This study investigates the Baini River Basin using the MixSIAR model to quantify nitrate contributions from wastewater effluent (M&S), sewer leakage (SL), agriculture (AG), soil nitrogen (SN), and urban runoff (UR), with flux calculation used to verify model reliability. Results show that wastewater effluent is the dominant source (67.5%), followed by soil nitrogen (13.7%), agriculture (12.6%), sewer leakage (4.7%), and urban runoff (1.1%). Source contributions show spatiotemporal variations. In the wet season, wastewater effluent accounts for 54.0%, while agriculture and soil nitrogen contribute 18.3% and 19.5%, respectively. In the normal season, wastewater effluent rises to 61.8%, while agriculture and soil nitrogen decline. Spatially, the main channel and downstream tributaries are dominated by wastewater effluent (55.4% and 57.1%), whereas agricultural input rises at the watershed boundary (25.7%). Isotopic and dissolved oxygen data indicate limited denitrification in the basin, and wastewater effluent and untreated sewage leakage are the main sources of riverine nitrogen. Therefore, sewer network maintenance, stormwater–sewage separation, and sewage overflow control should be strengthened.

以白坭河流域为研究对象, 基于MixSIAR模型, 定量地解析污水处理厂尾水、溢流污水、农业源、土壤氮及城市面源的硝酸盐贡献特征, 并通过通量核算, 验证模型可靠性。结果表明, 污水厂尾水是硝酸盐的主要来源(67.5%), 其次为土壤氮(13.7%)、农业源(12.6%)和溢流污水(4.7%), 城市面源的贡献较低(1.1%)。污染源贡献呈现显著的时空分异, 丰水期污水厂尾水排放占54.0%, 农业和土壤氮分别为18.3%和19.5%; 平水期污水厂尾水排放贡献提高至61.8%, 农业与土壤的贡献明显降低。主河道和下游支流区域均以污水厂尾水排放为主, 分别占55.4%和57.1%, 入境断面农业贡献则明显升高至25.7%。同位素特征及溶解氧数据表明流域反硝化作用较弱, 污水厂尾水与污水管网溢流导致的未处理污水直排是河流氮素的主要来源, 因此应加强管网维护, 推进雨污分流, 减少污水溢流对水体的污染。