2.3.2.2 Geology and hydrogeology


A regional geological model of the Namoi subregion was developed for the Assessment, based on the CDM Smith geological model (NTEC, 2013), the Water Resource Assessment for the Great Artesian Basin (Smerdon et al., 2012) and the Hydrogeological Atlas of the Great Artesian Basin (Ransley et al., 2015) (see companion product 2.1-2.2 for the Namoi subregion (Aryal et al., 2018a)).

The model focused on the strata of the Permian – Triassic Gunnedah Basin where the coal seams are present, and the Jurassic – Cretaceous Surat Basin where aquifers of the Great Artesian Basin (GAB) are present. The layering and depth profiles generated in the geological model were a key input into the numerical groundwater modelling (see companion product 2.6.2 for the Namoi subregion (Janardhanan et al., 2018)).

The geological history of deposition, deformation, uplift and erosion that has occurred in the Namoi subregion has implications for the connectivity between the geological units, coal resource developments and water-dependent assets. Information about the water-dependent assets for the Namoi subregion is presented in companion product 1.3 (O’Grady et al., 2015).

2.3.2.2.1 Geology

The main geological systems of interest for the Assessment, from oldest to youngest, are the Permian – Triassic Gunnedah Basin, the Jurassic – Cretaceous Surat Basin (see Figure 6) and the Cenozoic alluvium. These geological systems overlie the older Paleozoic rocks of the Lachlan Orogen. The stratigraphy column and the surface geology of the Namoi subregion are shown in Figure 7 and Figure 8 in companion product 2.1-2.2 for the Namoi subregion (Aryal et al., 2018a). The Cenozoic alluvium and Pilliga Sandstone in the GAB are the main groundwater sources in the Namoi subregion and do not contain coal. The coal resources under development in the Namoi subregion are primarily in the Gunnedah Basin. The main economic coal seams occur in the Black Jack Group and the Maules Creek Formation.

Coal and CSG are commercially extracted from the northern part of the Surat Basin in Queensland, however, there is no current or planned coal or CSG resource development from the Surat Basin in the Namoi subregion. More detail on the coal resources in the Namoi subregion can be found in companion product 1.2 for the Namoi subregion (Northey et al., 2014).

Figure 6

Figure 6 Geological basins of the Namoi subregion, including the Hunter-Mooki Thrust Fault System

Data: Geoscience Australia (Dataset 6, Dataset 7) and FROGTECH (Dataset 8)

Note: The stratigraphic units of the Gunnedah, Bowen, Sydney and Werrie basins are Permo-Triassic and the overlying stratigraphic units of the younger Surat Basin are Jurassic.

Geological history and lithology

The Gunnedah Basin was initiated in a back-arc setting in the early Permian when an extensional event resulted in the deposition of a thick succession of volcanics interlayered with lacustrine sedimentary rocks (Totterdell et al., 2009). In the west, the Gunnedah Basin sits unconformably on older basement rocks of the Lachlan Orogen and in the east it abuts the New England Orogen along the Hunter-Mooki Thrust Fault System (see Figure 15 in companion product 1.1 for the Namoi subregion (Welsh et al., 2014)). The main lithology associated with strata in the Namoi subregion is shown in Table 3 and a stratigraphic column is shown in Figure 7 in companion product 2.1-2.2 for the Namoi subregion (Aryal et al., 2018a). The geological strata relative to coal resource development are shown in Figure 7 and Figure 8.

Table 3 Main lithology of strata in the Namoi subregion


Period

Formations and groups

Host sedimentary basin

Main lithology

Quaternary

Alluvium

na

Clays, sands and gravels

Jurassic and Cretaceous

Rolling Downs Group

Surat Basin

Mudstone, siltstone and fine sandstone

Jurassic

Pilliga Sandstone

Surat Basin

Sandstone and conglomerate with minor mudstone, siltstone and coal

Jurassic

Purlawaugh Formation

Surat Basin

Sandstone thinly interbedded with siltstone, mudstone and thin coal seams

Jurassic

Garrawilla Volcanics

Surat Basin

Flows and intrusions of dolerite, basalt, trachyte, tuff and breccia overlying major depositional unconformity with the Gunnedah Basin.

