Mapping oil-contaminated sand and till with the spectral induced polarization (SIP) method

The laboratory and field results from an environmental application of the spectral induced polarization (SIP) method are presented. The phase spectra of the resistivity of uncontaminated glacial till, silt sand and gravel were measured in the laboratory. The effects of waste oil and motor oil on the phase and amplitude spectra of the resistivity were studied using artificially contaminated sand and till samples and mineral soil samples from real waste sites. Field IP and SIP measurements were also made at the waste sites. The laboratory phase spectra of sands and tills were straight or slightly concave upwards in a log-log plot. The phase angle varies between 0.1 and 20 mrad at 1 Hz frequency and increases towards higher frequencies with a slope of 0.15-0.25. In laboratory tests, motor oil and waste oil changed the phase and amplitude spectra of sand and till. At first, the amplitude and phase decreased due to oil contamination. Later, during continued maturation, both the amplitude and phase increased. After a few days or weeks of maturation, some of the contaminated samples showed a convex-upwards phase spectrum. The features observed in artificially contaminated samples were also detected in the sample material from real waste sites. Furthermore, the in situ results from the waste sites were in agreement with the laboratory results. In laboratory tests, the phase spectra of clean sand and till remained stable with time, whereas the phase spectra of oil-contaminated samples changed with increasing maturation time. This, together with the field results, suggests that differences between the spectra of clean and polluted soils, and also changes occurring in the phase spectra of contaminated soils with time, can be indicative of contamination.