Groundwater
Aquifer
Connectivity
in Queensland, Australia
Technical Communication 1
November 2014
About GasFields Commission Queensland
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sustainable coexistence between rural landholders, regional communities and the onshore gas industry
in Queensland, Australia.
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For more information visit the GasFields Commission website at www.gasfieldscommissionqld.org.au
About this Technical Communication
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The Commission’s technical communications aim to fill a gap in information between the simple fact
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and Queensland Government departments, independent technical specialists and scientific experts, and
Queensland’s onshore gas industry.
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© 2014 GasFields Commission Queensland.
Groundwater Aquifer Connectivity
2
Contents
Introduction
4
Confined and Unconfined Aquifers
4
What is Aquifer Connectivity?
5
What determines the degree of connectivity between geological formations?
7
How has aquifer connectivity been measured and monitored?
11
What are the expected impacts of induced aquifer leakage?
14
Conclusion
18
Glossary
19
References
20
Tables
Table 1:
Hydraulic features of geological formations in the Surat and southern Bowen Basins. 8
Table 2:
Typical vertical separation distances between the shallowest coal measures
and closest overlying aquifer for the Surat, Bowen and Galilee Basins.
Table 3:
Number of bores within LAAs impacted by induced aquifer leakage associated
with CSG production within the Surat CMA.
9
16
Figures
Figure 1: Location of Surat, Bowen and Galilee Basins in relation to the Great Artesian Basin. 4
Figure 2: A conceptual groundwater system.
4
Figure 3: Schematic representation of groundwater water pressure and vertical flow direction. 5
Figure 4: Conceptual equation for cross formation water flow.
6
Figure 5: Close up photograph of a section of a rock core sample from the Moolayember
Formation (a low permeability aquitard) in the Galilee Basin.
7
Figure 6: Direct contact zone between Bandanna Formation and Precipice Sandstone aquifer. 9
Figure 7: A knowledge improvement cycle for preparing and implementing an Underground
Water Impact Report (UWIR) for a CSG project.
11
Figure 8: Comparison of estimated annual groundwater extraction and induced flow rates in
the Surat CMA.
14
Figure 9: Extent of the Long-term Affected Areas in the Surat CMA
15
Figure 10: Make good obligations.
17
Groundwater Aquifer Connectivity
3
Introduction
The management of groundwater aquifers is
critical to ensuring coexistence of rural
landholders, regional communities and the
onshore gas industry. Aquifer connectivity is
important as it underpins our understanding of
the groundwater impacts from CSG
development. This paper explores the concept
of aquifer connectivity and the key factors that
regulate it. Importantly, discussion is also
provided on how connectivity in relation to CSG
production has been measured and monitored in
Queensland.
Confined and Unconfined
Aquifers
Figure 1 provides an overview of the principal
CSG geological basins in relation to the Great
Artesian Basin (GAB), one of the world’s largest
groundwater systems. The GAB includes the
geological formations of the Surat Basin and the
upper Triassic formations of the Bowen and
Galilee Basins. The deeper, older formations of
the Bowen and Galilee Basins are not within the
GAB.
Figure 2:
Figure 1:
Location of Surat, Bowen and Galilee
Basins in relation to the Great
Artesian Basin. Source: DoE, 2014
A conceptual groundwater system showing a confined aquifer, unconfined aquifer
and aquitard layers. Source: QWC (2012)
Groundwater Aquifer Connectivity
4
The GAB is a groundwater basin made of rock
layers (called formations) that form aquifers
(permeable layers that readily transmit water)
and aquitards (the confining layers that restrict
groundwater flow) (Figure 2). The aquifers of the
GAB are confined as they are overlain by
aquitards which restrict the vertical movement of
water and enable water pressure to develop in
the aquifer. On top of the GAB lie more recently
deposited rock layers. Near the surface some of
these materials, such as river gravels and sands
or basaltic rock from ancient volcanic activity,
form shallow, unconfined aquifers (permeable
layers that readily transmit water but are not
covered by an aquitard).
What is Aquifer Connectivity?
Connectivity describes the ease with which
groundwater can flow within or between
geological formations (QWC, 2012). It is
controlled by the resistance to flow between
geological formations, which is determined by the
rock type, its porosity and how well pores are
connected. Also, discontinuities such as
geological fractures (e.g. natural vertical cracks
in formations) or poorly constructed bores or
wells (e.g. bore or well casings that are not
properly sealed between formations) can
increase connectivity.
a) Undisturbed groundwater system
Figure 3:
Water flows from areas of higher water level or
water pressure to areas of lower water level or
water pressure. In aquifers, most flow is
horizontal (DoE, 2014). In aquitards, most flow is
vertical (DoE, 2014) due to the upwards or
downwards leakage of water from aquifers.
