Chapter 5: Groundwater in Near-Surface
Sediment Aquifers
Near-surface sediment deposits formed by either alluvial or colluvial processes (and some by a
combination) occur widely throughout the Broken Hill region. These poorly consolidated to
unconsolidated sediments are geologically young (Pliocene to Holocene age, Figure 3.2) and variably
overlie older Cenozoic sedimentary rocks and weathered parts of the Precambrian basement. The
thickness of the deposits and the nature of the sediment (e.g., composition, grain size, angularity etc.)
are spatially variable across the study area, and largely depend on factors such as location relative to the
source and the composition of the underlying bedrock. The alluvial and colluvial deposits represent
transported regolith horizons formed by erosional and depositional processes. Detailed regolith
mapping conducted at a number of sites (commonly around economically prospective mineral
tenements) has shown that considerable complexity and variation may occur in the nature and origin of
the regolith material and its associated landforms, e.g., Thomas et al., 2002; Hill, 2004a; Brown and
Hill, 2005.
In many parts of the study area the near-surface sediment cover is relatively thin, with weathered
bedrock < 5–10 metres below the surface (Conor, 2004). As the watertable generally occurs at much
deeper levels, these thin surficial sediment horizons clearly have no potential to form a significant
groundwater resource. However, there are several areas around Broken Hill where relatively significant
thicknesses of near-surface sediments have accumulated, e.g., 50–100 metres thick. These have mainly
been transported and deposited by streams which no longer exist as active surface features
(‘palaeochannels’). Modern drainages such as the Darling River also have associated alluvial deposits
within their floodplains. Additionally, fan-like accumulations of alluvial and colluvial materials are
developed along the flanks of many uplifted basement highs. These deposits have a variable clast and
matrix composition, including clay, silt, sand, gravel and calcrete. Importantly for this study, local-scale
groundwater flow systems can occur in the more permeable horizons of each of these near-surface
sediment deposits, and as such they represent a potentially important groundwater resource.
Three near-surface sediment aquifer systems are of greatest interest from a groundwater perspective in
the Broken Hill region (Figure 5.1). These are:
1. Alluvial sediments associated with the ancient and modern courses of the Darling River
(Section 5.1);
2. Palaeochannel deposits located to the south-east of Lake Frome, in South Australia (Section
5.2), and;
3. Alluvial/colluvial fan and slope deposits at the margins of bedrock uplands, specifically
associated with the Mundi Mundi Fault on the western margin of the Barrier Ranges (Section
5.3).
5.1. DARLING RIVER ALLUVIAL AQUIFERS
5.1.1. Location and Extent
The Darling Plains, south-east of the Barrier Ranges and Broken Hill, are mantled by Cenozoic
sediment deposits such as the Shepparton Formation and the Coonambidgal Formation (Figure 5.2).
The most significant groundwater-bearing deposits are the multiple generations of medium- to coarsegrained alluvium (e.g., sand, silt and gravel) formed along the Darling drainage system.
55
Figure 5.1: The three near-surface sediment aquifer systems outlined above are of greatest interest from a
groundwater perspective in the Broken Hill region (the various background colours on this map relate to different
geological units – the abundance of pale yellowish colours reflects widespread surface sediment deposits.
56
Figure 5.2: Physical extent of the Shepparton and Coonambidgal Formation aquifers in the Murray Geological Basin. The Shepparton Formation is more widespread across the
eastern part of the basin, whereas the Coonambidgal Formation is spatially restricted nearby major rivers (after Brodie, 1994; Kellett, 1994).
57
5.1.2. Regional Geology
The Pleistocene to Holocene deposits associated with the Darling River system partly overlie older
Cenozoic sediments of the Murray Geological Basin (Chapter 4). Hence, this section focuses on the
younger fluvial deposits, which are considered separately from the Murray Geological Basin because
their lateral depositional equivalents extend northwards along the Darling River corridor (beyond the
extent of the Murray Geological Basin). The younger Darling alluvial formations (Figure 5.3) include
the:
1. Shepparton Formation, widely distributed and low-lying deposits of unconsolidated clay, silt,
sand and gravel associated with the Darling River floodplain (but mapped within the Murray
Geological Basin);
2. Narrabri Formation, lateral equivalents of the Shepparton Formation mapped in the Darling
River basin to the north of Menindee;
3. Coonambidgal Formation, a series of narrow floodplain and channel alluvial deposits formed
by recently abandoned palaeochannels (‘ancestral streams’) as well as the current river channel
of the Darling River; and the
4. Pooraka Formation, colluvial and high-level alluvial deposits derived from erosion of
basement highs at the north-western margin of the Murray-Darling Basin (Brown and
Stephenson, 1991).
5.1.3. Shepparton Formation Aquifer
a. Geological Context
The Shepparton Formation consists of Late Pliocene to Quaternary fluvio-lacustrine deposits that form
flat-lying alluvial fans, riverine plains and terraces, and overflow lakes. This geological unit reflects
‘prior streams’ – remnants of abandoned channels and floodplains created by watercourses of higher
discharge than the modern Darling River (Brown and Stephenson, 1991). The Shepparton Formation
conformably overlies the fluvial sands of the Calivil Formation, whereas it both overlies and interfingers with the marine Loxton-Parilla Sands. The formation is largely concealed at surface by thin
aeolian dune sand deposits.
