Coupled iron, sulfur and carbon isotope evidences for arsenic enrichment in groundwater

Summary It is generally accepted that microbial processes play a key role in the mobilization and enrichment of arsenic (As) in groundwater. However, the detailed mechanism of the metabolic processes remain poorly understand. We apply isotopic measurements of iron (δ 56 Fe vs. IRMM-14), sulfur (δ 34 S SO4 vs. V-CDT) and carbon (δ 13 C DIC vs. V-PDB) to an experimental field plot in the Datong Basin, northern China. An array of monitoring wells was installed in a ≈1700-m 2 plot in which high concentrations of As, ranging from 4.76 to 469.5 μg/L, were detected in the groundwater. The measured range of δ 34 S SO4 values from 10.0‰ to 24.7‰ indicates the prevalence of microbial sulfate reduction within aquifers. The range of δ 56 Fe values measured in the groundwater suggests microbial Fe(III) reduction and the occurrence of isotopic exchange between Fe(II)aq and FeS precipitation. The low δ 13 C DIC values (up to −33.6‰) measured in groundwater are evidences for the microbial oxidation of organic matter, which is interpreted as the light carbon pool within the aquifer sediments. The high As (As > 50 μg/L) groundwater, which has higher δ 34 S SO4 and δ 56 Fe values and lower δ 13 C values, indicates the following: (1) microbial reduction of sulfate causes the mobilization of As through HS − abiotic reduction of Fe(III) minerals and/or formation of As-sulfur components; and (2) direct microbial reduction of Fe(III) oxides, hydroxides and oxyhydroxides cannot increase As concentrations to greater than 50 μg/L. Re-oxidation of Fe-sufide explains how sample C1-2 can have a high As concentration and low δ 34 S SO4 and high δ 56 Fe values. The results provide new insight into the mechanism of As enrichment in groundwater.