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⁻-δ¹⁵N accompanied by NO₃⁻ reduction with the distinct δ¹⁸O/δ¹⁵N 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.