Effects of the groundwater flowing and redox conditions on arsenic mobilization in aquifers of the Datong Basin, Northern China

•Groundwater flowing models underestimated the concentrations of groundwater As.•Redox gradients related to Fe-S-As cycling controlled the variations of groundwater As.•Abiotic reduction of Fe(III) minerals contributes ∼ 35 % of groundwater As. A multi-level field site was built at the As-affected area of the Datong basin. The one-year monitoring results showed that the three aquifers exhibit different redox conditions and hydrogeochemical behaviors. In the shallow aquifer, groundwater As concentration is up to 220.5 μg/L, which is related to the dissimilatory reduction of poor-crystalline Fe minerals. In the middle aquifer, groundwater sulfate reduction shows two sides effects on As mobilization that the reduced sulfide on the one hand promotes the release of sediment As via abiotic reductive dissolution of Fe minerals and on the other hand immobilizes groundwater As via the adsorption on the newly formed Fe(II)-sulfide minerals. In the deep aquifer, the reductive dissolution of crystalline Fe minerals was the dominant process causing the release of sediment As into groundwater with the highest As concentration up to 341.1 μg/L. Based on the one-year monitored data, a coupled flow and reactive transport model was developed. The model results showed that only considering the groundwater flow underestimated the concentration of groundwater As. When adding the hydro-biogeochemical processes, the good agreement was achieved between the observed and simulated groundwater As. In the shallow and deep aquifers, microbially mediated reduction of poor-crystalline and crystalline Fe minerals contributed approximately 85 % of groundwater As. In the middle aquifer, abiotic reduction of Fe minerals driven is responsible for around 35 % of the release of sediment As, and around 17 % of released As was re-adsorbed or co-precipitated onto the neo-formed Fe(II) minerals. Under different redox conditions, the hydrogeochemical behavior of As was dominantly controlled by the Fe-S-As cycling in the groundwater system.