Characteristics of hydrochemical and stable isotopes in the Upper and Middle Reaches of the Yarlung Tsangpo River

Abstract

There have been few studies on the spatiotemporal variation of the hydrochemistry and stable isotopes characteristics in the upper and middle reaches of the Yarlung Tsangpo River owing to its high elevation and alpine climate, and the primary controlling mechanisms have yet to be clarified. Statistical analysis, a Piper diagram, Gibbs diagrams, ion correlation analysis, and other methods were integrated to analyze water samples from the main stream and tributaries of the Yarlung Tsangpo River together with rainwater samples from certain areas obtained in May 2020. And both the formation mechanism of the hydrochemistry and the process of chemical weathering were explored. The results revealed that river water in the basin was of HCO3−–SO42−–Ca2+ type (i.e., weakly alkaline), evaporite weathering had little impact on chemical weathering in the upper and middle reaches of the Yarlung Tsangpo River, and chemical weathering in the basin was controlled jointly by weathering of silicate rock and carbonate rock. Overall, the mean value of deuterium excess of the samples from the upper and middle reaches of the Yarlung Tsangpo River was 7.22‰, i.e., lower than the global mean of precipitation (10‰), indicating that the sampled water was subject to evaporation enrichment to some extent. Owing to the unique climatic and geographic conditions of the Qinghai–Tibet Plateau, the upper and middle reaches of the Yarlung Tsangpo River were also affected by water–rock interaction (chemical weathering and physical erosion). In addition, the relationship of stable isotopes between rainwater and riverwater also suggested that precipitation is an important source for river water in the upper and middle reaches of the Yarlung Tsangpo River.