Aquaculture Groundwater Resource Atlas - Pilbara

Overview

The Pilbara region consists of granite - greenstone bedrock in the north, and the sedimentary and volcanic rocks of the Hamersley Basin in the south. The granite - greenstone areas lack regional groundwater resources. In the Hamersley Basin, large amounts of groundwater are used for mining related purposes, principally from calcrete and pisolite valley fill aquifers, and from solution cavities in the Wittenoom Dolomite. Groundwater is generally fresh, or brackish, except for the internally draining upper Fortescue Valley where groundwater below The Marsh is hypersaline.

The shallow calcrete and pisolite aquifers are the most easily exploited. Large amounts of groundwater are also discharged from dewatering operations.

References: Balleau (1973); Davidson (1975).

Distribution of aquifers in the Pilbara

Section showing aquifers in the Hamersley Range

Valley-fill alluvium, calcrete and pisolite

Valleys in the Hamersley Ranges are infilled with pisolite ironstone at the base, overlain by calcrete and alluvium. These valley–fill sediments overlie the Roy Hill Shale, Marra Mamba Iron Formation and the Wittenoom Dolomite, and range up to 60 metres in thickness. There are also concealed deep palaeochannels consisting of gravels underlying the calcrete and pisolite in the Fortescue Valley.

The water table may be 20 to 30 m below the surface, with the calcrete unsaturated in places. The water table also comes to the surface in springs at Millstream and at Weeli Wolli Springs. Groundwater is generally fresh, becoming brackish in parts of the Fortescue Valley. The groundwater is hypersaline in the internally draining Marsh in the upper part of the Fortescue valley near Roy Hill.

The groundwater resource associated with the Millstream calcrete, just upstream from Millstream and along the Fortescue River where the river provides direct recharge into the aquifer, is used for the West Pilbara town water supply scheme. Groundwater from calcrete and pisolite is used for Tom Price. Groundwater from the alluvium is used for Paraburdoo (and formerly Wittenoom) town water supplies.

Reference: Barnett and Commander (1986); Forrest and Coleman (1996).

Wittenoom Dolomite

The Wittenoom Dolomite occurs within the Hamersley Basin sequence, stratigraphically overlying the Marra Mamba Iron Formation, and being overlain by the shale of the Mt Sylvia Formation. The Wittenoom Dolomite commonly underlies the main valleys in the Hamersley Range, where it is subject to karst development (solution cavities). It is overlain by a thick sequence of valley filled sediments consisting of pisolite, calcrete and alluvium. The Wittenoom Dolomite is the most important aquifer in the province; it is highly transmissive and high yielding where there is karst development. Water levels may be fairly deep. The groundwater is generally fresh.

The aquifer has been developed for Tom Price and Marandoo water supply and has been investigated at other localities. There is likely to be significant development pressure on this aquifer for supply to iron ore operations.

Reference: Forrest and Coleman (1996); WRC Hydrogeology Report 61 by A.Wright (1997).

Hamersley Basin

The remaining Precambrian rocks of the Hamersley Basin are principally basaltic rocks, shales and iron formation in which groundwater is contained within fractures. The groundwater may be deep below the surface, and is generally fresh. The main use is for mining and mine dewatering from iron ore mines. Bores have also been drilled for road and railway construction. There will be increasing dewatering from the fractured rocks around iron ore mines as the pits become deeper.

Reference: Forrest and Coleman (1996); WRC Hydrogeology Report 61 by A.Wright (1997).

North Pilbara Granite Greenstone Terrane

The Archaean granite–greenstone terrane consists of circular areas of low lying granite, surrounded by ridges of greenstone, overlain by thin alluvial sediments in the river valleys. The water table is generally within 5 to 10 metres of the surface in the granitic areas, but may be quite deep below the greenstone hills. The major aquifers within the greenstones are thin quartz veins, and chert layers.

Groundwater is mainly fresh, ranging up to brackish towards the coast. Bore yields vary depending on intersection of fractures. Pastoral bores and wells draw water from both the fractured rock and the overlying weathered zone. Marble Bar town water supply is drawn from bores in acid volcanic rocks. Nullagine's town water supply is drawn from both shallow alluvium (less than 12 m deep) and fractured sandstones. Water has also been produced by dewatering from the iron ore mines in the Goldsworthy–Shay Gap–Yarrie area.

There are not considered to be any major regional groundwater resources in the Pilbara fractured rock. Development is likely to be on a local basis principally for mining and town water supply.

Reference: Forrest and Coleman (1996); WRC Hydrogeology Report 61 by A.Wright.