Biogeochemical cycling of arsenic in coastal salinized aquifers: Evidence from sulfur isotope study

Abstract

Arsenic (As) contamination of groundwater, accompanied by critical salinization, occurs in the southwestern coastal area of Taiwan. Statistical analyses and geochemical calculations indicate that a possible source of aqueous arsenic is the reductive dissolution of As-bearing iron oxyhydroxides. There are few reports of the influence of sulfate–sulfide redox cycling on arsenic mobility in brackish groundwater. We evaluated the contribution of sulfate reduction and sulfide re-oxidation on As enrichment using δ 34S [SO 4] and δ 18O [SO 4] sulfur isotopic analyses of groundwater. Fifty-three groundwater samples were divided into groups of high-As content and salinized (Type A), low-As and non-salinized (Type B), and high-As and non-salinized (Type C) groundwaters, based on hydro-geochemical analysis. The relatively high enrichment of 34S [SO 4] and 18O [SO 4] present in Type A, caused by microbial-mediated reduction of sulfate, and high 18O enrichment factor (ε [SO 4–H 2O] ), suggests that sulfur disproportionation is an important process during the reductive dissolution of As-containing iron oxyhydroxides. Limited co-precipitation of ion-sulfide increased the rate of As liberation under anaerobic conditions. In contrast to this, Type B and Type C groundwater samples showed high δ 18O [SO 4] and low δ 34S [SO 4] values under mildly reducing conditions. Base on 18O mass balance calculations, the oxide sources of sulfate are from infiltrated atmospheric O 2, caused by additional recharge of dissolved oxygen and sulfide re-oxidation. The anthropogenic influence of extensive pumping also promotes atmospheric oxygen entry into aquifers, altering redox conditions, and increasing the rate of As release into groundwater.