Nowadays, the growing concern about the environmental problems affecting the subsoil has focussed efforts on the detection and characterization of contaminated sites through geophysical prospecting methods. In the present study, a case of a contaminated site by hydrocarbons and their study by means of time domain-induced polarization tomography is presented. The response in chargeability of porous media due to this kind of pollutant allows its delimitation using this method. However, one of the limitations for the application of this technique is the presence of lithologies that contain electro-metallic salts. These salts can produce anomalies of chargeability and mask those due to nonaqueous phase liquids. The studies were conducted in an area contaminated by fuel leaks from supply tanks within a train maintenance facility. Those leaks occurred while the tanks were in use, but since their dismantling, the leak stopped. The geology of the area presented strong heterogeneities and the access was limited by train tracks. In order to locate and characterize the contaminant plume, measurements of resistivity and chargeability were carried out. A grid of monitoring wells in this area was also available from which information about free-phase pollutants was obtained, and a new drilling was carried out to verify an unexpected anomaly. The results obtained show that the location of the plume by the geophysical techniques employed can lead to ambiguity, as an anomaly that does not correspond to contaminated areas appeared but to the presence of clays rich in electro-metallic components such as Mg, Fe, Mn and Al.
Keywords: Chargeability; Electro-metallic salts; Induced polarization; NAPL; Resistivity.