Combining porewater profiles and batch tests to quantify redox reactions and heavy-metal(loid)s release potentially induced by managed aquifer recharge

Like other highly populated alluvial plains, the Po plain (Italy) is suffering from groundwater reserves' decline and widespread pollution since the last two decades. Managed aquifer recharge (MAR) is often used for aquifer replenishment and to mitigate groundwater contamination. Brescia, located in the middle of the Po plain, with its 200 k inhabitants and a wide industrial area exerts large pressures on the groundwater quality and could benefit of MAR. To evaluate the interactions among MAR water and aquifer matrix, two cores were drilled in an urban/industrial area to capture the lithological variability of the unconfined aquifer and to delineate redox gradients. To this purpose, vertical profiles of soil cores were collected via Rhizon samplers and analysed for grain size, TDS, pH, Eh, DOC, major ions, and trace elements. In addition, 3 batches with 160 g of sediment and 800 mL of deionized water were set-up in an inert atmosphere (N₂) and monitored for 3 months. Stratigraphical cores results were used to locate the most permeable layers to maximize MAR injection efficiency. The porewater composition of depth profiles revealed that possible pollution sources could be untreated wastewaters from leaking sewers or overflows rather than industrial contamination. The batches and sequential extraction demonstrated that no significant release of heavy-metal(loid)s was induced by MAR water and the already present contaminants were diluted below admissible limits. Pre- and post-experimental characterization of Fe-mineral phases via sequential extraction, suggested a >50 % ± 25 increase of crystalline Fe-oxides at shallow depths that can further decrease heavy-metal(loid)s mobility. The proposed framework could be employed in other environments to assess the feasibility of MAR prior to perform a full-scale trial and to assist tailored design solutions.