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

Abstract

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 （δ15N and δ18O） NO3- approach suggested that sewer leakage is a major source of nitrogen contamination in shallow aquifers. In addition, the exponential increase in NO3--δ 15N accompanied by NO3- reduction with the distinct δ 18O/δ 15N slopes of NO3- （~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.