In the undisturbed groundwater systems of the
Surat, Bowen and Galilee Basins, natural aquifer
leakage is usually upwards from deep formations
under higher pressure to overlying formations
under lower pressure (CSIRO 2012b and DoE,
2014) (Figure 3a), although this trend is not
consistent within the Surat Basin.
When the hydrological system is disturbed by
activities such as groundwater extraction, the
direction of natural vertical aquifer leakage can
be reversed if the water pressure in the deeper
formations is reduced (CSIRO 2012b and DoE,
2014) (Figure 3b). Aquifer leakage that occurs in
response to a disturbance to the groundwater
system is called induced aquifer leakage.
When considering vertical water leakage
between geological formations under natural and
disturbed conditions, the water pressure
difference (called the hydraulic gradient) is
important but is only part of the equation. A
sufficient degree of connectivity is also required
between the formations for water to flow (CSIRO
2012b) (Figure 4).
b) Disturbed groundwater system
Schematic representation of groundwater water pressure and vertical flow direction in a
hypothetical groundwater system under both a) undisturbed and b) disturbed conditions.
Water pressure in the confined aquifer is indicated by the potentiometric surface and by the
water table level for the unconfined aquifer. Source: National Groundwater Association
(NGWA) cited in DoE (2014). Note: 3a shows natural upwards vertical leakage from the higher
pressured confined aquifer to the lower pressured unconfined aquifer and 3bshows a decline in the
water pressure level of the confined aquifer near the well due to groundwater pumping and a reversal
in vertical leakage. Induced aquifer leakage occurs near the well as water pressure is higher in the
unconfined aquifer.
Groundwater Aquifer Connectivity
5
Sufficient
degree of
connectivity
Figure 4:
Adequate
hydaulic
gradient
Cross
formation
water flow
Conceptual equation for cross formation water flow. Source: CSIRO, 2012b
Connectivity is commonly expressed in simple
terms as low, medium or high. It can be different
depending on the direction of water flow. Since
geological formations are typically layered
reflecting geological history, the resistance to
flow will vary depending on whether flow is
vertical or horizontal.
While there will be no flow between formations
with high connectivity if there is no hydraulic
gradient between them, there will be flow
between formations with low connectivity if a
sufficiently large hydraulic gradient exists (QWC,
2012). However, the rate of flow is likely to be
extremely slow and cause a significant delay
between the time of creation of the hydraulic
gradient and the time when the flow between the
formations peak (CSIRO 2012b and QWC 2012).
Current studies have found that low connectivity
is a dominant geological characteristic of aquifers
across the Surat, Bowen and Galilee Basins
(Australian Government, 2014; CSIRO, 2012a
and b; Marsh et al., 2008; RPS, 2012 and QWC,
2012) due to:
 The low vertical permeability of the coal
measures consisting of primarily mudstone,
siltstone and intervening discontinuous coal
layers and separating aquitard layers; and
 The vertical separation of varying degree
between coal measures and aquifers.
The low degree of connectivity between coal
measures and aquifers plays an important role in
minimising the impacts of CSG development on
groundwater resources by limiting induced
aquifer leakage. Further discussion is provided
in the following sections on the key factors that
determine connectivity (vertical permeability and
separation), the ways that connectivity has been
measured and how it is predicted to limit the
impacts to groundwater resources.
Summary: What is aquifer connectivity?
1.
Connectivity describes the ease with which water can move between geological
formations.
2.
Low connectivity is a dominant feature of the Surat, Bowen and Galilee Basins
and means that water flow between formations will be extremely slow.
3.
Low connectivity minimises the groundwater impacts of CSG development by
restricting induced aquifer leakage.
Groundwater Aquifer Connectivity
6
What determines the degree of
connectivity between geological
formations?
Low vertical permeability and generally large
vertical separation distances act together to limit
aquifer connectivity between coal measures and
aquifers across most parts of the Surat, Bowen
and Galilee Basins.
Groundwater does not flow in ‘underground
rivers’; instead it flows slowly between the
connected pores and fractures or fault lines in
geological formations. The term “permeability” is
used to describe the ease with which water can
flow horizontally and vertically though a
geological formation.
In sedimentary materials, such as those found in
the Surat, Bowen and Galilee Basins, vertical
permeability is typically at least 100 to 1,000
times less than horizontal permeability (QWC,
2012). This is due to the layered structure of
sedimentary formations (Figure 5), which means
that it is easier for water to move horizontally
along the sedimentary layers than vertically
across them (QWC, 2012). Figure 5 shows very
fine grained sandstone (light brown) and
mudstone (dark grey) layers in a low permeability
aquitard in the Galilee Basin.
