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 Macquarie River. The lower alluvial aquifer of the Southern Paleovalley and the upper Quaternary aquifer of the Macquarie Marshes are distinct hydrostratigraphic units (Macaulay and Kellett, 2009). Groundwater salinity (as determined from electrical conductivity) of both the Upper and Lower alluvial aquifers of the Upper Macquarie is summarised in Table 9. In general, groundwater salinity is less in the Lower aquifer than in the Upper aquifer, and is lower upstream of Dubbo than downstream (Smithson, 2010).

EMGA Mitchell McLennan Pty Ltd (2012) describe groundwater electrical conductivities (EC) of the upper Talbragar alluvial deposits between 2009 and 2012 in the area immediately adjacent to the Cobbora Coal Project. EC was reported to range from approximately 1850 to 6950 mS/cm, with a mean of approximately 3650 mS/cm. The pH ranged from 6.2 to 8.0, with a mean of 7.1. MDBA (2012) depicts the alluvial deposits of the Castlereagh and Talbragar rivers as having groundwater salinity of mostly less than 1500 mg/L.

Table 9 Upper and Lower Macquarie alluvial aquifer salinity


Formation

Location

Range of electrical conductivity (mS/cm)

Mean electrical conductivity (mS/cm)

Range of salinity (mg/L)

Mean salinity (mg/L)

Upper alluvial aquifer

Upstream of Dubbo

300–1,718

896

192–1,100

573

Downstream of Dubbo

288–18,700

2,433

184–11,968

1,557

Lower alluvial aquifer

Upstream of Dubbo

251–832

479

161–532

307

Downstream of Dubbo

312–804

519

200–515

332

Basement

Sandstone

599–15,700

5,309

383–10,048

3,398

Source data: Smithson (2010)

The Southern Paleovalley contains a high yielding basal alluvial aquifer, from which good quality (EC <1000 mS/cm) groundwater is extracted for irrigation (Macaulay and Kellett, 2009). However, much of the area contains higher salinities, and levels of up to 26,900 mS/cm were observed at depths of about 10 to 15 m beneath the Macquarie Marshes. MDBA (2012) depicts the upper formations (down to 50 m depth) in the Lower Macquarie river basin as mostly having groundwater with salinity less than 1,500 mg/L, but with some zones, especially towards the Bogan River, having salinity over 3,000 mg/L and as high as 14,000 mg/L.

Cenozoic Basalts

MDBA (2012) depicts the Warrumbungle Basalt groundwater salinity as being less than 1500 mg/L, with the Liverpool Ranges being similar but with some higher salinity groundwater of up to 3000 mg/L within the western extremities.

1.1.4.3.2 Great Artesian Basin

Groundwater quality of the GAB varies but generally has salinity of 500 to 1500 mg/L in the Lower Cretaceous-Jurassic aquifers (Habermehl, 2002; Kellett and Stewart, 2013). The groundwater is of a Na-HCO3-Cl type, and generally suitable for domestic, town supply and stock use. However it is unsuitable for irrigation in most areas due to its high sodium adsorption ratio. Kuske et al. (2011) identified the Cadna-owie Formation within the Coonamble Embayment as having salinity mostly in the range of 250 to 1000 mg/L. Macaulay and Kellett (2009) identify water in the Drildool and Keelindi beds with electrical conductivities of 1145 mS/cm and 1070 mS/cm within the Central West subregion.

MDBA (2012) depicts the Oxley sub-basin as having groundwater salinity varying up to 3,000 mg/L in the upper reaches of the Castlereagh and Talbragar Rivers, and increasing downstream to levels of 3,000 to 14,000 mg/L in areas between Dunedoo and Gilgandra.

Areas of high conductivity, inferred to represent high salinities, are shown in Macaulay and Kellett (2009) along the Bogan River, in the west of the subregion. However, detailed discussion was not provided.

1.1.4.3.3 Gunnedah Basin

Varying groundwater quality conditions have been reported by EMGA Mitchell McLennan Pty Ltd (2012) following drilling in the vicinity of the proposed Cobbora coal mine in the upper reaches of the Talbragar catchment. Data are summarised in Table 10.

Table 10 Groundwater quality in the Gunnedah Basin – proposed Cobbora mine


Aquifer group

EC range (mS/cm)

EC mean (mS/cm)

pH range

pH mean

Triassic

827–9060

3993

5.8–7.7

6.5

Permian

556–9030

1618

5.3–12.2

6.5

Source data: EMGA Mitchell McLennan Pty Ltd (2012)

Last updated:
5 January 2018