
Silicates of aluminum with calcium and sodium. They are a member of the
feldspar family of minerals and are an important constituent of many plutonic
and volcanic rocks.
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Term
Description
Porosity
The ratio of the volume of void or air spaces in a rock or sediment to the total
volume of the rock or sediment. The capacity of rock or soil to hold water varies
with the material. For example, saturated small grain sand contains less water
than coarse gravel.
Potentiometric surface
An imaginary surface that everywhere coincides with the static level of the
water in a given water bearing formation. The surface to which the water from a
given interval will rise under its full hydraulic head.
Precipitation
The part of the hydrologic cycle when water falls, in a liquid or solid state, from
the atmosphere to Earth (rain, snow, sleet).
Quartzose / Quartzitic
Relating to primarily comprising the mineral quartz.
Recharge
The infiltration of water into the soil zone, unsaturated zone and ultimately the
saturated zone. This term is commonly combined with other terms to indicate
some specific mode of recharge such as recharge well, recharge area, or
artificial recharge.
Recharge Area
An area where permeable soil or rock allows water to seep into the ground to
replenish an aquifer.
Reservoir
A subsurface, porous, permeable rock body in which oil and gas is stored.
Residual head
The subtraction of one groundwater elevation from another to determine if the
remaining value is positive or negative (i.e., indicating potential for upward or
downward flow).
Runoff
The portion of precipitation (rain and snow) that ultimately reaches streams.
Salinity
An accumulation of soluble salts in the soil root zone, at levels where plant
growth or land use is adversely affected. Also used to indicate the amounts of
various types of salt present in soil or water (see Total Dissolved Solids).
Sandstone
A sedimentary rock composed of individual grains of sand cemented together.
Saturated Zone
The portion below the earth's surface that is saturated with water is called the
zone of saturation. The upper surface of this zone, open to atmospheric
pressure, is known as the water table.
Shale
A sedimentary rock formed by the deposition of successive layers of clay.
SI values
Mineral saturation indices values which indicate the potential for a mineral to
form (precipitate from solution), as noted by SI values greater than 0, or
dissolve/weather, as noted by SI values less than 0.
Silt
Mud or clay-sized sediment.
Siltstone
A fine-grained rock of consolidated silt.
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Term
Description
Smectite
A clay mineral group that is characterised by the ability to expand and absorb
large quantities of water due to their relatively open mineral structure (two
tetrahedral silicate sheets sandwiching a central octahedral sheet).
Spring
A flow of water above ground level that occurs where the water table intercepts
the ground surface.
Stratigraphy
The study of the sequence of layered geologic deposits based on their spatial
positions, depositional sequence in time, and correlations across different
localities.
Sub-cropping
Bedrock unit occurring at the bedrock surface but covered by surficial deposits.
Subsidence
The gradual settling or sudden sinking of the land surface owing to natural or
anthropogenic influences of materials in the subsurface.
Subsurface
Beneath the surface.
Surface Water
Water above the surface of the land, including lakes, rivers, streams, ponds,
floodwater, and runoff.
Sustainable Yield
See safe yield.
Syncline
A downward-curving fold, with layers that dip toward the centre of the structure.
Tectonic
Pertaining to the structure or movement of the earth’s crust.
Tertiary
The first period of the Cainozoic Era, from about 65 to 2 million years ago.
Topography
The configuration of a surface including its relief and natural and artificial
features.
Thermogenic
Formed by heating.
Total Dissolved Solids
(TDS)
Concentration of all substances dissolved in water (solids remaining after
evaporation of a water sample).
Transmissivity
A measure of the capability of the entire thickness of an aquifer to transmit
water. Also known as coefficient of transmissivity.
Triassic
The Triassic encompasses a time frame between about 248 and 213 million
years ago.
Unconformity
Surface of contact between two groups of unconformable strata, which
represents a hiatus in the geological record due to a combination of erosion and
a cessation of sedimentation.
Unconfined Aquifer
A permeable bed only partly filled with water and overlying a layer of lower
hydraulic conductivity. Its upper boundary is formed by a free water table where
pore pressure is equal to atmospheric pressure. Water in a well penetrating an
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Term
Description
unconfined aquifer does not, in general, rise above the water surface.
Unsaturated Zone
The body of soil and rock separating the water table and the land surface.
Watertable
The upper surface of groundwater of the level below which the soil is saturated
with water.
Well
An excavation or structure created in the ground by digging, driving, boring or
drilling to access water in the subsurface.
Wellbore
The physical hole that makes up the well, and can be cased, open or a
combination of both.
Yield
The quantity of water removed, or able to be removed from a well.
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