15In France, the monitoring of landfill cover after closure of the site is a local problem, since its 16 tightness must be ensured over time. Leaks in the cover are a problem, as they allow water to 17 infiltrate the stored waste. In order to locate such leaks, electrical resistivity tomography was 18 used on an experimental site in which defects had been intentionally made in the cover. 19Repeated measurements taken on this site showed that the weather conditions preceding the 20 measurements need to be taken into account, as they affect the water content in the cover 21 material. They also showed that there are optimal weather conditions for detecting defects in 22 the cover. A statistical analysis carried out on the electrical resistivity results for all surveys 23 and cover material samples showed that the material was heterogeneous; this variability was 24 mainly due to a difference in particle size (fines content) and in compaction. This study has shown the capacity of electrical resistivity tomography to detect defects and 26 heterogeneity in the cover material, indicating that it is a good means of monitoring the 27 quality of landfill cover both when it is put in place and subsequently.

International audienceOn closed landfills, impermeable covers are capping the waste in order to minimize water infiltration and accumulation of leachate inside the waste. In France, the cover composition depends notably on the kind of stored waste. In cases of hazardous waste, the cover must be composed of a drainage layer under the top soil and a geomembrane or a Geosynthetic Clay Liner (GCL) associated with an underlying 1.0 m thick low permeability material to ensure its tightness. However, this protection cover is sometimes damaged leading to an escape of landfill gazes and an unusual increase of leachate within the waste after rainy events. As leachate treatment is very expensive, it appears necessary to locate the weakness zones of the cover and assess their sizes in order to limit maintenance cost on landfills. In order to detect damages in the cover, the 2 following geophysical methods have been carried out on a French landfill: cartography with an Automatic Resistivity Profiling (ARP©), the Electrical Resistivity Tomography (ERT) and the Self Potential method (SP). The joint use of these methods has given us the opportunity to determine the contribution of each of them for cover's characterization. The ARP survey has put in evidence the lateral heterogeneity of cover materials on the whole landfill. The ERT has confirmed the variability of the cover composition but also provided information about the cover thickness and the damaging of the GCL. SP measurements have revealed a negative anomaly at the top of the landfill, possibly linked with a thin and damaged cover and therefore a greater proximity of waste. Finally, manual augers holes have enabled to associate electrical resistivity properties with different materials used in the cover and the damaged GCL. This study on a hazardous waste landfill shows that geophysical methods associated with manual auger holes have allowed to improve the knowledge of the cover. Thus, the damaged areas detected thanks to measurements performed on site may be useful for the landfill manager who can optimize the drilling survey necessary to check the nature of defects and then choose a suited remediation of the cover

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