## 2 Materials and Methods
### Sampling Sites and Methods
Point F is located on the right side of Dongshuimen Bridge, near a sewage outlet. The water pollution is serious, and the water surface is yellowish. Point G is 100 m from point F. There is a sewage outlet nearby. Point H is located in Sunshine 100 International New City, surrounded by small super high-rise residential areas. Point I is located in Changjiang Art Plaza, with the Chongqing Cigarette Factory and Experimental Middle School within 1 km; Point J is located in Changjiahui shopping park (a large commercial center), surrounded by high-rise residential areas. Figure 3: Functional layout of land use along the river. To facilitate the research and protection of urban environment and water quality, Chongqing municipality has unified the monitoring time of surface water quality in the Yangtze, Jialing, and Wujiang Rivers since 2011. The samples were taken 48 h after rain to avoid the dilution of water pollutants by rain, which would interfere with the research results. The sampling occurred from 07:00 to 10:00 a.m. to avoid volatilization caused by sunlight on the collected water samples. According to the sample volume regulation (Table 1) of the standard inspection index for drinking water, the water receiver was a 380 mL plastic bottle. Sampling occurred in hydrometric sections in the waterbed cross-section. Before sampling water, the water sampler was flushed three times with the substitute sample to ensure the water sample filled the container. After collecting the water sample, we measured the pH of the water sample immediately, then added concentrated sulfuric acid and adjusted the water sample to pH \(\leq\) 2, then refrigerated it at 4 \({}^{\circ}\)C to inhibit the metabolic process of microorganisms. The samples were sent to a laboratory for testing as soon as possible to prevent the deterioration of the sample. To prevent sudden changes in water quality at the intake point from affecting the measurement results, three samples were collected around each intake point for analysis. Research shows that such sampling method can reflect the average variation of concentration [18]. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Indicators** & **Methods** & **Instrument** & **Standard** \\ \hline pH & Glass electrode method & pH meter & GB 6920-86 \\ \hline \multirow{3}{*}{COD} & Rapid digestion & \multirow{3}{*}{SpeedDigester, DRB200} & Methods for monitoring \\  & spectrophotometry & & and analyzing water and \\ \hline \multirow{3}{*}{TP} & Basic potassium & UV/VIS & Methods for monitoring \\  & persulfate digestion & spectrophotometer, & and analyzing water and \\  & ultraviolet & UV-2450 SHIMADZU & wastewater \\  & spectrophotometry & & \\ \hline \multirow{3}{*}{TN} & Ammonium molybdate & UV/VIS & Methods for monitoring \\  & spectrophotometric & spectrophotometer, & and analyzing water and \\ \cline{1-1}  & method & UV-2450 SHIMADZU & wastewater \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Test method of each index. Figure 4: Water sampling sites at the south bank of Yangtze River in Chongqing. In addition to the water quality data set, we collected 15 other types of data (summarized in Table 1) on planning, infrastructure, land use, topography, and climate indicators to serve as our independent variables for the analysis.