Physico-Chemical and Oxygen-Hydrogen Isotopic Assessment of Bagmati and Bishnumati Rivers and the Shallow Groundwater along the River Corridors in Kathmandu Valley, Nepal

Abstract

The direct dumping of solid wastes into the rivers, discharge of industrial effluents together with direct discharge of domestic sewage have excessively polluted the major rivers Bagmati and Bish-numati. Groundwater along these river corridors is also affected from pollution of these rivers. Two major rivers: Bagmati and Bishnumati and shallow tube wells adjacent to these rivers were monitored for 2 years. Samples were analysed for the stable isotopes of hydrogen and oxygen (δD and δ18O) and selected physico-chemical parameters to investigate the possible interrelationship between river water and shallow groundwater along these river corridors. The physico-chemical values revealed that shallow groundwater and river water along the Bishnumati River corridor were heavily mineralized due to direct discharge of sewage wastes into this river. The isotope compositions of river water and shallow groundwater clustered together revealed possible interrelationship between them. Some of the isotopic compositions of groundwater and river water deviated below the Local Meteoric Water Line (LMWL) indicating that the water has undergone evaporation. The isotopic and chemical results suggested possible interrelationship between river water and groundwater. Fractional contribution of the river water to groundwater was calculated based on isotopic data using mass balance approach. Results showed that shallow groundwater SG1, along the Bagmati River corridor (in September 2013), was composed of approximately 30% - 40% Bagmati River water. Similarly, shallow groundwater SG5 of Bishnumati River corridor (in September 2013), was composed of approximately 45% - 50% river water. This result indicated that high portion of river water mixed-up with adjoining shallow groundwater along the river corridors. Further, the mix-up of the river water with groundwater can be harmful when rivers are polluted. These findings can be useful for a better understanding of hydrogeological processes at the river-aquifer interface and eventually benefit water management of the Kathmandu Valley in future.