## 4 Conclusions  
In this study, we attempted to visualize the behavior of water migration in the waste layer during water injection tests using ERT monitoring at Site A, which is mainly composed of non-combustible residues, and Site B, which is mainly composed of incineration ash. The main results obtained are as follows.  
* Electrical resistivity tomography (ERT) was used to capture the changes in the two-dimensional and time-series distribution of water migration in the landfill, and it was possible to detect the existence of water channels and the dynamics of saturated regions that could not be captured by point methods.
* Water injection test monitoring clearly captured the large-scale vertical migration of water in the waste layer and the behavior of unevenly distributed water channels. The water infiltration rate was 3.10 cm/min at the landfill site containing mainly non-combustible residues and 0.77 cm/min at the landfill site containing only incineration residue.
* X-ray CT analysis of undisturbed samples revealed differences in pore structure depending on the type of waste landfilled. The porosity of the landfill layer composed mainly of non-combustible residues was 0.328, while the porosity of the landfill layer composed of incineration ash was 0.195. Since incineration residue is easier to compact than non-combustible residues, its porosity is smaller and its water permeability rate is lower. The results of the X-ray CT analysis were consistent with the infiltration rate measured in the injection test.
* Analysis of the change in resistivity has been shown to be effective not only as an indicator of changes in water content but also as an indicator of changes in the electrical conductivity of pore water due to washout (i.e., the progression of landfill stabilization). In landfills composed mainly of incineration ash, the salt in the ash is flushed. Thisresults in significant changes in resistivity and makes it easier to capture the washout process with ERT.
* It was confirmed that the characteristics and structural differences of the landfill (e.g., the presence or absence of a final cover, layout of gas extraction wells, etc.) have a significant effect on the water migration behavior and the resistivity structure. This will be an important factor to consider in the design and maintenance of future landfills.  
Conceptualization, Y.I. and H.L.; methodology, Y.I. and H.L.; software, Y.I. and H.I.; validation, Y.I. and H.I.; formal analysis, Y.I.; investigation, Y.I.; resources, Y.I. and H.I.; data curation, Y.I.; writing--original draft preparation, Y.I. and H.I.; writing--review and editing, Y.I. and H.I.; visualization, Y.I. and H.L.; supervision, H.I.; project administration, H.I.; funding acquisition, H.I. All authors have read and agreed to the published version of the manuscript.  
This research was performed by the Environment Research and Technology Development Fund (JPMEERF20213003) of the Environmental Restoration and Conservation Agency provided by Ministry of the Environment of Japan.  
Not applicable.  
The data presented in this study are available on request from the corresponding author. The data are not publicly available due to institute ownership rights.  
We would like to express our gratitude to [PERSON] of Obayashi Corporation, [PERSON] of Mitsubishi Materials Corporation, and [PERSON] of Leaf Air Corporation for the undisturbed sampling and water injection experiments at the landfill sites in this research.  
The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.  
The following abbreviations are used in this manuscript.