Zonal control of total nitrogen flux into the sea in the Oujiang River Basin based on SWAT model and abatement cost optimization

To evaluate the effectiveness of comprehensive total nitrogen (TN) pollution control measures and identify their influencing factors in the Oujiang River Basin, this study developed a hydrological-water quality model based on the SWAT model. The study systematically revealed the spatiotemporal distribution and pollution source structure characteristics of TN fluxes into the sea, proposed a comprehensive risk index to classify TN pollution types, and quantified TN reduction amounts and economic costs of different schemes through multi-scenario simulations of TN mitigation measures. The results indicated that: (1) TN fluxes into the sea in the Oujiang River Basin exhibited a distinct spatial gradient of lower reaches > middle reaches > upper reaches, with high-value zones identified in the Songyinxi River sub-basin and the estuary area. Flood season contributed over 74% of the annual TN flux. (2) These high-risk source regions accounted for only 27.83% of the basin area but contributed 47.5% of the TN load, highlighting three dominant pollution types of crop cultivation-dominated, industry-dominated, and mixed sources. (3) Differentiated TN reduction strategies achieved annual TN reductions ranging from 42.36 t/a to 6 052.52 t/a, with annual costs ranging from 1.44×10⁴ yuan to 8.98×10⁶ yuan, representing cost savings of 12.25%-43.54% compared to basin-wide uniform control. (4) The TN load intensity of the Oujiang River Basin was relatively low among all river basins flowing into the East China Sea in Zhejiang Province. TN load showed a significant positive correlation with factors such as urban population, built-up area, urbanization rate, and tertiary industry output, with a variance contribution rate of 68.8%. The findings suggest that TN pollution in the Oujiang River Basin is closely linked to industrial structure and land use patterns. TN control pressures will remain in the future, while zoned and categorized differentiated management strategies can enhance the abatement efficiency and economic feasibility.