Carbonate weathering and geothermal inputs: dominant controls on the hydrochemistry of China's Nianchu River

Understanding the hydrochemical dynamics of alpine arid basins is essential for ensuring sustainable water resources on the Tibetan Plateau. The Nianchu River, a typical alpine and arid basin on the Tibetan Plateau, is the focal area of the "One River, Two Tributaries" Comprehensive Development Project in Tibet Autonomous Region of China. However, no systematic studies on hydrochemistry in this basin have been conducted. This study investigated the spatio-temporal variations and controlling mechanisms of water chemistry in the Nianchu River basin with a focus on the understudied influence of widely distributed geothermal springs. During the dry and wet seasons, 44 river water and 13 geothermal water samples were collected and analyzed for major ions. Results showed that the river water was weakly alkaline, with total dissolved solids and major ion concentrations (e.g., Ca²⁺, HCO₃^-, SO₄^2-) significantly higher in the dry season (mean EC: 372 μS/cm) than in the wet season (mean EC: 236 μS/cm), and peaking in the midstream tributary of the Longma River. In contrast, the geothermal springs were moderately acidic and exhibited markedly higher mineralization (mean EC: 1, 936 μS/cm), with Na⁺, K⁺, and Cl^- concentrations being 20.8, 22.5, and 44.8 times those in river water, respectively. The direct discharge of geothermal water was identified as a key driver that significantly elevated Na⁺, K⁺, and Cl^- levels and altered the hydrochemical facies of the river in affected reaches. The main conclusions are that: (1) the hydrochemical evolution of the Nianchu River is predominantly governed by carbonate and silicate weathering, while geothermal water chemistry is controlled by evaporation-crystallization and deep water-rock interactions; and (2) sulfuric acid participates in carbonate weathering alongside carbonic acid, particularly in the mid-lower reaches, enhances the release of Ca²⁺, Mg²⁺, and SO₄^2-. This study provides a holistic understanding of hydrogeochemical controls in a typical alpine basin and underscores the critical role of geothermal fluids, offering a scientific basis for protecting water resources under climate and anthropogenic pressures.