Trends of water composition and discharge in the Ramganga River, Ganga Basin over the last 40 years signal enhanced nitrate flux

Abstract

Studying long-term trends in river water chemistry is critical to assessing the impacts of human activities and climate change on water resources. This is of particular importance in the Indian subcontinent such as the Ganga River Basin – home to nearly 500 million people. As previous studies are mostly focused on seasonal to annual scale variabilities, the long-term riverine changes in the hydrochemistry of the rivers draining the Ganga Basin remain poorly constrained. Here we present the river chemistry and discharge data of Ramganga Basin, a major sub-catchment of Ganga River, for three hydrological stations between 1981 and 2020. Our results indicate that the total concentration of dissolved solids (TDS) within the Ramganga basin varies from 123 to 1170 mg/L and does not show a systematic spatiotemporal trend. However, we found that the fluxes of certain dissolved inorganic constituents from Ramganga Basin have significantly increased in the last 40 years with NO3– having the highest trend followed by Ca2+, Cl−, K+, SiO2, F−, and PO43−. The concentration-discharge (C–Q) analysis suggests a dominant dilution behaviour for most of the major solutes in the basin at longer timescales. We find that the cations and anions riverine loads are primarily dominated by Ca2+ and HCO3– ions, respectively, controlled by rock weathering processes, whereas the fluxes of NO3–, PO43−, and SO42− are controlled by anthropogenic processes. In summary, the long-term dataset for the Ramganga River revealed significant variability in river water chemistry, solute fluxes, and C–Q relationships over the last 40 years controlled by natural and anthropogenic processes.