Simulation of Nitrate Isotopic (δ15N and δ18O) by Coupling the Hydrology and Transport Processes Described by the SWAT Model

Abstract

To improve the reliability of methods to trace surface water pollutants in river basins, hydrological and water quality processes in the Fuxi River Basin were continuously monitored from 2013 to 2015, and the main pollution sources in the watershed and δ15N as well as δ18O in the rivers were measured simultaneously. The Soil and Water Assessment Tool (SWAT) model was used to simulate the NH4+ and NO3- migration processes in the hydrological processes of the land surface and rivers. On this basis, the processes of mixing, transformation, and fractionation of δ15N and δ18O in NO3- were coupled, and the simulation methods of δ15N and δ18O in the rivers were developed. The results showed that δ15N and δ18O in the rivers were mainly affected by the pollution sources in the river basin and the variation in runoff conditions during different hydrological periods. The contribution of the mixing process of different isotopes to the isotope abundance was 82.74%. The contribution of isotope fractionation in the process of nitrogen conversion was 16.26%. The influence of NH4+ and NO3- concentration deviation from the SWAT simulation on the simulation errors of δ15N and δ18O was 10.44%. The δ18O simulation errors were 18.72% larger than those of δ15N because of the higher variation range of δ18O in rainfall and the complexity of δ18O. The systematic errors and deviations of the simulated δ15N and δ18O results using the proposed method were less than 10% and 15%, respectively. The simulation method of δ15N and δ18O in the river basin has a clear physical meaning, which provides a useful approach for tracing nitrogen sources in rivers.