## 2 Materials and Methods
### Conceptual Model of ERT Method  
ERT [47,53] is an array resistivity measurement method. The layout of the detection electrode depends on the actual geological conditions and pollution distribution of the site. ERT can quickly measure the resistivity of two-dimensional geoelectrical cross-sections by densely arranging points and it can process the data in real time at the collection site to ensure the accuracy of the data. Based on the electrostatic field theory, the electric field is established with a power supply at two points: A and B. Then, we can measure the potential difference between M and N and calculate the resistivity between the two points (Figure 3). Due to the inhomogeneity of the electrical properties of the ground at the actual site, this method obtained the average value of the resistivity of rock and soil within the range of the electric field, which is called the apparent resistivity.  
\[\rho=K\,(\Delta U_{\text{MN}}/I), \tag{1}\]  
\[K=2\pi/(1/\text{AM}-1/\text{AN}-1/\text{BM}-1/\text{BN}), \tag{2}\]  
where \(\rho\) is the apparent resistivity, \(\Omega\cdot\text{m}\). \(\Delta U_{\text{MN}}\) is the voltage value between the two points of MN, V. \(I\) is the current value, and A. \(K\) is defined as the electrode device coefficient, which is affected by the spatial position of the electrode. AM is the distance between the two points A and M, m. AN is the distance between the two points A and N, m. BM is the distance between the two points B and M, m. BN is the distance between the two points B and N, m.  
\begin{table}
\begin{tabular}{c c c} \hline \hline  & \multicolumn{2}{c}{**Groundwater Resistivity (\(\Omega\cdot\text{m}\))**} \\ \cline{2-3}
**Well** & **June 2018** & **April 2019** \\ \hline GW1 & 6.053 & 6.536 \\ GW2 & 32.787 & 15.221 \\ GW3 & 29.070 & 13.477 \\ GW4 & 3984.064 & — \\ GW6 & 8.104 & — \\ GW7 & 1131.222 & 2840.909 \\ \hline \hline \end{tabular} Note. “—”: Some wells’ data are missing.  
\end{table}
Table 2: The resistivity of the water samples.  
Figure 2: A piper diagram for the groundwater geochemistry at the site.  
The electrical changes of the underground space medium were obtained by inputting direct current underground. The difference between the detection target and the surrounding medium was obtained and then we identified the potential pollution sources and the distribution of the pollution plumes. Due to the injection of contaminants, the electrical properties of the formation changed, which resulted in a huge difference from the surrounding formation. The ERT test results produced an abnormal signal and the pollution range was delineated according to the range of the abnormal signal, as shown in Figure 3.  
ERT has various arrangements, including the Wenner array, Schlumberger array, pole-pole array, dipole-dipole array, etc. Compared to other forms, the Wenner device is less affected by the unevenness of the surface, the electrode lateral structure is highly sensitive, the signal strength is good, and the result is relatively smooth. Therefore, this research used the Wenner arrangement method to detect the distribution of pollution. The measurement process was layered measurement. The first layer measurement was started from point A. Initially, AM = MN = NB = \(a\). After one measurement, the four electrodes A, B, M, and N were all moved to the next electrode along the survey line and measured again. We repeated the above steps until B reached the last electrode. A, B, M, and N returned to the first electrode when the measurement was complete. Then, the distance between AM, MN, and NB became 2\(a\). Electrodes A, B, M, and N measured the second layer at a distance of 2\(a\). When electrode B reached the end of the survey line, the measurement ended. We repeated the above process until the test of the entire profile was complete.