Determining the origin and fate of nitrate in the Nanyang Basin, Central China, using environmental isotopes and the Bayesian mixing model

Identifying sources of nitrate contamination has been a long-term challenge in areas with different land uses. We investigated the biogeochemical processes and quantified the contribution of potential nitrate sources in the Nanyang Basin, the source area of the South to North Water Diversion Project in China. Hydrogeochemical characteristics, the dual-isotope method (δ 15 N-NO 3 − and δ 18 O-NO 3 − ), and the Bayesian mixing model (SIAR) were combined. The results for 160 samples indicated that mean nitrate concentrations of residential area (162.83 mg L −1 ) and farmland (75.71 mg L −1 ) were higher compared with those of surface water (16.15 mg L −1 ) and forest (36.25 mg L −1 ). Hydrochemical facies and molar ratios of major ions indicated that the natural environment was greatly impacted by anthropogenic activities. Nitrification, ammonium volatilization, and mixing effects were the dominant processes in nitrogen transformation. The contributions of different sources to nitrate contamination were 45.41%, 35.81%, 17.87%, and 0.91% for sewage and manure, soil organic nitrogen, synthetic fertilizer, and atmospheric deposition, respectively. Undeveloped infrastructure and sewage disposal in rural areas were the main causes of nitrate contamination. Our results provide a theoretical basis for the development of measures to guarantee long-term water supply of the South to North Water Diversion Project.