Time-lapse resistivity imaging and self-potential monitoring of experimentally induced saline intrusion in coastal aquifer sands

Excessive groundwater abstraction in coastal areas exacerbates saltwater intrusion (SWI), a widespread global issue. Characterization of mechanisms delivering saltwater to wells can assist in developing suitable SWI mitigation strategies for reducing the risk of groundwater degradation. This paper presents findings from hydrogeological monitoring, time-lapse electrical resistivity tomography (ERT) and self-potential (SP) measurements to investigate SWI under natural and artificially perturbed conditions in a quasi-homogeneous pristine coastal sand aquifer, affected by large tidal ranges (>2 m). Time-lapse ERT surveys conducted under undisturbed conditions identified an upper saline recirculation cell (IRC) beneath the intertidal zone, arising due to seawater infiltrating into an underlying ∼20 m thick sand sequence containing fresher groundwater, with resistivity variations noted between spring and neap tides. Measurements taken during a 69-h constant-rate pumping test, discharging at 10.2 L/s, revealed that pumping drew saline water from the IRC towards abstraction wells. This resulted in saltwater contributions to discharge increasing from 1.4 to 4.1 %, consistent with the decrease in resistivity detected in ERT profiles between 3 m and 7 m below surface. Over the same period, SP signals fell by between 20 and 30 mV with greater declines occurring at locations nearer to the high-water mark. Monitoring data suggest that these changes in SP are primarily due to saline water intrusion from the IRC, rather than pressure changes resulting from pumping. Research findings provide further evidence that SP monitoring could act as a key geophysical early warning parameter for SWI, while ERT data further highlight the potential for monitoring SWI in shallow coastal aquifers. This study also demonstrates that optimal groundwater abstraction strategies in tidal-influenced coastal aquifers can be achieved by targeting deeper zones.