Figure 5:
Close up photo of a rock core sample
from the Moolayember Formation in
the Galilee Basin. Source: adapted
from Rawsthorn et al. (2009).
Regional scale permeability data for selected
geological formations found in the Surat and
southern Bowen Basins are provided in Table 1.
Permeability varies greatly between the different
formation types and basins as well as between
the horizontal and vertical directions.
Vertical permeability influences connectivity
between formations due to the effect it has on
the rate of vertical water movement. Typically,
the rate ranges from millimetres per year in
aquitards and coal measures to kilometres per
year in aquifers. The time required for water to
travel 100 metres vertically in geological
formations ranges from days and decades in
aquifers to millennia in aquitards and coal
measures. The limiting effect of these extremely
slow speeds of vertical water movement in
aquitards and coal measures on connectivity is
further compounded by vertical separation
between aquifers and coal measures.
The target coal measures are usually located at
considerable depths from the surface and the
thin coal seams are physically separated from
overlying or underlying aquifers by low
permeability interburden materials and normally
at least one or more low permeability, thick
aquitard layers (Australian Government, 2014;
CSIRO, 2012a and b; Marsh et al., 2008; RPS,
2012 and QWC, 2012). As vertical separation
between formations increases, the connectivity
decreases (QWC, 2012). Therefore, the total
depth of the target coal seam and the thickness
of low permeability geological formations
separating the coal seam from an aquifer are
important factors in restricting connectivity
between coal seams and aquifers.
Examples of typical vertical separation distances
for the shallowest target coal measures and their
closest overlying aquifer for the Surat, Bowen
and Galilee Basins are provided below (Table 2).
Groundwater Aquifer Connectivity
7
Table 1:
Typical hydraulic features of geological formations in the Surat and southern Bowen Basins.
Aquifer
Aquifer: Formations with a high permeability, such as sands, porous
sandstones or basalt, are called aquifers. The high permeability of
aquifers is attributed to the connected spaces or pores between sand
grains or fractures and cracks in rock that allow water to flow with little
resistance. Aquifers are important sources of water in many rural areas
and are accessed by bores.
Permeability1: tens of metres per year to several thousand metres per
year, similar in both vertical and horizontal directions.
Examples include: Condamine Alluvium in Surat Basin and Clematis
Sandstone in Bowen Basin.
Adapted from Co2crc (2014)
Aquitard
Aquitards: Formations with a low permeability, such as siltstone or
mudstone, are called aquitards. The low permeability of aquitards
reflects the poor connection between the microscopic pores of fine
grained and compacted geological materials, with resultant high
resistance to water flow. Aquitards are common and play a role in
maintaining groundwater pressure in aquifers by restricting aquifer
leakage.
Permeability1: 0.5 meters per year in horizontal direction
0.002 meters per year in vertical direction
Examples include: Westbourne Formation in Surat Basin.
Adapted from Co2crc (2014)
Coal Measure
Coal measures: Formations that contain many thin coal seams
separated by low permeability rock (interburden) are called coal
measures. They are often considered to be aquitards because of their
low vertical permeability.
Permeability1: 10 meters per year in horizontal direction
0.002 meters per year in vertical direction
Examples include: Walloon Coal Measures in Surat Basin and
Bandanna Formation in Bowen Basin.
Note 1:
Adapted from APLNG (2011)
Permeability is a measure of how freely water can flow through a rock stratum and can vary depending on
flow direction. This value indicates how far water would move in a year under a unit pressure gradient.
Values range from millimetres to kilometres/year. Source: permeability values from GHD (2012).
The vertical separation data provided in Table 2
highlight some important differences between the
basins:
1.
Coal measures targeted for CSG
development outside the Central
Condamine Area of the Surat Basin are
very deep; and
2.
Relatively thin aquitards immediately
overlie the Walloon Coal Measures in the
Surat Basin compared to the much thicker
aquitards in the Bowen and Galilee Basins.
Despite these unique characteristics of the Surat
Basin, a low degree of connectivity exists
between aquifers adjacent to the Walloon Coal
Measures (QWC, 2012). Vertical separation of
aquifers and coal measures by one or more low
permeability aquitards occurs across most parts
of the basins, but there are locations within each
basin where the aquitard is absent and direct
contact between coal seams and aquifers occurs
(i.e. no vertical separation exists) (RPS, 2012
and QWC, 2012). An example from the Bowen
Basin adjacent to the Spring Gully and Fairview
CSG fields is provided in Figure 6.
Groundwater Aquifer Connectivity
8
Table 2:
Typical vertical separation distances between the shallowest coal measures and closest
overlying aquifer for the Surat, Bowen and Galilee Basins.
