GROUNDWATER EVOLUTION WITHIN A CATCHMENT AFFECTED BY DRYLAND SALINITY, SOUTHEASTERN AUSTRALIA

The Willaura catchment in western Victoria comprises Palaeozoic basement overlain by Cenozoic basalts and alluvium, and is strongly affected by saline springs, scalds and lakes. In the main recharge area, along the flanks of the Grampians range, the groundwater is <50 years old, and becomes progressively older downflow to the southeast, reaching 8ka. The groundwater increases in salinity downflow, becoming more saline than seawater. The salinity reflects mainly the concentrations of Na+ and Cl-, and is derived predominantly from rainfall; 36Cl data from an adjacent area show that there is no significant input from rock weathering or connate water. Rainfall is concentrated within the soil zone by evapotranspiration. Along the flow path, the groundwater progressively receives saline infiltration through the soil, resulting in the downflow increase in salinity.

The stable isotope compositions of Willaura groundwater plot more or less on the local meteoric water line, but the groundwater becomes heavier downflow. This probably reflects the progressive addition of soil water that has undergone evaporation under high humidities within the soil. Under these conditions the gradient of the evaporation line approximates that of the meteoric water line, so the groundwater compositions will still plot on the meteoric line even though they have undergone evaporation. The stable isotope composition of groundwater in the recharge areas is close to the average composition of winter rainfall, indicating that most recharge occurs during winter and spring.

There is a clear trend of increasing groundwater pH downflow, probably reflecting cation exchange on clays, particularly Na+ exchange for Ca2+ in smectites. A decrease in Si/Cl ratio downflow may be due to the replacement of kaolinite by smectite, and perhaps also the growth of authigenic feldspars. Within the basalt aquifer, weathering of silicates increases the Ca/Cl, Si/Cl and HCO3/Cl groundwater ratios, but the relative contribution of the ions released by these reactions to the overall groundwater salinity is very small.