Quantitative Measure of Salt Interception Using In-River Transient Electromagnetic Geophysics

In the lower Murray-Darling Basin, most groundwater discharges to the floodplain of the River Murray. Most of the groundwater is of high salinity and therefore can transfer significant salt loads into the river. To mitigate saline groundwater intrusion into the river, salt interception schemes (SIS) have been commissioned since the early 1990s. The SIS intercept highsalinity groundwater flow adjacent to the river floodplain and the intercepted water is pumped to distant evaporation basins. The in-river transient electromagnetic (RTEM) geophysics technique can be used to infer saline groundwater discharge areas and to inform SIS locations. RTEM results have also been used, albeit qualitatively, in the monitoring and evaluation of the performance of SIS. A methodology for evaluating SIS performance has been developed based on the area above the cumulative frequency distribution (ACFD) of RTEM riverbed-only resistivities. In addition to RTEM maps and cross-sections, the ACFD characterises a river reach with a single number. Increases in ACFD, from pre- to post-SIS RTEM surveys, indicate the changing groundwater flow regime and the building of freshwater lenses.