Dual-isotope-based source apportionment of nitrate in 30 rivers draining into the Bohai Sea, north China

Excessive nitrate (NO₃^-) in rivers can lead to water quality deterioration, and can also be directly input into estuaries and oceans, thus posing a serious threat to the stability of their ecosystems. In this study, the concentration, isotopes and sources of NO₃^- in 30 rivers discharging into the Bohai Sea were comprehensively investigated. The mean concentration of NO₃^--N was 2.24 ± 2.11 mg L^-1, with obvious seasonal and spatial variations. In total, 104.24 kt of NO₃^--N was discharged into the Bohai Sea annually, to which the Yellow River Basin and Liao River Basin made the largest contributions. The range of δ¹⁵N-NO₃^- was -1.1‰ to +33.2‰ (mean value, +11.4 ± 5.0‰), with no significant seasonal or spatial differences; the mean value of δ¹⁸O-NO₃^- was +9.4 ± 7.2‰, with much higher values seen in June. Based on the MixSIAR model, manure (24.3 ± 7.5%) and sewage (19.1 ± 14.5%) were the primary sources of NO₃^- in the 30 rivers, followed by NO₃^- fertilizers (16.3 ± 12.5%), soil N (15.5 ± 11.9%), atmospheric deposition of NO₃^- (13.5 ± 5.7%) and NH₄⁺ fertilizers (11.4 ± 8.9%). This finding highlights the vital roles of sewage and manure management in riverine NO₃^-. Using a mathematical method, the contributions of various sources to each river were simulated. The results indicated that management of the Yellow River, Daliao River, Liao River, and Xiaoqing River is more urgently needed than that of other rivers to control Bohai NO₃^- pollution. We believe that this finding will provide guidance for scientific management of NO₃^- pollution in these 30 rivers and the Bohai Sea.