Aquaculture Groundwater Resource Atlas - Gascoyne – Murchison

Overview

The Gascoyne – Murchison region, which here includes the Ashburton River and part of the Yarra Yarra Catchments, contains a variety of Precambrian bedrock aquifers ranging from granite, to metamorphic and sedimentary rocks, overlain by alluvium and calcrete. Groundwater is used for stock water, mining and minor town water supply.

Groundwater salinity generally decreases northwards, from hypersaline in the upper parts of the Yarra Yarra and Murchison Catchments, to fresh to brackish in the Gascoyne catchment.

Calcrete aquifers have the most potential for shallow, large supplies, of fresh - brackish (in the Gascoyne Catchment) or brackish to saline groundwater (in the Murchison Catchment). Calcrete aquifers are mapped in the ‘Hydrogeology‘ dataset on the Water and Rivers Commission Hydrogeological Atlas.

References: GSWA Record 1992/6 by A. Laws; WRC Hydrogeology Report HR 129 by S. Johnson and D.P. Commander (1999); WRC Hydrogeology Report HR 191 by Y. Thomson (2001).

Alluvium

Alluvial and surficial sediments lie along the main river valleys of the Yilgarn Craton and adjacent Proterozoic basins. The alluvium overlies calcrete, palaeochannels, and fractured rock, and is probably up to about 30 metres thick. The alluvium probably consists of silts, sands and clays but mostly fine-grained material. It is not known to be a major aquifer, and is utilised by wells and bores for stock watering. The groundwater salinity is variable, being generally fresh in the valley sides with salinity increasing into the centre of the valleys.

Calcrete

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

Calcrete aquifers occur principally in the Murchison, from Yalgoo northwards into the Gascoyne and Ashburton, although calcrete bodies in the Ashburton are mostly eroded and are unsaturated. The salinity is variable, generally brackish to saline in the Murchison, and fresh to brackish in the Gascoyne. Groundwater from calcrete has been used at Cue for the town water supply, where it is brackish, and for limited irrigation of fodder crops.

Palaeochannel

Palaeochannels, which are the basal infilling of ancient river valleys, are poorly explored in the Murchison, and the only major development is at Windimurra for mining. Groundwater salinity is generally high, with hypersaline groundwater present in areas near salt lakes. It is not known if potentially high yielding palaeochannel sands are present near the Murchison and Gascoyne Rivers, where there may be potential for large supplies of brackish groundwater.

Fractured rock

Fractured rock aquifers include a large variety of metamorphic and igneous rocks ranging from the sedimentary rocks in the Ashburton, Bangemall, and Nabberu Basins to the granite–greenstone terrain of the Yilgarn Craton. Fractured rock aquifers are the main source of water for mining in the Murchison and East Murchison, and groundwater salinity is variable, ranging from brackish to hypersaline.