Nitrogen contamination of shallow groundwater in Katmandu Valley, Nepal

Abstract

To identify the sources of nitrogen contamination of urban shallow groundwater in Kathmandu Valley in Nepal, groundwater, river and sewage water samples were collected and analyzed for oxygen and hydrogen isotopes in water, nitrogen and oxygen isotopes ratio in nitrate and major dissolved ions. Groundwater samples were collected from 15 dug wells and 19 driven wells; river water samples from 8 locations in the major rivers; and sewage water samples from two locations in the urban area. Results showed that nitrogen contamination in the shallow groundwater is composed mainly of nitrate and ammonium ions. The nitrate-nitrogen and ammonia nitrogen are as high as 63.9 mg/L and 36.7 mg/L, respectively. The results of nitrate nitrogen isotope ratio suggest sewage as the source of nitrate contamination. The increase in hydrogen isotope ratio of shallow groundwater with chloride concentration indicates the occurrence of sewage water contamination to shallow groundwater. In addition to nitrate, nitrogen contamination is also revealed from the increasing trend of dissolved inorganic nitrogen DIN with hydrogen isotope ratio in the shallow groundwater. The level of concentration, however, is found as lower than expected concentration of DIN on the sewage-contaminated groundwater. A possibility of reducing nitrogen by denitrification under anoxic conditions in the shallow groundwater was also analyzed based on hydrogen isotope ratio and chloride concentration. The results showed that most of the groundwater has no rainwa-ter dilutions, and therefore, denitrification may be occurring in the shallow groundwater of the Kathmandu Valley.