The Shepparton Formation extends across the northern and eastern part of the Murray Geological Basin,
mainly associated with topographic lows. Unconsolidated to poorly consolidated silt and clay are
common, with intercalated lenses of fine- to coarse-grained, poorly sorted, and rounded to angular silty
sand and gravel (Brown, 1989; Cameron, 1997). Multicoloured mottled clays, buried soil horizons and
‘hard-bands’ indicate a high degree of sediment modification caused by weathering. Although these
sediments themselves represent previous river channels and floodplains, they are locally traversed and
incised by smaller palaeochannel sand and gravel deposits with associated sand and silt levee splays
(Brown and Stephenson, 1991; Cameron, 1997).
The thickness of the Shepparton Formation is highly variable, reflecting the onlapping nature of the unit
(Brown and Stephenson, 1991). Total thickness of the formation in the Menindee region is on average
30–40 m, ranging from 10–20 m over Miocene structural highs to 50–60 m in deeper palaeochannels,
such as those along the Blantyre and Menindee Troughs (Brodie, 1994).
Permeable horizons within the Shepparton Formation consist of fine- to coarse-grained shoe-string sand
bodies with varying degrees of lateral and vertical interconnection (Brown, 1989; Brown and
Stephenson, 1991). The formation may form a shallow, unconfined to semi-confined aquifer where the
sediment profile is relatively thick or the watertable is near-surface. Mostly, this occurs adjacent to the
Darling River where the watertable depth is typically less than 30 m (Figure 5.3). Saturated sediments
also occur along a corridor connecting the Neckarboo Ridge with the Talyawalka Creek system, as well
as the Baden Park Depression (Figure 5.3). The aquifer is also found in discrete northerly trending
alluvial fans which flank the remnant Lake Wintlow High.
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Figure 5.3: Distribution and characteristics of the Shepparton, Coonambidgal and Pooraka Formation aquifers within the north-west part of the Murray Geological Basin. The
freshest groundwater occurs adjacent to the rivers, creeks and lakes, reflecting leakage from the surface water to the unconfined aquifers (after Brodie, 1994; Kellett, 1994).
59
b. Groundwater Processes
Groundwater in the Shepparton Formation is largely recharged from surface waters (via river leakage or
flood inundation) within floodplains, in areas where the hydraulic conductivity of the surface material
permits. Further away from active surface drainages, especially where lacustrine clays overlie the
Shepparton Formation, relatively little recharge occurs. In north-west parts of the Murray Geological
Basin, groundwater flow in the Shepparton Formation is contiguous with that found in the underlying
Pliocene Sands aquifer (Figure 4.14), indicating a high-degree of hydraulic connectivity. The general
groundwater flow path trends to the south-west, towards the depositional centre of the basin (Figure
5.3). A groundwater mound developed near Menindee, where the watertable has been elevated by over
10 metres, disrupts this regional flow direction (Brodie, 1994).
c. Groundwater Quality
The Shepparton Formation partial aquifer has a similar salinity distribution to that of the underlying
Pliocene Sands aquifer. Groundwater is relatively fresh (700 mg/L) near the channel of the modern
Darling River, but water quality decreases rapidly away from the current watercourse, i.e., becomes
considerably more saline (Figure 5.3). Recharge resulting from river overflow produces groundwater
salinities of approximately 2,000 mg/L (Kellett, 1994), whereas areas which are less prone to flooding
(e.g., topographically high floodplain terraces) have groundwater salinities of ~7,000 mg/L. Away from
the floodplain, the regional salinity varies from 7,000 mg/L near the northern basin margins, to >20,000
mg/L in the south.
d. Aquifer Characteristics
Due to the silty nature of the Shepparton Formation, aquifer yields are generally less than 0.5 L/s.
However, higher yields occur in thicker accumulations of relatively coarse-grained material from some
palaeochannels. For example, pump testing estimated a yield exceeding 10 L/s for a recently drilled
production bore near Menindee (CMJA, 2008).
e. Groundwater Use and Management
In general, the Shepparton Formation is not extensively used as a groundwater resource in the Lower
Darling region, due to the variable yield and water quality. The sequence is also susceptible to damage
by waterlogging and deterioration of soil structure (Evans and Kellett, 1989). The Shepparton
Formation aquifer in western New South Wales is not actively managed, as it provides only limited
stock and domestic groundwater supplies.
In western New South Wales, the Shepparton Formation is part of the Western Porous Rock
groundwater management unit (GMU N612) which also includes the Murray Geological Basin aquifers
of the Renmark Group and the Pliocene Sands. Collectively, the limit of extraction for this GMU is
estimated at 663.8 GL/yr (CSIRO, 2008).
5.1.4. Narrabri Formation Aquifer
The shallow alluvial deposits of the Narrabri Formation are lateral equivalents of the Shepparton
Formation. These near-surface sediments extend northwards across the Darling Floodplain and contain
discontinuous sand aquifers connected to the Darling River and its tributaries. Like the Shepparton
Formation, the Narrabri Formation forms a shallow unconfined aquifer in some areas of the floodplain,
with recharge mainly occurring from surface water systems (Ife and Skelt, 2004). Groundwater flowpaths in the aquifer are inferred to follow the general gradient of the Darling catchment. In western New
South Wales, groundwater quality in the Narrabri Formation is generally poor, with the exception of
fresher water zones caused by dilution effects near the river (MDBC, 2007).