Triassic

Deriah Formation

Gunnedah Basin

Sandstone with volcanic fragments and mudclasts. Overlain by sandstone and mudstone

Triassic

Napperby Formation

Gunnedah Basin

Sandstone and siltstone interbedded with thick, massive or crossbedded sandstones; minor conglomerate

Triassic

Digby Formation

Gunnedah Basin

Conglomerate at base, overlain by sandstone. Siltstone/claystone at top

Permian

Black Jack Group

Gunnedah Basin

Conglomerateic sandstone, claystone, tuff and pyroclastic detritus. Coal bearing

Permian

Watermark Formation

Gunnedah Basin

Fining-up sequence of silty sandstone to siltstone/claystone, then a coarsening-up sequence

Permian

Porcupine Formation

Gunnedah Basin

Basal conglomerate passing upwards into bioturbated silty sandstones and minor siltstones with dropped pebbles

Permian

Maules Creek Formation

Gunnedah Basin

Basal claystone, sandstone, siltstone, numerous coal seams, conglomerate

Permian

Goonbri and Leard Formations

Gunnedah Basin

Claystones, siltstones and sandstones

Permian

Boggabri Volcanics

Gunnedah Basin

Rhyolitic to dacitic lavas and ashflow tuffs with interbedded shale

Source: Geoscience Australia and Australian Stratigraphy Commission (2017)

The onset of sedimentation in the basin is marked by localised deposition of colluvial and alluvial material of the Leard Formation in paleovalleys on the weathered surface of the basement volcanics. This is overlain by the lacustrine Goonbri Formation (Gurba et al., 2009).

This was followed by an influx of volcanolithic sediments, sourced from the nearby Boggabri Ridge (see Figure 10) and other local highlands by erosion, forming the lower Permian Maules Creek Formation. The Maules Creek Formation was deposited in a variety of fluvial settings ranging from alluvial fans to peat swamps and contains numerous coal seams (Gurba et al., 2009; Totterdell et al., 2009). Thrusting of the Maules Creek sub-basin in the east of the subregion, mainly during a compressional event at the end of the Permian or Early Triassic, resulted in a disparity of several hundred metres in the vertical elevation of the Maules Creek Formation across the Boggabri Ridge (Tadros, 1993).

The lower Permian Porcupine and Watermark formations were deposited under transgressive marine conditions. The contractional event that marks the boundary between the Maules Creek Formation and overlying Porcupine Formation may have resulted in further uplift of the Boggabri Ridge, which then provided the source of sediment for the Porcupine and Watermark formations (Totterdell et al., 2009).

This was followed by marine regression and the deposition of fluvial sediments, including peat swamp deposits, the precursor sediments of the relatively widespread Hoskissons Coal in the Black Jack Group (Totterdell et al., 2009). The upper Black Jack Group, which overlies the Hoskissons Coal, was deposited as part of an alluvial system from which sediments were derived from the New England Orogen to the east of the Namoi subregion. The Hoskissons Coal crops out to the west of the Boggabri Ridge.

A contractional event in the late Permian resulted in deformation of the Gunnedah Basin. Erosion prior to the deposition of Triassic sediments resulted in the formation of an essentially flat plain. The erosion may have removed almost all of the Gunnedah Basin strata above the Maules Creek Formation in the Maules Creek sub-basin, including the Black Jack Group (Geological Survey of NSW, 2002). The Digby and Napperby formations in the Early Triassic were deposited on this erosional surface (Totterdell et al., 2009).

Down-warping during the Late Triassic – Early Jurassic resulted in the accumulation of more alluvial sediments creating the Surat Basin, which unconformably overlies the western part of the Gunnedah Basin in the subregion (see Figure 6).

