## 1 Introduction
### Isotopic Fractionation  
The nitrogen and oxygen isotopes have been widely used in the effective identification of nitrate pollution sources of surface water. Nevertheless, during the nitrogen cycle, complex isotopic fractionation altered \(\delta^{15}\)N and \(\delta^{18}\)O compositions, thereby leading to the incorrect identification of nitrate source. The reaction processes which affect the isotopic composition include nitrification, denitrification, volatilization, assimilation, and mineralization [21; 22]. Nitrification refers to the process in which ammonium salt is oxidized to nitrate by nitrobacteria. Because the microorganisms give priority to utilize light isotopes (14N and 16O), lots of \(\delta^{15}\)N and \(\delta^{18}\)O are enriched in the nitrate. One-third of the oxygen of the newly generated nitrate should be derived from the dissolved oxygen in atmosphere and the other two-thirds are from the surrounding water bodies [4]. No isotopic fractionation occurs during the oxygen combination. The \(\delta^{18}\)O generated through nitrification process can be calculated by Equation (2) with the known \(\delta^{18}\)O-O\({}_{2}\) in atmosphere and \(\delta^{18}\)O-H\({}_{2}\)O in water, and the \(\delta^{18}\)O usually ranges from \(-\)10% to +10%. The \(\delta^{18}\)O value of nitrate in the water samples collected from agricultural area are within this range. In rivers run in forest area, the \(\delta^{18}\)O value is relatively high and is above +16% [11; 12]. If the measured \(\delta^{18}\)O value is within the calculated range of \(\delta^{18}\)O, it indicates that nitrate is greatly influenced by nitrification. If the measured value of \(\delta^{18}\)O is not within the calculated range, one possibility is that the isotopic compositions are slightly influenced by nitrification [18]. Another possibility is that the evaporation of soil water and the respiration of oxygen in atmosphere have led to the rise of \(\delta^{18}\)O value, that is, the measured value of \(\delta^{18}\)O is higher than the calculated one [23]. High temperature will facilitate the respiration of microorganisms, especially in summer, resulting in the enrichment of \(\delta^{18}\)O-O\({}_{2}\). \[\delta^{18}\text{O}=2/3\delta^{18}\text{O-H}_{2}\text{O}+1/3\delta^{18}\text{O -O}_{2} \tag{2}\]  
Denitrification occurs under anaerobic conditions or low concentration of dissolved oxygen (<2 mg/L) [24]. During denitrification, 14N and 16O are respectively converted to N\({}_{2}\)O and N\({}_{2}\) by microorganisms, resulting in an enrichment of 15N and 18O in the remaining nitrate. The enrichment  
Figure 1: Distribution of \(\delta^{15}\)N and \(\delta^{18}\)O from different potential nitrate sources. factors range from \(-40\%\) to \(-5\%\) for nitrogen isotope [25] and from \(-18\%\) to \(-8\%\) for oxygen isotope [26]. The extent of fractionation resulted from denitrification is influenced by the surrounding circumstances. Besides, denitrification will lead to a negative correlation between \(\delta^{15}\)N value and nitrate concentration. According to the Rayleigh equation, the logarithmic values of nitrate should be linearly correlated with \(\delta^{15}\)N and \(\delta^{18}\)O values, respectively. If \(\delta^{15}\)N displays a significant negative linear correlation with \(\delta^{18}\)O, the denitrification is responsible for nitrate transformation [18]. Denitrification will lead to the decreased nitrate concentration but increased values of \(\delta^{15}\)N and \(\delta^{18}\)O. \(\delta^{15}\)N and \(\delta^{18}\)O are correlated positively by a factor between 1.3:1 and 2.1:1 [4; 27; 28]. Denitrification has a great enrichment effect on \(\delta^{15}\)N, while \(\delta^{18}\)O is rarely affected by denitrification. It indicates that denitrification process is accompanied by some other reactions concerning nitrate [29].