Isotopic and geochemical studies in the Upper Ganga Basin, Uttarakhand, India: Implications on Dissolved Inorganic Carbon systematics

In the recent scenario of global warming, the release of organic and inorganic carbon from the melting glaciers has been a subject of scientific research since it can have a pronounced effect on the riverine carbon cycle and primary productivity. Apart from being one of the largest Himalayan glaciers, Gangotri glacier provides water to the Bhagirathi River (Ganga River) which is the most important perennial river in India in terms of economy and livelihood. In the last decade, the melting and recession of Gangotri glacier have increased significantly leading to the formation of glacial lakes and debris-covered areas. As a result, primary productivity and microbial activity have increased in the subglacial areas which release a great amount of soil CO₂ that has not been documented previously in the literature. In the present study, the Bhagirathi River, which is the proglacial melt-stream of the Gangotri glacier has been sampled during the Post-monsoon period. A total of 27 samples including river, groundwater, geothermal spring, and reservoir were collected and have been analyzed for pH, surface temperature, Electrical conductivity (EC), major ions, and stable isotopes of carbon. From the study of major ion abundance patterns and mixing ratios, it has been inferred that carbonate weathering is predominant in the basin, though the major rock type of the area are silicates. The (HCO₃^- ≈ Dissolved Inorganic Carbon, DIC) values of river water show no correlation with altitude (mean = 42.8 mg/L), while δ¹³C values show a decreasing trend with a decrease in altitude, with an overall range between -10 and - 5‰. As altitude decreases, organic matter activity increases, and thus more CO₂ is washed out from the Soil Organic Matter (SOM), which makes the δ¹³C values of the river depleted. The δ¹³C of groundwater (mean = -11.8‰) and reservoir water (mean = -9.4‰) are depleted than river water due to mixing of soil carbon in them, and δ¹³C of geothermal spring water (mean = -3.6‰), shows enriched values since these are places of active CO₂ degassing. The source of DIC in the river water is mainly carbonate weathering in the upstream part and soil CO₂ in the downstream part of the study area. Quantifying pCO₂ values of the river water and calculating carbon flux from the river would provide important information on whether the Bhagirathi River is acting as a carbon source or sink to the atmosphere.