Basin Name
Surat (Central
Condamine
Area)
Coal Measure – depth to
formation (m)
Overlying Aquitard – average
thickness / vertical separation
(m)
Closest Overlying
Aquifer
Walloon Coal
Measures (WCM)
30-130
Transition zone between
Condamine Alluvium
(CA) and WCM
30
Condamine Alluvium
Walloon Coal
Measures
700
Upper aquitard of WCM
15
Springbok
Sandstone
Bowen
Bandanna
Formation
300-800
Rewan
300
Clematis Sandstone
Galilee
Betts Creek Beds
700 +
Rewan
3001
Clematis
Sandstone/Dunda
Beds
Surat
Note 1:
Assumed same as Bowen Basin. Geoscience Australia (2014) note a maximum thickness of 840 metres
for this formation in the Galilee Basin.
Sources: AGL, 2013; Arrow Energy, 2012a; Comet Ridge, 2014; DNRM, 2014; RPS, 2012 and QWC, 2012
Figure 6:
Direct contact zone between Bandanna Formation and Precipice Sandstone aquifer in the
Fairview and Spring Gully CSG production area in the Bowen Basin near Roma. Direct
contact occurs between the formations due to erosion of the Rewan Formation (aquitard)
and Clematis Sandstone aquifer prior to deposition of the Precipice Sandstone sediments.
Source: QWC, 2012
Groundwater Aquifer Connectivity
9
In this specific location a higher degree of
connectivity and leakage from the aquifer
induced by CSG development was expected due
to the absence of the Rewan Formation aquitard.
However, CSG production commenced in this
area in 1996 and water pressure monitoring over
this time has not shown a discernible effect on
water pressures in the Precipice Sandstone
aquifer due to depressurisation of the Bandanna
Formation coal seam from CSG activity. This
means that this aquifer has not been adversely
impacted by CSG production (QWC, 2012).
Direct contact zones also occur between the
Condamine Alluvium (CA) and the Walloon Coal
Measures (WCM) in the Surat Basin and the
Hutton Sandstone and the Betts Creek Beds in
the Galilee Basin (RPS, 2012 and QWC, 2012).
However, groundwater impact assessments of
CSG production in areas with direct contact
zones conducted by the Queensland Water
Commission, now the Office of Groundwater
Impact Assessment (OGIA), in the Surat Basin
(2012) and AGL in the Galilee Basin (2013) do
not predict significant levels of induced aquifer
leakage. The lack of measured and predicted
impacts on aquifers in these circumstances may
reflect a low level of aquifer connectivity, or
suggest that CSG production in these areas will
not produce a sufficient hydraulic gradient
(pressure difference between geological
formations) to induce groundwater to flow from
aquifers, known as induced aquifer leakage.
Summary: What determines the degree of connectivity between geological
formations?
1.
Connectivity between geological formations is dependent upon their vertical
permeabilities and vertical separation.
2.
Vertical permeabilities of aquitards and coal measures are very low due to their
layered sedimentary structure.
3.
Coal measures are usually located at great depths and separated from aquifers
by low permeability interburden materials and some aquitard.
4.
Low vertical permeabilities and separation distances between geological
formations create a high resistance to flow and a low degree of aquifer
connectivity in the Surat, Bowen and Galilee Basins.
Groundwater Aquifer Connectivity
10
How has aquifer connectivity
been measured and monitored?
Aquifer connectivity is determined by a process
of measurement and modelling (DoE, 2014).
There are numerous methods of measurement
available: hydraulic assessment (pump tests and
application of groundwater responses);
laboratory tests (permeability analysis of rock
cores); geophysical assessment of stratum; and
geochemical analyses (water chemistry
analysis). Data obtained are used to develop a
conceptual model of the system which is then
built into a computer-based mathematical
representation of the groundwater system. The
mathematical model is tuned to all available
measured groundwater responses and then
applied to explore changes imposed by water
extraction and management activities. While the
mathematical model is only a partial description
of the groundwater systems, confidence in these
models improves as new data become available
from the monitoring program.
The Australian Government Department of
Environment commissioned a background review
(DoE, 2014) to describe the methods used to
measure and model connectivity in any
groundwater system, and the understanding of
aquifer connectivity within the Surat, Bowen and
Galilee Basins. This review was conducted on
the advice of the Independent Expert Scientific
Committee on Coal Seam Gas and Large Coal
Mining Development (IESC). The review
highlights a continuing need for additional
measurements and analysis to improve
conceptual understanding of connectivity and
thus improve the prediction of impacts through
modelling.
In Queensland, regional scale assessment of
connectivity and aquifer leakage from CSG
development has been conducted in the Surat
Cumulative Management Area (Surat CMA). The
most comprehensive study regarding the
measurement of aquifer connectivity in CSG
development areas is the Underground Water
Impact Report (UWIR) for the Surat Cumulative
Management Area. The Surat CMA UWIR was
prepared by the former QWC in 2012 and is now
administered by the OGIA.
