Complex Electrical Resistance Tomography Of A Subsurface Pce Plume

A controlled experiment was conducted to evaluate the performance of complex electrical<br>resistivity tomography (CERT) for detecting and delineating free product dense non<br>aqueous phase liquid (DNAPL) in the subsurface. One hundred ninety liters of PCE were<br>released at a rate of 2 liters per hour from a point 0.5 m below ground surface. The spill<br>was conducted within a double walled tank where saturated layers of sand, bentonite and<br>a sand/bentonite mixture were installed. Complex electrical resistance measurements<br>were performed from 4 vertical electrode arrays, each with 10 electrodes spaced between<br>3 m and 0.5 m depth. Data were taken before the release, several times during, and then<br>after the PCE was released. Magnitude and phase were measured at 1 and 64 Hz. Data<br>from before the release were compared with those during the release for the purpose of<br>imaging the changes in conductivity resulting from the plume. Conductivity difference<br>tomographs showed a decrease in electrical conductivity as the DNAPL penetrated the<br>soil. A pancake-shaped anomaly developed on the top of a bentonite layer at 2 m depth.<br>The anomaly grew in magnitude and extent during the release and borehole television<br>surveys data confirmed the anomaly to be free-product PCE whose downward migration<br>was stopped by the low permeability clay. The tomographs clearly delineated the plume<br>as a resistive anomaly. Images showing phase changes caused by the spill are also<br>presented. The phase changes at 64 Hz suggest that the DNAPL spill increased the<br>induced polarization (IP) effect of the clay layers.