## 3 Results and Discussion
### Hydrochemical Verification  
The test results of the groundwater monitoring wells are shown in Table 4 and the pollutant distribution map was drawn according to these test results, as shown in Figure 7.  
According to the test results, the concentrations of trans-1,2-dichloroethylene, 1,2,3-trichlorobenzene, and hexachlorocyclohexane were all higher than the standard (National Standards of the People's Republic of China: Standard for groundwater quality (GB/T \begin{table}
\begin{tabular}{c c c c} \hline
**Groundwater Well** & **Trans-1,2-Dichloroethylene** & **1,2,3-Trichlorobenzene** & **Hexachlorocyclohexane** \\ \hline GW1 & 703 & 23 & 1 \\ GW2 & 1.1 & \textless{}1.0 & 0 \\ GW3 & 1.7 & 41.2 & 13 \\ GW4 & 61.3 & 316 & 72 \\ GW7 & 9.4 & 103 & 783 \\ GW8 & 5.5 & 102 & 65 \\ \hline \end{tabular}
\end{table} 
Table 4: The monitoring results of water samples, unit: \(\mu\)g/L. Figure 7: A schematic diagram of the concentration distribution of organic pollutants in the groundwater: **(a)** the concentration distribution of trans-1,2-dichloroethylene; **(b)** the concentration distribution of trans-1,2-dichloroethylene; **(c)** the concentration distribution of hexachlorocyclohexane. 14848-2017). (This standard will be used in the rest of the article). Trans-1,2-dichloroethylene was mainly concentrated in GW1, GW4. The water of GW4 (61.3 \(\upmu\)g/L) and GW1 (703 \(\upmu\)g/L) was beyond level V, which means the pollution was very heavy. GW1, GW3, GW4, GW7, GW8 all displayed high concentrations of 1,2,3-trichlorobenzene. GW1 (23 \(\upmu\)g/L), GW3 (41.2 \(\upmu\)g/L), GW7 (103 \(\upmu\)g/L), GW8 (102 \(\upmu\)g/L) were level IV, and GW4 (316 \(\upmu\)g/L) was level V. Hexachlorocyclohexane gathered in GW3, GW4, GW7, and GW8. The highest concentration was GW7 (708 \(\upmu\)g/L), which was level V. GW3 (13 \(\upmu\)g/L), GW4 (72 \(\upmu\)g/L), GW8 (65 \(\upmu\)g/L) were all level IV. The whole study field was polluted and needs remediation. The water sample test results show that the groundwater at the site was mainly composed of three pollutants: trans-1,2-dichloroethylene, 1,2,3-trichlorobenzene, and hexachlorocyclohexane. Due to the limited number of wells, there were some errors in the detection of contaminant distributions across the site. Nevertheless, the overall trend was correct. The HCH concentrations were highest near GW7 and the trans-1,2-dichloroethylene concentrations were highest near GW1. Overall, when the groundwater flowed upstream, the concentration of pollutants dropped sharply, so the concentration of pollutants near GW12 was 0. The main pollutant near GW7 was hexachlorocyclohexane, which could be due to a leakage occurring in the production process of hexachlorocyclohexane. The total highest concentration was 783 \(\upmu\)g/L. Furthermore, the resistivity of the groundwater reached 1131.222 \(\Omega\)-m and 2840.909 \(\Omega\)-m in June 2018 and April 2019, respectively, which also illustrates the high concentration of pollution in groundwater. It diffused mainly in the southeast direction, along with the groundwater flow, and the concentration in the southwest region was basically zero. Probably due to the overall detection depth of ML-I being too shallow and in parallel with the groundwater flow direction, only the 25-33 m region at a 3 m depth showed contamination. The area of 0-20 m on the ML-II profile, with more than 500 \(\Omega\)-m resistivity, could be the remains of the acid tank. The east of acid tank was more than 150 \(\Omega\)-m. This could be caused by a high concentration of HCH. We suspected that there was another source to the west of the HCH production building (Figure 6). Theoretically, pollutants are most concentrated near to the source and then spread out. Pollutant concentrations decline most slowly downstream of groundwater flows, while concentrations decline rapidly upstream of groundwater flows. On the profile ML-II, this anomaly could be observed.