Using Multivariate Statistics, Geochemical Modeling, and Multi-Isotopes to Identify the Formation Mechanism of Cold and Thermal Groundwaters in the Western Tibetan Plateau.

Groundwater is an indispensable material foundation for social and economic development and the most important renewable resource. Water scarcity in the western Tibetan Plateau severely constrains sustainable development. Although cold and thermal groundwaters have been investigated, limited understanding of their genesis hinders utilization. This study aims to analyze the hydrochemical processes of groundwater in the western Tibetan Plateau by integrating multiple statistical methods, geochemical modeling, and machine learning. Results show that the cold groundwater total dissolved solids (TDS) ranges from 57.58 to 766.43 mg/L, mainly of the HCO3-Ca type, while the thermal groundwater TDS ranges from 285.4 to 4746 mg/L, mainly of the HCO3-Na type. Both groups of recharges originate from meteoric water/meltwater, subsequently undergoing water-rock interaction (dissolution of carbonate, silicate, and halide minerals) and cation exchange. Cold groundwater is characterized by silicate weathering and slight calcite precipitation, while thermal groundwater exhibits stronger. The six key influencing factors are ranked according to their contributions: evaporation-concentration (28.6%) > comprehensive factor (23.8) > cation exchange (19%) > human activities (14.3%) > carbonate dissolution (9.5%) > gypsum dissolution (4.8%). The silicon-enthalpy mixing model quantitatively constrains the thermal groundwater system, revealing a deep reservoir temperature of 136.2°C-235.9°C and a significant cold-water mixing ratio of 50%-83.1%. These findings support rational development and protection of groundwater resources in the Tibetan Plateau.