Woolley et al. (2004) showed that alluvial deposits of sand, silt and clay occur to depths of about 60
metres (below surface) downstream of Wilcannia in the Darling River Floodplain. Drilling across the
floodplain near Wilcannia intersected low-salinity groundwater (400–1,400 µS/cm) in sandy units
correlated with the Narrabri Formation (Woolley et al., 2004). The fresh groundwater zone was
interpreted to be 40–50 m thick, with a maximum width of 5 km. Subsequent work led to the
60
development of a production borefield in this area which now supplies town water for Wilcannia at the
rate of about 1 ML/day.
Woolley et al. (2004) suggested that recharge of the alluvial aquifers in the Wilcannia area was
dominated by river flooding, both during deposition and in recent times. If so, then these groundwater
systems are largely disconnected from river flow within the main channel of the Darling. This raises the
possibility of further developing groundwater resources from similar alluvial aquifers in other parts of
the Darling Floodplain closer to Broken Hill (Chapter 9).
Apart from the production borefield for Wilcannia, the Narrabri Formation does not produce significant
amounts of groundwater within the Lower Darling region. This is due to the discontinuous nature of the
formation’s sandy lenses (Evans and Kellett, 1989), low bore yields and elevated salinity levels away
from surface water bodies. The potential for lateral ingress of saline groundwater into the Narrabri
Formation with pumping is considered to be high (MDBC, 2007). Because of these factors, the
Narrabri Formation is not actively managed in western New South Wales.
5.1.5. Coonambidgal Formation Aquifer
a. Geological Context
Across the Darling Floodplain, the Shepparton Formation is unconformably to disconformably overlain
by younger alluvial sediments. These are termed the Coonambidgal Formation, and were deposited in
fluvial and fluvio-lacustrine, channel and floodplain environments of existing and recently active rivers
(Cameron, 1997; Brown and Stephenson, 1991). Hence, sedimentation is confined to floodplains and
eroded into older alluvial landforms (such as the Shepparton Formation).
The distribution of the ancestral streams of the Coonambidgal Formation is closely associated with the
present-day rivers, and the former channels often carry floodwaters (Brown and Stephenson, 1991).
The geomorphology of the Darling River changes along its length; between Wilcannia and the junction
with the Darling Anabranch, anastomising distributary channels (including Talyawalka Creek) occupy a
wide floodplain (20–50 km-wide). This converges to a more linear, narrow floodplain (2–3 km) further
downstream towards Wentworth. A similar trend is evident for the Darling Anabranch, where the
number and size of floodplain lakes decreases southwards.
The variations in river geomorphology along the Darling River reflect two main generations of channel
diversion (Bowler et al., 1978). The current anabranch systems of Talyawalka Creek and the Darling
Anabranch define the palaeo-Darling channel of the Late Pleistocene. These channels contain a large
meander scroll pattern and a relatively coarse-grained bed-load. This is significant from the perspective
of exploring for groundwater supplies in coarse palaeochannel sands. In contrast, the present course of
the Darling River was initiated about 11,000 years ago, and tends to be confined within a narrower
trench with smaller meander wavelengths and relatively clay-rich bed-loads (Bowler et al., 1978).
The Coonambidgal Formation variably consists of grey or red-brown silt, silty clay, poorly sorted sand
and gravel. Sand bodies are typically upward-fining, with coarse-grained sand and gravel at the base,
grading upwards into silty sand and silty clay (Brown and Stephenson, 1990). The formation can be
separated from the underlying Shepparton Formation by the Blanchetown Clay. This clay layer was
deposited in a large body of freshwater known as Lake Bungunnia (Figure 5.4), which covered part of
the western Murray Basin in the Late Pliocene and Early Pleistocene (Rogers, 1995).
The thickness of the Coonambidgal Formation varies across the study area. Channel sands beneath the
Riverine Plain in the eastern half of the Murray Geological Basin are up to 20 m thick. Sediments of the
Monoman Formation (the basal member or lateral equivalent of the Coonambidgal Formation) within
the Murray River Gorge in South Australia are reportedly at least 15–25 m thick (Brown and
Stephenson, 1991). This coarse-grained quartz sand and gravel alluvial layer may have been partly
derived from reworked Loxton-Parilla Sands (Brown and Stephenson, 1990). At Menindee, the
Coonambidgal Formation typically extends to depths of 20–30 m (Jewell, 2007a). Here, the deposits
consist of interbedded clays, silts, clayey sands and sands, and are generally of low to moderate
permeability. These sediments represent the local watertable aquifer, which forms the hydraulic link
between the surface water systems of the Darling and the underlying aquifers (Jewell, 2007a).
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b. Groundwater Processes
The alluvial deposits overlying the sediments of the Murray Geological Basin are recharged by leakage
from surface water bodies associated with the Darling River. Recharge is mainly a function of
proximity to the river and frequency of flood inundation. Over higher-level alluvial terraces which are
further away from the watercourses, recharge occurs less frequently and less rapidly (Cunneen, 1980).
Unlike the river system, Coonambidgal alluvial deposits underlying the Menindee Lakes are not thought
to be significantly recharged by lake leakage due to the presence of a pervasive layer of low
permeability lacustrine clay (Allen, 2006).
Figure 5.4: Distribution of the Blanchetown Clay Formation (represented as the dark brown geological unit
shown here) and the location of the ancient Lake Bungunnia to the south of Broken Hill.
