Nitrogen Contamination and Denitrification Occurrence in Shallow Groundwater of Urbanized Area at Kathmandu Valley, Nepal

The high contaminated shallow groundwater by nitrate (NO3-) and ammonium (NH4+) in the Kathmandu Valley, Nepal were investigated along with their nitrogen sources and distribution mechanisms. Thirty-six shallow groundwater samples were collected from the tube- and dug- wells during the monsoon season (August) of 2009 and 2010. High levels of NO3- (maximum: 63.9 mg/L) and NH4+ (maximum: 36.7 mg/L) concentrations were detected in shallow groundwater. NO3- represented a significant contribution to the total dissolved inorganic nitrogen in most the dug wells; these wells were characterized by unprotected inner walls, indicating the possible influx of nitrified NO3- from the unsaturated zone to the insides of the wells. The dual isotopic (delta N-15 and delta O-18) NO3- approach suggested that sewer leakage is a major source of nitrogen contamination in shallow aquifers. In addition, the exponential increase in NO3- -delta N-15 accompanied by NO3- reduction with the distinct delta O-18/delta N-15 slopes of NO3- (similar to 0.5) indicated denitrification. Finally, we established a negative nonlinear relationship between NO3- and the dissolved organic carbon, suggesting that organic carbon strongly affects denitrification reactions in shallow groundwater.