A national inventory of seawater intrusion vulnerability for Australia

Study region: Twenty-eight coastal aquifer case study areas across Australia. Study focus: Seawater intrusion causes degradation of groundwater resources in coastal areas. The characterization of seawater intrusion is difficult and expensive, and there is therefore a need to develop methods for rapid assessment of seawater intrusion as part of large-scale screening studies in order to guide future investment. We use a steady-state analytic approach to quantify seawater extent and propensity for change in seawater extent under different stresses, in combination with findings from a previous qualitative investigation, which relies on a data-based assessment of regional trends. New hydrological insights for the region: The combination of methods identified areas of highest risk to SWI including unconfined aquifers at Derby (WA) and Esperance (WA), and confined aquifers at Esperance (WA) and Adelaide (SA). The combination of analytic and qualitative approaches offers a more comprehensive and less subjective seawater intrusion characterization than arises from applying the methods in isolation, thereby imparting enhanced confidence in the outcomes. Importantly, active seawater intrusion conditions occur in many of Australia's confined coastal aquifers, obviating the use of the analytical solution, and suggesting that offshore groundwater resources provide significant contributions to these systems.