# Analysis of the Water Quality Improvement in Urban Stream Using MIKE 21 FM
## 3 Selection and Current Status of the Target Area  
### Current Status of the Target Area  
Gulpocheon is a stream 20.73 km long, with a basin area of 131.75 km\({}^{2}\) with Galsandong, Bupyeonggu, as a starting point and Gimposi, Gyeonggido, as an endpoint. The upstream of Gulpocheon is the typical urban stream that passes through Incheon and Bucheonsi, and agricultural lands are widely distributed downstream [32]. The water quality of Gulpocheon has deteriorated due to the inflow of domestic sewage and industrial wastewater, slow flow, stream covering, inflow pollutants, and structural problems. In particular, small-sized factories near the stream, large-scale industrial complexes established midstream and upstream, and domestic sewage from increasing population are the primary sources of pollution of Gulpocheon. Furthermore, the structural problems of the stream, including straightened channels and slow flow rate, degrade the self-purification capacity of the stream. Hence, the water quality of Gulpocheon remains the lowest in the country [33]. Before the supply of the ozone-treated reclaimed water, Gulpocheon was supplied by Hangang untreated water from the Seoul Pungnap water intake station as the base flow of the upstream. However, although more than 75,000 tons of water needed to be supplied to Gulpocheon daily, only 20,000 tons of water, i.e., less than one-third of the supply plan, was supplied in a day due to the cost issues [34]. In particular, because of the precipitation characteristics of South Korea, the streamflow is extremely low in December to February compared with June to August, and it affected the water quality; thus, a continuous and sufficient supply is required. The ozone treatment facility connected with the sewage treatment plant alongside the supply pipeline connected to the Gulpocheon upstream was completed inside Gulpocheon in March 2019. The flow supply was enabled to utilize the reclaimed water. The annual concentration of water quality factors was compared to analyze the water quality improvement before and after the supply of the ozone-treated reclaimed water. After collecting the actually measured water quality data, it was applied to the numerical analysis model. The water quality monitoring spots within the target area are presented in Figure 1. There are Cheongcheoncheon and Gyesancheon as tributary streams in Gulpocheon. The water monitoring spots correspond to the upstream and midstream of Gulpocheon. For analyzing the degree of improvement of the water quality, the stream water quality environmental standard factors [23], i.e., BOD, DO, TN, and TP, were investigated. The correlation between the annual seasonal change and water quality factors was taken into account. Since the ozone treatment facility was completed in March 2019 and operated after that, the water quality of main spots in Gulpocheon 1 year before and after March 2019 was comparatively analyzed. The water quality information from April 2018 to March 2020, provided in the Water Environment Information System [23], was used for the water quality monitoring data. The flow data were additionally collected and utilized for the MIKE 21 FM model building. As shown in Figure 1, the supply points of the ozone-treated reclaimed water are the upstream and midstream (Gyesancheon) of Gulpocheon, and 30,000-45,000 m\({}^{3}\)/d and 13,000-20,000 m\({}^{3}\)/d of flux are provided to the upstream and midstream (Gyesancheon in  
Figure 1: Monitoring and analysis spots of Gulpocheon. coming point), respectively, since March 2019. In 2019, 30,000 m\({}^{3}\)/d of flux was discharged in the Gulpocheon upstream on average, and 15,000 m\({}^{3}\)/d of flux was discharged in the Gyesancheon (midstream of Gulpocheon). The discharge flow is adjusted by changing the base flow because the stream flux rapidly increases in the summer rainy season. For this reason, the water quality change sometimes rapidly takes place in the summer season. Figure 2 presents the 2019 supply data of the base flow in the Gulpocheon upstream and Gyesancheon.