Geological conditions have the dominant control in salinity and chemical composition of groundwater in the Hunter subregion. The saline water associated with the Permian coal measures and the intervening marine sequence is thought to have a controlling influence on the overall water quality of the Hunter River (Kellett et al., 1989). Groundwater quality is generally brackish to saline (Mackie Environmental Research, 2006). Salinity within the hard rock aquifers associated with the Hunter coal seams is typically in the range 4000 to 12,000 μS/cm, but electrical conductivity (EC) has been recorded at over 26,000 μS/cm. The pH values range from 5.8 to 9.2 with a mean around 7.1 (NSW Department of Planning, 2005).
However, low salinity was recorded in water hosted by the coal seams within the Koogah Formation in the area of Bickman Coal Mines, which is the most northern mining location in the Hunter subregion (Aquatera, 2009). Here the mean total dissolved solid concentrations in the main coal seams were between 468 and 893 mg/L (see Aquatera, 2009, p. 2). Groundwater salinity decreased with depth of coal seam within the Koogah Formation in the area of Bickman Coal Mines.
Coal seams in the Hunter Valley also have high sulfur contents, particularly in the Greta Coal Measures mined in the Cessnock Coalfield, which may be a potential source of elevated water acidity.
Kellett et al. (1989) identified eight provinces in the Upper Hunter Valley, each with a unique hydrochemical signature. In addition to the southern New England Fold Belt (which is not included in the Hunter subregion), these include:
- The province formed by fractured rocks of the Triassic Narrabeen Group in the south and west of the assessment area. The groundwater is of low to moderate salinity (mean total soluble salts (TSS) = 600 mg/L) and dominated by ions of sodium (Na+), magnesium (Mg2+), chlorine (Cl-). Relatively low pH (around 6) may reflect high levels of iron (Mackie Environmental Research, 2006).
- The four provinces with the Permian fractured rocks forming the Central Lowlands and foot slopes, including:
- The groundwater of the Newcastle Coal Measures which is moderately saline (mean TSS = 1070 mg/L), dominated by Na+, Mg2 +, Cl-, HCO3- (bicarbonate). The fundamental difference between the Newcastle Coal Measures and the underlying Permian rocks is that groundwater quality is not largely influenced by marine environment during deposition stage
- Groundwater in the Wittingham Coal Measures (west) within the Jerrys Plains Subgroup containing the Cenozoic intrusion (the WI1 province) is of moderate to high salinity (mean TSS = 2300 mg/L), dominated by Na+, Cl-, HCO3-. The lower average salinity in this province and different chemical composition to groundwater in the eastern part of the Wittingham Coal Measures are believed to be a consequence of both a longer period of flushing by meteoric water and prior thermal mobilisation of connate marine fluids peripheral to the Cenozoic intrusives
- Groundwater in the Wittingham Coal Measures (east and south-east) where intrusive units are absent (the WI2 province) is the most saline in the Hunter River valley (mean TSS = 5700 mg/L). Hydrochemical facies grade from Na+, Cl-, HCO3- in the WI1 province to Na+, Cl- in the WI2 province, and mean
concentration is over ten times higher in the WI2-type groundwater - The GM province incorporates the largest proportion of marine sedimentary rocks in the Upper Hunter Valley, which consists of groundwater of the Maitland Group, Greta Coal Measures, and Dalwood Group. The chemistry of groundwater is dominated by Na+, Cl-, HCO3- and and is highly saline (mean TSS = 4300 mg/L). The strong marine signature is perpetuated through the upper seams of the Greta Coal Measures because these beds were saturated by oceanic water while they were still actively growing peats.
- Alluvial aquifers of the Hunter floodplains were divided into two provinces: alluvial aquifers upstream from the Hunter-Goulburn River confluence (HFP 1) and alluvial aquifers downstream from this confluence (HFP 2). This boundary reflects a distinct change in aquifer-built material (mean grain size and sorting) and as such aquifer provenance (HFP 1 – the coarse grained lithic sediments derived from the Carboniferous rocks; HFO 2 – the fine-grained dominantly sands eroded from the Triassic rocks). Though chemical composition of groundwater in these provinces is similar, groundwater salinity increases from mean TSS from 650 mg/L in the HFP 1 province to 840 mg/L in the HFP 2 province. This suggests intersection of the stream with groundwater from the Permian coal measures which is known to be saline.
Locally groundwater quality in the alluvial aquifers can be influenced by a number of factors. Kellett et al. (1989) also found that Cenozoic basalt contributed salt to the headwaters of the creeks in the Hunter. The pH of waters in the upper catchment is consistent with that of groundwater in contact with basalt and suggests that aluminosilicates may be weathering to produce dissolved silica and bicarbonate.
It was also observed that upward fluxes of groundwater from regolith aquifers to alluvial aquifers could lead to stratification of groundwater quality in the latter with high salinity levels at the base of the alluvial aquifers. As alluvial aquifers are intensively used, groundwater abstraction can enhance upward fluxes from underlying Permian units, particularly during drought periods, leading to groundwater quality deterioration. For example, during the drought of 2001 to 2004, when groundwater levels in alluvial aquifers dropped by 5 m, mean salinity in alluvial aquifers increased by 43% (NSW Department of Planning, 2005).
Mining operations in some locations led to the groundwater gradient reversing from alluvial to Permian aquifers (see Australasian Groundwater and Environmental Consultants Pty Ltd, 2013, p.37, Figure 14). This has led to a reduction in groundwater salinity in the alluvial aquifers (see Australasian Groundwater and Environmental Consultants Pty Ltd, 2013, p.38, Figure 15). As the changes in the groundwater gradient did not lead to any significant changes in groundwater levels, this is likely to be indicative of the compensating effect of alluvial aquifer recharge associated with river flow.
Mean values of groundwater salinity (electrical conductivity) observed by Beale et al. (2000) in 718 bores across all geologies of the Hunter subregion are shown in Table 11. The values indicate that groundwater salinity is elevated in the majority of the Hunter subregion. The exceptions are related to the alluvial aquifers in the west and the aquifers in the Narrabeen Group in the south-east of the subregion.
Table 11 Salinity of groundwater in various geological formations and provinces in the Hunter subregion
Source: Table 9 in Beale et al. (2000)