The Garrawilla Volcanics generally form the base of the Surat Basin (Totterdell et al., 2009) and crop out to the west and south of Gunnedah (refer to Figure 8 in companion product 2.1-2.2 (Aryal et al., 2018a)). The Purlawaugh Formation unconformably overlies the Garrawilla Volcanics and is interpreted to have been deeply eroded prior to the deposition of the overlying Pilliga Sandstone (Totterdell et al., 2009). Erosion of the Pilliga Sandstone prior to the deposition of Cenozoic sediments has resulted in a layer of saprolite that is of variable thickness and extent across the Namoi subregion.

During the Cenozoic more alluvium was deposited over the subregion, forming the Upper and Lower Namoi alluvial aquifers. This period was also marked by volcanic activity and the remains of the resulting volcanic structures are seen today as hills around the margin of the Namoi river basin, including the Liverpool and Nandewar ranges (Wellman and McDougall, 1974). The weathered remnants of these Cenozoic volcanic flows now form the rich and fertile soils of the Liverpool Plains.

More detail on the general geology of the Namoi subregion, including the stratigraphy, is in companion product 1.1 (Welsh et al., 2014) and Section 2.1.2 in companion product 2.1-2.2 (Aryal et al., 2018a) for the Namoi subregion. Other summaries of the sedimentary basins in the subregion include Hawke and Cramsie (1984), Korsch and Totterdell (2009), Tadros (1993) and Totterdell et al. (2009).


Figure 7

Figure 7 Schematic diagram of the south-east of the Namoi subregion from Quirindi to Gunnedah showing underlying geology relative to coal resource development

The coal resource developments in the coal resource development pathway (CRDP) are the sum of those in the baseline coal resource development (baseline) and the additional coal resource development (ACRD).

Figure 8

Figure 8 Schematic east-west diagram of the Namoi subregion from Gunnedah to Wee Waa showing underlying geology relative to coal resource development

The coal resource developments in the coal resource development pathway (CRDP) are the sum of those in the baseline coal resource development (baseline) and the additional coal resource development (ACRD).

CSG = coal seam gas

The Werris Creek Mine is located in the Werrie Basin (Figure 6), targeting the coal-bearing Willow Tree Formation (see Figure 9). The Werrie Basin is a north-north-west trending synclinal structure to the east of the Hunter-Mooki Thrust Fault System and is therefore a structurally isolated (non-contiguous part) from the Gunnedah Basin (Figure 9). The Hunter-Mooki Thrust Fault System terminates the basin at each end and as the fault is considered to be an impermeable boundary, the deeper groundwater systems of the Werrie Basin are hydrogeologically isolated from the Gunnedah Basin. While the Werrie Basin is not considered to be part of the Gunnedah Basin, the age and stratigraphic similarities between them suggest a close association and probably a direct physical link during deposition (Geological Survey of NSW, 2002). The coal measures in the Willow Tree Formation were once connected to the early Permian Greta Coal Measures of the Hunter Valley. Since deposition, periods of uplift and erosion have left only the isolated remnants of coal measures at the Werris Creek Mine (Whitehaven Coal, n.d.). The alluvial cover is connected over the Hunter-Mooki Thrust Fault System.

Figure 9

Figure 9 Indicative cross-section of the Werrie Basin

Source: derived from Pratt (1996)

For approximate location of cross-section, please see Figure 10.

Structure and subdivisions of the Gunnedah Basin

This section provides information on the subdivisions and structures of the Gunnedah Basin relative to the coal resource development pathway (CRDP) for the Namoi subregion. The Gunnedah Basin consists of several major structural elements (Figure 10), as initially proposed by Tadros (1988). The architecture of the geological basement underlying the Gunnedah Basin comprises three north-north-westerly oriented sub-basins lying between basement ridges, over which the strata thin. The coal resources of the Gunnedah Basin in the Namoi subregion are located in the Maules Creek and Mullaley sub-basins, as shown in Figure 10 and Table 4. Section 2.3.4 contains more detailed information about the CRDP for the Namoi subregion.

