Geochemistry and hydrologic processes in the evolution of hypersaline brines at a ground water discharge area: Raak Plains, Murray Darling Basin, Australia

It is now recognized that significant amounts of groundwater leakage from natural groundwater discharge zones has the potential to carry saline brine into down-gradient regional groundwaters. At Raak Plain, in the Murray Basin of south-eastern Australia, groundwater is discharged by evaporation through a complex system of wind-deflated playa lakes. Three methods are compared to determine the impact of the entire system on regional groundwater. A simple water balance, using chloride and stable isotope profiles to calculate recharge and discharge, is used to determine the amount of water leaving Raak Plain via the groundwater flow path. Large uncertainties, including large spatial variations in discharge and a high dependence on accurate recharge rate estimates, highlight the inadequacy of the simple model for the Raak Plain system. An evaporation/leakage model based on an oxygen-18 mass balance for a single salt lake also indicates large variations in leakage from individual salt lakes, further discrediting a simple model approach. Both methods do agree however that a large proportion of inflow is lost from the system and Cl/Br mass ratios across the site also indicate brine sinking