## 1 Introduction  
In the maintenance and management of final disposal sites for municipal solid waste (MSW), it is essential to appropriately monitor the stabilization condition of landfilled waste. The washout of pollutants due to rainwater infiltration and the occurrence of water retention within the landfill are critical factors affecting the degree of landfill stabilization [1]. Delays in landfill stabilization can have a direct impact on practical problems such as longer maintenance periods and cost increases. In Japan, almost all combustible general waste, except plastic, is incinerated as an intermediate treatment, while plastic, glass, and ceramic waste that cannot be recycled is crushed. As a result, the main landfill wastes are incineration ash and non-combustible residues. Numerous studies have been conducted on the washout of pollutants in landfills. It is well-known that organic substances and heavy metals are present in high concentrations in the leachate in the early stages of landfilling [2], and, therefore, methods are required to understand these dynamics. Traditionally, monitoring wells have been installed to investigate internal hydrological conditions and water quality, but the information obtained is primarily point-based and one-dimensional, and there are limitations in understanding the two-dimensional and spatial infiltration behavior. To address this issue, electrical resistivity tomography (ERT), a physical exploration technique, has gained attention as a method capable of visually representing changes in underground water content on a two-dimensional scale [3]. ERT involves installing multiple electrodes on the ground surface, repeatedly injecting current and measuring potential differences, and estimating the spatial distribution of resistivity through inverse analysis. Resistivity is a parameter that indicates how difficult it is for electric current to flow through soil. The current flow depends on the material composition and water content of the porous material, as well as other factors. In recent years, inverse analysis techniques have evolved significantly, including specialized 3D annular electrode configuration for the detection of leachate channel [4] and 4D inverse analysis considering temporal continuity [5], optimal electrode placement based on sensitivity distribution [6], and improved computational efficiency through the introduction of unstructured meshes and wavelet compression [7], enabling faster and more accurate estimation of resistivity distributions. In addition, for phenomena where temporal changes proceed in one direction, methods have been proposed that introduce positivity constraints or asymmetric time constraints to suppress noise-induced artifacts and extract physically consistent changes in subsurface structure [8]. ERT has demonstrated its effectiveness in hydrological and geological studies [9; 10; 11; 12]. Particularly in the field of waste landfill sites, it has been applied to underground structure surveys [13], detection of not only leachate distribution in landfill sites [14; 15] but also washout pathways in unmanaged landfill sites [16], evaluation of rainwater infiltration reduction by geomembranes [17], sensitivity analysis of liner failure [18], and quantification of the influence of geomembranes as boundary conditions on inverse analysis [19], the relationship with landfill gas of CH\({}_{4}\) emission [20], and the temperature during waste biodrying process [21]. Notably, [PERSON] et al. demonstrated that ERT monitoring at a landfill could describe leachate infiltration and their flow pathway [22]. However, there are few cases where not only the results of ERT surveys have been compared across multiple sites with different landfilled wastes, but also the application of temporal and spatial high-resolution ERT monitoring has been used for exploring a water channel in landfill.