Chemical Weathering Dynamics and Riverine Geochemistry of the Ganga River Basin: Spatial and Seasonal Controls on Elemental Fluxes

Chemical weathering of continental rocks plays a key role in regulating river water chemistry and global biogeochemical cycles. This study investigates the spatial and seasonal variability in major and trace element geochemistry of rivers in the Ganga basin to constrain dominant weathering processes and their downstream evolution. A total of 200 water samples and sediment samples were collected across different hydrological regimes and physiographic settings. The results indicate that dissolved loads are primarily controlled by the weathering of silicate and carbonate lithologies. In the Himalayan headwaters, river chemistry is dominated by carbonate and Ca²⁺–Mg²⁺-rich silicate weathering, with Ca²⁺ + Mg²⁺ and HCO₃⁻ contributing approximately 85% of total cations and anions. In contrast, downstream reaches exhibit a systematic decrease in Ca²⁺ + Mg²⁺ contributions and an increase in Na⁺ + K⁺ proportions (up to ~50%), suggesting enhanced influence of silicate weathering and/or alkaline soil inputs. Trace elements such as  Pb, Hg, Th, Sr, Rb, Mo, U, Ba, and V reveal spatially variable source contributions across different catchments. Sodium-normalized trace metal ratios and Ca/Na* relationships indicate additional contributions from carbonate or Ca²⁺–Mg²⁺-rich silicates, particularly during high-discharge periods. Strontium isotope ratios (⁸⁷Sr/⁸⁶Sr) of the Ganga River reflect chemical weathering and sediment sources in the Himalayan region. The river drains diverse lithologies of the Himalaya, causing spatial and seasonal isotopic variations. Higher ⁸⁷Sr/⁸⁶Sr values indicate silicate weathering of radiogenic continental crust, especially during monsoon periods. Lower ratios reflect inputs from carbonate rocks and recycled sediments. Sr isotopes highlight the role of Himalayan weathering in controlling riverine Sr flux.These observations highlight the combined influence of lithology, hydrology, and seasonal discharge on riverine geochemistry and provide new constraints on chemical weathering processes and trace element fluxes from the Ganga basin to the ocean.