Near the river, aquifers of the Coonambidgal Formation are typically incised through the less permeable
Shepparton Formation and Blanchetown Clay, allowing surface water to directly recharge the
underlying Loxton-Parilla Sands (Allen, 2006). These act as a direct hydraulic link between the modern
Darling River, the Shepparton Formation partial aquifer and the regional Pliocene Sands aquifer system.
During periods of flood inundation in the Darling River a downward vertical hydraulic gradient exists
62
from the upper Quaternary deposits to the Renmark Group aquifer, which promotes freshwater recharge
of the deeper groundwater systems (Jewell, 2007b).
c. Groundwater Quality
Groundwater salinity is less than 1,000 mg/L in the Coonambidgal Formation aquifer underlying the
Darling River channel and parts of the Talyawalka Creek channel (Figure 5.3). The freshest
groundwater occurs proximal to the main watercourses, and a freshwater mound is interpreted to occur
(on average) up to 1.5 km away on either side of the Darling River. However, the dimensions of this
fresh groundwater body can change depending on recent river flow levels (Jewell, 2006). Further from
the Darling River, salinity levels in the Pliocene to Quaternary aquifers increase. Within the upper
terraces bordering the main watercourse, groundwater tends to be of good stock quality. Likewise along
the Darling Anabranch, salinity increases downstream from 900 mg/L to 4,000 mg/L.
d. Aquifer Characteristics
Groundwater in the Coonambidgal Formation is generally unconfined, and the watertable is typically
12–15 m below the surface (Jewell, 2007a). Overall, saturated aquifer thickness is limited and
constrains aquifer yields to less than 5 L/s (Brodie, 1994). For example, Allen (2006) estimated a
specific yield of 3 % and a thickness of 1.5 m for the aquifer near the Darling River. The Coonambidgal
Formation sand aquifer has moderate to very high hydraulic conductivity, whereas the silty/clayey sands
have lower permeability, but significant storage capacity (Jewell, 2007a; 2007b).
e. Groundwater Use and Management
In reporting of groundwater status, the Coonambidgal Formation is assigned to the composite Murray
Trench sub-system of the Murray Geological Basin (Skelt et al., 2004a). The proximity to readily
available surface water means that this aquifer is not widely used at present. Currently, groundwater is
mainly extracted for limited stock and domestic supplies, but demand from irrigators bordering the
Darling River is predicted to increase in the future (CSIRO, 2008).
In the Murray Geological Basin, the Coonambidgal Formation has been specifically defined as the
Lower Darling Alluvium groundwater management unit (GMU N45). The extraction limit assigned to
the aquifer along the length of the Darling downstream of Wilcannia is 9.3 GL/yr, with current
extraction estimated at 2 GL/yr (CSIRO, 2008b). North of Wilcannia, the aquifer has been assigned to
the Upper Darling Alluvium groundwater management unit (GMU N46), which also includes the
alluvial deposits of the Paroo and Warrego Rivers. The long term groundwater extraction limit for this
GMU is 19.5 GL/yr (CSIRO, 2008b).
5.1.6. Pooraka Formation Aquifer
The Pooraka Formation is a composite unit of colluvial and high-level alluvial deposits adjacent to
basement ranges at the north-western margin of the Murray Geological Basin (Figure 5.3). The lower
slopes and outwash areas of these ranges are characterised by moderate to low-angle sloping or broadly
undulating stony plains with sparse dendritic drainage systems. Residual lag deposits occur around
basement outcrops, including the Proterozoic sediments of the Benda Range, the Precambrian highgrade metamorphic complex of the Barrier Ranges and the Devonian sandstones of the Manara Hills.
The Pooraka Formation consists of diverse sediment types; gravel, sand, red-brown, poorly-sorted
clayey sand and clay. These may be cemented locally by ferruginous or silicified matrices.
Near Broken Hill and along the north-west margin of the Murray Geological Basin, the colluvial cover
is relatively thick and the Pooraka Formation becomes the shallow aquifer. Hydrogeological mapping of
the area (Brodie, 1994), inferred the aquifer to be low-yielding (<0.5 L/s) and predominantly of brackish
to saline quality (>3000 mg/L), e.g., Figure 5.3. Regionally, the watertable is deep, some 60–70 m
below the land surface. However, episodic recharge events from ephemeral streams draining the Barrier
Ranges can result in localised mounds of fresher groundwater, although these likely dissipate over-time
if subsequent recharge does not occur. This is most evident for Stephens Creek and Turkey Plain Creek
which flow onto the Darling sand plain.
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5.1.7. Statistical Analysis of Darling Alluvial Bores in the Broken Hill Region
Based on data in the NSW Groundwater Works database (NSW Department of Water and Energy), 198
registered bores are inferred to access groundwater within the Shepparton and Coonambidgal
Formations (and their equivalents) in the study area. From the information provided in the database it is
not possible to discriminate the individual aquifers, and hence the following statistics refer to general
trends across both aquifers.
Of the 198 bores, only 7 contain yield data, ranging from 0.2 to 2.5 L/s. Nearly half of the bores (84 of
the 198) have salinity information, however these are provided in broad categories of 0–500 mg/L, 0–
1000 mg/L, 0–14,000 mg/L, 1000–10,000 mg/L and 10,000–100,000 mg/L, rather than specific
measurements. The spatial distribution of groundwater salinity ranges are shown in Figure 5.3. The
distribution of bores and corresponding salinity values indicates that water quality is highest proximal to
surface water bodies, such as the Darling River. A general positive relationship between increasing
depth and salinity is also apparent. These findings are consistent with previous investigations of the
Darling floodplain (Brodie 1994; Kellett, 1994; Jewell, 2007a; 2007b), and support our conceptual
understanding of groundwater systems in this region.
