Shallow groundwater levels are shown on Figure 25 for the alluvium in the Gwydir subregion and surrounds. It is apparent that shallow groundwater generally flows from the high ground in the east towards the west. Refer to Figure 23 for the extent of groundwater systems and management areas discussed below.
Lower Gwydir Alluvium
Barrett (2009) and Carr and Kelly (2010) describe groundwater levels in the Lower Gwydir Alluvium and include a comprehensive review of available hydrographs. This work was undertaken prior to the 2010 floods. Key findings about the Lower Gwydir Alluvium include:
- Groundwater is generally unconfined in the 10 to 30 m thick shallow groundwater system (Narrabri Formation).
- Groundwater in the more productive, deeper (35 to 80 m) groundwater system of the Gunnedah Formation is confined to semi-confined.
- There is no laterally continuous horizon marking the boundary between the two systems. Some authors (e.g. Carr and Kelly, 2010) question whether they are accurately represented as separate hydrostratigraphic (or lithostratigraphic) units.
- Both formations may contain several aquifers that vary in thickness and lateral extent. Aquifers are most extensive in the east near Moree but become more irregular downstream to the west where they grade into finer sediments.
- A high degree of vertical hydraulic connectivity between all depths of the aquifer system is reported by Carr and Kelly (2010) at a regional scale, while clay lenses may limit connectivity locally.
- The dominant aquifer recharge process is leakage from watercourses. This is evident in Figure 25 where groundwater mounding about the Lower Gwydir distributary, downstream of Moree, suggests recharge to the Narrabri Formation, via stream leakage, is occurring. Additional recharge contributions also occur from rainfall infiltration, irrigation and groundwater inflow from the east. Pumping from the deeper, more productive formations induces vertical leakage from overlying aquifers.
- Hydrographs show that groundwater heads have generally fallen with increased groundwater extraction from the deep aquifer since the 1980s, indicating over-extraction has been occurring. This has induced leakage from overlying shallower aquifers and drawdowns in the watertable. Although there was some recovery in the wetter years from 1996 to 2001, declines have subsequently continued. In the far west, declines of over 8 m were recorded since the 1970s (Barrett, 2009). At the edge of the groundwater source, some bores are reported to have shown gradual rising water levels in places.
- Ransley et al. (2012b) show that the thickest alluvium in the area occurs as an east-west trending paleochannel incorporating the Lower Gwydir Alluvium (Figure 25). It meets the Lower Namoi paleochannel south-west of the subregion. This paleochannel is anticipated to have an impact on groundwater flow at the subregional scale.
Figure 25 Groundwater levels (post-2000) in the Narrabri Formation showing areas where groundwater heads are higher than in the upper parts of the Great Artesian Basin. The outlines of paleochannels are shown
Only paleochannels that fall within or partially within the Gwydir subregion are shown. Other paleochannels are known to exist (e.g. in the Border Rivers) but maps of them could not be found in the literature.
Lower Namoi Alluvium
The Lower Namoi Alluvium is only considered briefly here. A more detailed discussion is included in the context statement for the Namoi subregion (Welsh et al., 2014).
- Similarly to in the Lower Gwydir Alluvium, groundwater is present in the shallower Narrabri Formation and the deeper Gunnedah Formation in this area. However, the Cubbaroo Formation is also present at the base of the alluvial sequence in the deepest paleochannel (see Figure 25), consisting of carbonaceous sand and gravel with interbedded clays from which groundwater is also extracted (Kelly et al., 2007).
- In some areas there is limited hydraulic separation between the alluvial formations although there is minimal connectivity in other places (CSIRO, 2007a; Kelly et al., 2007; Parsons Brinckerhoff, 2011).
- Recharge mechanisms are similar to those in the Lower Gwydir Alluvium, although the Cubbaroo and Gunnedah formations are also recharged in part by upward leakage from the GAB (Ransley et al., 2012b) in the main paleochannel. As water levels decline in the alluvium, there is a greater potential for upward leakage.
- Hydrographs show that groundwater levels have generally declined since monitoring commenced in the late 1960s, although there was some recovery from 1996 to 2001 when wetter seasons resulted in reduced extraction and increased recharge. Strong drawdowns are reported in the area of the paleochannel between Narrabri and Wee Waa where deep aquifer pumping has also resulted in leakage from shallow aquifers and reductions in their water levels. South of Wee Waa, Smithson (2009) reports that pumping has resulted in the previously confined deeper aquifer (Gunnedah Formation) becoming unconfined over a considerable area (450 km2).
