Natural arsenic source, migration, and flux in a catchment on the Southern Tibetan Plateau.

Arsenic-affected aquifers are broadly found in floodplains of South Asia and Southeast Asia, which are supplied by rivers that originate from the Tibetan Plateau. Earlier investigations have ascertained the Arsenic (As) enrichment of river water in the Tibetan Plateau. However, the source, migration, and flux of As in catchment-scale have not been well constrained. In this study, determinations have been made of As and other chemical components in geothermal spring, rock, river water, and suspended particulate material in the Xiangqu River basin, a tributary of the Yarlung Tsangpo River. The study has shown that the main stream waters contain a high concentration of dissolved As (>10 μg/L), whereas the majority of tributaries present a relatively low dissolved concentration (<10 μg/L), with the highest dissolved As levels occurring during the low-flow period (April to June) and the lowest during the high-flow period (July to September). Moreover, the study has found that with the geothermal spring discharge in the upper reaches being the principal source of dissolved As, the proportion of As derived from rock weathering increases during the high-flow period, and the decrease of dissolved As concentration in the main stream is dominated by the adsorption process in the upper reaches and the mixing of tributary water in the lower reaches. The particulate As is temporally stable throughout the sampling period. Due to the high erosion rate during the high-flow period, the annual As flux is 76.8 t/yr and As is primarily transported as particulate (79%). Furthermore, the contribution of weathering (58-62%) is more than that of geothermal spring discharge (38-42%) to the total As in river water. Overall, this study has highlighted a non-conservative As behavior in the upper reaches of river flows in a geothermal field on the Tibetan Plateau.