## 5 Source Identification of Nitrate in Surface Water by Nitrogen and Oxygen Isotope-Based Methods  
The conventional methods based on land-use types, hydrochemistry, multiple isotopes (\(\delta^{2}\)H, \(\delta^{18}\)O, \(\delta^{11}\)B, \(\delta^{13}\)C, and \(\delta^{34}\)S) and dual isotopes of nitrate are combined to reduced isotopic fractionation caused by objective factors like spatial-temporal change [5; 12; 57]. [PERSON] et al. [18] carried out a study on the rivers on the Loess Plateau and found that nitrate was the major existing form of nitrogen in these rivers. Land-use type has an obvious impact on nitrogen concentration. The nitrate concentrations of rivers affected by coal are an order of magnitude higher than those of samples collected in forest or agricultural areas. As the original values of \(\delta^{15}\)N and \(\delta^{18}\)O differ according to the land-use types, correlation analysis should be conducted on them under each land-use type. The solute tracers in hydrochemistry like soluble ions (Cl\({}^{-}\) and Br\({}^{-}\)) link with the ratio difference between different ions (content ratios of NO3\({}^{-}\)/Cl\({}^{-}\), Cl\({}^{-}\)/Br\({}^{-}\), Cl\({}^{-}\)/Na\({}^{+}\), and SO4\({}^{2-}\)/Ca\({}^{2+}\)) are conductive to the accurate nitrate source identification based on \(\delta^{15}\)N and \(\delta^{18}\)O [4; 12; 66]. Cl\({}^{-}\) is difficult to have physical, chemical, or biological reactions and thus is a good indicator for reflecting the input and dilution of domestic sewage. [PERSON] et al. [67] reported that whether the nitrate of surface water was mainly from fertilizer or animal manure and sewage could be known by analyzing the molar ratio of NO3\({}^{-}\)/Cl\({}^{-}\). [PERSON] et al. [68] found the reason for the increased \(\delta^{15}\)N in water by analyzing the correlation between nitrate concentrations and \(\delta^{15}\)N values. That is, the nitrification of microorganisms decreases nitrate concentration but increases \(\delta^{15}\)N. If they both increase, it may be because there are two or several nitrate sources with high in nitrate concentrations and \(\delta^{15}\)N values. [PERSON] et al. [69] attempted to identify the sources of nitrate in water samples collected from a farm. They found that there was little difference between \(\delta^{15}\)N and \(\delta^{18}\)O of nitrate from the two possible sources, i.e., septic tanks and manure. Yet septic tanks could be excluded according to the difference between the compositions of \(\delta^{2}\)H and \(\delta^{18}\)O-H2O of septic tank effluent and groundwater. The change of isotope \(\delta^{13}\)C of dissolved inorganic carbon in river reflects the geochemical behaviors and biogeochemical cycle of carbon. The major sources of \(\delta^{13}\)C are decomposition of soil organic matters, dissolution of carbonate minerals, respiration of aquatic plants, and atmospheric CO2. As the carbon pools are quite different, the \(\delta^{13}\)C in water can provide important information for the evolution, biogeochemical cycle, and environmental change of carbon [70]. As the multi-sources of pollutant and reaction mechanism can cause the superposition of isotopic signals, the source of nitrate and its transformation ways can be traced through \(\delta^{15}\)N, \(\delta^{18}\)O, and \(\delta^{13}\)C. [PERSON] et al. [71] identified the source and transformation ways of nitrate in the upstream of Chaobai River, using the \(\delta^{15}\)N and \(\delta^{18}\)O as well as \(\delta^{13}\)C, and found that soil nitrogen was the major source of nitrate in rainy season, while sewage and manure were the major sources in dry season. Although denitrification did not occur, mineralization and nitrification played important roles in nitrogen biogeochemistry.