1.1.4.3 Groundwater quality

1.1.4.3.1 Alluvial aquifers

The main alluvial aquifers of the subregion are those associated with the Gwydir, Namoi and Border (Dumaresq and Macintyre) rivers. Limited groundwater quality data were identified for these groundwater systems in the literature, but being used intensively for irrigation, stock and domestic purposes, and town water supplies indicates that they are of relatively good quality. Maps included in Murray–Darling Basin Authority (MDBA, 2012a) show that salinity in the alluvium of the Gwydir subregion generally increases from east to west. Outside of the main alluvial aquifer areas, much of the groundwater has total dissolved solids (TDS) concentrations greater than 14,000 mg/L west of the Lower Gwydir Alluvium. Such high salinity water is of limited use for anthropogenic purposes.

Lower Gwydir Alluvium

MDBA (2012a) includes a map based on Murray-Darling Basin Commission (MDBC, 2000) data which shows that groundwater in the Lower Gwydir Alluvium (see Figure 23) has relatively low salinity, with TDS concentrations across most of the area being in the 0 to 1500 mg/L category. Green et al. (2011a) indicate that much of this water is within the 0 to 500 mg/L category, with salinity increasing towards the far west.

Border Rivers Alluvium

Groundwater in the NSW Border Rivers Alluvium and NSW Border Rivers Tributary Alluvium (Figure 22) is shown in MDBA (2012a) to be relatively fresh, with TDS concentrations mostly in the range 0 to 1500 mg/L. This generally matches work presented by Please et al. (2000) who identified TDS concentrations in this area of less than 1000 mg/L. Please et al. (2000) indicated that salinity increases significantly downstream west of these areas. In the east, groundwater had mixed cationic composition (sodium dominant) but whereas shallow groundwater was dominated by chloride anions, deeper groundwater was dominated by bicarbonate. Further west water was generally sodium and bicarbonate dominated, while high salinity areas in the far west had sodium chloride type waters. Similar findings were reported from a more recent study by Baskaran et al. (2009).

Although groundwater supports irrigation, stock and domestic use, Please et al. (2000) indicate that it does have a high sodium hazard rating in the area in places, particularly close to the Peel Fault Zone (about 45 km east of Goondiwindi) where sodium concentrations are elevated in the alluvial system. This may be due to upward leakage of groundwater from the GAB which is known to have elevated sodium concentrations.

Lower Namoi Alluvium

Maps included in Green et al. (2011b) indicate that groundwater in the east of the Lower Namoi Alluvium is fresh (TDS concentration of 0­ to 500 mg/L) and therefore suitable for a variety of uses. Quality decreases to the west, with some areas being defined as saline (TDS concentrations in the range 7,000 to 14,000 mg/L). An increase in groundwater salinity, possibly due to increased saline drainage through the alluvium, has been reported in irrigation areas (Smithson, 2009). Barrett et al. (2006, as cited in Parsons Brinckerhoff, 2011) indicate that groundwater salinity generally increases away from the main recharge areas in the east, and from the mean Namoi River electrical conductivity (EC) of 560 μS/cm to over 30,000 μS/cm in the Narrabri Formation.

McLean (2003) analysed changes in groundwater salinity between the mid-1980s and 1999 and concluded that the salinity of groundwater in the eastern part of the Lower Namoi Alluvium had increased by 100 μS/cm over the preceding 20 years, and in the western portion of the aquifer by several thousand μS/cm. These increases have been attributed to changes in direction of potential flow paths caused by pumping practices (Kelly et al., 2007). The changes in water quality due to pumping were localised and showed no general trends (McLean, 2003).

The results from a recent hydrogeochemical sampling and characterisation project of alluvial groundwater in the Lower Namoi (Parsons Brinckerhoff, 2011) indicated that:

1.1.4.3.2 Surat Basin

Habermehl (2002) states that groundwater quality of the GAB is variable but salinity is generally in the range 500 to 1500 mg/L in the Lower Cretaceous-Jurassic aquifers (of these, the most widely used aquifer in the east is the Pilliga Sandstone (NSW Government, 2009)). The NSW Government (2009) indicates that groundwater quality in the deeper aquifers is more variable although salinity is generally lower than that found in the confining sequence. Groundwater salinity reportedly increases away from the recharge areas in the east (200 mg/L TDS) along groundwater flow paths to the north and west to over 2000 mg/L in places.

Kuske et al. (2011) report that the Cadna-owie Formation (which includes the Pilliga Sandstone) within the Coonamble Embayment has salinity mostly in the range of 250 to 1000 mg/L.

Groundwater in the Lower Cretaceous-Jurassic aquifers is typically of a sodium-bicarbonate-chloride type (chloride becoming more dominant away from the intake beds), and generally suitable for domestic, town supply and stock use (although elevated fluoride concentrations in places – mostly Queensland – may cause issues for stock watering). However, the NSW Government (2009) indicates that it is unsuitable for irrigation in most areas due to its high sodium adsorption ratio. They state that as an exception in the subregion, a combination of lower sodium absorption ratio groundwater and sandy soils supports irrigation in the recharge areas on the eastern margin of the GAB (in the Eastern Recharge Groundwater Source discussed above).

The upper confining units of the Surat Basin (managed under the New South Wales GAB Shallow Groundwater Source Water Sharing Plan to a depth of 60 m) generally consist of low permeability claystone, mudstone, calcrete and shale with minor conglomerate and sandstone (Green et al., 2012). The aquifers they contain are described as sporadic and often low yielding, producing brackish to saline water of limited use.

1.1.4.3.3 Gunnedah-Bowen Basin

Limited information could be found on the quality of groundwater in the Gunnedah-Bowen Basin in the Gwydir subregion. Where the rocks of this basin outcrop in the extreme south-east of the river basin they are shown to contain relatively fresh groundwater (MDBA, 2012a), having TDS concentrations in the range 0 to 1500 mg/L.

Some information from investigations associated with CSG development is available for local areas in the Namoi subregion to the south. Although not directly applicable to the Gwydir subregion, the results provide an indication of what groundwater chemistries may be anticipated. From the limited published data available, the majority of groundwater samples from the non-coal seam formations in the Gunnedah Basin of the Namoi subregion are fresh to slightly brackish, while groundwater samples from the Hoskissons Coal Member are predominantly brackish. An increasing salinity trend was observed in the following order:

Narrabri/Gunnedah<Surat Basin<Napperby/Digby Formation<Black Jack Group<Hoskissons Coal Member (Golder Associates, 2010; Golder Associates, 2011).

Major ion analysis of a very limited number of groundwater samples from the Gunnedah Basin in the Namoi subregion found that groundwater from the Hoskissons Coal Member generally had high concentrations of sodium and bicarbonate and low concentrations of sulfate and chloride. Groundwater from other formations, including the Napperby and Digby formations, was characterised by a range of water types reflecting the heterogeneity of their sedimentary environments (Golder Associates, 2010, 2011). Further information including typical groundwater quality ranges for target coal seams is included in Welsh et al. (2014).