Aquaculture Groundwater Resource Atlas - Goldfields

Overview

Groundwater in the Goldfields occurs in alluvium, calcrete, palaeochannels and fractured and weathered bedrock. The calcrete, which occurs north of Menzies, has the most potential for shallow, large supplies, of brackish to saline groundwater. Saline or hypersaline groundwater from palaeochannels or bedrock aquifers is utilised by the mining industry. Stock supplies, in the north of the area where the salinity is suitable, are drawn by low yielding bores and wells in alluvium and weathered bedrock.

Palaeochannels in the eastern and northeastern Goldfields

Diagrammatic section across a palaeochannel

References: Allen (1996); Commander and others (1992); Johnson (1997); Johnson and others (1999); Morgan 1965; Sanders (1969 1971 1972,1973); Sanders and Harley (1971); Water Authority of Western Australia (1994, Goldfields Groundwater Area Management Plan) 1:250 000 Hydrogeological series maps for Boorabbin, Widgiemooltha, Kalgoorlie, Kurnalpi, Laverton, Leonora, Sir Samuel.

Alluvium

Alluvium of clayey sands and clay occurs in the main valleys, reaching a maximum thickness of 60 m in the north of the area. The alluvium is generally used for stock water supply in the northern part of the area where the groundwater salinity is relatively low. South of Menzies, the groundwater is too saline for stock. Groundwater in the alluvium is fresh on the valley sides and increases to hypersaline near salt lakes.

Bore yields in alluvium are likely to be low, but the alluvium also contributes water to bores which are screened both in the palaeochannels and in the overlying alluvium.

Calcrete

Calcrete is a chemically precipitated limestone, occurring in pods within the alluvium, north of Menzies, close to the centres of the valleys, and commonly adjacent to salt lakes. It ranges in thickness up to about 10 to 15 metres, and the water table is generally within 5 m of the surface. Bore yields from calcrete can exceed 1000 kilolitres/day where there is development of solution voids below the water table.

Groundwater salinity is variable. At Wiluna, where it is used for town water supply, the groundwater has low salinity. Elsewhere, especially adjacent to salt lakes, groundwater in calcrete may be brackish or saline. There is significant development potential for brackish to saline water.

Palaeochannel

The palaeochannels are the major aquifers in the area utilised by the mining industry. They represent the infilling of former rivers. The palaeochannels range in thickness up to 70 metres and the main palaeochannels are up to a kilometre wide. The tributary palaeochannels are substantially smaller. In the northern part of the area (Leonora to Wiluna) their depths range from 60 metres to 130 metres. In the southern part of the area (around Kalgoorlie) they are shallower, ranging from about 20 to 60 metres.

Palaeochannels principally comprise quartz sand with gravel at the base. Bore yields may be up to 1000 kilolitres/day or more. The tributaries north of Menzies contain low salinity water only in their upstream parts. The distribution of potable water is restricted in the northern part of the area. In the southern part of the area tributaries have not been identified as groundwater sources. The trunk palaeochannels contain saline to hypersaline groundwater, with salinity increasing below salt lakes but decreasing through access of lower salinity water from the tributaries.

The palaeochannel resources in the Roe and the Carey Palaeodrainages are extensively developed for mining. The saline groundwater resources are used principally for gold mining and for nickel processing at Cawse and Bulong. In the north, the Murrin Murrin nickel operation utilises brackish to saline water from tributaries.

Fractured rock

Groundwater occurs in the fractured and weathered Archaean bedrock. It is generally pumped only for dewatering around mines, though there are some water supply borefields drawing water from fault and fracture zones. Bore yields are highly variable, depending mainly on fractures, although there are some formations, such as weathered ultramafic rocks which have development of secondary porosity.

Groundwater salinity generally depends on topographical position and depth. Groundwater is fresh near catchment divides and hypersaline beneath the main valleys, and also becomes more saline with depth.

Owing to the difficulty of locating groundwater supplies from fractured rock aquifers, development is likely to be restricted to the vicinity of mines. Some decommissioned open pit mines may have substantial potential from inflows