Paper No. 274-9
Presentation Time: 10:20 AM
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 SO42- 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 SO42--S and SO42--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 δ34SSO4, δ18OSO4, δ13CDOC values and concentrations of As, Fe, S2- and NH4-N increased with the decrease in δ13CDIC values, Eh, and SO42-/Cl-. The identical δ34SSO4 values between groundwater and sulfides indicated that dissolved SO42- 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 δ34SSO4 and δ18OSO4 values, and lower δ13CDIC and Eh values. Isotopic enrichment factors of sulfur and oxygen in dissolved SO42- showed the occurrence of bacterial disproportionation of sulfur intermediates (BDSI). This study indicated that both chemical oxidation of BRS-produced S2- 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.