Results from three large-area ground TEM surveys over highly salinised floodplains on the River Murray, South Australia

Three high-resolution, time-domain electromagnetic (TEM) surveys were run over two highly salinised floodplains of the River Murray in South Australia between 2021 and 2023. These surveys were run to quantify changes in conductivity distribution, both spatially and temporally, as these floodplains underwent intermittent flooding, both man-induced and natural, with the former intended to promote salt removal in the floodplain soils. The first survey was completed in May 2021, providing the baseline for later surveys. The second survey in May/June 2022 was designed to capture subsurface conditions on the floodplains after a controlled inundation earlier in 2022. The third survey, conducted in June/July 2023, was intended to capture subsurface conditions on the floodplains after the major natural flood event that covered most of the River Murray system, peaking in December 2022. By exactly repeating traverses from the 2021 baseline survey, and then exactly repeating processing steps, including inversions, the results should mainly reflect changes in ground conditions, much of which will be dominated by changes in salinity related to the two flooding events. In this paper we concentrate on results from the less heavily salinised Katarapko Creek floodplain. Overall, it appears that the study areas on the Katarapko floodplain show relatively extensive improvement in the shallower parts of the floodplain over the course of the study. Interestingly, it was noted that at the response at depth over much of the survey area showed that the conductivity actually did not change and in fact in some areas appear to have increased slightly by the time the 2023 data sets were collected. There were also several areas where the most obvious conductivity change appears to related to lateral recharge of near-bank groundwater created by the higher hydraulic gradient from the river/creek adjacent the floodplain.