Using major ions and stable isotopes to improve conceptualisation of a spring-aquifer system in the Galilee Basin, Australia

Developing conceptual models is a critical step in hydrogeological studies that should utilise multiple lines of evidence and data types to minimise conceptual uncertainty, particularly in data-sparse systems. This study used new and existing major ion and isotope (O, H, Sr, C) data sets to refine a previous hydraulic-head-based conceptual model of the Galilee Basin (Australia). The analyses provide evidence for the locations of recharge and discharge areas and determine hydrochemical processes along flow paths to improve understanding of potential source waters to the Doongmabulla Springs Complex (DSC) and to infer mixing within, or exchange between aquifer units. There was good agreement between previously inferred recharge and discharge areas defined using hydraulic head data and interpretations from hydrochemical evolution along groundwater flow pathways, at least where data were available. Major ion and isotope data suggest that the DSC likely receives water from both a relatively shallow, local flow path and a deeper regional flow path. This observation is relevant to previous concerns about threats to the DSC, as mine-induced drawdown may impact the relative contributions to spring discharge from different recharge sources and aquifers. Silicate weathering in the deeper Clematis Formation and Dunda Beds, and evapotranspiration in the overlying Moolayember Formation have strong control on the total dissolved solids content. These findings suggest that the Clematis Formation and Dunda Beds are hydrochemically distinct from the Moolayember Formation, with limited exchange between these aquifers, which has important implications for model conceptualisation and ongoing monitoring of mining activities in the Galilee Basin.