The Surat CMA groundwater model is widely
accepted as the most appropriate tool for
assessing the regional groundwater impacts of
CSG development (OGIA, 2013) and was used
by the Commonwealth Scientific and Industrial
Research Organisation (CSIRO, 2012b) in the
Water Resource Assessment for the Surat
Region. The reliability of any model however
depends on the accuracy of values, such as
vertical permeability, used to build the model.
The measured vertical permeabilities used to
develop the Surat CMA model are considered to
be sufficient for general characterisation of the
geological formations (DoE, 2014). Ongoing
monitoring is being undertaken to gather
additional data to improve confidence in the
conceptual and computer model of the
groundwater system (Figure 7).
Measure
Review
Report
Figure 7:
Model
Monitor
A knowledge improvement cycle for
preparing and implementing an
Underground Water Impact Report
(UWIR) for a CSG project.
Project based research monitoring is being
conducted by the OGIA for the Surat CMA. One
example is the Condamine Interconnectivity
Research Project (CIRP) which employs several
techniques to obtain locally measured values of
connectivity to confirm the accuracy of values
used in the Surat CMA model for the Condamine
Alluvia (CA) and the underlying Walloon Coal
Measures (WCM).
Groundwater Aquifer Connectivity
11
The CIRP is being conducted by the OGIA with
support from Arrow Energy, research
organisations and the Department of Natural
Resources and Mines (DNRM). The purpose of
this research is to characterise the relationship
between the groundwater systems of the CA and
WCM by compiling multiple lines of evidence
from analysis of existing water quality data sets,
field studies such as water level mapping and
pump testing and computer modelling of
geological conditions. Preliminary results from
the CIRP show that the upper WCM and
transition layer have low vertical permeabilities
(OGIA, 2014). While a hydraulic gradient exists
between the WCM and the CA, there are
significant differences in formation water
chemistry, which indicates no significant cross
formation flow. This means that there is low
connectivity between the CA and WCM (OGIA,
2014).
production or testing activities in the northern
Bowen or Galilee Basins.
However, based on the DoE’s (2014) findings, it
is possible that impacts may be under-predicted
where connectivity has been measured over
small areas. To address this potential issue,
CSG activities under an UWIR are subject to
ongoing monitoring, annual reviews and regular
revision to give confidence in the reliability of
conceptual and computer models.
To build on the current knowledge and
understanding of aquifer connectivity in the
Surat, Bowen and Galilee Basins, regional scale
measurement, monitoring and research
programs are underway and include:
 Ongoing regional scale water pressure
monitoring and expansion of the monitoring
network as required under the Surat CMA
UWIR;
 Six separate research projects by the OGIA
“We prepared the Surat Underground
Water Impact Report in 2012 using
available knowledge. We are currently
carrying out research projects to
improve knowledge of the groundwater
flow system, which includes the
connectivity between formations. The
new knowledge will be incorporated
into the construction of a new regional
groundwater flow model which will be
used to update the Surat Underground
Water impact Report in December
2015.”
– Randall Cox, General Manager,
Office of Groundwater Impact
Assessment
investigating connectivity, geological
structures and modelling, groundwater
modelling techniques and springs in the Surat
CMA. The new knowledge gained from these
studies will be used to build a new regional
groundwater flow model and update the Surat
CMA UWIR in December 2015; and
 A program of scientific bioregional
assessments to better understand the
potential water impacts of coal seam gas and
large coal mining developments being
undertaken by a collaboration of Australian
Government organisations (Department of the
Environment, Bureau of Meteorology, CSIRO
and Geoscience Australia). These
assessments include measurement and
modelling of aquifer connectivity and are
scheduled for completion in 2016.
No adverse aquifer impacts are predicted in the
UWIRs prepared by CSG operators for
Groundwater Aquifer Connectivity
12
Summary: How has aquifer connectivity been measured and monitored?
1.
Connectivity is determined by direct measurement and computer modelling.
2.
Field and laboratory data are used to develop a conceptual model and then to
build a computer-based mathematical representation of the groundwater system.
3.
Regional scale estimations of aquifer connectivity are being improved through
various studies by OGIA and other agencies.
4.
Regulatory controls exist to assess the accuracy of aquifer connectivity
predictions.
5.
Data from ongoing monitoring and measurement will build confidence in the
conceptual and computer models used to predict groundwater impacts from CSG
development.
Groundwater Aquifer Connectivity
13
CSG extraction in the Surat, Bowen and Galilee
Basins is expected to induce groundwater
leakage from some aquifers due to
depressurisation of the target coal measures.