5.2. PALAEOCHANNEL AQUIFERS IN THE CURNAMONA PROVINCE – CALLABONNA
SUB-BASIN
5.2.1. Location and Extent
To the north-west of Broken Hill, palaeochannel systems (ancient rivers and creeks which are no longer
active surface features) are incised into older rock formations of the Proterozoic Curnamona Province
(Figure 5.1). These palaeodrainages contain sediments of variable Cenozoic age, and include
widespread alluvial and lacustrine deposits and linear, meandering channels filled with relatively
permeable and porous sands and gravels.
5.2.2. Regional Geology
During the Early Cenozoic, well-sorted sand horizons (known locally as the Eyre Formation) formed
thin and laterally continuous layers across the northern part of the Curnamona Province (Callabonna
Sub-basin). In the south, the equivalent sediments of the Eyre Formation – angular, poorly sorted,
fluvial sands and interbedded clays and silts – were deposited in major stream channels of restricted
areal extent (Blunt, 1978). These channel systems were incised into the Precambrian basement rocks
and the marine mudstones of the Late Cretaceous Marree Subgroup (McKay and Miezitis, 2001; Fabris
et al., 2004).
The formation of palaeochannels was largely initiated by Palaeocene uplift of the Olary Block (Blunt,
1978). Tertiary sands deposited in the palaeochannels were derived from uranium-rich metamorphic and
granitic rocks in the surrounding Precambrian uplands, e.g., the Willyama Supergroup, and the
Benagerie Ridge and Mt Painter Inliers (Blunt, 1978; Fabris et al., 2004). In some places, the
palaeochannel sediments contain economically mineable concentrations of uranium, such as the
Beverley and Honeymoon deposits.
Following initial fluvial sedimentation, interbedded clay, sand and dolomite units of the Miocene
Namba Formation were disconformably deposited atop the older sediments, forming an effective seal
which capped the palaeochannel systems (McKay and Miezitis, 2001; Fabris et al, 2004).
Palaeochannel sequences and basement rocks were subsequently covered by up to 30 m of Quaternary
surficial sediment, predominantly clays and dune sands (Reif, 2000).
Structural deformation in the Miocene caused uplift of the Flinders Ranges, forming the western margin
of the Lake Frome Embayment. Similarly, the Olary and Barrier Ranges formed barriers and watershed
divides to the south and east of the region (Blunt, 1978). The Curnamona Province contains several
Tertiary palaeochannel systems, which originally flowed towards the north (Blunt, 1978; Hou et al.,
2007). These are the Yarramba, Billeroo, Wyambana, Lake Namba and Lake Charles palaeochannels
(Figure 5.5; Hou et al., 2007).
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Figure 5.5: Several palaeochannels in the Curnamona Province occur within 150 km of Broken Hill, such as the
Yarramba and Lake Charles systems (note the radial distance lines from Broken Hill shown here). This map highlights
the variable yields (left) and mostly saline groundwater (right) which typically occurs within the palaeochannels of this
region (after Hou et al., 2007).
65
5.2.3. Regional Hydrogeology
Recharge to the palaeochannel aquifers mainly occurs by rainfall infiltration in basement outcrops at the
margins of the Callabonna Sub-basin, and possibly by accessions from tributary channels during
episodic flow events. The relatively low hydraulic conductivity of rocks surrounding the palaeochannels
– Precambrian basement and Tertiary silts and clays – effectively confines groundwater flow to the
permeable horizons of the main palaeochannel deposits.
5.2.4. Yarramba Palaeochannel Aquifer
a. Geological Context
The Cenozoic sediments of the Yarramba Palaeochannel lie either within a deeply incised stream
channel at the base of weathered Cretaceous and Proterozoic rocks of the Curnamona Province (Curtis
et al., 1990; Reif, 2000); or in the vicinity of Lake Frome, overlying sedimentary rocks of the Great
Artesian Basin (GAB). The palaeochannel system is characterised by a broad, flat valley floor bounded
by steep sided valley walls (Southern Cross Resources, 2000). The channel follows a sinuous course,
extending for ~155 km along the eastern side of the Benagerie Ridge. The channel varies from 1.5–6
km-wide (average of 3 km) and contains up to 55 m of channel-fill sediments of the Eyre Formation
(Blunt, 1978). These Eocene sediments are uncemented and poorly consolidated and were originally
sourced from the concealed Benagerie Ridge and the Olary Block (Curtis et al., 1990; Reif 2000; Fabris
et al., 2004). The Eyre Formation sequence can be subdivided informally into three discontinuous
trough-type cross-bedded units typical of a high-energy, braided river environment (Figure 5.6) (Blunt,
1978; Curtis et al., 1990).
The palaeochannel is disconformably capped by ~40 m of interbedded clay, sand and dolomite of the
Miocene–Pliocene Namba Formation. The entire palaeochannel has subsequently been buried by ~30
m of surficial Quaternary clay (Rief, 2000), such that about 70 m of younger sediment cover (Namba
Formation and Quaternary Clays) overlies the Eyre Formation (Blunt, 1978).
