BIOGEOCHEMICAL PROCESSES OF ARSENIC MOBILIZATION IN THE WESTERN HETAO BASIN, CHINA: EVIDENCES FROM MULTIPLE ISOTOPE APPROACHES

High As groundwater showed a great variation in dissolved SO₄^2- under reducing conditions. Sulfate reduction would coprecipitate dissolved As, while sulfur intermediates may lead to reduction of Fe(III) oxyhydroxides and subsequently release of adsorbed As. Therefore, sulfur behaviors would significantly affect transport and transformation of groundwater As. However, the role of S cycling in As behavior is not well-understood, which may help to characterize biogeochemical processes controlling groundwater As. This study provides the observation of isotopic enrichment factors of SO₄^2--S and SO₄^2--O and evaluation of the role of S cycling in groundwater As using multiple isotope approaches. Eighty-eight water samples were taken for chemical and isotopic analysis in the western Hetao basin, Inner Mongolia. Results showed that, from the alluvial fans to the flat plain, groundwater δ³⁴S_SO4, δ¹⁸O_SO4, δ¹³C_DOC values and concentrations of As, Fe, S^2- and NH₄-N increased with the decrease in δ¹³C_DIC values, Eh, and SO₄^2-/Cl^-. The identical δ³⁴S_SO4 values between groundwater and sulfides indicated that dissolved SO₄^2- was mainly sourced from chemical weathering of sulfides in the alluvial fans. Bacterial sulfate reduction (BSR) was evidenced in the flat plain by relatively higher δ³⁴S_SO4 and δ¹⁸O_SO4 values, and lower δ¹³C_DIC and Eh values. Isotopic enrichment factors of sulfur and oxygen in dissolved SO₄^2- showed the occurrence of bacterial disproportionation of sulfur intermediates (BDSI). This study indicated that both chemical oxidation of BRS-produced S^2- and BDSI, which were coupled with reduction of Fe(III) oxyhydroxides, led to the release of the adsorbed As into groundwater. It was suggested that biogeochemical processes associated with S cycling should be important in mobilizing As in aquifer systems.