Geophysical methods in saline groundwater studies: locating perched water tables and fresh-water lenses

A fast and informative tool was required for the delineation of stratigraphy and hydrology and for monitoring salinisation effects around the Stockyard Plains Disposal Basin, which is used as a storage lake for salt water pumped from aquifers close to the Murray River near the town of Waikerie, South Australia. Such salinisation effects include the formation of a perched saline water-table close to the disposal basin and consequent threats to local fresh-water lenses. Ground penetrating radar, direct current (DC) resistivity, time-domain EM (TEM) and low induction number frequency-domain EM (FEM) were compared in terms of efficiency and effectiveness at two survey locations near the Stockyard Plains Disposal Basin. Low induction number FEM measurements were fast to implement but suffered from a non-linear response with ground conductivity in high conductivity areas. Radar methods were ineffective due to high signal attenuation in the highly saline environment. Fast time-sampling TEM successfully depicted a perched saline water table related to leakage from the disposal basin, and was found to be the most useful technique for delineation of hydrogeology due to its high vertical resolution. DC resistivity was the slowest technique, but was found useful in imaging a fresh-water lens. Results from this study suggest that TEM and DC resistivity methods are useful tools for both fresh-water detection and hydrogeology monitoring in saline groundwater environments.