Multiple isotope (O, S and C) approach elucidates the enrichment of arsenic in the groundwater from the Datong Basin, northern China

Summary Hydrochemical data, the δ 34 S and δ 18 O values in dissolved sulfate SO 4 2 - and the δ 13 C signature in dissolved inorganic carbon (DIC) have been analyzed and used to deduce the predominant biogeochemical/geochemical processes controlling the mobilization of arsenic within aquifers of the Datong Basin. High dissolved Fe(II), HS − and NH 4 + concentrations and low NO 3 - , SO 4 2 - and ORP values were observed in the high-arsenic groundwater. The coupled occurrence of high dissolved Fe(II), HS − , NH 4 + and HCO 3 - concentrations in groundwater indicates that microbially mediated Fe(III) and SO 4 2 - reduction and organic matter oxidation has occurred. Wide ranges of δ 34 S and δ 18 O values were measured in dissolved SO 4 2 - (ranging from 7.0‰ to 36.8‰ and from 3.2‰ to 13.0‰ for δ 34 S and δ 18 O, respectively) and depleted δ 13 C values of DIC (varying from −6.9‰ to −22.0‰) were detected. The wide ranges of the δ 34 S SO4 and δ 18 O SO4 values and the correlation between the low δ 13 C DIC values and the high δ 34 S SO4 values indicate that the microbial reduction of SO 4 2 - and organic matter biodegradation have occurred or are occurring. The correlation between δ 34 S SO4 and δ 18 O SO4 values and Fe concentrations indicates that Fe(III) oxides/hydroxides reduction and iron sulfide formation may be the main processes controlling the enrichment of arsenic in groundwater. Furthermore, the negative correlations between the δ 13 C DIC values and the δ 34 S SO4 values and Fe concentrations demonstrate that arsenic mobility are influenced by processes: (1) the reduction of arsenic-bearing crystalline Fe(III) oxide/hydroxides and SO 4 2 - coupled to the oxidation of organic matter in aquifer sediments or unsaturated zone and (2) the reduction of amorphous Fe(III) oxyhydroxide without the significant reduction of sulfate.