Electrical resistivity of frozen and petroleum-contaminated fine-grained soil

Both positive and negative resistivity anomalies associated with contaminated soil have been reported in the recent literature. By positive we mean expected increases of resistivity with decreasing water content, increasing hydrocarbon content, and freezing. In this investigation, we tested these findings by measuring the electrical resistivity of samples of fine-grained soils contaminated with petroleum and by performing a field survey of an abandoned petroleum storage site where drilling and sampling confirmed soil contamination. In the laboratory, the cylindrical silt samples demonstrated a three-order-of-magnitude decrease in resistivity as water content increased. A corresponding resistivity increase was observed when water content was held at a constant level and petroleum percent was increased incrementally to saturation. Resistivity and temperature were also measured along a vertical profile as both the frost and petroleum front advanced beneath the surface of a large, unsorted silty-sand sample. Resistivity of the uncontaminated silty sand increased from 400 to 4000 Omega in as the soil froze; resistivity further increased to 10,000 Omega in when the soil was both frozen (-3.0 degreesC) and contaminated. In the field we contoured resistivity, determined by Wenner array profiling within the containment berm of a storage tank where spills occurred sporadically during the 25-year operation period. Increasing values of apparent resistivity compare favorably with documented residual petroleum contamination and corroborates our laboratory measurements. We conclude that residual petroleum on soil particle surfaces results in a permanent increase in soil resistivity and that positive anomalies should be expected in most common situations. Published by Elsevier Science BN.