The geoelectric properties of hydrocarbon contaminant plumes vary and their geophysical<br>responses suggest that the ‘insulating layer’ model cannot be uniformly applied to all hydrocarbon<br>contaminated sites. Changes in the aquifer pore fluids chemistry, causes the variability of the<br>geoelectrical signature. In this study, we investigate the electrical properties of a complex hydrocarbon<br>plume resulting t?om 50 years of leakage into a glacio-fluvial geologic setting using ground penetrating<br>radar (GPR) and electrical resistivity methods. The results show the following: (1) regions of low<br>apparent resistivities are coincident with attenuated GPR reflections (shadow zones) but are limited to<br>the edges of the surveyed areas; (2) a central region of high apparent resistivities with bright GPR<br>reflections below the water table; (3) an upper GPR reflector subparallel to the water table, several feet<br>above the current free product level and coincident with the top of an gray, oil-stained layer, and (4) the<br>existence of a thin conductive layer coincident with a dark gray to black layer saturated with free<br>product occupying a zone immediately above the water table. These results suggest that the light<br>hydrocarbon free-product, and associated dissolved plumes are dynamic systems whose chemistry is<br>changing in time and position within the plume. As a result, this influences the variability of the<br>geoelectrical signature. Therefore, geophysical results must be interpreted in the light of a coherent<br>model based upon combined geological, geochemical, and geophysical understanding of the evolution<br>of these systems.


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