A field experiment was conducted over a Light Non-Aqueous Phase Liquid (LNAPL) contaminated site located near a gas station where a tank had leaked its content. In this area, a dolomitic bedrock is overlain by 5 to 9 m of clayey sands. In order to delineate the plume and measure piezometric heads and depths to bedrock, ten boreholes were drilled and four cone penetration tests were conducted. Soil and water samples were collected and analyzed. Although these samplings highlight the presence of hydrocarbons in some boreholes, the plume is poorly outlined due to the limited number of drillings.
To predict the response of hydrocarbon-contaminated areas in this specific context, a laboratory study was undertaken. It shows that an increase in resistivities of about 40% should be found in highly polluted areas.
To assess the contribution of geo-electrical investigations in delineating hydrocarbon-contaminated areas, a 3D cross-diagonal resistivity survey was performed using a roll-along technique. The electrical dataset was inverted within Res3DInv to build a resistivity-depth model of the ground. High resistivities suggest a bedrock geometry which is consistent with the drilling results. However a detailed analysis reveals that rockhead resistivities tend to be significantly higher in areas where gas was detected through samples analyses than in uncontaminated areas.
Moreover resistivities plotted on the water table showed a zone of higher resistivities near the tank than in uncontaminated zones. This increase in resistivities is interpreted as a possible effect of the fresh mobile LNAPL plume. Confronting boreholes and geophysical investigations could therefore lead to a better delineation of the spill extent.