## 3 Results and Considerations
### Results of Water Injection Test at Site B  
Figure 6 shows the measured resistivity distributions before and after the water injection test at Site B. Before the water injection test, the overall resistivity at Site B was lower than at Site A (Figure 6a). The main components of waste are inert, non-combustible residue at Site A, and incineration ash at Site B, as mentioned in Section 2.1. Thus, it was considered that the large amount of soluble chemicals such as salt and water content remained high at Site B even after 7 years of landfilling and the progress of washout was still limited compared to Site A. Additionally, the water infiltration rate during the initial water injection test was 3.10 cm/min at Site A and 0.77 cm/min at Site B. This indicates that Site B, which only accepts incineration residue, has not only higher salinity but also lower permeability and slower washout conditions than Site A, which only accepts non-combustible residues.  
The resistivity structure before water injection shows that, although there was no soil cover layer as in Site A, resistivity regions of 15 \(\Omega\cdot\)m or higher were distributed horizontally from the ground surface to a depth of 1 m. Below 1 m, very low-resistivity regions of 5 \(\Omega\cdot\)m or lower were widely distributed, especially in the 12 to 20 m horizontal direction (Figure 6a). No significant change in resistivity was observed in the surface layer at a depth of 0.5 m from the ground surface as a result of the water injection test. As in Site A, this is thought to be due to the fact that the water injection test did not affect the change in water content at the soil surface due to the structure of the injection trench. It is clear that the positioning of the survey line installation and the adjustment of the exploration depth in the ERT measurements have a significant impact on the analytical resolution. Three hours after the first water injection, a significant decrease in resistivity was observed at depths greater than 1 m in the 7-17 m horizontal area where the injection trench was located (Figure 6b). And 67 h later, prior to  
Figure 5: Resistivity change ratios after the third water injection test at Site A: The red arrow represents the ERT survey line. The white dotted arrow at the left side represents a significant change in the vertical direction. The other arrows at the center and right side represent water moving downward and the effect of a nearby gas extraction well.  
the second water injection test, resistivity increased and showed a tendency to approach the pre-water injection resistivity structure (Figure 6c). These resistivity changes were particularly pronounced in the region from 11 m to 16 m horizontally and from 1.5 m to 4 m in depth. Visual inspection of infiltration during water injection also showed a tendency for faster infiltration in the area between 12 and 17 m of the trench, indicating that the amount of infiltrating water moving in this area was large and that it may have become a water channel. While maintaining a lower resistivity than before water injection, the second and third water injection tests also showed a similar trend of a decrease in resistivity immediately after water injection and a gradual increase thereafter (Figure 6d-g), returning to the same resistivity distribution as before water injection after 240 h (Figure 6h).  
Figure 6: Measured resistivity distributions before/after water injection test at Site B. The red arrow represents the ERT survey line.  
These results indicate that water movement can be visualized even at Site B, and that ERT monitoring is an effective method for detecting and visualizing water channels non-destructively, regardless of differences in landfill waste type and landfill age.