HIGH ARSENIC GROUNDWATER IN THE SONGNEN BASIN, CHINA

High As groundwater has been universally found in arid-semiarid inland basins of north-western China. The Songnen basin is one of the biggest basins for agricultural production in China, where high As groundwater occurs in both shallow aquifer and deep aquifer. Little has been known about groundwater chemistry and geochemical processes in these aquifers. This abstract reports groundwater chemistry and its relation to As mobilization. ¹⁸O and D isotopes indicated that groundwater mainly originated from meteoric water, being subject to slight evaporation. High As groundwater occurred in reducing conditions, with neutral pH values. Sulfate and NO₃^- concentrations were generally low, with ranges between <0.1 mg/L and 531 mg/L (median 36.3 mg/L) and <0.1 and 37.9 mg/L (median 2.87 mg/L), respectively. Arsenic concentration ranged from <0.1 to 321 μg/L, which was dependent of Eh values, Fe concentration and S^2- concentration. Groundwater As generally increased along the groundwater flow path. In the discharge area, both shallow and deep aquifers hosted high As groundwaters, at depths between 20 and 30 m, and 60 and 90 m, respectively. Dissolved organic carbon (DOC) concentration was usually high, with the range between 4.0 and 33 mg/L. Carbon isotope (¹³C) of DOC and DIC suggested that microbial degradation of organic carbon should be the active process controlling groundwater chemistry. Therefore, microbe-mediated Fe oxide reduction was expected to be the major mechanism for As mobilization in the aquifers.

Acknowledgement

The study has been financially supported by NSFC (Nos. 41222020 and 41172224), the Chinese Universities Scientific Fund (No. 2652013028), and the Chinese Scholarship Council (CSC).