CHEMICAL DIVERSITY OF NATURAL WATERS IN THE ACID SALINE SYSTEMS OF SOUTH WESTERN AUSTRALIA

The hydrogeochemistry of ephemeral lakes in south Western Australia (WA) provides a rare example of an extremely varied natural acid saline system. We have investigated 58 lakes and associated shallow groundwaters on the Archean Yilgarn Craton over three field seasons. Chemical evaluation of the fluids reveals a new spectrum of diverse hydrogeochemical conditions not typically observed in evaporative or acid systems. Unlike other known natural acid lakes elsewhere in the world, the WA lakes are not fed by volcanic or hydrothermal waters, or waters in contact with weathering massive sulfide deposits. And, unlike typical saline systems, the pH of the fluids is as low as 1.5. Approximately 40% of the lakes, and 84% of the measured groundwaters, are extremely acidic (pH < 4). The waters range from nearly fresh to hypersaline with total dissolved solids > 28%. Water analyses have included field measurements of temperature, pH, and salinity, and laboratory characterization of major ions (IC), major and trace elements (XRF, ICP-MS, ICP-OES), and stable isotopes (dD, d¹⁸O). The fluids are typically Na-Mg-Cl-SO₄ brines with variable yet locally high amounts of Ca, K, Al, Fe, Si, and Br. The fluid compositions are unusual. For example, in some fluids, the amount of Al >> Ca, the amount of Br > K, and comparison of total S to SO₄^- values suggest the presence of other uncommon S-bearing species. The spatial and temporal variations in water chemistry are considered in relation to differences in interacting systems including regional geology, host rock mineralogy and weathering, sedimentology, and biology. The geology and hydrogeochemistry in WA appears to be quite different from previously studied acid saline systems in Victoria. Detailed characterization of the WA fluids will allow for expansion of existing models for brine evolution and a better understanding of how variations in lake chemistry can influence resulting lake deposits.