Practical method, based on measurements of soil (geo)electrical properties, gas content and δ13C(CO2), for monitoring the remediation by bio-stimulation of hydrocarbon-contaminated soils

International audienceHydrocarbon contaminated soils represent an environmental issue as it impacts on ecosystems and aquifers. Where significant subsurface heterogeneity exists, conventional intrusive investigations and groundwater sampling can be insufficient to obtain a robust monitoring of hydrocarbon contaminants, as the information they provide is restricted to vertical profiles at discrete locations, with no information between sampling points. In order to obtain more spatial or volumetric information on subsurface modifications, complementary methods can be used like geophysics. Among geophysical methods, geoelectrical techniques like electrical resistivity (ER) and induced polarization (IP) seem the more promising, especially to study the effects of biodegradation processes. Indeed, laboratory and field geoelectrical experiments to characterize soils contaminated by oil products have shown that mature hydrocarbon-contaminated soils are characterized by enhanced electrical conductivity although hydrocarbons are electrically resistive [1-3]. This high bulk conductivity is due to bacterial impacts on geological media, resulting in changes in the chemical and physical properties and thus, to the geophysical properties of the ground [4]. Moreover, microbial activity induced isotopic fractionation of carbon. Indeed, the light chemical bonds are easier to break, so the molecules with 12C will react preferentially. Thus, the produced CO2 is depleted in 13C and its ratio δ13C(CO2) will decrease. The French research project BIOPHY proposes to use electrical methods and gas, chemical and microbiological analyses to develop an operational and non-destructive method for in situ biodegradation monitoring of hydrocarbons in order to optimize soils treatment. In this presentation, we will show a field study located in the South of Paris (France), where petrol and diesel leaked from a tank in 1997. Before the in situ remediation by bio-stimulation, a characterization phase has been performed. Eight new boreholes have been drilled in addition to the 17 boreholes already present in the site. A geophysical campaign including ER and IP profiles, as well as resistivity and gamma ray logs have been performed to better identify the contaminant plume. Results have shown a more conductive and chargeable zone at around 3 meters depth, which match the polluted zone defined by geochemical borehole analyses. In order to stimulate biodegradation, a trench will be dug to supply oxygen to the water table and thus stimulate aerobic metabolic processes. Time domain and spectral induced polarization surveys will be performed regularly to monitor the stimulated biodegradation and the progress of depollution until the soil cleanup. The production of CO2 and the isotopic ratio δ13C in the produced CO2 will be measured by infrared laser spectroscopy. We expect to show an increase of the CO2 concentration and a decrease of δ13C due to the microbial activity. We expect to see an evolution of electrical conductivity and IP responses in correlation with gas, microbiological and chemical analyses. Our results will argue for combining geophysics with biogeochemistry studies and gas content and isotopic measurements to monitor zones of biodegradation and will allow us to give a practical method of monitoring and checking in situ remediation. [1] Sauck, 2000. "A model for the resistivity structure of LNAPL plumes and their environs in sandy sediments." Journal of Applied Geophysics 44: 151-165 [2] Abdel Aal et al., 2006. "Induced-polarization measurements on unconsolidated sediments from a site of active hydrocarbon biodegradation." Geophysics 71(2): H13-H24 [3] Che-Alota et al., 2009. "Case History Temporal geophysical signatures from contaminant-mass remediation." Geophysics 74(4): B113-B123 [4] Atekwana and Atekwana, 2010. "Geophysical Signatures of Microbial Activity at Hydrocarbon Contaminated Sites: A Review." Surveys in Geophysics 31: 247-28