delta S-34 and delta O-18 of dissolved sulfate as biotic tracer of biogeochemical influences on arsenic mobilization in groundwater in the Hetao Plain, Inner Mongolia, China

摘要

Environmental isotopology of sulfur and oxygen of dissolved sulfate in groundwater was conducted in the Hetao Plain, northwestern China, aiming to better understand the processes controlling arsenic mobilization in arsenic-rich aqueous systems. A total of 22 groundwater samples were collected from domestic wells in the Hetao Plain. Arsenic concentrations ranged from 11.0 to 388 mu g/L. The delta S-34-(SO4) and delta O-18-(SO4) values of dissolved sulfate covered a range from +1.48 to +22.4 parts per thousand and +8.17 parts per thousand to +14.8 parts per thousand in groundwater, respectively. The wide range of delta S-34-(SO4) values reflected either an input of different sources of sulfate, such as gypsum dissolution and fertilizer application, or a modification from biogeochemical process of bacterial sulfate reduction. The positive correlation between delta S-34-(SO4) and arsenic concentrations suggested that bacteria mediated processes played an important role in the mobilization of arsenic. The delta O-18-(SO4) values correlated non-linearly with delta S-34-(SO4), but within a relatively narrow range (+8.17 to +14.8 parts per thousand), implying that complexities inherent in the sulfate-oxygen (O-SO42-) origins, for instance, water-derived oxygen (O-H2O), molecular oxygen (O-O-2) and isotope exchanging with dissolved oxides, are accounted for oxygen isotope composition of dissolved sulfate in groundwater in the Hetao Plain.