Features and effects of some acid-saline groundwaters of southern Australia

Abstract

Many salt lakes have formed in southem Australia where regional saline groundwaters form discharge zones. Some of these groundwaters are characterized by acidity and high salinity. These are commonly found where underlying Tertiary sediments are sulfide-rich. When waters from the resulting formations come to the surface or interact with oxidized meteoric water, acid groundwaters result from oxidation of the sulfide and/or the dissolved ferrous iron. In this paper examples of such waters around Lake Tyrrell, Victoria and Lake Maurice, South Australia, are reviewed. The acid-saline groundwaters typically have dissolved solids of 3060 mg/L and pH commonly <4.5. Many contain high concentrations of Fe and other metals, leached from local sediments. The combination of acidity and salinity also releases radium, a short-life daughter nuclide of uranium and thorium, by a process of cation exchange. Around salt-lakes, these acidic waters often emerge at the surface in marginal spring zones where the low density (s.g. -1.04) regional water flows out over the denser (s.g. -1.16) lake brines. In the spring zones examined, large amounts of iron are commonly precipitated. In a few places minerals of the alunite-jarosite family are formed which can trap many other metals, including radium which generates radiometric anomalies. Such anomalies may however be short-lived. The age of the present salt lakes is <40000 years but in that time very large amounts of many metals are estimated to have been deposited into the lakes but, apart from iron, this appears to be dispersed and to not have formed significant accumulations. Evaporation concentrates the least soluble salts (gypsum and some halite) on the surface of the lakes. The lake brines contain most of the soluble salts and form a colunm within the porous sediments which is held in place by hydrostatic forces around the salt lakes. These brines are near neutral in pH.