## 2 Materials and Methods  
### Sampling Sites and Methods  
The total length of the river section considered in this study was 15 km. Satellite images were used as the basic information source, and remote sensing images were interpreted and vectored to obtain vector data of the south bank area. Based on the current land use, urban functional zoning, and the field investigation results, we drew a schematic diagram of land use and functional zoning is, as shown in Figure 2. Combined with the construction land planning of the main urban area in 2014, the main types of land used in the riverside section of south bank were divided into residential land, commercial service facility land, green land and square land, and public management and public service facility land. Large residential areas included Coral Water Bank, Haitang Xiaoyue, Wuyi Riverside, Sunshine 100 International New City, etc. Large commercial areas included Dongyuan 1891, Changjiahui shopping park, etc. (Figure 3). The study area included seven sewage outlets and two drinking water protection areas, which are the first and secondary protected area, respectively, located on the left and right side of Dongshuimen Bridge, respectively. Figure 1: The water system distribution map of the main urban area of Chongqing. Figure 2: Land use and functional classification. In this study, we focused on total phosphorus (TP), total nitrogen (TN), and chemical oxygen demand (COD) due to their recognized impacts on aquatic ecosystems and typical association with urban areas across the region (regulations on the division of drinking water source protection zones in Chongqing, 2002). First, the waters in the study were monitored for one year. After a long-term sampling test, we found that within the studied shoreline length, the data at each sampling point did not change significantly in some time periods. Secondly, due to the three periods of high water, low water, and flat water, the main body of economic development along the Yangtze River differs, resulting in different sources of pollution during different water periods. Therefore, we discussed problems according to different water periods. The study area has high construction density, which is a typical urban center/riverside area, different from the whole city (the area studied is relatively large). Finally, considering the uncertainties of long-term observation data sampling, three samples were obtained at the same sampling point each time. If the error difference between two groups of samples was 5%, resampling was conducted. If the error difference was less than 5%, the average value of the three groups was taken as the final measurement result. For these four reasons above, water samples were collected from 10 different areas along Nanbin Road along the Yangtze River in May and October 2019. The two sampling times were selected because May and October are before and after the Yangtze River flood season. Analyses were undertaken by accredited laboratories using standard methods and procedures: HJ 495--2009, TP and heavy metals-GB 11893-89, and HJ 636-2012 in China. According to Water Quality-Guidance on Sampling Techniques of Rivers, HJ/T 52-1999, the sewage discharged into most rivers can be fully vertically mixed within 1 km. In this case, a sampling point can be arranged at any depth in the vertical direction. Ten water extraction points were selected in this study, as shown in Figure 4. Point A is a gathering place for hotels and other commercial service facilities, located under the Yangtze River bridge. Point B is a sewage outlet in Huangge Wandu, surrounded by high-rise residential areas and several super high-rise hotels. Point C is located in Yanyu Park; nearby are high-rise residential groups and supporting businesses along the street; a sewage outlet exists near point D, which is a secondary protection area for drinking water. Point E is at the lower left of East Watergate. A sewage outlet is nearby and a university is 1 km away (Chongqing Institute of Socialism).