Using Hydrochemistry, Multi-Isotope, and MixSIAR Model to Analyze Nitrate Sources of Groundwater: A Case Study of the Yongning River Banks

Groundwater nitrate (NO3−) pollution, caused by anthropogenic activities, poses a global threat to water security. The mixing of multiple nitrate pollution sources and the associated biogeochemical reactions may create a complex chemical background, which renders traditional hydrochemical methods and single δ15N isotope analysis approaches limited in accurately identifying pollution sources and quantifying their contribution ratios. Accordingly, we adopted an integrated framework incorporating hydrochemistry, isotopes, and the MixSIAR model. Within this framework, results from different components mutually validate each other, helping to achieve more accurate source identification and contribution quantification. Results revealed severe nitrate contamination with striking spatial heterogeneity: concentrations were significantly higher in the eastern region (9.3–1890.7 mg·L−1, Mean: 472.8 mg·L−1) than in the western region (8.5–204.1 mg·L−1, Mean: 52.0 mg·L−1). Hydrochemical and δ18O-NO3− evidence identified nitrification as the dominant nitrogen transformation process. Critically, the MixSIAR model quantified drastically different source contributions between the two regions. In the eastern industrial zone, industrial wastewater was the predominant source (61.3%), followed by manure and sewage (18.5%). In contrast, in the western agricultural area, natural and agricultural sources dominated, with soil nitrogen contributing 43.9% and chemical fertilizer 31.7%. The findings pinpoint specific pollution drivers for each region, offering a robust scientific basis for formulating differentiated and effective nitrate pollution control strategies.