Hydrochemical characterization and pCO₂ dynamics in the surface waters of Himalayan River: A case study of river...

Chemical weathering processes are becoming increasingly important in studies on carbon cycling because they are responsible for increased solute fluxes in the proglacial zone, can effectively sequester atmospheric CO₂ and raise carbon budgets for lateral transport via rivers. Here, we examined the hydrochemical and hydrogeochemical processes, solute sources and factors controlling riverine pCO₂ of the Alaknanda River and its tributaries for three sampling seasons, viz. pre-monsoon (May 2021), post-monsoon (October 2021) and winter (January 2022). The surface water is enriched with Ca²⁺ and Mg²⁺ as the dominant cations, while HCO₃^- and SO₄^2- were the major anions. Gibbs's plot confirmed rock weathering as the leading mechanism in controlling the hydrochemistry of the basin. The chemical composition of river water was mainly regulated by the weathering of carbonate end members: dolomite, limestone and feldspar along with small inputs from silicate weathering. The mean pCO₂ values in the mainstream (1702.7 µatm), Pindar (2267.9 µatm) and Mandakini (1136.1 µatm) revealed the streams were oversaturated with CO₂ having a higher rate of exporting excess CO₂ gas to the atmosphere. The study showed the persistence of high pCO₂ closed system characteristics associated with increased suspended sediment concentration resulting from carbonate weathering, dominance of HCO₃^- over SO₄^2- and thereby results in high values of C ratio. Principal component analysis of the water chemistry suggests that weathering contributes about 41%, while humans contribute about 13% of the ionic load to the river. This study is one of its kind to understand the system characteristics of Alaknanda River water.