Hydrochemical evolution of groundwater in a semi-arid environment verified through natural tracer and geochemical modelling, northwest Australia

The semi-arid Hamersly Basin in Australia is the hub for extensive mining, which requires the relocation of a significant volume of groundwater for dry mining operations. Understanding water balance components is crucial for managing and conserving water resources. This study adopts a joint approach using isotopic and hydrochemical techniques to identify and quantify water sources and recharge dynamics to explain the integral functioning of a typical floodplain aquifer. The observed chloride and stable isotopes suggest a mixing of recharge from high-rainfall cyclonic events and highly evaporated low-rainfall events at a ratio of 60:1. The highly evaporated water from light rainfall events would remain in the soil profile until mixed with precipitation from high-rainfall events recharge the underlying aquifers. The recharge rates by multiple methods range from 0.3 mm/y to 14.4 mm/y. Groundwaters have a unique hydrochemical signature and are characterised by high alkalinity and dissolved oxygen. The total dissolved solutes (TDS) range from fresh to brackish, however, most of the groundwater tends to have a TDS <1000 mg/L. The δ²H and δ¹⁸O concentrations of water samples vary over a narrow range despite a wide range of Cl concentrations. The data are consistent with salt concentration by evapotranspiration within the unsaturated zone, which becomes mixed with infiltration of rainfall from large cyclonic events. The hydrochemical pathway modelling for the major ion distribution shows that groundwater has evolved by evapo-concentration of rainfall prior to recharge in the unsaturated zone. This is followed by an increase in dissolved CO₂ and the precipitation of carbonate minerals. Although the initial dissolved CO₂ is acquired due to the decomposition of organic matter during passage through the unsaturated zone, the 10-fold higher CO₂ (pCO₂∼ −2.5) compared to atmospheric levels in the aquifer suggests the addition of further alkalinity due to aluminosilicate weathering. The negative correlation between δ³⁴S_SO4 and the SO₄/Cl ratio suggests the addition of sulphate to groundwater with relatively depleted δ³⁴S_SO4 values. The source of sulphate is likely to be the oxidation of pyrite from the bedrock, which is characterised by high arsenopyrite concentration. The results suggest that climatic conditions impart a unique signature on the groundwater quality. The method can be utilised to constrain water balance components such as recharge for floodplain aquifers globally.