## 3 Results and Considerations
### Pore Structure of Landfilled Waste  
Figure 7 shows cross-sectional images and X-ray CT analysis results for the waste samples from Site A and Site B. Visually, the samples collected from Site A contained incineration ash, non-combustible materials, and plastic waste, with numerous fine fibers observed in some areas. Samples from site B were primarily incineration ash but had a significant amount of metal fragments. The incineration ash had solidified, making it difficult to cut even with a motorized cutter. Therefore, the sample from Site B became smaller than that from Site A. At Site A, where non-combustible residues are the main component of the landfill waste, various materials such as fibers, plastics, incineration ash, and metals existed in the pores. In contrast, at site B, where incineration residues are the main component of the landfill waste, the pores were scattered compared to Site A, indicating a more compact porous medium. Three-dimensional image data obtained through X-ray CT analysis can be used to quantitatively evaluate the porosity and tortuosity of a porous structure in waste. First, image processing was applied to the acquired CT images. Thresholding was performed to binarize the solid and void regions. The ratio of void pixels to total pixels on a horizontal slice of the binary images was calculated to determine porosity. Next, tortuosity is evaluated by calculating the ratio of the geometric length of the flow path within the pore network to the straight-line distance. The flow path can be estimated using the fluid analysis of the porous structure. Thus, the geometric length of the flow path is determined by producing the flow vector and time step. On the other hand, the straight-line distance can be obtained by projecting the flow vector onto the depth vector. Figure 7: Undisturbed samples and their CT images: (**a**) non-combustible waste layer at Site A; (**b**) incineration ash waste layer at Site B. The tortuosity of the fluid flow path is quantified by dividing the geometric length of the flow path by the straight-line distance from the starting point of the flow to the endpoint. Figure 8 shows the distribution of void ratio in the depth direction. In the non-combustible residue landfill layer, the average value was 0.328, but it decreased with increasing depth. In the incineration ash landfill layer, the average value was 0.195. The incineration ash at Site B was landfilled at least 7 years ago and had solidified naturally, resulting in low porosity. No change in porosity with depth was observed in the incineration ash landfill layer. Figure 9 shows the tortuosity histogram. tortuosity is an indicator of the meandering nature of flow paths, with higher values indicating greater meandering. The tortuosity of the incineration ash landfill layer had a sharp histogram with a peak value of 1.07, while the non-combustible residue landfill layer had a broad histogram with a peak value of 1.44. When comparing the tortuosity of non-combustible residues and incinerated ash, significant differences were observed, suggesting that tortuosity could serve as one of the indicators for assessing the presence of water pathways. Figure 8: Porosity distributions in the z-direction, estimated from three-dimensional CT images. Figure 9: tortuosity distributions estimated from three-dimensional CT images. [PERSON] et al. reported that low resistivity values were detected in areas with dense fault planes, suggesting that these areas serve as pathways for seepage water [16]. This supports the idea that areas of relatively high porosity may act as water pathways. Site A had higher resistivity values overall, despite the shorter time elapsed since landfill closure compared to Site B. It suggests that this difference is due to not only variations in the quality of the landfill waste, but also progress in pollutant washout by rainwater infiltration.