This section discusses the expected impacts of
induced aquifer leakage from CSG production
and the management measures available to
ameliorate their predicted extent and magnitude.
The Water Act 2000 defines two measures of
future groundwater level impact on an aquifer:
the Immediately Affected Area (IAA) and Long
Term Affected Area (LAA). An IAA for an aquifer
is the area within which water level (or pressure)
reductions are predicted to exceed a trigger
threshold within three years. A LAA for an
aquifer is the area within which water pressure
reductions are predicted to exceed a threshold at
any time in the future. Threshold values have
been set at 5 metres for consolidated aquifers
(such as sandstones) and 2 metres for
unconsolidated aquifers (such as sand aquifers).
UWIRs prepared by CSG operators for projects
in the northern Bowen and Galilee Basins (AGL,
2013, Arrow Energy 2012a,b,c, 2014b, Blue
Energy, 2012, CDM Smith, 2013a, b, c, d and
Comet Ridge, 2014) do not predict induced
aquifer leakage to cause a decline in water
pressures for aquifers or springs that exceed
trigger thresholds during CSG production. That
is, existing and currently proposed CSG
development in these basins is not expected to
cause enough leakage to create IAA or LAAs in
aquifers.
In the Surat CMA, induced aquifer leakage is
predicted to impact some aquifers. A reduction
in source aquifer water pressure of more than 5
metres may affect up to 1% of water supply
bores in the Surat CMA, and a reduction of water
pressure of up to 1.5 metres in the spring source
aquifer could occur at the location of 5 spring
complexes (QWC, 2012). If a bore in an IAA is
assessed as experiencing or likely to experience
impaired function due to CSG activities, the CSG
operator is legally obliged to enter into a make
good agreement with the landholder so the
landholder is not disadvantaged by their
activities.
Between now and 2050 it is estimated that CSG
production in the Surat CMA will extract on
average approximately 95,000 ML of
groundwater per year (QWC, 2012). The total
amount of induced leakage from all formations
that overlie or underlie the WCM will be
approximately 50% of the total amount of water
extracted for CSG production (Figure 8). At the
regional scale, the predicted annual volume of
induced leakage is approximately 20% of total
groundwater extraction in the Surat CMA for nonpetroleum and gas purposes (e.g. agriculture,
stock and town water supply). Understanding
the expected extent of induced aquifer leakage in
terms of geographic area and water pressure
reduction is important for estimating impacts to
water supply bores and springs.
250000
Groundwater extraction for CSG
production
200000
ML per year
What are the expected impacts of
induced aquifer leakage?
Induced aquifer leakage to the WCM
150000
100000
Non-CSG groundwater extraction
from formations adjacent to the
WCM (see note)
50000
Non-CSG groundwater extraction
from the Surat CMA
0
Figure 8:
Comparison of estimated annual
groundwater extraction and induced
flow rates in the Surat CMA.
Source: original data sourced from
QWC (2012). Note: Based on
extraction rates for formations adjacent to
the WCM including the Condamine
Alluvium, Springbok Sandstone, Marburg
Sandstone, Hutton Sandstone and
Eurombah Formation.
No IAAs or LAAs are predicted for the
Condamine Alluvium. Induced leakage from the
CA to the WCM is estimated at 1,100 ML per
year over the next 100 years (QWC, 2012). This
volume of water is equivalent to a layer of water
Groundwater Aquifer Connectivity
14
0.2 millimetres deep per year across the
Condamine Plain which has an area of 7,000 km2
(Dafney and Silburn, 2013). Induced leakage
from the CA is predicted to result in an overall
decline in water levels over the next 100 years by
approximately 0.5 metres on average across the
area which is less than the major reduction that
has already occurred from extraction for irrigation
and town water supply (QWC, 2012).
Induced leakage will primarily affect the
Springbok Sandstone and Hutton Sandstone
aquifers which overlie and underlie the WCM
respectively. Small areas of IAAs are identified
by the QWC (2012) for the Springbok Sandstone
at locations south and west of Chinchilla. These
Figure 9:
IAAs are not large, and there are no water supply
bores that access these aquifers within the IAAs
(QWC, 2012). The LAAs for these two aquifers
are more extensive, and springs and water
supply bores in these LAAs are expected to be
impacted by reductions in water levels (or
pressure) (QWC, 2012) (Figure 9). Small LAAs
from CSG production are also predicted for the
Gubberamunda Sandstone and Precipice
Sandstone and Clematis Sandstone. LAAs for
the Precipice Sandstone and Clematis
Sandstone west of Moonie are related to
conventional oil and gas production, not CSG
production (QWC, 2012).
