## 3 Research Progress in Analytical Techniques of Nitrogen and Oxygen Isotopes
The source of nitrate in surface water varies both in seasons and spatial distribution. [PERSON] et al. [55] reported that precipitation and soil organic substance were the major sources of the nitrate in the northern part of Han River, but manure and sewage were the main sources in the southern part of Han River. With \(\delta^{15}\)N and \(\delta^{18}\)O isotopes, [PERSON] et al. [56] studied the surface water within the urban area of Beijing and found that the main sources of nitrate were manure and sewage. After investigating the pollution source of the nitrogen compound along the Olt River, [PERSON] et al. [57] found that inorganic nitrogen pollution was mainly caused by agricultural and domestic wastewater. Based on \(\delta^{15}\)N and \(\delta^{18}\)O, [PERSON] et al. [58] found that sewage was the main nitrate source of surface water, and assimilation, nitrification, and nitrogen fixation were the major driving factors under the complicated urban circumstances. The \(\delta^{15}\)N and \(\delta^{18}\)O has been widely applied in the source identification of nitrate in surface water within the above-mentioned small-scale basins and within other large-scale basins. Table 1 presents the data of the source identification of nitrate in surface water in large-scale basins all over the world. As suggested by the data, stable isotope method has provided useful information for analyzing the source and fate of nitrate as well as for the removal of nitrate. The nitrate source of surface water varies according to time and space. Besides, the nitrogen sources will go through several biochemical reactions (ammonium, nitrification, and denitrification) before entering rivers. However, these biochemical reactions are easy to be affected by land-use types, climate, hydrological conditions, and migration ways. At present, most of the researches only analyzed nitrate sources by comparing the \(\delta^{15}\)N and \(\delta^{18}\)O values of surface water with the \(\delta^{15}\)N and \(\delta^{18}\)O values from different sources. They ignored the influence of nitrogen sources on isotopic fractionation during migration and transformation, and failed to calculate the effect extent of fractionation on the results. Because of the complexity of the river system, simple isotopic geochemistry research is not enough to reveal the geochemical cycle process of river nitrate. Thus, it is of great significance to study the extent of these influences for identifying the nitrate pollution sources for tracing the migration and transformation ways of nitrogen. Furthermore, other indicators like hydrological conditions, water quality, and land-use types are necessary if detailed analyses on the source of nitrate in large-scale basin and on the pollution characteristics are to be conducted. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Watersheds** & **Area (km2)** & **Main Sources** & **References** \\ \hline Mississippipi River Basin, & \(2.9\times 10^{6}\) & Stream nitrogen assimilation & [59] \\ United States & & & \\ \hline Changjiang River Basin, & \(1.8\times 10^{6}\) & Nitrification (including “modified fertilizer”) and & [19] \\ China & & urban sewage effluent & [60] \\ \hline Yellow River Basin, & & \(\begin{array}{c}\text{Sewage/mannure effluents in the upstream;}\\ \text{mannure/sewage effluents and ammonium/urea-}\\ \text{containing fertilizer in the middle and lower reaches}\\ \end{array}\) & [61] \\ \hline Songhua River Basin, & \(5.6\times 10^{5}\) & Soil organic nitrogen, nitrogen fertilizers, and & \\ China & & sewage in the high flow season; soil organic nitrogen & [61] \\ \end{tabular}
\end{table} 
Table 1: Application of nitrogen and oxygen isotopes for the identification of nitrate source and fate in surface water in large-scale basins.