Differences in groundwater and chloride residence times in saline groundwater: The Barwon River Catchment of Southeast Australia

The residence times of groundwater and chloride and the processes contributing to the development of saline (total dissolved solids (TDS) up to 45,379 mg/L) groundwater within the Barwon River Catchment of southeast Australia were investigated using major ion, stable isotope (δ¹⁸O, δ²H, and δ¹³C) and radioactive isotope (³H, ¹⁴C, ³⁶Cl) geochemistry. The elevated groundwater salinity in the region is primarily due to evapotranspiration and recycling of solutes in saline lakes with minor contributions from weathering of halite, silicate and calcite minerals. Groundwater residence times estimated from ¹⁴C vary from modern to ~ 20 ka; for groundwater with lower ¹⁴C activities, the estimated residence times vary significantly depending on the assumed flow model and the ¹⁴C activity of recharge. Chloride residence times downgradient of Lake Murdeduke (a saline through-flow lake in the centre of the catchment) are greater than the corresponding groundwater residence times due to the recycling of Cl within the lake. Precise estimates of chloride residence time could not be determined using ³⁶Cl due to R³⁶Cl in precipitation being lower than that of groundwater. This is most likely due to R³⁶Cl values in rainfall having been higher in the past than they are at present due to climate variability. δ¹⁸O, δ²H, and δ¹³C values also suggest that the region has experienced increasingly more evaporative conditions with time. The results of this study demonstrate that, while Cl is a useful tracer of hydrological processes, it must be applied carefully in arid and semi-arid regions of the world. In particular, recharge rates calculated using chloride mass balance may be underestimated where recycling of Cl has occurred.