Mapping Water Content Dynamics in MAR-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 systems, particularly Soil-Aquifer Treatment (SAT) 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 (USZ) 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 USZ 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 USZ and biofilm development in two SAT systems. This was achieved using cross-hole electrical resistivity tomography and assessing biofilm formation through extracellular polymeric substances quantification in solid samples collected from the USZ during the recharge episodes.

The study compared two SAT systems: one consisting of fine sand and another with a reactive barrier incorporating of sand, woodchips, compost, biochar, zeolites, and clay. Two recharge episodes were analyzed: one with a continuous flow rate and the other using a pulsed flow rate (while maintaining the same average flow rate across both episodes). Resistivity measurements, associated with the properties of the porous medium and the fluids circulating through it, were collected in the initial dry state and during the recharge, revealing the 3D distribution of the USZ 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 SAT system. Measurements at the beginning and end of each recharge period 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 SAT 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 SAT systems.