In the east of the Namoi subregion the Boggabri Ridge separates the Maules Creek sub-basin from the central Mullaley sub-basin. To the west of the Mullaley sub-basin is the Rocky Glen Ridge and the Gilgandra sub-basin underlies the western portion of the Namoi subregion (Figure 10) (Tadros, 1993). In the Gunnedah Basin the depositional architecture of the Maules Creek and Mullaley sub-basins were compartmentalised, which also suggests the sub-basin strata may be compartmentalised in some areas with respect to regional groundwater flow systems.

Figure 10

Figure 10 Subdivisions of the Gunnedah Basin in the Namoi subregion showing the locations of mines in the coal resource development pathway

The coal resource developments in the coal resource development pathway (CRDP) are the sum of those in the baseline coal resource development (baseline) and the additional coal resource development (ACRD).

Source: derived from Gurba et al. (2009). This figure is not covered by a Creative Commons Attribution Licence. © CO2CRC Limited

Data: Bioregional Assessment Programme (Dataset 2)

The most easterly subdivision of the Gunnedah Basin is the early Permian Maules Creek sub-basin, which is bound to the east by the Hunter-Mooki Thrust Fault System and to the west by the Boggabri Ridge (Figure 10). East of the Boggabri Ridge the strata dip gently to the east, becoming steeper near the Hunter-Mooki Thrust Fault System. Several coal resource developments in the CRDP are located in the Maules Creek sub-basin, where the target coal seams of the Maules Creek Formation crop out or are close to the surface.

The Boggabri Ridge is oriented generally in a north–south direction and is truncated to the east of Gunnedah by the Hunter-Mooki Thrust Fault System (see Figure 10). West of the Boggabri Ridge the strata dip gently towards the basin axis. Early Permian sediments onlap the eastern and western sides of the Boggabri Ridge and thus the ridge would have separated the eastern half of the Gunnedah Basin into the Maules Creek and the Mullaley sub-basins during the early Permian. The Boggabri Ridge is not a continuous high and gaps exist (Tadros, 1993).

The Mullaley sub-basin, which extends over the entire length of the Gunnedah Basin, is the largest and most prominent of the sub-basins. It is divided by a prominent transverse high, the Walla Walla Ridge, and by several other structural highs into a series of north-north-westerly troughs including the Ballata, Bohena, Bando and Murrurundi troughs (see Figure 9 in companion product 2.1-2.2 for the Namoi subregion (Aryal et al., 2018a)). Coal mine and CSG developments in the Mullaley sub-basin primarily target the Hoskissons Coal, opportunistically mining the coal seams of the Maules Creek Formation where present.

West of the Mullaley sub-basin is another north–south trending ridge: the Rocky Glen Ridge (see Figure 10). This ridge is a prominent basement high partially onlapped by Permian strata. Aeromagnetic and gravity data confirm that it coincides with the western edge of the Werrie Basalt and Boggabri Volcanics in the Gunnedah Basin (Geological Survey of NSW, 2002).

The BA has compiled these features into a composite three-dimensional geological model to form the basis for the groundwater model (companion product 2.6.2 (Janardhanan et al., 2018) for the Namoi subregion).

The subdivision of the Gunnedah Basin into troughs and structural highs has the potential to compartmentalise deeper groundwater flow systems within the basin, thereby possibly compartmentalising potential groundwater impacts from coal resource developments. The Boggabri Ridge is the primary structure that may compartmentalise the Maules Creek and Mullaley sub-basins such that impacts in the Gunnedah Basin strata may not propagate over this structural high. However, as noted by Tadros (1993), the ridge is not continuous and so there may be areas where the groundwater systems may be connected. There is also the potential of flow around the Boggabri Ridge where the Boggabri Volcanics are close to the surface and are weathered or fractured (Schlumberger Water Services, 2012a). Additionally, the alluvial cover over the Boggabri Ridge is connected and so groundwater impacts may propagate via the alluvium.

