Evaluation of wastewater nitrogen transformation in a natural wetland (Ulaanbaatar, Mongolia) using dual-isotope analysis of nitrate Original Research Article

The Tuul River, which provides water for the daily needs of many residents of Ulaanbaatar, Mongolia, has been increasingly polluted by wastewater from the cityʹs sewage treatment plant. Information on water movement and the transformation of water-borne materials is required to alleviate the deterioration of water quality. We conducted a synoptic survey of general water movement, water quality including inorganic nitrogen concentrations, and isotopic composition of nitrogen (δ15N-NO3−, δ18O-NO3−, and δ15N-NH4+) and water (δ18O-H2O) in a wetland area that receives wastewater before it enters the Tuul River. We sampled surface water, groundwater, and spring water along the two major water routes in the wetland that flow from the drain of the sewage treatment plant to the Tuul River: a continuous tributary and a discontinuous tributary. The continuous tributary had high ammonium (NH4+) concentrations and nearly stable δ15N-NH4+, δ15N-NO3−, and δ18O-NO3− concentrations throughout its length, indicating that nitrogen transformation (i.e., nitrification and denitrification) during transit was small. In contrast, NH4+ concentrations decreased along the discontinuous tributary and nitrate (NO3−) concentrations were low at many points. Values of δ15N-NH4+, δ15N-NO3−, and δ18O-NO3− increased with flow along the discontinuous route. Our results indicate that nitrification and denitrification contribute to nitrogen removal in the wetland area along the discontinuous tributary with slow water transport. Differences in hydrological pathways and the velocity of wastewater transport through the wetland area greatly affect the extent of nitrogen removal.