The three sand units of the Eyre Formation are referred to as the basal, middle and upper sands, with
each separated by intervening clay layers (Figure 5.6). Significant differences in sediment type occur
over relatively short distances within the palaeochannel, such that the thickness and hydraulic properties
of the aquifers and confining layers are highly variable. For example, the upper sand unit is relatively
clay-rich and has low transmissivity (<100 m²/d), with sand grains commonly fine, and of sub-angular
to sub-rounded shape. These characteristics are indicative of a low-energy meandering river system,
and imply moderate porosity and permeability within the unit (Blunt, 1978; Southern Cross Resources,
2000).
The middle sand unit contains less clay then the upper sand, and is relatively coarse-grained and more
poorly sorted. This has resulted in lower porosity but higher permeability throughout the unit (Blunt,
1978). The middle clay (when present) effectively acts as an aquitard, hydraulically separating the
basal and middle sands. The average unit thickness is 3 m, but ranges from 0–20 m (Southern Cross
Resources, 2000).
The basal sand deposits are typically sub-angular and poorly sorted, with fine- to coarse-grain size. In
some deeper sections of the palaeochannel, a basal conglomerate of angular quartz pebbles within a
coarse sand matrix occurs. The basal sand unit thus has very high porosity and permeability (Blunt,
1978) and both the basal and middle sand units have high transmissivity (230–790 m²/d and 100–
800m²/d respectively; Southern Cross Resources, 2000). The storage coefficient of the basal sand is
about ~1 x 10-4.
Lateral hydraulic connection exists within the three sand units however vertical connection is limited
due to the presence of the clay horizons. Where present, the clay layers significantly retard the
movement of water between separate aquifers; thus, lateral groundwater movement is dominant.
However, in some areas where the clay layers are absent (due to truncation or lensing-out), groundwater
flow can occur vertically between sand units (Southern Cross Resources, 2000).
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Figure 5.6: Schematic cross-sections of the Yarramba Palaeochannel, highlighting the lateral and vertical
variations in the extent of the three main sand units and the intervening clay layers (source: Blunt, 1978).
An extensive thickness of shale belonging to the Cretaceous Marree Formation lies between the Eyre
Formation aquifers and the GAB Cadna-owie Formation aquifer and forms an effective aquitard which
minimises groundwater flow between the two systems (Southern Cross Resources, 2000). Variations in
potentiometric head indicate that any vertical flow would occur from the underlying Cadna-owie
Formation upwards to the Eyre Formation (Rief, 2000).
Recharge to the channel aquifers is likely to occur by infiltration of rainfall in the Olary Ranges to the
south, by lateral through-flow from fractured basement rocks, or by rare accessions from tributary
channels.
Groundwater movement is confined to the Eyre Formation within the palaeochannel due to the
impermeable nature of the Precambrian basement rocks and the overlying clay-rich Namba Formation
(Rief, 2000). Thus, groundwater within the Eyre Formation represents a ‘closed hydraulic system’
which ensures that water is neither lost nor gained along the flow path, except for minor contributions
from tributary channels (Southern Cross Resources, 2000). The sandy aquifers of the Eyre Formation
(basal, middle and upper sands) are considered at or near hydraulic equilibrium; groundwater flow is
restricted within the palaeochannel margins to a north-east and south-west direction. At the local scale
however, the direction of flow is controlled by the orientation of the channel (Southern Cross
Resources, 2000).
b. Groundwater Quality
Groundwater within the palaeochannel sands is naturally high in salinity, radionuclides and total
dissolved solids (TDS). Salinities range from 10,000–19,000 mg/L, with the highest levels developed in
the basal sand aquifer (16,000–19,000 mg/L TDS). Concentrations of radionuclides in these
palaeochannel groundwaters exceed guidelines for safe drinking water, with an average of 60-times the
level of uranium and 340-times the level of radium (Reif, 2000).
Significant uranium deposits within the Yarramba palaeochannel occur at Honeymoon (within the
coarse-grained basal sand), at East Kalkaroo (within the basal sand), Yarramba (along the clay-sand
interface in the middle unit) and Oban (associated with a restricted redox interface near the base of the
channel) (Curtis et al., 1990; Fabris et al., 2004). These uranium deposits are classified as sandstone
(roll-front) deposits, and have formed by the interaction of uranium-rich oxidising fluids with reduced
matter in the sediments, e.g., organic matter (Fairclough et al., 2006).
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c. Aquifer Characteristics
The rate of flow within the basal and middle sand units is 10–16 m/yr. Flow rates within the upper sand
are <1 m/yr due to the lower hydraulic connectivity of meandering river sediments compared to the
higher energy braided stream sediments (Southern Cross Resources, 2000). Total aquifer through-flow
within the palaeochannel is about 90 ML/yr (Southern Cross Resources, 2000).
d. Groundwater Use and Management
Due to the high concentrations of radionuclides in the Yarramba Palaeochannel, groundwater is mostly
unsuitable for potable supply, stock watering or other agricultural uses. However, groundwater in the
upper sand unit is used intermittently for stock watering in some areas to the north of the uranium
deposits (Southern Cross Resources, 2000).