Table 4 Target coal seams for coal resource developments in the Namoi subregion


Coal resource development

Sub-basin

Target stratigraphic unit

Target coal members

Boggabri Coal Mine and Boggabri Coal Expansion Project

Maules Creek

Maules Creek (minor basal seam from underlying Leard Formation)

Braymont, Merriowen, Jeralong

Caroona Coal Projecta

Mullaley

Black Jack

Hoskissons Coal

Gunnedah Precinct

Mullaleyb

Black Jackb

Hoskissons Coalb

Maules Creek Mine

Maules Creek

Maules Creek

15 members within Maules Creek Formation (i.e. Herndale to Templemore seams)

Narrabri North and South

Mullaley

Black Jack

Hoskissons Coal

Narrabri Gas Project

Mullaley (Bohena trough)

Black Jack Group and Maules Creek

Hoskissons Coal, Bohena

Rocglen Mine

Maules Creek

Maules Creek

Belmont, Upper Glenroc, Lower Glenroc

Sunnyside Mine

Mullaley

Black Jack

Hoskissons Coal, Upper Melville, Lower Melville

Tarrawonga Mine and Tarrawonga Coal Expansion Project

Maules Creek

Maules Creek

Braymont, Bollol Creek, Jeralong

Vickery Coal Project (including Vickery South Coal Project)

Maules Creek

Maules Creek

Gundawarra, Welkeree, Kurrunbede, Shannon Harbour (upper and lower), Stratford, Bluevale (upper and lower), Cranleigh (upper, middle and lower)

Watermark Coal Project

Mullaley

Black Jack

Hoskissons Coal, Melvilles

Werris Creek Mine

Werrie Basinc

Willow Tree Formation (early Permian, time-equivalent to Maules Creek Formation)

Willow Tree Formation (time-equivalent to the Maules Creek coals and Greta Coal Measures)

aThe Caroona Coal Project was discontinued in 2016. However, in accordance with companion submethodology M04 (as listed in Table 1) for developing a coal resource development pathway (Lewis, 2014), the project is included in the Assessment.

bNo information available. Assumed to be in Mullaley sub-basin targeting the Hoskissons Coal of the Black Jack Group due to proximity to the Sunnyside Mine.

cWerris Creek Mine is located in the Werrie Basin and targets the Willow Tree Formation. These coal measures were once connected to the Greta Coal Measures of the Hunter Valley but periods of uplift and erosion have left only the isolated remnant of coal at the Werris Creek Mine.

Overburden thickness of the Permian coal measures

The Permian coal measures are the primary targets for coal mining and CSG development in the Namoi subregion. Overburden thickness is one of several significant factors that affect the degree of connectivity between coal measures and water-dependent assets. The overburden of the Permian coal measures is the combined thickness of the Cenozoic sediments, the Surat Basin stratigraphic sequences where present, and the Triassic Digby and Napperby formations. The Watermark and Porcupine formations also form the overburden of the coal seams in the Maules Creek Formation.

The overburden thickens rapidly to the west of the Boggabri Ridge in the troughs of the Mullaley sub-basin with overburden thicknesses of between 600 and 1200 m above the coal seams in the Maules Creek Formation and 700 m for the Hoskissons Coal. The Gunnedah Basin sequence is thickest in the Bohena Trough to the west of the Boggabri Ridge. Overburden thickness is considerably less in the Maules Creek sub-basin, with the deepest coal seam targeted at the Maules Creek Mine approximately 400 m from ground surface, indicating a maximum overburden thickness in the Maules Creek sub-basin of approximately 400 m.

Potential for structural connectivity within the Gunnedah Basin

Faults can act as localised barriers to groundwater flow if they juxtapose an aquifer against an aquitard or if the faults are filled with significant amounts of clay. Conversely, they can act as conduits to groundwater flow if there is sufficient fracture connectivity. It is difficult to characterise individual faults as either potential conduits or barriers to groundwater flow as detailed and site-specific geoscientific studies are required. Consequently, the implications for groundwater flow near faults under stress are largely unknown and this uncertainty was the main reason why faults were not specifically modelled in the groundwater numerical modelling for Namoi.