NSW Border Rivers Alluvium
- Useable groundwater is restricted to sediments deposited in a narrow valley associated with the Dumaresq and Macintyre rivers. The valley broadens downstream of Keetah Bridge where groundwater generally becomes too saline for use.
- Similar to the Lower Gwydir and Lower Namoi alluvium, an unconfined Narrabri Formation aquifer (10 to 30 m thick) and deeper, confined Gunnedah Formation aquifer (up to 70 m thick) are identified in this area. The coarser sediments of the Gunnedah Formation form a paleochannel that meanders through the valley fill (CSIRO, 2007b).
- The Gunnedah Formation aquifers fine upward into Gunnedah Formation clays of variable permeability that are between 2 and 15 m thick and separate the aquifers from those of the Narrabri Formation (CSIRO, 2007b).
- CSIRO (2007b) indicate that recharge to the Narrabri Formation is mostly from stream losses, with minor contributions from rainfall and excess irrigation, recharge to the deeper Gunnedah Formation being mostly via cross-formational flow, upward leakage from underlying aquifers and infiltration of rainfall and runoff. However, Kellett and Stewart (2013) note that recharge from the Dumaresq River and Macintyre Brook are not strongly manifested in the groundwater surface (see Figure 25) which generally suggests either no significant interaction with streams or discharge to them in many areas (although the area near Goondiwindi is an exception where mounding is evident in the groundwater surface). There is evidence for some upward leakage from the underlying Surat Basin in this area as outlined in the section on groundwater quality below.
- Milne-Home et al. (2007) showed that although average water levels in the Border Rivers Alluvium fell by about 2.5 m in the east near Yelarbon from 1990 to 2005, they increased in the west by about 2.5 m south of Goondiwindi, suggesting that patterns of use vary across the water resource.
Groundwater level contours for water in the uppermost units of the Surat Basin presented by Kellett et al. (2012) are shown on Figure 26. The water levels mostly represent water in the Drildool and Keelindi beds in the area, but in the east of the Gwydir subregion the watertable is in the exposed rocks of the Pilliga Sandstone. The groundwater surface tends to be topographically controlled. It suggests minimal interaction between the upper units of the GAB and the Namoi and Gwydir rivers but distortions in the contours near the Border Rivers suggest some interaction in this area.
- The NSW Government (2009) indicates that groundwater recharge mostly occurs along the southern and eastern edge of the GAB via rainfall and streamflow, in an area known as the ‘intake beds’. The eastern intake beds outcrop in the east of the Gwydir subregion where water is extracted for irrigation near North Star from the Pilliga Sandstone.
- From the intake beds, groundwater levels are commensurate with general flow to the north and west.
- The vertical hydraulic gradient between the Surat Basin and overlying alluvium generally indicates potential for upward leakage to the alluvial deposits in the subregion except for the locations shown on Figure 25 where the head in the alluvium is higher.
- Groundwater discharges from the GAB via upward leakage to springs and soaks, shallower formations (particularly in paleochannel areas), and extraction bores (artesian and sub-artesian). Mound springs occur to the west of the subregion (e.g. near the Bogan River), while flowing bores occur basinward from Coonamble (Brownbill, 2000). The NSW Government (2009) indicates that more than 80% of the NSW portion of the GAB (of which the Surat Basin is a part) has experienced reduction in groundwater pressure due to extraction (free-flowing and sub-artesian bores). The GAB-associated springs recorded within the subregion (e.g. those along Jardines Creek and Ottleys Creek near Croppa Creek) are reportedly not mound springs. They are fed by ‘rejected recharge’, where recharge water is restricted from entering the GAB aquifers or flowing basinward, for example by geological structures (NSW Government, 2012).
Source: modified from Figure 5.19 in Kellett et al. (2012)
No literature relating to groundwater levels and flow in the Gunnedah-Bowen Basin was identified during this study of the Gwydir subregion.
As outlined in preceding sections, while the alluvium in the Gwydir subregion is typically separated in literature into the overlying Narrabri Formation and underlying Gunnedah Formation (with the Cubbaroo Formation locally present at the base of the Namoi paleochannel in the far south), the contact between them is difficult to determine lithologically and some question the validity of this separation. Some degree of hydraulic connectivity is often reported between the aquifers of both formations (e.g. Carr and Kelly, 2010), with leakage from the Narrabri Formation commonly cited as a source of recharge to the Gunnedah Formation aquifers.
