| Summary: | The World Health Organization (WHO) and the U.S. Environmental Protection Agency (EPA) provide guidelines on the maximum levels of nitrate nitrogen (NO<sub>3</sub>-N) contained in drinking water since excess nitrate ingestion may harm human health. Thus, monitoring and controlling the NO<sub>3</sub>-N concentration is of paramount importance, especially in sources of drinking water such as the Nakdong River in South Korea. This study addresses NO<sub>3</sub>-N pollution in the Nakdong River in South Korea, where such pollution mostly comes from diffuse sources in the catchment due to the agricultural use of fertilizers. The objective of this study is to suggest guidelines for designing strategies to control NO<sub>3</sub>-N in this river using a process-based model developed with HEC-RAS. The model was built based on water quality parameters (water temperature, dissolved oxygen, ammonia nitrogen, etc.) related to NO<sub>3</sub>-N dynamics incorporating hydraulic and meteorological data. This model simulated NO<sub>3</sub>-N dynamics downstream under 55 scenarios while focusing on a section near locations of drinking water intakes. The scenarios were constructed based on variations in water quantity and quality upstream. The simulation results showed that the peak concentration of NO<sub>3</sub>-N downstream could be directly controlled by limiting the NO<sub>3</sub>-N concentration upstream. Additionally, control of the flow rate upstream could also lead to a reduction in the overall average concentration of NO<sub>3</sub>-N downstream, but this predominantly occurred when the NO<sub>3</sub>-N concentration was decreasing. In conclusion, the design and implementation of strategies for the control of NO<sub>3</sub>-N downstream should be carried out after performing a quantitative analysis of the impact of different control measures for different downstream conditions using a water quality model.
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