Most structural activity, including faulting in the Namoi subregion, occurred in the lower to middle Permian (Tadros, 1993). Higher in the sequence, the younger Surat Basin strata is less deformed.

Tectonic activity has produced linked thrust faults in the east of the subregion with fewer faults identified in the west of the Namoi subregion. The most significant fault system within the Namoi subregion is the Boggabri Thrust located within the Boggabri Volcanics of the Boggabri Ridge, which approximately underlies the Namoi River between Gunnedah and Narrabri. Activation of this fault raised the Maules Creek sub-basin strata several hundred metres higher than equivalent strata in the Mullaley sub-basin (Geological Survey of NSW, 2002).

Several faults have been interpreted in the Maules Creek and Mullaley sub-basins that generally orientate in a north-north-westerly or south-westerly direction. In many instances, minor faulting appears to be associated with uplifting after the Permian (Geological Survey of NSW, 2002). Closely spaced drilling in the Maules Creek sub-basin east of the Boggabri Ridge identified a number of north-westerly striking normal faults with vertical displacements ranging from 80 to 120 m (Tadros, 1993). These fault structures have the potential to cause a large disconnection across geological units. It is anticipated that some of these faults may have a significant effect on groundwater flow characteristics at a local scale and would need to be assessed in local-scale model simulations (Schlumberger Water Services, 2011).

2.3.2.2.2 Hydrogeology

The Namoi subregion consists of two major aquifer systems – the Namoi Alluvial aquifer (Upper and Lower Namoi) and the Pilliga Sandstone aquifer. The most widely used aquifer in the Namoi subregion is the Namoi alluvium compromising the Quaternary Narrabri and Gunnedah Formations. These units contain significant resources of high-quality groundwater that is heavily utilised for irrigation, town water supply, and stock and domestic use. The Pilliga Sandstone is part of the Surat Basin and is a major regional aquifer consisting of medium- to coarse-grained sandstone and conglomerate with minor interbeds of fine-grained sediments. Minor groundwater resources occur in the Gunnedah Basin, however, these systems are only rarely utilised for stock and domestic purposes where the alluvium is absent. A detailed description of the key aquifers is in Section 1.1.4 of companion product 1.1 for the Namoi subregion (Welsh et al., 2014).

Of primary interest in understanding groundwater movement is the identification of potential connectivity of aquifers between basins (the Gunnedah and Surat basins) and, ultimately, connectivity with alluvial aquifers. Understanding the contact of aquifers and where vertical pressure gradients exist will assist in identifying potential pathways for fluids migrating upwards from underlying basins. The greatest potential for connectivity is through direct physical contact of aquifers either through overlap of their extents or from their juxtaposition along faults.

Cross-formational flow occurs where a vertical pressure gradient exists between two formations and there is sufficient permeability to permit flow between them. The rate of transfer between units is therefore a function of the connectivity in terms of permeability between the two units and the magnitude of the pressure gradient between them. This has been shown to occur in the Pilliga Sandstone, the regional potentiometric surface in the GAB, where hydraulic heads are above the heads in the overlying Narrabri Formation (see companion product 1.1 for the Namoi subregion (Welsh et al., 2014)), potentially resulting in flow from the GAB into the alluvial aquifers. Where formations have lower hydraulic conductivity and act as aquitards, such as the Purlawaugh and Napperby formations, the rate of vertical transfer between formations is slower, even if the pressure gradient is strong.

