Hydrochemical and Isotopic Characteristics of the Yepu River Basin in Southern Tibet: A Preliminary Investigation

The Tibetan Plateau, known as the "Asian Water Tower," plays a critical role in regional water resource sustainability. This study focuses on the water cycle processes and hydrogeochemical mechanisms in the Yepuqu Basin, southern Tibet, through systematic sampling of spring water, river water, snowmelt, and rainwater (21 samples collected in June2023). By integrating hydrochemical analysis, hydrogen-oxygen isotope tracers (δD, δ¹⁸O), and deuterium excess (d-excess) parameters, the research investigates water cycle pathways, solute sources, and rock weathering dynamics. Key findings include: (1) Water chemistry predominantly follows the Ca-HCO₃ type (88%), with Ca-HCO₃·SO₄ as a secondary classification (12%). Isotopic runoff separation reveals distinct recharge patterns: spring water derives 84% from snowmelt and 16% from rainfall, while river water combines 68% from snowmelt/groundwater and 32% from rainfall. Downstream analysis shows a gradual decrease in snowmelt and groundwater contributions along the river course. (2) Solutes originate primarily from atmospheric deposition, carbonate weathering, and silicate dissolution. Rainwater contributes 7.6% of total cations and 4.2% of ${\rm{SO}}_4^{2-}$ in springs, compared to 6.7% and 2.5% in rivers. Notably, sulfuric acid weathering dominates cationic contributions, accounting for 53% in springs and 52% in rivers—surpassing carbonate weathering inputs. This phenomenon is attributed to sulfide oxidation in coal-bearing strata, which generates substantial acidity within the basin. (3) Springs exhibit highly variable d-excess values (7.5‰−22.4‰) indicating isolated hydrogeological units with diverse recharge pathways. In contrast, river waters display clustered d-excess signatures (11.2‰−12.7‰), reflecting stable recharge sources. The downstream decline in river d-excess values suggests increasing groundwater contributions along the flow path. This study pioneers the quantification of sulfide oxidation as the dominant driver of rock weathering in Tibetan Plateau basins. The d-excess parameter is demonstrated to be a robust indicator for identifying aquifer structures and water-rock interactions. These findings advance the understanding of cryospheric hydrogeochemical processes and provide a scientific foundation for sustainable water resource management in high-altitude regions. The integration of multi-isotope tracers with hydrochemical proxies establishes a replicable framework for diagnosing water cycle dynamics in complex alpine environments. The methodology resolves critical uncertainties in distinguishing atmospheric, cryospheric, and lithospheric contributions to riverine systems. By elucidating the coupling mechanisms between sulfide-rich strata weathering and water quality evolution, this work highlights the vulnerability of Tibetan water resources to geological and climatic perturbations. The dominance of sulfuric acid weathering underscores the need to reassess carbon sink calculations in high-altitude basins, traditionally attributed to carbonate dissolution. This research enhances predictive models of water resource responses to glacier retreat and permafrost degradation, offering actionable insights for policymakers engaged in transboundary water governance across the Third Pole region.