Environmental isotopes as indicators of inter-aquifer mixing, Wimmera region, Murray Basin, Southeast Australia

Complex groundwater flow systems in confined aquifers that result from geological structures, stratigraphic changes, or the absence of efficient aquitards are difficult to constrain using physical parameters alone. Despite a relatively simple aquifer configuration, the distribution of groundwater total dissolved solids (TDS) concentrations, δ13C values, 87Sr/86Sr ratios, and 14C activities (a14C) in groundwater in the Wimmera region of the southern Murray Basin implies that considerable inter-aquifer flow has occurred. Given the presence of both silicate and carbonate aquifers, δ13C values and 87Sr/86Sr ratios are the key parameters that demonstrate inter-aquifer flow. Locally, between 40 and 95% of water from one aquifer has infiltrated the underlying aquifer homogenising many aspects of the groundwater geochemistry. Groundwater residence times estimated from a14C range from modern to > 30 ka and the distribution of 14C residence times confirm that inter-aquifer flow is regional scale and long term. Recharge of the deepest aquifers occurs across a broad region and not solely at the basin margins. Vertical leakage rates are ~ 6–10 × 10−3 m/year and long-term recharge rates 0.1–0.2 mm/year (< 1% of annual rainfall). Groundwater from this region is a locally valuable resource and failure to recognise that inter-aquifer flow occurs threatens the sustainability of this resource.

Research Highlights
► Southern Murray Basin groundwater shows inter-aquifer mixing.
► Sr and C isotopes imply mixing ratios are up to 90%.
► Additional carbonate dissolution modifies C and Sr isotopes.
► C-14 dating shows that mixing is regional scale and long-term.
► Vertical flow is driven by high K in deeper units.