Hydrogeology and groundwater resources of the Lake Amadeus and Ayers Rock region, Northern Territory

A hydrogeological map-set of the Lake Amadeus-Ayers Rock region in central Australia has been prepared as a pilot project for a national groundwater resources inventory. The maps in the set were compiled on a computer graphics system at 1:250000 scale from layers representing topography, bedrock geology, surficial geology, hydrodynamics, hydrochemistry, and water-bore locations. The quantitative information was obtained from a microcomputer database, and the geological and topographic information was digitised from specially revised maps. As the hydrogeological data are sparse it was decided to publish the main maps at 1:500000 scale. For three areas where more detailed information was available, inset maps at 1:100000 scale are presented. The region mapped is arid; annual rainfall is about 250 mm and potential evaporation close to 3000 mm. Groundwater recharge is of the order of I mm/year. Important aquifers are surficial Cainozoic sand and calcrete, and bedrock sandstone units including the Palaeozoic Pacoota Sandstone and Mereenie Sandstone. Groundwater varies in quality from 0.4 g/L to more than 200 g/L total dissolved solids. Groundwater converges from the north and south towards a southeast-trending chain of playas, where it discharges by evaporation. Hydrochemical evolution is apparent along groundwater flow paths. Fresher water, generally bicarbonate-rich, occurs close to the hilly recharge areas and in the calcrete. This evolves to highly saline chloride-rich brines in the playas, by processes of solution, precipitation of minerals, and evaporative concentration. Groundwater supplies have been developed for the Yulara tourist resort, the largest settlement in the area, although the water, derived from Cainozoic sands, is desalinated for use in domestic supplies. For a proposed tourist resort at Kings Canyon, a water supply is being developed from bedrock units, mainly the Pacoota Sandstone. Single bores are scattered through the region, supplying cattle stations and Aboriginal settlements. In this hydrogeologically complex region, extrapolation from conceptually known areas is difficult. The detailed assessment of ground water resources will require additional drilling.

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