Documenting the sources of water that contribute to streamflow is important for understanding and managing river catchments. Especially in semi-arid areas, groundwater inflows commonly contribute a significant proportion of river flow. For many rivers, the distribution of groundwater inflows and the composition of the groundwater is poorly known, which hampers the understanding of catchment functioning. Intermittent rivers, such as the Wimmera River, cease to flow during summer months and commonly consist of disconnected groundwater-fed pools. These pools provide a unique opportunity to investigate the distribution and geochemistry of groundwater inflows to the river. In this study, major ion concentrations, stable isotope ratios and 222Rn were analysed from shallow pools (depth < 1.5m) along the Wimmera River.
The pool waters have EC values of 2430-15,330µS/cm, Cl concentrations of 800-6210 mg/L, and Na concentrations between 440 and 2960 mg/L. The water is sodium chloride type, similar to the regional groundwater. δ2H and δ18O values are -24‰ to +37‰ and -3.3‰ to +10.3‰, respectively and define an array with a slope 4.5. The stable isotope values and major ion geochemistry demonstrate that the pools are largely fed by groundwater that has subsequently evaporated. 222Rn activities up to 2380 Bq/m3 also imply that the pools are derived from groundwater.
Understanding the distribution and geochemistry of groundwater inflows permits a better analysis of groundwater-river interaction using chemical mass balance at times when the river is flowing. Future work will involve using geochemical tracers including Sr isotopes and 3H to assess whether the groundwater originates close to the river or is part of the regional groundwater system.