BRINE CHEMISTRY OF GROUNDWATER DISCHARGE ZONES IN THE MURRAY BASIN, AUSTRALIA

Control of rising groundwater levels in Australia's Murray Basin requires the pumping of groundwater, and natural salt lakes seem ideal areas in which to dispose of the pumped water. A thorough understanding of the interaction of chemical and physical processes in particular geologic settings should be the basis for selection and management of appropriate disposal sites. In this work we use chemical modeling to predict the equilibrium evolutionary path of evaporating brines in two operating disposal basins and two natural salt lake complexes in the Murray Basin. Evaporative brines from an engineered and a natural, salt lake, groundwater disposal basin showed close correspondence with the predicted equilibrium evaporation pathways for the relevant inlet waters. There was no evidence of re-solution of salts during evaporation in the salt lake disposal basin. The;predicted evaporite suites for these two disposal basins differ because the inlet waters are derived from different sedimentary regions of the basin. In the two natural salt lake complexes studied, the comparison of field samples with predicted evaporation of regional groundwater indicates that brine evolution is a complex process of evaporation and re-solution of salts, and this is strongly influenced by the lithostratigraphic setting. Re-solution is favored where thick clayey sediments overlie the regional, unconfined aquifer, and such a site is favorable for disposal of pumped groundwater.