Nitrate source apportionments in the South Han River basin and its main tributaries using multi-isotopes and Bayesian approaches

The nitrate apportionment approach provides valuable insights into efficient water quality management. Water samples were collected from the South Han River (SHR), and the three tributaries of the SHR showed relatively high nutrient levels. Water samples were collected from the North Han River (NHR), another contaminated stream (Gyeongan Stream, GA), and wastewater treatment plants (WWTP). The objectives of this study were to identify the sources impacting water chemistry in the SHR and its tributaries, especially nitrate contaminants, and to estimate the apportionment of nitrate sources. Plots of SO 4 /Cl and NO 3 /Cl showed relatively low ratios for the tributary, GA, and WWTPs samples compared to those of the SHR and NHR. Interestingly, samples from the lowest reaches of a tributary showed considerably high SO 4 /Cl and low NO 3 /Cl, with distinct δ 34 S values, indicating a specific contaminant source in its watershed and subsequently influencing SHR water chemistry. The contributions of each source to nitrate in SHR and tributaries were estimated to be 58.9% to 93.3% and 0.2% to 95.7% for soil N and 15.9% to 40.3% and 4.1% to 99.4% for manure/domestic sewage, respectively, using a Bayesian isotope mixing model. These source contributions were not clearly consistent with the land-use patterns. δ 11 B related to δ 15 N-NO 3 values for SHR samples indicate that domestic wastewater exerts a greater influence on water chemistry than agricultural inputs. We suggest a comprehensive approach for identifying nitrate contaminants in water using multiple isotopes, including nitrate isotopes, with a Bayesian isotope mixing model.