Quantification of Post-monsoon CO₂ Degassing Flux from the Headwaters of the Ganga River: Emphasis on Weathering Pattern of the Basin

Research on the carbon-cycling process in high-altitude streams is crucial for understanding whether carbon acts as a source or sink for the atmosphere during present times of global climate change. In this study, we have quantified the post-monsoon CO₂ flux (FCO₂) from the Bhagirathi and Alaknanda rivers, which are two pristine watersheds in the Upper Ganga Basin in India with the help of analytical hydrochemistry and PHREEQC v.3.7.3 software. Our results show FCO₂ values of 88 gCO₂m⁻²d⁻¹ and 175 gCO₂m⁻²d⁻¹ from the upstream reaches of Bhagirathi and Alaknanda Rivers, respectively, which is significantly greater than the fluxes observed in the downstream reaches (18 gCO₂m⁻²d⁻¹ and 4.1 gCO₂m⁻²d⁻¹, respectively). This difference in FCO₂ is attributed to the major variation in gas transfer velocity (kCO₂) along elevation, with the upstream section exhibiting approximately eight times higher kCO₂ than the downstream section. The steeper bed slope leads to increased turbulence and energy dissipation at higher altitudes, enhancing the kCO₂ values. The partial pressure of CO₂ in the rivers was found to be approximately 2.5 times greater than the atmosphere. Our findings suggest that form-drag turbulence instead of bed friction, prevalent in the high-gradient reaches of the rivers, is the main driver of CO₂ degassing into the atmosphere. This study shows that Ganga headwater streams are sources of CO₂ to the atmosphere and underscores the need for monitoring other Himalayan streams for CO₂ flux.