Hydrochemical processes and pCO ₂ dynamics in a tropical mountainous catchment: insights from the Bhavani River, Attappadi CZO

Weathering and solute transport shape river chemistry and influence global biogeochemical cycles, especially in tropical mountainous regions. This study examines hydrogeochemical processes and controls on riverine pCO2 in the Bhavani River, located within the Attappadi Critical Zone Observatory in the southern Western Ghats, India. Using long-term data and seasonal sampling across different hydrological phases, we identify dominant solute sources, weathering mechanisms, and in-stream CO2 dynamics. Multivariate and concentration-discharge (C–Q) analyses reveal chemostatic to moderately chemodynamic behaviour of major ions, influenced by climate and reservoir operations. Hydrochemical signatures indicate a CaMgHCO3 dominated water type, with silicate weathering as the major geochemical process. Seasonal variability shows that monsoon dilution lowers ion concentrations and pCO2, while pre- and post-monsoon periods enhance weathering and CO2 buildup. Riverine pCO2 ranges from 314.6 to 1507.3 µatm, with a long-term increasing trend. Silicate weathering and CO2 consumption rates are 8.86 t km–2 year–1 and 1.39 × 105 mol km–2 year–1, respectively.