Characteristics and Potential Health Risk of Inorganic Nitrogen in Phreatic Water in the Central and Southern Parts of Yinchuan Plain (Northwest China)

Industrial and farming processes have introduced considerable amounts of inorganic nitrogen into phreatic water, leading to groundwater nitrogen pollution. To thoroughly explore the distribution characteristics, sources, and effects on human health of inorganic nitrogen, 113 groundwater samples were gathered from the phreatic aquifer in the central-south part of the Yinchuan Plain for physicochemical analysis. The source, transformation, and influencing factors of inorganic nitrogen in phreatic groundwater were identified by using hydrochemical data, multivariate statistical analysis, and nitrate stable isotopes, and the effects of inorganic nitrogen on human health were also assessed through a health risk assessment model improved with triangular fuzzy number. The findings found that the primary inorganic nitrogen forms present in the groundwater of the study area were NO₃–N and NH₄–N. High NH₄–N concentrations clustered in the alluvial-lacustrine plain, while NO₃–N was concentrated in towns and drainage ditches within the alluvial-pluvial plain and near the entrance of the Qingtongxia Gorge. Fertilizer use, manure and sewage, and soil nitrogen were identified as the primary sources of inorganic nitrogen, but natural factors such as redox conditions and hydrogeological characteristics played a vital role in regulating the pollution process. Nitrification rather than denitrification dominated the biogeochemical processes of nitrogen. The hazard quotients associated with exposure to NO₃–N for adults and children were from 0.038 to 0.062 and 0.056 to 0.083, respectively, while NH₄–N were with 0.011–0.017 and 0.016–0.023, respectively, suggesting a higher risk of NO₃–N than NH₄–N. Meanwhile, the non-carcinogenic risk values under the combined exposure route were similar to those under the oral ingestion exposure route. These results illuminate the critical relationship between inorganic nitrogen sources and public health risks, advocating for integrated approaches to mitigate groundwater contamination in the region.