Chemical Weathering Rates, C-Q Relationships, Fluxes, Dam Impact and pCO2 Potential in the Ganga Headwaters: Insights From Weekly Time-Series Data

Abstract

Predicted alterations to the hydrological cycle due to higher temperatures at the end of this century will impact riverine processes such as weathering, erosion, and sediment transport. The Himalayan River basins, with their steep slopes, heavy rainfall, and increasing river engineering projects, are excellent sentinels for monitoring climate change and human impacts on rivers. However, few attempts have been made to capture the river mountainous catchment interaction over shorter time scales (weeks to months) to capture pulses of enhanced chemical weathering rates and other riverine processes. Here, we present a weekly time-series record of dissolved inorganic constituents near the mouth of the Alaknanda and Bhagirathi rivers—the two headwater rivers of the Ganga River—with a weekly resolution during 2018–2019. We report new estimates of discharge-weighted concentrations and fluxes. We found chemical weathering rates of 98.2 ± 54.0 and 32.2 ± 20.4 t/km2/year and CO2 consumption yields by silicate weathering of 3.7 ± 1.5 × 105 and 1.8 ± 1.2 × 105 mol CO2/km2/year for the Alaknanda and Bhagirathi basins, respectively, which are significantly higher compared to the global chemical weathering rate and CO2 consumption yield of ∼24 t/km2/year and ∼1 × 105 mol CO2/km2/year, respectively. We find that the concentration-discharge relationship shows both chemostatic and dilution trends, with the Tehri dam strongly influencing the hydrology of the Bhagirathi River. The alkalinity-DIC framework reveals that the Himalayan weathering acts as a net source of atmospheric pCO2 over a timescale of 105–107 years. Further, we show that a sampling campaign spreaded well throughout the year is imperative in reducing uncertainties in flux estimation.