Evolution of chemistry along the Bagmati drainage network in Kathmandu valley

The Bagmati River in Kathmandu valley, Nepal, was studied to understand the influence of human and geochemical processes on changes in river chemistry (nutrients, organic matter, and major cations and anions) along the drainage network. Population density appeared to drive variation in the chemistry of surface waters at 10 stations in the Bagmati River. For all constituents studied, concentrations increased with distance downstream and many parameters showed strong relationships with human population density adjacent to the river. The composition of river water suggests that sewage effluent entering the river has a major effect on water quality. Concentrations of most solutes were highest during summer and lower during the winter monsoon season. The contribution of chemical weathering processes to water quality of the Bagmati appears to be minor within the Kathmandu valley. Dominant cations and anions when expressed in equivalents per liter were Na+ >> Ca2+ > K+ >> Mg2+ and Cl- >> SO42- > PO43- >> NO3- along the entire Bagmati drainage system. Ammonium contributed almost all nitrogen in the total dissolved nitrogen fraction and the concentration of nitrate was negligible, probably due to rapid denitrification and limited nitrification within the stream channel under conditions of relatively low oxygen. Decreases in sulfate along the stream channel may also be due to the reduction of sulfate to sulfide due to heavy organic matter loading. Water quality is unacceptable for any use and the whole ecosystem is severely affected due to human activities within the urban areas of the drainage basin.