Alluvial aquifers and watertable groundwater flow systems

The alluvial aquifer systems in the Namoi subregion occur along the river valleys and floodplains and play an important role for agriculture and groundwater-dependent ecosystems. Traditionally the alluvial material has been described as a stacked aquifer system with the uppermost Narrabri Formation (up to 30 to 40 m deep) underlain by the Gunnedah Formation (usually 40 to 100 m deep, 170 m at its deepest) with the Cubbaroo Formation in the Lower Namoi associated with the main paleochannel (Barrett, 2012). Within the alluvial sequence, gravel- and sand-rich layers generally occur near the base, overlain by fine-grained floodplain silts and clays that have been deposited in a lower energy environment. However, Kelly et al. (2007) emphasised that the alluvial sequence as described generally oversimplifies the complexity of the sequence and the interplay between clay, sand and gravel beds. Most groundwater models for the Namoi river basin have split the Namoi alluvium into the Narrabri Formation and the Gunnedah Formation in order to characterise the upward-fining alluvial sediments. However, it is recognised that in some areas there may be no hydraulic separation between the Narrabri and Gunnedah formations, and they act as a single aquifer. This variability in connectivity within the alluvium is the result of deposition in a complex fluvial system and can occur on a very small scale, making it difficult to characterise and incorporate in groundwater models.

The alluvial system is subdivided geographically and for management purposes into the Upper and Lower Namoi alluvium, with the Upper Namoi alluvium referring to the upper catchment where there is generally less alluvial deposition, and the Lower Namoi alluvium which, being the lower part of the catchment, is characterised by extensive and thicker alluvial development. The town of Narrabri marks the boundary between the Upper and Lower Namoi alluvium (Figure 11).

Narrow geological constrictions along the length of the Upper Namoi Valley have had a significant effect on how the alluvial sediment was deposited, which influences groundwater movement in the Upper Namoi alluvium. In certain places, the basement rocks form narrow valleys, which restrict groundwater movement through the alluvial aquifers and may compartmentalise groundwater impacts from mining activities. The main constrictions occur at Gin’s Leap, north of Boggabri (between Zone 4 and Zone 5), Coxs Creek at Mullaley (between Zone 2 and Zone 9) and at Breeza on the Mooki River(between Zone 3 and Zone 8) (Barrett, 2012) (Figure 11).

The alluvial sediments form a continuous unit across the Hunter-Mooki Thrust Fault System, extending beyond the eastern boundary of the Namoi subregion, similar to the surface water catchment. Parts of the Upper Namoi Zone 4 and Zone 12 groundwater management areas extend over the Hunter-Mooki Thrust Fault System, indicating connectivity in the alluvium extends over the fault.

The hydraulic characteristics of the Narrabri and Gunnedah formations are generally well documented, however this is not the case for the deeper strata (see companion product 2.1-2.2 for the Namoi subregion (Aryal et al., 2018a)). The hydraulic properties of the alluvial aquifers are highly variable depending on the presence of sand or clay lenses. However, hydraulic conductivity generally increases with depth and in the paleochannels.

Regionally, groundwater gradients in the alluvial aquifers indicate flow from the east in a north-westerly to westerly direction (see Figure 26 in companion product 1.1 for the Namoi subregion (Welsh et al., 2014)). Groundwater mounds tend to develop around the Namoi River and its anabranch, Pian Creek.

In the northern Namoi paleochannel in the Lower Namoi, groundwater flow is from the Narrabri Formation into the Gunnedah Formation due to groundwater pumping for irrigation mostly from the Gunnedah Formation over the Lower Namoi Valley, but especially in the paleochannel. Groundwater pumping from the Gunnedah Formation induces significant downward leakage from the Narrabri Formation and changes in local groundwater flow direction (Smithson, 2009).

In general, groundwater levels in the alluvial aquifers respond to rainfall variability and the associated variability in groundwater use. At Narrabri, the watertable level is about 4 to 12 m below ground level, becoming progressively deeper towards the west, and is about 25 to 34 m below ground level near Walgett. Long-term groundwater level declines at the western end of the valley, where usage is low, are most likely related to extraction higher in the valley, limiting throughflow (Smithson, 2009).

Figure 11

Figure 11 Groundwater management zones of the Upper and Lower Namoi groundwater sources within the preliminary assessment extent of the Namoi subregion

Data: Bioregional Assessment Programme (Dataset 3)

Surat Basin

The central and western parts of the Namoi subregion are underlain by the Coonamble Embayment, the south-eastern extent of the Surat Basin. The main Surat Basin aquifer is the Pilliga Sandstone, which crops out in the central part of the Namoi subregion, marking the eastern boundary of the Surat Basin.

