Hydrogeological assessment of Soil Aquifer Treatment-MAR: linking vadose zone processes to coastal groundwater recovery

Soil Aquifer Treatment-Managed Aquifer Recharge (SAT-MAR) using treated wastewater offers a promising option to enhance groundwater resources in climate-stressed Mediterranean coastal aquifers, yet its performance strongly depends on the poorly known behavior of the vadose zone. This study develops an integrated hydrogeophysical framework to characterize and model SAT-MAR functioning at the Korba coastal site (Chiba watershed, Cap Bon, Tunisia), where secondary treated wastewater is infiltrated through three basins to support a heavily overexploited aquifer threatened by long-term drawdown and seawater intrusion. Electrical Resistivity Tomography (ERT) and Time Domain Electromagnetics (TDEM) were used to image the shallow subsurface, revealing a vertically structured sequence of fine sand overlying more resistive sandstone or coarse sand that controls infiltration pathways, storage, and potential preferential flow. Soil sampling, hydraulic conductivity tests, and laboratory analyses were combined with geophysical results to parameterize an unsaturated flow model (Hydrus) beneath a 50 × 30 m basin, showing that treated wastewater requires on the order of 30 hours to traverse the approximately 20 m thick vadose zone, providing significant residence time for filtration and biogeochemical attenuation before reaching the water table. Regional groundwater responses to different recharge configurations were then evaluated with a MODFLOW model of the shallow aquifer, indicating that increased SAT-MAR recharge at Korba and replicated sites produces measurable recovery of hydraulic heads in depressed areas while contributing to stabilization of the coastal gradient. By explicitly linking geophysically derived vadose zone architecture, unsaturated flow dynamics, and saturated aquifer behavior, this work demonstrates how hydrogeophysical integration improves process understanding and supports the design and scaling of SAT-MAR schemes aimed at coastal groundwater recovery under global change.