Assessing ammonium pollution and mitigation measures through a modified watershed non-point source model.

Watershed water quality modeling is a valuable tool for managing ammonium (NH ₄ ⁺ ) pollution. However, simulating NH ₄ ⁺ pollution presents unique challenges due to the inherent instability of NH ₄ ⁺ in natural environment. This study modified the widely-used Soil and Water Assessment Tool (SWAT) model to simulate non-point source (NPS) NH ₄ ⁺ processes, specifically incorporating the simulation of land-to-water NH ₄ ⁺ delivery. The Jiulong River Watershed (JRW) is the study area, a coastal watershed in Southeast China with substantial sewage discharge, livestock farming, and fertilizer application. The results demonstrate that the modified model can effectively simulate the NPS NH ₄ ⁺ processes. It is recommended to use multiple sets of observations to calibrate NH ₄ ⁺ simulation to enhance model reliability. Despite constituting a minor proportion (5.6 %), point source inputs significantly contribute to NH ₄ ⁺ load at watershed outlet (32.4∼51.9 %), while NPS inputs contribute 15.3∼17.3 % of NH ₄ ⁺ loads. NH ₄ ⁺ primarily enters water through surface runoff and lateral flow, with negligible leaching. Average NH ₄ ⁺ land-to-water delivery rate is about 2.35 to 2.90 kg N/ha/a. High delivery rates mainly occur at agricultural areas. Notably, proposed NH ₄ ⁺ mitigation measures, including urban sewage treatment enhancement, livestock manure management improvement, and fertilizer application reduction, demonstrate potential to collectively reduce the NH ₄ ⁺ load at watershed outlet by 1/4 to 1/3 and significantly enhance water quality standard compliance frequency. Insights gained from modeling experience in the JRW offer valuable implications for NH ₄ ⁺ modeling and management in regions with similar climates and significant anthropogenic nitrogen inputs.
Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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