GROUNDWATER-FLOW REGIME WITHIN THE GAMBIER EMBAYMENT OF THE OTWAY BASIN, AUSTRALIA - EVIDENCE FROM HYDRAULICS AND HYDROCHEMISTRY

A conceptual groundwater flow model for the Cainozoic sequence of the Gambier Embayment of the Otway Basin, South Australia, is developed using hydraulic head distribution, environmental isotopes and hydrochemical data. Local flow cells occur within the upper sequence of the Gambier unconfined aquifer and are recognised by local relief of the water table and as rapid lateral variations in chloride, C-14, deltaH-2 and deltaO-18 concentrations, indicating that regional flow is not dominant in the upper portion of the unconfined aquifer. These localised systems are topographically controlled and have developed in response to undulating stranded coastal dune ridges of the Bridgewater Formation. Recharge areas correspond to dune highs while discharge is focused towards the lower lying interdunal regions. Total dissolved solids concentrations are controlled by evapotranspiration processes during recharge and ion ratios are modified by water rock interactions within the groundwater system. Groundwaters are more dilute in the southern section of the basin where the surplus of precipitation over evapotranspiration in the winter months is greater than that in the northern section of the basin. A regional flow system with interaquifer mixing has developed within the confined Dilwyn aquifer system. Recharge to the Dilwyn aquifer has been identified downgradient from the basin margin via downward leakage from an intermediate flow system within the Gambier unconfined aquifer to the deeper regional flow system of the confined Dilwyn aquifer. Interaquifer mixing appears to be geologically controlled; in the south by the elevated Gambier axis and in the north by the Kanawinka Fault. Downgradient of the zero head difference line, lateral flow appears to be the dominant transport mechanism within the Dilwyn aquifer as indicated by constant chloride concentrations, monotonic decrease in radiocarbon concentration and more negative deltaH-2 and deltaO-18 composition. the Dilwyn aquifer as indicated by constant chloride concentrations, monotonic decrease in radiocarbon concentration and more negative deltaH-2 and deltaO-18 composition.