## 3 Results and Considerations  
### Results of Water Injection Test at Site A  
The layout of the ERT survey line and the water injection trench is shown in Figure 3. The water injection test at Site A was able to capture the time-dependent changes in resistivity structure caused by water injection using the 66 ERT monitoring sessions. The resistivity cross-sections before and 6 h after the first water injection test are shown as the change in resistivity before and after the water injection test (Figure 4). From the ground surface to a depth of 0.5 m, high resistivity of 100 \(\Omega\)\(\cdot\)m or higher was horizontally distributed (Figure 4a). This layer is the top cover soil layer above the waste layer, which is a gravel-mixed soil mined in the vicinity of Site A. A high resistivity area of 80 \(\Omega\)\(\cdot\)m was also distributed at depths below 3.5 m. The landfill structure of Site A suggests that this is due to the intermediate cover soil layer below the waste layer. On the other hand, the waste layer (0.5 to 3 m depth) sandwiched between the top and intermediate cover soil layers showed a heterogeneous resistivity distribution. In the 2 to 7 m and 8.5 to 13 m horizontal regions, the upper layers at depths of 0.5 to 1.5 m showed high resistivity of more than 100 \(\Omega\)\(\cdot\)m, while the lower layers at depths of 1.5 to 3 m showed a significant decrease in resistivity to less than 30 \(\Omega\)\(\cdot\)m. This means that the high water content in the deep layers has not changed even four years after the landfill was filled. Conversely, high resistivity of more than 150 \(\Omega\)\(\cdot\)m was dominant in the lower layers at depths of 1.5 to 3 m in the 16 to 18 m horizontal region. Six hours after the first water injection test, the resistivity of the entire waste layer decreased significantly (Figure 4b). In particular, in the 7 to 8.5 m horizontal direction, the resistivity was over 50 \(\Omega\)\(\cdot\)m before water injection but decreased to 30 \(\Omega\)\(\cdot\)m after water injection. However, in the 13-18 m horizontal zone, the decrease in resistivity was less than in other areas. It was considered because of the presence of a gas ventilation pipe in the vicinity, about 3 m away from this region (Figure 3). Several vertical gas extraction wells consisting of a 200 mm diameter PVC pipe and a protective layer of quarry stone (1 m \(\times\) 1 m) are installed in Site A. Since these gas extraction wells are connected to the collection and drainage pipes at the bottom of the landfill and also provide a drainage path for internal water, the pipes and protective layer have not only high porosity but also extremely low saturation, resulting in higher permeability and drainage than the waste layer. In addition, it is difficult for heavy equipment to compact the waste around the gas extraction well, and the porosity of the waste layer tends to be high. This suggests that in this area, injected water moved toward the gas extraction well and drained away, resulting  
Figure 3: Aerial image showing the second and third waste layers of Site A with the ERT survey line and gas extraction wells at Site A.  
in higher resistivity. These results indicate that the water entering from the injection trench moved heterogeneously in a subsurface direction and that multiple water channels exist in the waste layer. On the other hand, there was almost no decrease in resistivity in the surface cover soil layer during water injection. This is because both sides of the water injection trench were covered with impermeable panels, so that the injected water moved only in the vertical direction, and, thus, the moisture content in the intermediate soil cover layer did not increase. To quantitatively visualize the resistivity change, a cross-sectional distribution plot of the resistivity change rate (\(\Delta\rho\)) at 1, 4, and 95 h (Run 40, 45, and 65, respectively) after the third water injection test was obtained (Figure 5), with Run 38 immediately before the third water injection test as \(\rho_{init}\).