Kellet and Stewart (2013) indicate that intense weathering of exposed GAB rocks prior to deposition of overlying alluvium resulted in development of a basin-wide saprolite layer. This layer has a low permeability basal portion that is considered to reduce connectivity with overlying systems. Kellet and Stewart (2013) state that in some places the saprolite has been removed by erosion making hydraulic connection possible. Such areas occur beneath paleochannels, of which there are reportedly many in the Surat Basin, commonly occurring beneath or next to modern stream channels. Depending on the relative head levels between the alluvium and GAB, there may be upward or downward flow of water in these areas (see Figure 25).
In the Gwydir subregion, Kellett and Stewart (2013) indicate that areas where upward flow of water from the GAB to the overlying alluvium are highly likely to include:
- the Border Rivers (Macintyre and Dumaresq rivers and Macintyre Brook) upstream of Goondiwindi (there is evidence of upward recharge in groundwater chemistry data near the Peel Fault Zone in this area as outlined in the section on groundwater quality below)
- the Gwydir River upstream of the Mehi River Anabranch
- the Namoi paleochannel (in the south of the Gwydir subregion).
Kellett and Stewart (2013) do not identify any areas in the Gwydir subregion where they consider groundwater flow from the alluvium to the underlying GAB formations to be highly likely.
It is emphasised that relative heads and thus potential flow directions are not fixed and may change over time in response to stresses such as groundwater extraction (e.g. for irrigation or coal seam gas development).
While a degree of hydraulic connection between alluvial aquifers and the topmost units of the GAB thus appears likely in some areas in the subregion, connection with deeper, productive GAB aquifers may still be limited in these places by the hydraulic properties of shallower formations. Recent drilling reported by Barrett (2009), for example, indicates that the Gunnedah Formation is directly underlain by the Bungil Formation of the Surat Basin (correlated to the Drildool beds of the Coonamble Embayment in Figure 23) beneath the Lower Gwydir Alluvium. This unit is described by Radke et al. (2012) as consisting of fine-grained sandstones, siltstones and mudstones. While it is likely that there will be hydraulic connectivity between its upper units in contact with the Gunnedah Formation where they contain water, it is described as a ‘tight aquitard’ (confining bed) in the north of the Coonamble Embayment and probably effectively separates the alluvial systems from deeper GAB aquifers. Geological maps of the area variably indicate much of the alluvium in the west of the subregion to be underlain by the Rolling Downs Group, the Griman Creek Formation or the Blythesdale Group (that is understood to include the Bungil Formation, the Mooga Sandstone and the Orallo Formation).
A preliminary regional-scale desktop assessment of the potential for hydraulic connectivity between the GAB and underlying basins was undertaken as part of the Great Artesian Basin Water Resource Assessment (CSIRO, 2012). Connectivity can occur where aquifers, partial aquifers and leaky aquitards are juxtaposed below and above the base of the GAB (noting that faults may also act as conduits in places through otherwise low permeability formations). Figure 27 shows that hydraulic connectivity between the GAB and underlying formations is relatively poor across much of the subregion except in the south-east where aquifers at the base of the GAB are indicated to overlie partial aquifers of the Gunnedah Basin. In this location, the Surat Basin has a heightened potential connectivity with the underlying Gunnedah Basin strata.
Figure 28 shows the locations of springs and other groundwater-dependent ecosystems in the former Border Rivers-Gwydir Catchment Management Authority areas reported by the NSW Office of Water (2009) (note that the Gwydir subregion only occupies the western parts of the areas shown). These systems are areas of groundwater discharge. The main features in the western parts of the subregion are wetlands and waterholes. Some springs are present in the east in the area of the intake beds to the GAB. The NSW Government (2009) highlights that these are not mound springs (GAB-fed systems in the confined, artesian areas of the GAB) but rather ‘rejected recharge’ from the intake beds.
Source: modified from Figure 5.7 in CSIRO (2012b)
Source: Figure 10 in NSW Office of Water (2009a)
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- 1.1.1 Bioregion
- 1.1.2 Geography
- 1.1.3 Geology
- 1.1.4 Hydrogeology and groundwater quality
- 1.1.5 Surface water hydrology and water quality
- 1.1.6 Surface water – groundwater interactions
- 1.1.7 Ecology
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