5.2.5. Lake Charles Palaeochannel
The Charles Lake Palaeochannel is a tributary of the Yarramba Palaeochannel. It is located 70–80 m
below the surface (overlain by younger sediments and regolith material) and contains 20–30 m of Eyre
Formation sands (McKay and Miezitis, 2001). Due to the lack of uranium deposits little information is
known about this palaeochannel system. However, as the Lake Charles Palaeochannel is down-gradient
of the Yarramba Palaeochannel, and further away from the Barrier Ranges watershed, it may contain
lower soluble radionuclide concentrations (due to precipitation of uranium closer to the source area), but
probably also has higher salinity levels.
5.2.6. Billeroo and Curnamona Palaeochannels
a. Geological Context
The Billeroo Palaeochannel is a north-trending system with a lateral extent of 5–10 km, extending ~80
km and containing up to 50 m of sediment infill (Curtis et al., 1990; Southern Cross Resources, 2000).
The overall morphology is similar to the Yarramba Palaeochannel, i.e., broad, flat valley shape;
however the channel banks are gently dipping (Southern Cross Resources, 2000). A significant
tributary system, the Curnamona Palaeochannel, enters the Billeroo Palaeochannel immediately
downstream of the Goulds Dam Uranium Deposit.
The Billeroo Palaeochannel sequence is informally subdivided into lower, middle and upper members.
The lower member consists of fine- to coarse-grained sand and gravel which is mostly angular and
poorly sorted. The unit is quartz-rich and contains locally extensive silt and clay horizons. The middle
unit comprises of fine- to medium-grained sand, silt and clay. The upper member of the Billeroo
Palaeochannel consists of interfingered medium- to coarse-grained, well sorted quartzose sand which, in
places, grades to fine sand and silt horizons (Curtis et al., 1990; Southern Cross Resources, 2000). In
some areas, the Billeroo Palaeochannel sediments are concealed beneath 90 m of Namba Formation
clays. Uranium mineralisation in this palaeochannel is restricted to the lower member, and also
associated with variations in the redox potential of the sediment infill.
b. Groundwater Processes
Groundwater in the Billeroo Palaeochannel flows from south to north, originating in the Olary Ranges
and heading towards the east of Lake Frome. The Curnamona Palaeochannel contributes significant
groundwater input to the Billeroo Palaeochannel.
5.2.7. Statistical Analysis of Curnamona province Palaeochannel Bores in the
Broken Hill Region
Based on data from the South Australian Drillhole Enquiry System, known as Obswell
(https://info.pir.sa.gov.au/des/page/desHome.html), about 180 water bores are drilled into palaeochannel
systems across the Curnamona Province. Records of these bores are of variable quality, but contain
some information on the groundwater resources, such as total dissolved solids (TDS), bore yields and
standing water levels (SWL). Basic statistical analyses of these data are provided in Table 5.1. These
data support the conceptual understanding of groundwater resources as outlined above, characterised by
highly elevated salinity levels which average >10,000 mg/L, and low to moderate bore yields of 1–3
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L/s. These characteristics clearly indicate that groundwater resources in the palaeochannel systems of
the Curnamona Province could not supply groundwater of sufficient quality or quantity for Broken
Hill’s urban water requirements.
Table 5.1: Statistics for water bore data for palaeochannel systems in the Curnamona Province.
TDS
(MG/L)
YIELD
(L/S)
PH
SWL (M)
Number
137
141
4.8
130
Mean
10355
2.86
7.3
40.8
Median
10330
1.52
7.2
49
Minimum
528
0
6
-2
Maximum
34116
27.9
10
92.3
Std. Dev.
5855
3.89
0.7
19.5
5.3. MUNDI MUNDI ALLUVIAL FAN AQUIFERS
5.3.1. Location and Extent
The Mundi Mundi Fault is a major geological structure which forms a prominent curvilinear escarpment
separating the upland Barrier Ranges in the east from the lower-lying Mundi Mundi Plains to the west
(Figure 5.7). Some sections of the Mundi Mundi Fault are less than 50 km from Broken Hill, and also
within a few kilometres of the Umberumberka Creek reservoir. The stratigraphy and structure of the
alluvial fan deposits of the Mundi Mundi Fault have been investigated as part of previous landscape and
regolith studies around Broken Hill, e.g., Wasson, 1979; Gibson, 1997. These fans comprise mixed
colluvial and alluvial sediments of sufficient thickness and permeability to form localised aquifer
systems in some places.
5.3.2. Regional Geology
The Mundi Mundi Escarpment trends north to north-easterly along its ~100 km-long strike extent and
dips at about 30° towards the east (Gibson, 1997). A number of ephemeral creeks dissect the
escarpment, flowing north-westerly from the Barrier Ranges and onto the adjacent plains. Bedrockderived sediments eroded from the upland ranges are transported in the creeks and deposited on the
down-slope side of the escarpment due to the changing flow conditions. Wasson (1979) identified three
major alluvial fan systems in the area, associated with the Umberumberka Creek, the Mundi Mundi
Creek and the Eldee Creek (Figure 5.7). Smaller sediment fans also occur between the larger fans and
consist of abundant colluvium proximal to the escarpment. The smaller fans are generally steeper
(average slopes up to 1°) and have a relatively greater proportion of coarse-grained sediment than the
larger fan systems. Five stratigraphic units of variable thickness and sediment type (gravel, sand and
mud) are recognised within the Mundi Mundi Alluvial Fans (Wasson, 1979).
The Umberumberka Fan is the largest and flattest of the alluvial fans along the Mundi Mundi Fault,
with an axial length of approximately 20 km and a relatively gentle slope ~0.15°. Gibson (1997) used
seismic reflection data collected by the Australian Geological Survey Organisation to interpret up to 200
metres of regolith material overlying bedrock adjacent to the Mundi Mundi Fault (Figure 5.8).
