A STABLE ISOTOPE INVESTIGATION OF GROUNDWATER SURFACE-WATER INTERACTIONS AT LAKE TYRRELL, VICTORIA, AUSTRALIA

The stable isotope composition of regional groundwaters around Lake Tyrrell, Victoria, indicates that the groundwaters are derived entirely from meteoric water that has undergone some degree of evaporation during recharge. Seepage waters at the western margin of Lake Tyrrell, Victoria, have a characteristic chemical and isotopic signature of regional groundwater flowing from the northeast and east. On a transect of shallow groundwaters across the lake, the isotopic composition is increasingly enriched in the heavy isotopes toward the centre of the Lake, but define a steeper slope on a delta-D-delta-O-18 plot than that of the regional groundwaters. The stable isotopic compositions of the shallow groundwaters within the seepage zone are typical of those due to evaporative enrichment currently occurring with a small component of vertical mixing with the deeper reflux brines. Reflux brines below the lake are relatively more enriched in O-18 than the brines forming now at the lake surface. The reflux brines appear to have undergone several cycles of evaporation and admixture of regional groundwater as have waters within the mixing zone between regional groundwater and reflux brines.