Lake George is a fluctuating closed lake in the eastern highlands of Australia. Groundwater in the 932 km2 catchment is mainly of low salinity, but high salinity groundwater is evident beneath the lake bed. Porewater analyses reveal a salinity profile in a clay aquitard 50 m thick, beneath the lake bed, that has the characteristics of diffusion but is also influenced by mixing with (a) lakewaters infiltrating downwards , and (b) groundwaters rising upwards under pressure. Hydrostatic balance is apparently achieved at a depth of 10m below the lake bed; this coincides with the maximum porewater salinity of 40-42 000 mg/L total dissolved solids (TDS). Salt accumulates in lake-full periods and concentrates by evaporation of lake waters. A net loss of salt is evident in recessive phases of the lake. During drying and refilling episodes, the lake water surface becomes the water table and vice versa. Salt is concentrated in the capillary zone during dry periods and is also transmitted downwards by diffusion. In recessive phases there is infiltration of lake water which has a freshening effect on the top of the salinity profile, but also transmits some salt. The processes of salt accumulation and diffusion in the aquitard may have operated for much of the Quaternary. In the Lake George basin , five types of water are characterised: surface water in creeks and in the lake; groundwater in the catchment, and in sandy aquifers beneath the lake and bed; and porewater in the clay aquitard underlying the lake bed. Creek water and catchment groundwater is fresh to brackish and generally of the HCO3-Cl or CI-HCO3 type with Na and Mg as major cations. The lake water is alkaline, of varying salinity up to 45 000 mg/L TDS, and of the Cl-Na type. Varying ionic concentrations in these waters are the result of evaporative concentration and precipitation of carbonate minerals. Beneath the lake bed are saline aquifer groundwaters (up to 48000 mg/L TDS) and aquitard porewaters (up to 42000 mg/L TDS); these are Cl-Na waters with appreciable Mg. Sulphate is also retained in the groundwater which evolves chemically through interaction with the aquifer matrix. Hydrochemical evolutionary pathways are different for groundwaters and for surface waters. In surface waters (creek and lake), dolomite and calcite saturation is achieved early but these waters are undersaturated with gypsum. In the groundwaters (catchment and lake bed), saturation with dolomite and calcite is achieved early but equilibrium relationships are more complex. Shallow groundwaters, down to 15 m beneath the lake bed, show evidence of mixing with infiltrating lake waters, and this has retarded mineral precipitation. The deeper, saline ground waters are close to saturation with gypsum . Stable isotope data also indicate mixing down to 15m below the lake bed between evaporated lake waters infiltrating downwards and saline groundwaters under upwards pressure. Chlorine-36 determinations indicate that younger groundwaters at 10-50 m below the lake bed overlie ground waters at 100 m that are tens of thousands of years old.