Geological mapping and drilling reports by mineral exploration companies have indicated that up to 100
metres of reddish coloured alluvium and colluvium of Pliocene to Holocene age (e.g., sheetwash
deposits) may occur close to the Mundi Mundi Escarpment. These sediments probably overlie older
Cenozoic sedimentary rocks (e.g., of the Eyre and Namba Formation) and weathered Proterozoic
bedrock (Gibson, 1997)
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Figure 5.7: Spatial extent of the three largest alluvial fan systems (named above) adjacent to the Mundi Mundi
Fault escarpment. These fans are here defined by the irregular finger-like alluvial deposits emanating from the
fault escarpment and extending out across the adjacent plains. This map also shows the surface geology of the
surrounding area and groundwater salinity data for local water bores.
70
Figure 5.8: Interpreted cross-section through the Mundi Mundi Fault based on seismic data, showing the
inferred thickness of several hundred metres of Pliocene to Holocene alluvium associated with deposition of
alluvial fans on the Mundi Mundi Plain to the west of the fault scarp (source: Gibson, 1997).
5.3.3. Regional Hydrogeology
Scant scientific or technical information relating to the groundwater resources in the Mundi Mundi
Alluvial Fans is available. However, given the estimated thickness (up to ~100 m) of the sediment
profile and the presence of permeable sediment deposits (e.g., abundant coarse-grained sediment
fragments), localised groundwater flow systems are predicted to occur within the Mundi Mundi Alluvial
Fans. The adjacent uplands (relatively impermeable bedrock of the Barrier Ranges) represent a
proximal recharge area which potentially supplies good inflows of fresh groundwater. Drilling records
and seismic data also suggest that the underlying rocks are considerably less permeable and porous than
the alluvial fan sediments, providing a likely barrier against vertical groundwater flow from the system,
e.g., Conor, 2004. Hence, groundwater probably flows down-gradient towards the north-west, away
from the fault escarpment, with localised discharge points associated with breaks-of-slope or surface
depressions along the plains.
5.3.4. Statistical Analysis of Mundi Mundi Bores in the Broken Hill Region
Based on bore data provided by the NSW Department of Water and Energy, about 30 registered stock
and domestic bores to the north-west of Broken Hill are inferred to tap groundwater resources within the
Mundi Mundi Alluvial Fans (Figure 5.7). However, as the spatial extent and thickness of the alluvial
fan systems is poorly mapped, some of these bores possibly extract groundwater from deeper aquifers
which underlie the alluvial fans (although it is not possible to be certain as detailed drilling logs are not
available).
The drilled depth of the water bores ranges from 24 to 214 metres, with ~60 % of bores less than 100
metres deep. Depth of the standing water level (SWL) and the groundwater yield are recorded for 20
and 16 bores respectively. There is considerable variation in the depth of the SWL, ranging from 6–73
metres, with a mean of 46 metres (Table 5.2). These characteristics suggest the existence of localised
groundwater flow systems which respond rapidly to major recharge events.
Bore yields are mostly low and range from 0.3-1.5 L/s although one bore has a recorded yield of 73 L/s
(this may be spurious data however, and has been treated as an outlier in Table 5.2). Salinity values are
available for 24 of the 32 bores, however these were provided in classes of 0–500 mg/L, 0–1,000 mg/L,
0–14,000 mg/L, 1,000–10,000 mg/L and 10,000–100,000 mg/L. The distribution of salinity ranges is
shown in Figure 5.7. The majority of groundwater sampled was either potable or suitable for stock use.
One particular bore was sampled at two depths (107.9 and 181.4 m), with salinities of ‘stock’ quality
and ‘good’ quality respectively. This may indicate that groundwater quality increases with depth,
although the supporting data are limited. Of the six bores drilled in probable alluvial fan sediments <2
km from the Mundi Mundi Fault (i.e., into the thickest part of the alluvial fan systems), the total
dissolved solids concentration is <3,000 mg/L. This suggests that groundwater systems in the alluvial
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fan sediments are recharged from the adjacent fractured rock uplands, and that the highest yields of
good quality water will occur close to the escarpment, i.e., within a few kilometres.
Table 5.2: Statistical summary of water bore data from the Mundi Mundi Alluvial Fan Systems
YIELD
YIELD
SWL
minus outlier
(m)
Number
17
16
20
Mean
5.1
0.9
46.1
Median
0.9
0.9
49.4
Minimum
0.3
0.3
6.1
Maximum
73
1.5
73.3
Std. Dev.
17
0.4
17.2
Based on favourable hydrogeological conditions and sediment deposit characteristics, local groundwater
systems containing potable to stock quality water resources are predicted to occur within the Mundi
Mundi Alluvial Fans. Water bore data indicates that accessible groundwater occurs at moderate depths
below the surface, although yields are mostly low and probably only suitable for private stock and
domestic purposes. Although one local bore yielded significant quantities of groundwater when drilled,
its long-term sustainability is unknown. In summary, the available evidence indicates that the Mundi
Mundi Alluvial Fans represent a prospective target for local stock and domestic groundwater supplies.
However, without further detailed hydrogeological studies, the potential of these groundwater systems
to provide a significant contribution to Broken Hill’s urban water supply remains unknown.
72