Insights from distribution and fractionation of the rare earth elements into As enrichment in the Singe Tsangpo River Basin

The geogenic As enrichment occurs extensively in the major river basin from the Tibetan Plateau, while the knowledge involved with the underlying mechanisms is far from completion. The present study utilized the geochemical behaviors of rare earth elements (REE) to study the hydrogeochemical evolution and As enrichment in the Singe Tsnagpo River basin, a typical As-rich river basin in the Tibetan Plateau. The river water was characterized by significant positive Eu anomalies and slight negative Ce anomalies, indicating the hydro-geochemical control of oxidative weathering of sourcing rocks and the contribution of felsic rocks. The PHREEQC modeling results suggested that the carbonate weathering contributed to the complexation of REE in the river water, where REE(CO3)(+) and REE(CO3)(2)(-) were the predominant complex species. Besides, the reversing scenarios of HREE/LREE enrichment in the river water/sediments suggested a critical control of iron (hydr)oxides on the REE fractionation due to the preferential adsorption of LREE compared with HREE. Interestingly, the variations in Y/Ni and Cr/V ratios from the river sediments suggested a different contribution of sourcing rock weathering along the river flow path, where ultramafic rock showed a substantial contribution to the river sediments in the lower reaches and granite source is predominant in the upper reaches. It was also notable that the concurrent enrichment of REE and As in the river waters showed a response to the substantial enhancement of chemical weathering in the upper reaches of the STR basin, which was evidenced by the corresponding increases in the electrical conductivity and the delta O-18 values in the river waters. The present study thus provides new insights into utilizing REE as environmental tracers for studying hydrogeochemical evolution and As enrichment in the STR basin, which could also apply to similar alpine arid and cold river basins.