Geoelectrical investigations (DC) on a contaminated site during biostimulation: monitoring results and resolution analysis
Caterina, David[Université de Liège - ULg > Département Argenco : Secteur GEO3 > Géophysique appliquée >]
Nguyen, Frédéric[Université de Liège - ULg > Département Argenco : Secteur GEO3 > Géophysique appliquée >]
Université de Bonn
[en] Biostimulation ; Contaminated site ; geophysics
[en] In Belgium, as in many other countries, relatively anarchic economical and industrial development of the past century has resulted in a significant number of contaminated sites. When one of these sites poses a risk to human or ecosystem, measures need to be taken to clean it up. Among these measures, methods using in situ bioremediation are beginning to become more important because of their ease of implementation and their relatively low cost. However, it is often difficult to ensure their effectiveness except by carrying out extensive drilling and sampling, which can be long and expensive while offering only punctual information. Thus, it becomes necessary to use other techniques to overcome these shortcomings. Recently, an increasing interest is being born to use geophysical methods as tools for remediation monitoring. As part of our work, we conducted several electrical resistivity tomography campaigns on a bus station located in Bassenge (Belgium) which has undergone a contamination of hydrocarbons (gasoline) for several years and on which a biostimulation remediation device was set up in order to clean it up. The aim of our investigations was to study the electrical response of the contaminated area during the remediation phase and whether electrical resistivity tomography allowed to monitor its effectiveness. After a year of monitoring, the time lapse images obtained show a significant decrease of electrical resistivities (up to -40%) during biostimulation at the location of the main contaminant plume and an increase again of resistivities from the time the biostimulation was stopped. The electrical response during the biostimulation is in agreement with the models presented by several authors in the literature. The increase again of resistivities after the stimulation is however more surprising and can be explained by several physico-chemical (sorption-desorption processes) or biological (decrease of conductive biofilms) assumptions. The results obtained tend to suggest that it is possible to use electrical resistivity tomography as a tool for qualitative control during the remediation of a contaminated site. However, for a more quantitative use of resistivity models, it is important to assess their reliability through the use of resolution indicators. We therefore developed a methodology to address this issue based on the creation of synthetic models representing simplified cases of field resistivities and we applied it on our case study. The results obtained provided us important information about the reliable parts of the resistivity models. These findings may lead in the future to the development of mathematical models that can link quantitatively geophysical properties to the level of (de)contamination of a site.
Fonds pour la formation à la Recherche dans l'Industrie et dans l'Agriculture (Communauté française de Belgique) - FRIA