Transport and transformation of arsenic in coastal aquifer at the scenario of seawater intrusion followed by managed aquifer recharge.

Release of contaminants from aquifers at the coastal area is of increasing concern, but remains unclear due to the complex groundwater dynamics and hydrochemistry. Specifically, frequently occurring seawater intrusion and the subsequent engineering measures of managed aquifer recharge (MAR) could alter the groundwater regime, which might affect the fate and behaviors of contaminants. In this work, we investigated the transport and transformation of arsenic (As) in the coastal aquifer at the scenario of seawater intrusion followed by the injection-based MAR process. Results showed that seawater intrusion induced 10.3% more release of aqueous As in aquifers, which was attributed to the competitive desorption as a result of elevated anion concentration and pH, and the reduction of As(V) to As(III) due to the reduced redox potential and enriched As-reducing bacteria. Furthermore, seawater intrusion inhibited the recrystallization of iron (hydr)oxides and instead facilitated its conversion to iron sulfide with lower affinity to As. The subsequent MAR introduced oxygenated recharge water into aquifers and increased the redox potential, leading to the dissolution of iron sulfide followed by formation of amorphous iron (hydr)oxides. However, the competitive desorption of As with rich HCO3- under increased pH dominated continuous increase in the aquifer aqueous As during MAR process. A constructed numerical model for describing As transport based on the experimental data showed that As transported along the interface between seawater and freshwater, and MAR enhanced the release of As and expanded the spread range of As. Our findings reveal that both seawater intrusion and subsequent MAR could cause the release, transport, and transformation of As, which provides new insight on the understanding of geochemical process of As in coastal aquifers.