Hydrochemical processes and water balance trends based on stable isotopes of typical alpine lakes in the Qinghai-Tibet Plateau

Due to the warming climate, the hydrological cycle of the Qinghai-Tibet Plateau (QTP) is accelerating, leading to continuous growth of alpine lakes. In case of overflow, this can pose significant risks to downstream water quality and resources. However, there is a lack of comprehensive investigations on the chemical composition and water balance of lakes in QTP. This study examines the evolutionary mechanisms of alpine lakes in the Yanhu Lake Basin (YHB) located in central QTP. It investigates stable isotopes (δ¹⁸O and δD), hydrochemistry, and utilizes the isotope balance model to determine the evaporation to inflow (E/I) ratio of lake water. The findings reveal that lake water exhibits a ClNa hydrochemical type with enrichment in total dissolved solids (TDS), boron (B), and lithium (Li) elements. On the δD- δ¹⁸O plot, lake water falls below the Local Meteoric Water Line (LMWL) towards rightward direction with a slope of 4.75 and an intercept of 11.83, indicating pronounced evaporation characteristics. The presence of more positive isotope signals and ratios among major ions suggests that evaporation processes primarily govern lake water's hydrochemistry. The modeled E/I ratio ranges from 0.19 to 0.65 across YHB, implying a positive lake water balance which contributes to consistent expansion in surface area. This highlights the need for local government to enhance monitoring efforts, closely observe fluctuations in Yanhu Lake's water level, expedite implementation of lake water diversion projects, and conduct comprehensive assessment on their potential impact on local ecological environment.