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

Hydrochemical data, the δ34S and δ18O values in dissolved sulfate (SO24) and the δ13C 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, SO24 and ORP values were observed in the high-arsenic groundwater. The coupled occurrence of high dissolved Fe(II), HS-, NH˙4 and HCO3 concentrations in groundwater indicates that microbially mediated Fe(III) and SO24 reduction and organic matter oxidation has occurred. Wide ranges of δ34S and δ18O values were measured in dissolved SO24 (ranging from 7.0‰ to 36.8‰ and from 3.2‰ to 13.0‰ for δ34S and δ18O, respectively) and depleted δ13C values of DIC (varying from ―6.9‰ to ―22.0‰) were detected. The wide ranges of the δ34SSO4 and δ18OSO4 values and the correlation between the low δ13CDIC values and the high δ34SSO4 values indicate that the microbial reduction of SO24 and organic matter biodegradation have occurred or are occurring. The correlation between δ34SSO4 and δ18OSO4 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 δ13CDIC values and the δ34SSO4 values and Fe concentrations demonstrate that arsenic mobility are influenced by processes: (1) the reduction of arsenic-bearing crystalline Fe(III) oxide/hydroxides and SO24 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.