Nitrate source identification and health risk assessment of surface waters in southwestern China

Nitrate pollution in surface waters poses a dual challenge to ecosystem sustainability and human health, particularly in vulnerable plain basins with agricultural and urbanized regions. This study developed an integrated framework combining statistical and isotopic analyses, receptor modeling (Positive matrix factorization and MixSIAR), and probabilistic health risk assessment to investigate nitrogen pollution in a typical plain river basin of southwestern China. Results revealed that nitrate was the primary nitrogen pollutant in surface water, with higher concentrations observed in urbanized, agricultural, and confluence areas. Nitrification posed a significant influence on the nitrate concentration, whereas the effect of denitrification was considered negligible. Among diverse pollution sources, sewage discharge was the predominant contributor (dry season: 62.3 %, wet season: 65.2 %), followed by soil nitrogen and agricultural fertilizers. In addition, nitrate posed negligible non-carcinogenic risks to adults, with the maximum values of THI<1.00 (dry season: 0.44, wet season: 0.50). However, in the wet season, 1.90 % of the watershed posed potential health risks to children due to intense nitrification. A pronounced risk increase was identified in areas characterized by intensive anthropogenic activities and at river confluence zones. These findings revealed that nitrate contamination and associated health risks were substantially elevated in urban, agricultural, and confluence zones. This highlights the urgent need for strengthened sewage management, optimized fertilizer application, and targeted monitoring in high-risk zones. The proposed integrated framework provides a reliable approach for nitrate source identification and risk evaluation in plain basins, while providing effective guidance for local governments and policymakers in nitrate mitigation and sustainable development of water resources.