Tracing the sources of organic carbon and nitrate in aquatic environment using stable isotopes and hydrological modeling

Abstract

In this thesis, stable isotope-based organic matter and nitrate sources within the categories of land use were attempted. Further, the impact of excess anthropogenic nutrient inputs on elemental stoichiometry (Redfield Ratio, nutrient condition) and nutrient cycle in the water column was evaluated. For this purpose, water column samples were collected and analyzed in the river catchment and the estuary environment. Carbon, nitrogen, and oxygen stable isotope ratios (δ13C, δ15N, δ18O) were used to trace the origin of organic matter and nitrate. Furthermore, the hydrological modeling technique was used to cross-validate the stable isotope ratio approach and quantitatively estimate the land-use type-specific OM and nitrate flux. The overall results of the thesis focused on source identification and flux estimation within the land-use boundaries to support the water quality diagnosis in aquatic environments. To verify the applicability of the stable isotope ratio approach for nitrate source tracing according to land-use type patterns, the nitrate dual isotope ratios (δ15NNO3 and δ18ONO3) in the Gumho River was measured. Further, these results were compared to a watershed model (Hydrological simulation program for FORTRAN-HSPF). Differential ranges depending on the surrounding land use types (forests, urban and industrial complexes, agriculture, and wastewater treatment plants (WWTP)) were identified by nitrate dual isotopic composition in surface water samples. This result indicated soil N, sewage, synthetic fertilizers, and WWTP effluents are the main sources. The results of the HSPF indicated that the total nitrate load in the Gumho River might be controlled by hydrological factors, such as water discharge. Further, nitrate loads in the mainstream represented a significant increase after the inflow of urban tributary. A comparison of the contribution of each of the four land use types results based on the two approaches represented relatively high correlations for urban areas, forests, and WWTP, meanwhile, agricultural areas show low correlations. This was attributed to differences in the specific properties of surface runoff, ground infiltration, and groundwater runoff among the land-use types, indicating the need to consider the area and use of the land type. Further, integrating the two complementary techniques may provide land-use type specific nitrate flux in the water column. To trace the non-point source in the estuary environment, stable isotope ratios in water column samples from Sihwa Lake were analyzed. For this purpose, particulate and dissolved organic matter and nitrate inputs from different land types (urban, industrial, and wetland) were identified according to typhoon events. The carbon and nitrogen concentrations and stable isotope ratios of particulate (δ13CPOC and δ15NPN) and dissolved matter (δ13CDOC) in the water column significantly differed before and after typhoon events. In the pre-typhoon, relatively natural sources (soil, algae) were dominant. In contrast, after typhoons, the contribution of anthropogenic sources such as road dust and fossil fuel combustion increased significantly. The nitrate concentration and dual isotopic composition in the same period also showed significant differences between pre-and post-typhoon. In the pre-typhoon, relatively natural sources such as soil, dust, and wetlands dominated, while anthropogenic sources such as road dust and sewage increased after typhoons. These results indicate that massive freshwater discharge may inflow large amounts of anthropogenic organic matter and nutrients within the land-use boundaries into the estuary environment. In this thesis, the elemental stoichiometry (Redfield ratio) was compared by nutrient ratios to identify the impact of excess anthropogenic nutrient input. Before the typhoon, the water column was mostly N and P-limited conditions, except in industrial areas where anthropogenic sources were prominent. Meanwhile, due to large amounts of anthropogenic nutrient inputs, the water column was relatively N and P-rich in areas adjacent to the land surface in the post-typhoon. When weather and water temperature conditions are met, these conditions can lead to harmful algal blooms. Further, this condition may cause changes in algal community composition and algal size in aquatic ecosystems depending on nutrient imbalances. These results indicate that excess discharge of anthropogenic nutrients needs to be monitored to preserve water quality and marine ecosystems. In this thesis, the stable isotope ratios of organic matter (POM and DOM) and nitrate were applied to identify specific pollutants within land use categories (urban areas, industrial complexes, wetlands, agricultural lands, and forests). Further, it was identified that excess anthropogenic nutrient inflow might cause an elemental imbalance in the water column, resulting in changes in the nutrient cycle and environmental pollution (algal bloom). The research methods and results in this thesis can be used to trace the pollution sources in various land use types. Further, this approach could provide the scientific data with the land-use type-specific pollutant flux, which may support water quality management. The stable isotopic compositions of nitrate in the aquatic system may altered by enhanced human activities and various biogeochemical processes. This phenomenon may hinder precisely tracing pollutant sources within diverse land-use patterns. Multiple stable isotope ratios (δ11B, δ18O, δ2H, δ15N) will be applied in the future plan to overcome these limitations. Through this approach, urban sewage and livestock wastewater may be distinguished, and further specific nitrogen cycle processes (e.g., nitrification and denitrification) in the aquatic system may be identified. Further, a systematic approach in this thesis and multi-year monitoring data accomplished by the KME and the KMOF will be integrated for additional future research. If the water quality deterioration by potential pollutant discharge is confirmed, the stepwise approach will be used to trace the specific pollutants flux within the land-use boundaries. Ultimately, through further research plans, scientific data may be provided to support the water quality control strategy of the government department, such as the total water pollution control policy.