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

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 (delta Fe-56 vs. IRMM-14), sulfur (delta S-34(SO4) vs. V-CDT) and carbon (delta C-13(DIC) vs. V-PDB) to an experimental field plot in the Datong Basin, northern China. An array of monitoring wells was installed in a approximate to 1700-m(2) plot in which high concentrations of As, ranging from 4.76 to 469.5 mu g/L, were detected in the groundwater. The measured range of delta S-34(SO4) values from 10.0 parts per thousand to 24.7 parts per thousand indicates the prevalence of microbial sulfate reduction within aquifers. The range of delta Fe-56 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 delta C-13(DIC) values (up to -33.6 parts per thousand) 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 mu g/L) groundwater, which has higher delta S-34(SO4) and delta Fe-56 values and lower delta C-13 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 mu g/L. Re-oxidation of Fe-sufide explains how sample C1-2 can have a high As concentration and low delta S-34(SO4) and high delta Fe-56 values. The results provide new insight into the mechanism of As enrichment in groundwater. (C) 2014 Elsevier B.V. All rights reserved.