Nitrate Removal and Geogenic Contaminant Mobilization during Oxic–Anoxic Cycling of Managed Aquifer Recharge Basin Soils

Abstract More frequent and intense precipitation events challenge water capture and recharge. Managed aquifer recharge (MAR) diverts excess surface water into aquifers to secure groundwater supplies; when using nonconventional waters such as stormwater, elevated nitrate loads can pose risks. Carbon-based permeable reactive barriers (PRBs) composed of woodchips installed in infiltration basins promote reducing conditions and stimulate denitrification. However, these conditions may also mobilize arsenic (As) and manganese (Mn), and the effects of repeated redox oscillations on mobilization in PRB-treated MAR basins remain poorly understood. Using batch reactor experiments, our results show that woodchip addition accelerated nitrate removal compared to nitrate-only reactors but also promoted HCl-extractable Fe(II) formation and increased aqueous As and Mn. Metal mobilization peaked during the first cycle of anoxic conditions and then declined during oxic periods with mobilization attenuating over time. Manganese release likely resulted from microbial reduction of Mn(III/IV) oxides, while As cycling was consistent with reductive dissolution of Fe and Mn oxides followed by readsorption during reoxidation. Results highlight a trade-off whereby adding woodchips may remove nitrate but can inadvertently mobilize geogenic metals and accelerate the onset of reducing conditions. Findings underscore the need to design, operate, and monitor MAR systems with a consideration of nitrate and geogenic contaminants simultaneously.