Mapping zones of higher groundwater and solute movement using airborne geophysics in a heterogeneous aquifer ? A case study in weathered crystalline basement rocks, southwestern Australia

Abstract

In southwestern Australia, the prediction of land salinity and stream flow trends requires the accurate construction of groundwater surfaces. This information is essential in the determination of groundwater and solute gradients. Airborne electromagnetic (AEM) data have the potential to be a useful tool in providing a `snapshot' of current regolith salt content and possible gradient directions. The focus of major previous research on groundwater and solute movement has been on linear breaks in electrical conductivity produced by dykes and faults. Interpreting electrical conductivity information within granitic regolith, with respect to groundwater recharge and salt mobility has, to a large extent, been ignored. Comparisons of the granitic regolith salt content and electrical conductivity measurement at Broomehill show that two types of regolith salt profile dominate. These profiles show marked differences in mineralogy and chloride content, and interpreted hydraulic properties. Unfortunately they are not readily distinguished in inverted AEM data for the area, making predictions of regolith salt content problematic. If AEM data are to be used in quantifying salt exported from catchments, then their interpretation should proceed with an appreciation of regolith heterogeneity and models of groundwater flow.