Using geochemistry to identify and quantify the sources, distribution, and fluxes of baseflow to an intermittent river impacted by climate change: The upper Wimmera River, southeast Australia

Documenting the distribution, sources and fluxes of baseflow discharge into rivers is important for their management and for maintaining ecosystem health. This study uses major ion geochemistry, ²²²Rn, and ³H to differentiate between the input of low-salinity near-river waters (bank storage and return waters and/or interflow) and regional groundwater in an intermittent river from southeast Australia that is undergoing long-term changes in flow resulting from climate change. Baseflow discharge calculated by ²²²Rn mass balance was up to 1.3 m³/m/day in the high flow period in July 2019 and up to 0.1 m³/m/day at low flow conditions in November 2019. The distribution of ²²²Rn activities implies higher baseflow fluxes in the upper and middle reaches that have relatively steep topography and higher hydraulic gradients. The lower reaches received less baseflow due to subdued topography and fine-grained sediments. The observation that Cl concentrations did not increase uniformly downstream, however, implies that much of the baseflow may comprise bank return flow or interflow. This conclusion is also consistent with water mass balance calculations and the observation that ³H activities (1.85-3.00 TU) in the river were higher than in the groundwater (<0.45 TU). Intermittent streams are likely to be less well connected to regional groundwater, and thus near-river water stores will be more important in sustaining streamflow during dry periods than regional groundwater. These rivers and their ecosystems may be less susceptible to the impacts of groundwater extraction and the near-river waters will provide a buffer zone from potentially contaminated regional groundwater. However, these near river stores are vulnerable to short-term climate variability, and changes to flow regimes resulting from climate change may significantly impact water supplies and ecosystem health.