Mapping water content dynamics in SAT systems using 3D electrical tomography

Abstract

The growing demand for high-quality water requires sustainable strategies to promote reuse and recycling. Managed Aquifer Recharge strategies, particularly Soil-Aquifer Treatment systems, have demonstrated effectiveness in improving water quality by reducing contaminants through biodegradation, retention, and sorption. The coexistence of solid, liquid, and gas phases in the unsaturated zone enhances adsorption, and retention of pathogens and colloids, while the availability of organic carbon and terminal electron acceptors sustains this zone as critical for biodegradation processes. During recharge, the hydration-drainage front in the unsaturated zone follows a depth-dependent pattern. However, soil heterogeneity causes water to infiltrate through preferential pathways, gradually hydrating the surrounding medium. This behavior is further influenced by system management (recharge strategy, applied flow rate, and/or installation of reactive barriers), which also promotes the development of biofilms. These biofilms facilitate water retention and act as localized microreactors for contaminant biodegradation. We investigate the influence of recharge strategies (pulsed versus continuous) and the presence of a reactive barrier on the hydration-drainage front in the unsaturated zone and biofilm development in two Soil-Aquifer Treatment systems. This was achieved using cross-hole electrical resistivity tomography, combined with the assessment of biofilm formation through extracellular polymeric substances quantification in solid samples collected from the unsaturated zone during the recharge episodes. The study compares two Soil-Aquifer Treatment systems: one composed of fine sand and another incorporating a reactive barrier consisting of sand, woodchips, compost, biochar, zeolites, and clay. Two recharge episodes were analyzed: one operated under continuous flow and the other under pulsed flow, while maintaining the same average flow rate. Resistivity measurements, associated with the properties of the porous medium and the fluids circulating through it, were acquired under the initial dry conditions and throughout recharge, revealing the 3D distribution of the unsaturated zone volume during hydration (at the start of recharge) and drainage (when recharge ceased). Over time, these measurements also indicated the potential formation of biofilms in the Soil-Aquifer Treatment system. Measurements at the beginning and end of each recharge episode capture the 3D evolution of water content. Results showed that water infiltration occurred through preferential pathways or fingers, creating significant heterogeneity in water content in both Soil-Aquifer Treatment systems. The reactive barrier enhanced water retention during dry periods, supporting biofilms development. Furthermore, the pulsed recharge strategy promoted biofilm growth more effectively than the continuous recharge strategy. These findings provide insight into optimizing recharge strategies and media composition to manage system dynamics and, consequently, enhance contaminant removal in Soil-Aquifer Treatment systems.