(Im)mobilization of iron, manganese, and arsenic during managed aquifer recharge in Bangladesh: Push-pull tests under oxidative and reductive conditions

Managed Aquifer Recharge (MAR) systems have supplied drinking water to rural communities in southwestern Bangladesh since 2009. Although MAR enhances water availability, there are concerns about the potential mobilization of iron (Fe), manganese (Mn), and arsenic (As) during storage. Fourteen push-pull tests (PPTs) were performed under oxidative and reductive conditions at four MAR sites. These tests involved injecting filtered and O₂-saturated pond water for oxidative conditions, and sucrose-amended anoxic stored MAR water for reductive conditions, via a well in the stored MAR water. During oxidative PPTs, repeated aeration, injection, and abstraction cycles resulted in rapid consumption of dissolved oxygen (DO) with first-order rate constants of ∼52 to 72 day^-1 across all sites. DO was mainly consumed by adsorbed and dissolved Fe, with no apparent signs of pyrite and organic matter (OM) oxidation. The consistently high rate constant across the cycles suggests that heterogeneous Fe oxidation dominates. DO oxidizes Fe(II) to form Fe-(oxyhydr)oxides, resulting in the temporary removal of dissolved Fe (∼98 %), Mn (∼70-80 %), and As (60-70 %) at sites GMF11 and JJS91 due to sorption onto newly formed Fe-(oxyhydr)oxides. At sites MGS and MF05, increased As concentrations were noted due to the desorption of As from the Fe-(oxyhydr)oxides surface during abstraction. During reductive PPTs, the sucrose degraded over time, resulting in increased bicarbonate (HCO₃) and acetate concentrations and decreased pH and (sucrose-derived) DOC in abstracted water. These conditions led to the reductive dissolution of Fe-(oxyhydr)oxides, mobilizing Fe, Mn, and As, resulting in concentration peaks up to 70 mg/L Fe, 3.5 mg/L Mn, and 120 µg/L As. At MGS and MF05, similar trends for Fe and Mn were observed, while As levels did not increase. Peak concentrations were observed after about one day at JJS91, and two days at the other sites. Regular infiltration of O₂-saturated water may limit mobilization of Fe, Mn, and As, while the occurrence of reduced conditions should be prevented, as they could result in mobilization of these geogenic metals and endanger the provision of safe drinking water.