## 2 Materials and Methods  
### Site Setting  
The real-world site is a chemical plant that was used to produce pesticides, such as hexachlorocyclohexane (HCH). The site consists mainly of HCH production and storage areas, including HCH production buildings, storage tanks, acid tanks, and loading and unloading platforms. The plant was established in 1938 as a large chloralkali chemical company. In the 1950s, the plant began the mass production of raw HCH powder. This was prepared by passing chlorine into pure benzene under light. In 1983, production was halted due to the law. In 1986, the plant resumed the production of HCH as a raw material for the synthesis of trichlorobenzene, which continued until 1995. From 1995 to 2000, the plant purified about 7000 tons of HCH that it held in stock, and then the alpha-HCH and beta-HCH were used to produce trichlorobenzene by co-heating them with lime milk, while the gamma-HCH was used to produce lindane for export by extraction, centrifugation, and crystal precipitation. In addition, the plant has historically produced pentachlorophenol and benzene chloride. The distribution of major facilities is shown in Figure 1a. During production, HCH has risk of leakage. Due to the long service life, the storage tanks may become damaged. There is also a high risk of leakage from the handling of the products and raw materials. Therefore, there may be multiple sources of pollution within the survey area. The site is located in the north temperate monsoon subhumid continental climate zone. The landform type of the site is a flood plain, which is relatively flat and the altitude is about 43 m. The stratum structure of the site is evenly distributed. The surface layer is dominated by miscellaneous fill down to about 2.1 m, consisting of construction waste, cohesive soil, etc. Underneath this is brown-yellow silty clay (with a thickness of about 1.4 m) and brown-yellow medium sand (with a thickness of about 4.4 m), then brown-yellow coarse sand (with a thickness of about 1.3 m), brown-yellow silty clay (with a thickness of about 1.3 m), brown-yellow fine sand, and gravel sand (with a thickness of about 6.0 m) (Figure 1b). The range and examples of resistivity of common stratigraphic materials are shown in Table 1 The shallow groundwater of the site occurs in the Quaternary Holocene  
Figure 1: Schematic diagrams for (**a**) the original plant layout, survey line distribution, and sampling well distribution; (**b**) the north-south hydrogeological cross-section of the site and stratum description. pore phreatic aquifer. The main medium of the aquifer from top to bottom is fine sand, medium-coarse sand, gravel-containing medium-coarse sand, and gravel (see Figure 1b below). The water level is 6.0-7.0 m. The thickness of the aquifer is 15.0-20.0 m and the permeability coefficient is about 7.0-9.0 m/d. Due to the existence of the cohesive soil layer in the vadose zone, atmospheric precipitation and evaporation have little impact on groundwater dynamics. Lateral runoff is the main source of replenishment and the main discharge method is also lateral runoff discharge. The flow direction of the groundwater is northwest to southeast and the hydraulic gradient is about 5-7.5%. We tested the groundwater levels in six wells on the site, as well as the pH, dissolved oxygen, and conductivity (which we tested twice: once in June 2018 and once in April 2019) (Table 2) of the water samples on the site. The wells' positions are shown in Figure 1a. The pH value of the groundwater in the study area ranged from 3.38 to 6.98. The groundwater in this area was mainly composed of Na\({}^{+}\), Ca\({}^{2+}\), and Cl\({}^{-}\) ions (Figure 2). In addition, the groundwater also contained a certain amount of dissolved phase iron and manganese. The range of Eh was between \(-\)39.7 and 1060 mV, the groundwater was in a reducing environment, and the variation range of DO was between 1.82 and 8.71 mg/L.