Hydrogeochemistry and Sulfur Isotopes of High Arsenic Groundwater of Datong Basin, China

Anomalous geogenic arsenic (3.4 to 670 ug/l) occurs in shallow groundwater in the Cenozoic Datong basin. It is located in an arid/semi-arid region of Northwestern China and is associated with the Shanxi rift system. Hydrochemical and sulfur isotopes were used to get a better understanding of the conditions that control arsenic mobility from the Quaternary aquifers into groundwater. The low contents of nitrate in groundwater (commonly below detection level) and the correlation between arsenic and proxies of redox conditions (Eh values and U, Mo and SO4 concentrations) indicate that the occurrence of high arsenic groundwater at Datong is related to the reducing conditions of the aquifer system. The dominant aqueous species of arsenic are arsenite (As(III)) and arsenate (As(V)) in the study area and the ratio of As(III)/As(V) is controlled by redox conditions. The wide range of δ34S[SO4] values (from -2.5 to +36.1 per mil) and the relation between δ34S[SO4] and Eh values and U, Mo contents of the groundwater samples indicate possible microbial reduction of sulfate in the aquifer system. Although the positive correlation between δ34S[SO4] and total arsenic content supports the hypothesis that reducing conditions were responsible for arsenic mobilization from aquifer into the groundwater, concomitant enrichment of δ34S[SO4] was not observed with the increase of the total arsenic. Moreover, there are two samples with high total arsenic but low δ34S[SO4] values (-2.5 and +8.5 per mil, respectively), which could not be explained by microbial reduction which should result in continuous enrichment of δ34S[SO4] values of residual sulfate. Therefore, microbial oxidation of sulfide with depleted δ34S[SO4] values under anaerobic condition may occur and microbial anaerobic oxidation process in the presence of Fe-Mn oxides/hydroxides should be another important process for the mobilization of arsenic in shallow groundwater at Datong.