## 5 Source Identification of Nitrate in Surface Water by Nitrogen and Oxygen Isotope-Based Methods
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Watersheds** & **Area (km2)** & **Main Sources** & **References** \\ \hline Illinois River Basin, & \multirow{2}{*}{7.8 \(\times\) 10\({}^{4}\)} & A mixture of relatively undenitrified tile drain & \\ United States & & waters, highly denitrified groundwater, and & [62] \\ \cline{1-1} Oldman River basin, & & treated wastewater & \\ Canada & & Soil nitrification in the western tributaries; manure & \\ \cline{1-1} Lake Winnipeg, Canada & & and/or sewage in the eastern tributaries & \\ \cline{1-1} Guadalhorce River Basin, & & \\ Spain & & & \\ \cline{1-1} \cline{4-4} Taihu Lake, China & & \(2.3\times 10^{3}\) & Sewage/manure, and soil organic N in winter; atmospheric precipitation and sewage/manure & [54] \\ \cline{1-1} \cline{4-4}  & & inputs in summer & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Cont. Boron (B) isotope transports simultaneously with nitrate in surface water. Generally, B comes from fertilizer, manure, and household goods [23]. There is obvious difference in the compositions of \(\delta^{11}\)B from different sources. Because B is not affected by transformation processes, the combined use of \(\delta^{11}\)B and the dual isotopes of nitrate have great benefits of the correct identification of nitrate source. [PERSON] et al. [66], by using \(\delta^{15}\)N, \(\delta^{18}\)O and \(\delta^{11}\)B, found that the major sources of the nitrate of surface water in the Flanders area of Belgium were manure and domestic sewage. [PERSON] et al. [72] used \(\delta^{11}\)B to evaluate the contribution of domestic sewage to nitrate. The \(\delta^{11}\)B is an effective indicator for distinguishing animal waste (between +6.9\(\%\) and +42.1\(\%\)) from domestic sewage (between \(-7.7\%\) and +12.9\(\%\)). \({}^{17}\)O can be introduced into identifying nitrate source. Unlike \(\delta^{18}\)O, the value of \(\delta^{17}\)O will not change during biochemical processes. Hence, \(\delta^{17}\)O is more sensitive and reliable in tracing the nitrate from atmospheric precipitation than the conventionally used \(\delta^{18}\)O. The \(\delta^{17}\)O can be used to accurately recognize the nitrate from atmospheric precipitation (\(\delta^{17}\)O \(>\) 0) and that from other pollution sources (\(\delta^{17}\)O = 0) [73]. [PERSON] et al. [74] applied \(\delta^{17}\)O to quantify the rate of precipitation into lake. The total amount of nitrate in lake decreased during the observation period, indicating that nitrate from atmospheric precipitation was assimilated quickly in summer. More than 90% of the assimilated nitrate would be transformed into nitrate through mineralization during the nitrogen cycle. [PERSON] et al. [75]--by using the \(\delta^{15}\)N, \(\delta^{18}\)O, and \(\delta^{17}\)O of nitrate, the \(\delta^{2}\)H and \(\delta^{18}\)O-H\({}_{2}\)O; and the \(\delta^{34}\)S and \(\delta^{18}\)O of sulfate--found that river pollution derived from agricultural activities, precipitation, nitrification, and urban sewage within Mariano lagoon basin in Italy. In summary, the complex isotope fractionation during each process in the nitrogen cycle may change the \(\delta^{15}\)N and \(\delta^{18}\)O values of nitrate pollution sources, which may lead to the deviation of nitrate pollution source identification. However, by combining with other isotopes or conventional methods, e.g., the solute tracer in hydrochemistry, the isotopes are still powerful to accurately analyze the nitrate pollution source and fate of surface water. In addition, some other methods--such as correlation analysis, end-member analysis [76], principal component analysis, classification-tree method [77], uncertainty analysis [78], and grey water footprint [79]--could be used to assist the source identification of nitrate pollution.