Understanding the Hydrological, Carbon and Nitrogen cycle in the Ganga river basin: Insights from the stable isotopes

Kumar, Anurag (2021) Understanding the Hydrological, Carbon and Nitrogen cycle in the Ganga river basin: Insights from the stable isotopes. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Anurag Kumar (14RS031)) 14RS031.pdf - Submitted Version Restricted to Repository staff only Download (2MB)

Abstract

The Ganga river basin (GRB) of India is one of the most densely inhabited river systems in the world where the natural and anthropogenic variables govern the biogeochemical cycle in the basin. Understanding the sources and the forcing factors of the different biogeochemical cycles in the GRB is a fundamental challenge to the researchers as they govern the water, carbon and nitrogen budget of the Ganga river. Therefore in the present study, the stable isotopes have been used as a tracer to understand the sources and processes controlling the water, dissolved inorganic carbon (DIC) and nitrate dynamics of the Ganga river. The understanding of spatial distribution of water isotopes in the large river system is useful to comprehend the hydrological processes in the basin. However, the spatial and temporal isotopic archives of different water inventories from the GRB are not well documented, hindering the understanding of hydrological processes. Therefore, to resolve this issue, the isotopic distribution of river water in the GRB has been modeled. To comprehend the spatial isotopic variability, the δ¹⁸O and ƍD values of river water samples (n=540) from the GRB were used to derive a secondorder polynomial equation representing the river water isotope as a function of latitude and elevation. A strong correlation was observed between the measured and modeled isotope values (δ¹⁸O and δD) of river water with slope and correlation coefficient square (r²) of 1.0 and 0.7, respectively. The results indicate that the model captures ~75% of isotopic variability in the river water of GRB. The residuals from the model provided an excellent Gaussian fit indicating the competence of the model in capturing the spatial isotopic variability of the river water without any biases. The residual values of the samples also reflected the influence of factors other than latitude and elevation on the isotopic composition of river water. The residual of sample along the river stretch indicates that the upper stretch of the river has influence of catchment effect in the river water. The river Ganga also exerts a strong influence on the productivity of the Hooghly estuary, which is sensitive to terrestrial organic matter and nitrogen flux from the rivers. Nitrate among the bioavailable nitrogen forms, is most labile and highly soluble in water. Therefore, the study of nitrate in river water becomes useful to understand the nitrogen dynamics in the basin. In the thesis work, for the first time, the isotopes of dissolved nitrate (δ¹⁸O and δ¹⁵N) of the Ganga river has been measured to understand the sources and processes mediating the nitrogen pool in the Ganga river. The results suggest that nitrification and assimilation processes play an important role in controlling the dual isotopic composition of dissolved nitrate in river water. A large amount of organic matter and ammonium fertilizer is brought into the river Ganga through surface runoff during the monsoon period. These nitrogen-bearing compounds are converted to nitrate via nitrification.

Actions (login required)

View Item