Arsenic mobilization in a freshening groundwater system formed within glaciomarine deposits

Arsenic release to groundwater and conditions favoring As mobility are investigated in a system of aquifers formed within unconsolidated Quaternary sediments. The studied groundwater system is comprised of unconfined aquifers formed in glaciofluvial sediments with Ca–Mg–HCO3 groundwater, and confined aquifers formed within glaciomarine sediments with high As (above 10 μg/L) Na–HCO3 or Na–Cl groundwater. A positive relationship of As concentrations with the Na/(Ca + Mg) ratio of groundwater indicates that As release occurs in glaciomarine sediments concurrent to cation exchange reactions related to groundwater freshening. Arsenic is mobile in confined aquifers as a result of groundwater basic pH which prevents arsenate from adsorbing to mineral surfaces, and reducing conditions that favor speciation to arsenite. Selected extractions applied to sediment core samples indicate that As occurs in sediments predominantly in sulfide minerals and in Mn oxides and/or Fe oxyhydroxides. General positive relationships between As and the reduced species Fe2+, NH3 and dissolved S2− suggest that As release occurs at increasingly reducing conditions. Despite likely As release via Fe oxyhydroxide reductive dissolution, Fe remains at relatively low concentrations in groundwater (up to 0.37 mg/L) as a result of possible Fe adsorption and Fe reprecipitation as carbonate minerals favored by basic pH and high alkalinity. The presence of S2− in some samples, a negative relationship between δ34S of SO4 and SO 4 2 - concentrations, and a positive relationship between δ34S and δ18O of SO4 indicate that groundwater in confined aquifers is undergoing bacterial SO4 reduction.