Analyzing geophysical signature of a hydrocarbon-contaminated soil using geoelectrical surveys

Abstract

In February 1994, a major oil blow-out occurred as a consequence of an accident at an exploration drill-hole at Trecate (North-West, Italy). More than 15 years after the incident, a series of geophysical and hydrogeological surveys were conducted for investigating the hydrocarbon-contaminated soil. The hydrocarbon is still spatially distributed as residual phase in the unsaturated zone. A free phase exists between the transition zone and the vadose zone, in what is defined as a smear zone. Geoelectrical methods have shown their potential in contaminant detection, and are increasingly used for contaminated sites characterization. We carried out geoelectrical surveys over the most contaminated area of the site, in order to analyze the changes in geophysical signature due to the presence of contaminants, and furthermore to understand the role of biomass activity in degrading the hydrocarbon and altering petrophysical properties of the soil. Acquisition of time- and frequency-domain resistivity data allowed us to approach the same features with different methods, providing a better understanding of the problem. We further analyzed the role of soil properties on electrical resistivity; porosity, pore water specific resistivity and the influence of cation exchange capacity on the bulk resistivity at the site. The electrical resistivity observed at the site shows a clear relationship with water saturation, porosity and soil texture as well as with resistive hydrocarbon species. We also monitored soil polarizability in the same frequency range of electrical resistivity measures, allowing better delineation of the degrading processes underway in the contaminant, which constitutes an additional substrate for the proliferation of anaerobic degraders.