Abstract
In landfill management projects, such as stabilization and management of groundwater pollution caused by leachate, it is very important to visualize permeating water paths non-destructively. In addition, incinerator ash containing cesium at levels below 8,000 Bq/kg has been placed in controlled landfill sites since the 2011 Fukushima nuclear accident. These sites have been built so that rainwater can easily permeate through the fill to accelerate stabilization. On the other hand, cesium is very soluble in water. Therefore, it is also necessary to clarify the water paths in the fills to prevent cesium from escaping into the environment. However, landfills are composed of various types of waste that are heterogeneously distributed, so a suitable visualization technique has not yet been proposed. We propose that changes in leachate content and quality might be reflected in the resistivity of fills. Therefore, we measured resistivity accompanying leachate level changes both in a laboratory setting and in field resistivity prospecting. In the laboratory experiment, a simulated landfill layer consisting mainly of incinerator ash was prepared in a rectangular tank. The resistivity prospecting methods were applied at high and low leachate levels. After measurement, a number of undisturbed core samples were taken from the fills based on resistivity profiles. We measured the hydraulic conductivity and main chemical components for each sample to elucidate parameters that affected resistivity. In the field experiment, resistivity was measured before and after forced drainage in an industrial landfill site to check the results of the laboratory test. The leachate table was not deeper than 1 m. At first, the resistivity at this level was measured along 4 prospecting lines, and drainage work by pumping up leachate from a collecting pit was performed for about 30 days. After that, resistivity along the same lines was measured, three-dimensional inverse analysis was applied to the apparent resistivity data, and 3-D profiles were generated. Differential profiles between areas with high and low leachate levels were also calculated. The results showed that it is difficult to identify a permeable zone using a simple resistivity profile. However the water path in the fills can be visualized. Because the resistivity's rate of change in the leachate table levels is closely related to the permeability of the fills in the laboratory experiment and to the conductivity of the leachate in the field experiment. Furthermore, it became clear that water permeated almost vertically in fills toward the filtration layer with the drainage pipes.