Extent of the Long-term Affected Areas in the Surat CMA Source: QWC (2012)
Groundwater Aquifer Connectivity
15
Table 3:
Number of bores within LAAs impacted by induced aquifer leakage associated with CSG
production within the Surat CMA.
Aquifer
Total No. of bores in
formation
No. of impacted bores
in LAA
Percentage of impacted
bores in formation
223
104
47%
2,828
23
<1%
Precipice Sandstone
292
0
0%
Gubberamunda
Sandstone
908
1
<1%
4,251
128
3%
Springbok Sandstone
Hutton Sandstone
Total
Source: original data sourced from QWC (2012)
There are 71 spring complexes (a complex is a
group of springs) and 43 watercourse springs in
the Surat CMA. A total of five spring complexes
associated with the LAAs of the Precipice, Hutton
and Gubberamunda Sandstones are predicted to
experience water pressure declines above the
trigger threshold. The estimated declines in
water pressures are not expected to occur earlier
than 2017 and may affect the ecological and
cultural heritage values of the five potentially
impacted spring complexes. The OGIA are
currently conducting the Spring Knowledge
Project to improve the scientific understanding of
springs to determine the most appropriate
mitigation measures for the five potentially
impacted spring complexes.
Water bores in the Surat CMA provide water for
agriculture, industry, urban, stock and domestic
uses. There are a total of 4251 bores extracting
groundwater from CSG affected aquifers within the
Surat CMA area. However, only 128 are predicted
to experience a reduction in water pressures from
aquifer leakage due to CSG production (Table 3).
At a regional scale, the number of bores predicted
to be impacted by induced leakage is less than 1%
of the total number of all private bores (21,192) in
the Surat CMA.
Water levels in bores tapping the Springbok
Sandstone will be most heavily affected, mainly
between 2060 and 2075. Lowering of water
levels beyond the trigger thresholds places the
impacted bores at a greater risk of a reduced
water supply; however, controls exist under
Chapter 3 of the Water Act 2000 to prevent bore
owners being disadvantaged by CSG production,
such as ‘make good obligations’.
If a bore in an IAA is assessed as experiencing
or likely to experience impaired function due to
CSG activities, the CSG operator is legally
obliged to enter into a make good agreement
with the landholder. The make good agreement
outlines the make good measures to be
implemented by the CSG operator so the
landholder is not disadvantaged by their
activities. The make good obligations of CSG
operators outlined in the Queensland
Department of Environment and Heritage
Protection’s (2013) guideline to make good
obligations are shown in Figure 10.
Groundwater Aquifer Connectivity
16
Figure 10: Make good obligations. Source: DEHP (2013)
Significant CSG development in the Surat CMA
is expected to cease by 2050, but the effects of
groundwater extraction and induced aquifer
leakage will be experienced after development is
finished. Natural recovery timeframes for
affected geological formations have been
estimated at 50% recovery by 30 to 80 years
after peak CSG development has occurred
(QWC, 2012). However, the application of
mitigation measures, including aquifer reinjection
and groundwater use efficiency improvements, is
expected to significantly reduce this period.
Reinjection of treated CSG water to depleted
formations may play an important role in
mitigating the effects of CSG production on
groundwater systems by increasing groundwater
pressures in the injected formation and reducing
the hydraulic gradient with adjacent formations.
Feasibility studies have shown that this
technique can work under the right hydraulic and
geochemical conditions (Klohn Crippen Berger,
2011 and Santos, 2013), and trials are underway
to determine the potential for broader application
of this impact mitigation strategy (APLNG, 2014;
Arrow Energy, 2013; QGC, 2013 and Santos,
2013).
The QWC (2012) model also does not account
for the expected benefits to be delivered by the
Great Artesian Basin Sustainability Initiative
(GABSI) (i.e. a water bore efficiency
improvement program) to the Surat Basin
groundwater systems. The CSIRO (2012b)
estimate that groundwater pressure decrease
from CSG development is less than the potential
groundwater pressure increase assuming the
GABSI is run to completion. For the
Gubberamunda Sandstone for example, the
QWC (2012) model predicts that by 2070 water
pressures over most of the aquifer reduce by less
than 0.2 metres. However, over this same
period, the CSIRO (2012b) expect that water
pressures in the Gubberamunda Sandstone will
increase by between 5 and 25 metres if the full
benefits of the GABSI are achieved. The CSIRO
(2012b) conclude that the estimated decrease in
groundwater pressures from CSG development
in the Surat Basin is relatively small, especially if
the GABSI is fully implemented.
Groundwater Aquifer Connectivity
17
Summary: What are the expected impacts of induced aquifer leakage?
1.
Induced aquifer leakage is predicted to impact a small number of spring
complexes and water bores. Widespread, negative groundwater impacts are not
expected due to low connectivity.
