Use of Hydrochemistry, Stable Isotope, Radiocarbon, 222Rn and Terrigenic 4He to Study the Geochemical Processes and the Mode of Vertical Leakage to the Gambier Basin Tertiary Confined Sand Aquifer, South Australia.

The mode of vertical recharge to aquifers is important to the application of appropriate recharge estimation methods. This study identifies the origin, geochemical evolution and mode of vertical leakage to the Gambier Basin confined aquifer, south east of South Australia. The recharge zone spans areas of the Glencoe-Nangwarry-Nagwarry (GNN). The Hundreds of Glencoe and Nangwarry are in South Australia, and the Parish of Nagwarry adjoins Nangwarry in western Victoria. The plot of stable isotopes of water molecules, δ²H versus δ18O, indicates that local rainfall with minor surface evaporation is the source of recharge. The results of hydrochemical analysis indicate that the sources of ions in the recharge zone groundwater are derived from carbonate and silicate weathering with cation exchange. The majority of water types (66% of samples) within the South Australian part of the recharge zone show Ca-Na-HCO₃-Cl due to carbonate dissolution processes, and about 83% of samples within the Victorian part of the recharge zone show Na-Ca-HCO₃-Cl water types, indicating cation exchange or mixing with other waters. The influence of faults on vertical leakage was studied at eight sites located in the Nangwarry and Nagwarry area using electrical conductivity logging, measuring the concentration of radiocarbon activity, δ18O, ²²²Rn and terrigenic ⁴He in the vertical profiles. Results show that regardless of land use in the study area, the interconnection of the unconfined Tertiary limestone aquifer with the Tertiary confined sand aquifer occurs, via both diffuse and preferential flows. Thus, the application of conventional vertical leakage estimation methods using Darcy's equation or the application of tracer techniques may be inappropriate unless the duality of the flow system is considered. [ABSTRACT FROM AUTHOR]