Hydrogeochemical and isotopic characteristics of spring water in the Yarlung Zangbo River Basin, Qinghai-Tibet Plateau, Southwest China

The Yarlung Zangbo River Basin(YZRB)is situated in the southern part of the Tibetan Plateau and remains in a mostly natural state.To understand the chemical characteristics of spring water and its controlling factors in the YZRB,68 sets of spring water samples were analyzed using hydrochemical and isotopic techniques.The spring water was found to be slightly alkaline with total dissolved solids(TDS)below 1000 mg L−1.Major ions were Mg2+,Ca2+,SO42−,and HCO3−.The spring water types in this basin were determined to be HCO3-Ca·Mg and SO4·Cl-Ca·Mg.Ion exchange and dissolution of carbonate,gypsum,and silicate were identified as the prevalent hydrogeochemical processes contributing and defining spring water chemistry in this basin.Saturation indices(SI)of most major minerals studied in this region were below zero,indicating that these minerals remain under-saturated in the spring water in this area.Overall,the rank of different processes in terms of their contribution to the chemical composition of spring water in the YZRB was carbonate weathering>evaporate dissolution>silicate weathering>precipitation input.The content of 18O in spring water ranged from−22.22‰to−14.08‰with a mean of−18.15‰.Samples collected below and close to the local and global meteoric water lines indicated that spring water in this area is derived from meteoric water with chemistry affected by evaporation.