2.
Water level reductions increase the risk of either bore impairment or harm to the
ecological or cultural heritage values of springs.
3.
Measures, such as Make Good Agreements, are in place to make sure bore
owners are not disadvantaged and to determine the most appropriate impact
mitigation techniques for springs.
4.
Predicted groundwater impacts for the Surat CMA are expected to be reduced by
aquifer reinjection and groundwater use efficiency improvements (i.e. the
GABSI).
5.
Ongoing groundwater monitoring and updating of regional groundwater models
are continuing to improve the scientific understanding of groundwater impacts
from CSG development.
Conclusion
Aquifer connectivity describes the ease with
which water can flow between geological
formations. Water will only move between
formations if there is a large enough hydraulic
gradient to overcome the resistance to flow that
is provided by their vertical permeabilities and
vertical separation. The results of field and
laboratory measurements and computer
modelling show that low aquifer connectivity is a
dominant geological characteristic of aquifers
across the Surat, Bowen and Galilee Basins.
The low degree of connectivity reflects a high
resistance to cross formation flow due to the low
vertical permeabilities of coal measures and
aquitards and the often considerable vertical
separation distances between aquifers and coal
measures.
CSG development is expected to induce aquifer
leakage, but the low degree of aquifer
connectivity means that widespread, negative
impacts are not predicted. Current forecasts of
groundwater impacts from CSG production are
limited to some aquifers in the Surat and
southern Bowen Basins. Predicted groundwater
impacts could potentially be reduced by
reinjecting treated CSG water into aquifers and
improving groundwater use efficiency. Also,
where the function of bores in the Immediately
Affected Area is impaired by CSG activities, CSG
operators are legally obliged to make good the
bores so that landholders are not disadvantaged
by their activities. Data from ongoing monitoring
and research projects will provide new
knowledge of aquifer connectivity and continue to
build confidence in the predicted effects of CSG
development on groundwater resources.
Groundwater Aquifer Connectivity
18
Glossary
Term
Definition
Aquifer
A saturated underground geological formation or group of formations, that can store
water and yield it to a bore or spring. A saturated formation that will not yield water in
usable quantities is not considered an aquifer.
Aquitard
A geological formation that prevents significant flow of water, e.g., clay layers or tight
deposits of shale; geological material of a lower permeability.
Basin (Geological)
An area in which the rock layers dip from the margins towards a common centre; the site
of accumulation of a large thickness of sediments.
Confined Aquifer
A saturated aquifer bounded between low permeability materials like clay or dense rock.
Fault
A crack in a geological formation caused by up shifting, or tectonic movement and uplift,
of the earth’s crust, in which adjacent features of the formation are displaced relative to
one another and parallel to the plane of fracture.
Formation
A sediment or rock, or group of sediment or rocks. Geologists often group rocks of similar
types and ages into named formations.
GABSI
Great Artesian Basin Sustainability Initiative
Geological
Formation
See Formation
Groundwater
(or underground water) Water found in the cracks, voids or pore spaces or other spaces
between particles of clay, silt, sand, gravel or rock within the saturated zone of a
geological formation.
Hydraulic Gradient
The difference in water pressure or water level across one or more formations over a unit
distance. The hydraulic gradient indicates which direction groundwater will flow and how
rapidly.
Induced Aquifer
Leakage
Aquifer leakage that occurs in response to a disturbance to the groundwater system.
Connectivity
The ease with which groundwater can flow within or between geological formations.
Immediately
Affected Area (IAA)
The area within which water level (or pressure) reductions are predicted to exceed a
trigger threshold within three years. Threshold values have been set at 5 metres for
consolidated aquifers (such as sandstones) and 2 metres for unconsolidated aquifers
(such as sand aquifers).
Long Term Affected
Area (LAA)
The area within which water level (or pressure) reductions are predicted to exceed a
threshold at any time in the future. Threshold values have been set at 5 metres for
consolidated aquifers (such as sandstones) and 2 metres for unconsolidated aquifers
(such as sand aquifers).
Measures
A series of coal bearing rocks.
Make Good
Agreement
See Water Act 2000
Permeable
Capable of transmitting water through porous rock, sediment or soil.
Permeability
The property of a soil, sediment or rock indicating how easily water will be transmitted
through it under a gradient.
Sedimentary basin
A geological basin containing a sequence of dominantly sedimentary rocks.
Unconfined Aquifer
An aquifer with no overlying low permeability layers that restrict water movement into the
aquifer. The water level in an unconfined aquifer is known as the water table.
Vertical
Permeability
The property of a formation indicating how easily or rapidly water will be transmitted
vertically.
Groundwater Aquifer Connectivity
19
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