Preliminary study of hydrochemical and isotopic characteristics of the Yepuqu basin, southern Xizang

Known as the "Asian Water Tower", the study of water cycle in typical basins of the Qinghai-Xizang Plateau plays a critical role for water resources conservation. This study takes the Yepuqu basin in southern Xizang as the research object, through systematic sampling of spring water, river water, snowmelt, and rainwater (a total of 21 samples collected in June 2023),we comprehensively employed hydrochemical analysis, hydrogen-oxygen isotope tracers (δD, δ¹⁸O), and deuterium excess (d-excess) techniques to investigate water circulation pathways,solute sources and rock weathering mechanism. Key findings include: (1) The dominant water chemical type is Ca-HCO₃·SO₄ (82%), followed by Ca-HCO₃ (12%). Isotopic hydrograph separation reveals distinct recharge patterns: spring water recharge derives 84% from snowmelt and 16% from rainfall, while river water recharge comprises 68% from snowmelt and groundwater and 32% from rainfall. Furthermore, the contribution ratio of snowmelt and groundwater decrease progressively along the river flow direction. (2) The solutes are primarily derived from atmospheric precipitation and the weathering of carbonate and silicate rocks. Rainwater contributes 7.6% and 4.2% to the total cation concentration and ${\rm{SO}}_4^{2-}$ in spring water, respectively, and 6.7% and 2.5% in river water, respectively. Notably, sulfate plays a significant role in the rock weathering, accounting for 53% in springs and 52% of the total cation content in spring and river water, respectively, which exceeds the contribution from carbonate weathering. This is mainly attributed to acid production via sulfide oxidation in coal-bearing strata. (3) Springs exhibit highly variable and scattered d-excess values (7.5‰ to 22.4‰), indicating that most springs belong to independent hydrogeological units. In contrast, the d-excess values of river water are concentrated (11.2‰ to 12.7‰), suggesting more stable recharge sources. The downstream decline in river d-excess values reflect an increasing proportion of groundwater recharge. This study quantitatively identifies the dominant contribution of sulfide oxidation-induced acidification to rocking weathering in typical Qinghai-Xizang Plateau basins for the first time. It elucidates the hydrogeological significance of the d-excess parameter in identifying hydrated structural units and water-rock interactions. These findings advance the theoretical framework of water-rock interaction in alpine regions and provide a scientific basis for sustainable water resource management and development on the plateau.