Results from the Great Artesian Basin Water Resource Assessment (CSIRO, 2012) indicated that in the Coonamble Embayment, the Rolling Downs Group is a tight aquitard, the Pilliga Sandstone is an aquifer and the Purlawaugh Formation is an aquiclude. Other documentation describes the Purlawaugh Formation as a negligibly transmissive unit (CH2MHill, 2013) and an aquifer (Schlumberger Water Services, 2011). These differences may be attributed to there being a limited groundwater monitoring network in the Purlawaugh Formation (Schlumberger Water Services, 2011) and the reported difficulty in picking the boundary between the Purlawaugh Formation and underlying rocks (Hawke and Cramsie, 1984). This may result in the Purlawaugh Formation not always being accurately logged and indicates a better understanding of its hydraulic behaviour may assist in determining the potential for connectivity between the Gunnedah and Surat basins in the Namoi.

Groundwater in the Pilliga Sandstone aquifer flows from south-east to west and north-west (Figure 12). From east to west, the watertable lies in the Pilliga Sandstone then passes into thin bands of Keelindi and Drildool beds and then into undifferentiated Rolling Downs Group to the west, outside of the Namoi subregion (CSIRO, 2012). Hydrographs from bores screened in the GAB show that artesian groundwater levels in the GAB have fallen over time but have been relatively stable since the 1970s (Schlumberger Water Services, 2012b). According to the New South Wales Government (NSW Government), increases in artesian bore pressure are being observed in many areas as a result of the cap and piping program under the GAB Sustainability Initiative (NSW DPI, 2017).

Figure 12

Figure 12 Potentiometric surface of groundwater in the uppermost Great Artesian Basin aquifer

Source: modified from CSIRO (2012)

The Pilliga Sandstone potentiometric heads do not generally show any obvious surface water – groundwater interaction along the Namoi River and its anabranch, Pian Creek. One possible exception is the area just downstream of Narrabri at the confluence with Bohena Creek.

The weathering of exposed Surat Basin sediments prior to the deposition of overlying Cenozoic alluvium resulted in a basin-wide saprolite layer of low permeability in the basal portion (Kellett and Stewart, 2013). This is considered to reduce connectivity with the alluvium, except in some places where the saprolite has eroded – namely, the paleochannels in the Upper and Lower Namoi alluvium.

Potentiometric heads in the Pilliga Sandstone are above watertable levels in the Narrabri Formation, indicating the potential for upwards leakage in places. This is in contrast to the situation in the early 1990s, when Williams (1997) showed the water levels in the alluvium were generally lying above the regional Surat Basin watertable. This suggests that there has been a reversal in the head differential between the two flow systems due to groundwater pumping for irrigation.

It is likely that water in the Pilliga Sandstone aquifer is providing baseflow to tributaries of the Namoi River at the eastern extent of the outcropping Surat Basin units. These baseflows are fed by rejected recharge, which occurs where water is restricted from entering the aquifer, mainly due to geology, and is discharged at the surface.

Gunnedah Basin

The Gunnedah Basin hosts the porous rock aquifers within the Digby Formation and Clare Sandstone (part of the Black Jack Group), and the Permian coal beds. The Gunnedah Basin strata do not constitute a single aquifer – rather, the sequence consists of many overlapping units that are stacked together and commonly separated by lower-permeability layers. The porous rock aquifers and coal beds within the Gunnedah Basin are considered to have a significantly lower hydraulic conductivity than the alluvial aquifers.

There is very little published information relating to the Gunnedah Basin aquifers, so groundwater levels and flow paths are largely unknown.

Basement

Lower Permian volcanic rocks of the Lachlan Orogen underlie the Gunnedah Basin strata. There is little published information relating to the potential for Lachlan Orogen aquifers.

Last updated:
19 December 2018