The electrical resistivity and the self-potential methods applied to contaminated sites

The purpose of this work is to demonstrate that electrical resistivity and self-potential methods can provide spatial information about the fluid conductivity and the redox potential of a contaminant plume. These methods can therefore be used to optimize the implantation of piezometers. The application of the electrical resistivity tomography to the Entressen landfill (Provence) provided a map of the fluid conductivity of the aquifer, which helped us to determine the extension of the contaminant plume. The self-potential signals, which are passively measured over contaminated sites, are associated with two main mechanisms, which are the fluid flow (streaming potential) and the presence of organic-rich contaminants in an aquifer ('electro-redox' phenomenon). Strong electrical potential anomalies have been evidenced above contaminated sites, but no physical explanations have been proposed. Therefore, we have developed a natural (bio)-geo-battery model, in which bacteria allow the transfer of electron between the reduced and the oxidized parts of the aquifer. The resulting current density produces self potential anomalies recordable at the ground surface. In order to validate this model, we have realized self potential experiments on two contaminated sites (Entressen in Provence and Trecate in Italy), and on a sandbox with sulfato-reducing bacteria. Both the field and laboratory results are in accordance with the geo-battery model, showing a linear coupling between the self potential signals and the redox potential. (author)