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

Hydrochemical data, the delta S-34 and delta O-18 values in dissolved sulfate (SO42-) and the delta C-13 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 NH4+ concentrations and low NO3-, SO42- and ORP values were observed in the high-arsenic groundwater. The coupled occurrence of high dissolved Fe(II), HS-, NH4+ and HCO3- concentrations in groundwater indicates that microbially mediated Fe(III) and SO42- reduction and organic matter oxidation has occurred. Wide ranges of delta S-34 and delta O-18 values were measured in dissolved SO42- (ranging from 7.0 parts per thousand, to 36.8 parts per thousand, and from 3.2 parts per thousand to 13.0 parts per thousand, for delta S-34 and delta O-18, respectively) and depleted delta C-13 values of DIC (varying from -6.9 parts per thousand, to -22.0 parts per thousand) were detected. The wide ranges of the delta S-34(SO4) and delta O-18(SO4) values and the correlation between the low delta C-13(DIC) values and the high delta S-34(SO4) values indicate that the microbial reduction of SO42- and organic matter biodegradation have occurred or are occurring. The correlation between delta S-34(SO4) and delta O-18(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 delta C-13(DIC) values and the delta S-34(SO4) values and Fe concentrations demonstrate that arsenic mobility are influenced by processes: (I) the reduction of arsenic-bearing crystalline Fe(III) oxide/hydroxides and SO42- 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. (C) 2013 Elsevier B.V. All rights reserved.