Time-lapse electrical imaging of water infiltration in the context of soil aquifer treatment

In this study, we investigated the ability of online geoelectrical monitoring to characterize water dynamics in the context of soil aquifer treatment (SAT). In spring 2014, we conducted several infiltration experiments at the Shafdan SAT facility in Israel, where treated wastewater is recurrently infiltrated through large ponds into the sandy coastal plain aquifer. Temporally dense electrical resistivity tomography (ERT) measurements were collected to investigate vadose zone flow processes and the influence of heterogeneous sedimentation on the infiltration pattern. Measurements of water content as well as cross-hole ground-penetrating radar (GPR) profiles and lithological analyses from boreholes were used to support the ERT results. The observed electrical responses reflect the high temporal dynamics in subsurface water content associated with the applied infiltration-drying cycles. Individual apparent resistivity monitoring data captured general temporal dynamics, providing an easy and cost-effective means for infiltration monitoring. Time-lapse ERT results reveal different temporal dynamics in regions of the subsurface as a result of spatially varying hydraulic properties. We propose to use, based on a simple quantitative parametric description, the imaged temporal behavior of electrical conductivity to discriminate zones with different hydraulic properties, which represents a new approach for aquifer zonation based on the observed process dynamics. Our results demonstrate that time-lapse ERT is a powerful tool for the monitoring of SAT water infiltration with high spatiotemporal resolution. The provided information may be used for assessing and optimizing applied infiltration-drying cycles and, thus, for more effective SAT management.