A CHEMICAL-MODEL FOR THE EVOLUTION OF AUSTRALIAN SODIUM-CHLORIDE LAKE BRINES

We present a chemical model which calculates the composition of waters as they are progressively concentrated from various starting compositions to determine the important processes leading to the development of Na-Cl type brines typical of Australia. The model can be extended to other continental settings which are characterised by low rainfall, low topographic relief and highly weathered regolith. The model extends previous brine evolution models to include clay mineral-solution reactions as a means of controlling aqueous silica concentrations and we suppress K+ concentrations via clay mineral interactions. Our model also evaluates the importance of elevated partial pressures of CO2 which mimics situations encountered in groundwaterfed systems. Several important controls on major ion evolution were observed: (1) Initial Ca/Alkalinity ratios must be greater than 0.5 to generate Cl-rich brines of circum-neutral pH. One-thousand fold concentration of average Australian river water (Ca/Alk = 0.31) generates alkaline, high pH, brines while the same ionic strength brine derived from mean Australian rainwater (Ca/Alk = 1.2) generates a brine which is two orders of magnitude lower in alkalinity. (2) Elevated pCO2 values which occur in subsurface waters suppresses precipitation of carbonate minerals. This allows Ca to build up more quickly thereby accelerating the onset of gypsum precipitation. (3) Aqueous silica concentrations are controlled by equilibrium with clay minerals in the "dilute" waters but reach amorphous silica saturation in concentrated brines because of the reduced activity of the neutral SiO2(aq) species in high ionic-strength brines. (4) Continual dissolution and reprecipitation of salts in the groundwaters or efflorescent crusts are responsible for minor- and trace-element ratios very different from that of the initial starting waters.