A mixed-method approach for the investigation of riverine nitrate pollution at catchment scale

It is important to have an insight into the fate of riverine nitrate at the catchment level. The idea is to apply different methods for one same study site, in order to overcome their limitations and take advantage of their strong points for the research. An approach with necessary steps were proposed to be conducted: 1) evaluation of the current nitrogen status in the water bodies; then 2) identification of the major nitrogen input sources; and when possible 3) development of a model which is able to simulate the nitrogen dynamics within the river catchment. A mixed-method approach, combining the experimental works (sampling campaigns and labs), statistical analysis (multivariate tool PCA, dual stable isotope tool), and modelling (isotope mixing model SIMMR, ecohydrological model SWAT) was proven to be an effective approach to investigate the fate of nitrogen, especially nitrate, at the river catchment level. In this dissertation, the Fuhse River catchment was chosen as the study site for investigating the status and dynamics of the riverine nitrate. The results from hydrochemical statistical analyses showed that water of the Fuhse River was an evolved type with mix abundances of ionic concentrations. It was classified into the main hydrochemical group of Ca-Cl-SO4, which represented that the water was influenced by anthropogenic activities. After that, the dual isotope method and the mixing model SIMMR indicated the soil nitrogen, manure and sewage as the predominant nitrate sources in this river water, which might lead to the nitrate of the river exceeding the natural threshold concentration. Finally, the incorporation of isotope results into SWAT has improved the robustness and the creditability of model. The nitrate loads were successfully simulated by the model