SOLUTE BUDGET FOR AN ARID-ZONE GROUNDWATER SYSTEM, LAKE AMADEUS, CENTRAL AUSTRALIA

Lake Amadeus, a large playa in central Australia, is the locus of groundwater discharge from a regional groundwater flow system extending over a catchment of 66 000 km2. Most of the groundwater flow occurs in shallow aquifers in Cainozoic sediments, but some flow takes place in deeper aquifers in fractured Proterozoic and Palaeozoic rocks. The regional groundwaters evolve from fresh and brackish waters of the HCO3-Cl and Cl-HCO3-SO4 type to hypersaline playa brines of the Cl-Na type. The brines are saturated with respect to calcite, dolomite, gypsum and glauberite, and are close to saturation with respect to halite. The groundwaters evolve to brines by concurrent processes of dissolution of aquifer matrix, evaporative concentration and mineral precipitation. Groundwater inflow is estimated as 17% of the total water inflow to the playa, the remainder being rainwater. However, nearly 99% of the salt in the brine pool is derived from groundwater. A solute budget for shallow groundwaters calculated on the basis of chloride concentration factors shows that the playa brines are nearly 300 times more concentrated than the recharge groundwaters. Hydrochemical processes in the flow system lead to the virtually complete loss of HCO3, Ca and SiO2, which are taken up in mineral precipitation. Other ions remain in solution, but there are net losses of Mg,SO4 and K relative to chloride, which is conserved.