Enrichment of arsenic in surface water, stream sediments and soils in Tibet

Abstract

Groundwater in sedimentary deposits in China, Southern, and Southeast Asia down gradient from the Tibetan plateau contains elevated As concentrations on a regional scale. To ascertain the possibility of source region As enrichment, samples of water (n=86), stream sediment (n=77) and soil (n=73) were collected from the Singe Tsangpo (upstream of the Indus River), Yarlung Tsangpo (upstream of the Brahmaputra River) and other drainage basins in Tibet in June of 2008. The average arsenic concentration in stream waters, sediments and soils was 58±70μg/L (n=39, range 2–252μg/L), 42±40mg/kg (n=37, range 12–227mg/kg), and 44±27mg/kg (n=28, range 12–84mg/kg) respectively for the Singe Tsangpo and was 11±17μg/L (n=30, range 2–83μg/L), 28±11mg/kg (n=28, range 2–61mg/kg), and 30±34mg/kg (n=21, range 6–173mg/kg) respectively for the Yarlung Tsangpo. A dug well contained 195μg/L of As. In addition to elevated As levels in surface and shallow groundwater of Tibet, hot spring and alkaline salt lake waters displayed very high As levels, reaching a maximum value of 5,985μg/L and 10,626μg/L As, respectively. The positive correlation between [As] and [Na]+[K] in stream waters indicates that these surface water arsenic enrichments are linked to the hot springs and/or salt lakes. Further, 24% of As in stream sediment is reductively leachable, with bulk As displaying a positive correlation with stream water As, suggesting sorption from stream water. In contrast, the fraction of reductively leachable As is negligible for soils and several rock samples, suggesting that As in them is associated with un-weathered minerals. Whether the pronounced As anomaly found in Tibet affects the sedimentary As content in deltas downstream or not requires further study.