Electrical resistivity for detecting subsurface non-aqueous phase liquids: A progress report

Soils and groundwater have been contaminated by hazardous substances at many places in the United States and many other countries. The contaminants are commonly either petroleum products or industrial solvents with very low solubility in water. These contaminants are usually called non-aqueous phase liquids (NAPLs). The cost of cleaning up the affected sites in the United States is estimated to be of the order of 100 billion dollars. In spite of the expenditure of several billion dollars during the last 15 years, to date, very few, if any major contaminated site has been restored. The presence of NAPL pools in the subsurface is believed to be the main cause for the failure of previous cleanup activities. Due to their relatively low water solubility, and depending on their volume, it takes tens or even hundreds of years to deplete the NAPL sources if they are not removed from the subsurface. The intrinsic electrical resistivity of most NAPLs is typically in the range of 107 to 1012Ω-m, which is several orders of magnitude higher than that of groundwater containing dissolved solids (usually in the range of a few Ω-m to a few thousand Ω-m). Although a dry soil is very resistive, the electrical resistivity of a wet soil is on the order of 100 Ω-m and is dependent on the extent of water saturation. For a given soil, the electrical resistivity increases with decrease of water saturation. Therefore, if part of the pore water is replaced by a NAPL, the electrical resistivity will increase. At many NAPL sites, both the vadose and phreatic zones can be partially occupied by NAPL pools. It is the great contrast in electrical resistivity between the NAPLs and groundwater that may render the method to be effective in detecting subsurface NAPLs at contaminated sites. The following experiments were conducted to investigate the change of the electrical resistivity of porous media when diesel fuel (NAPL) replaces part of the water