Mapping of complex hydrocarbons contaminant using geoelectric and electromagnetic methods

A combined geophysical survey was carried out in a oil refinery to study the complex hydrocarbon contamination of the soil and the near surf ace aquifer, resulting from several years of leakage of hydrocarbon products. A near surf ace level characterized by a strong reflector was weIl detected using Ground Penetrating Radar; lateral changes of the strength of the reflection and the subsequent drilling confirmed the existence of a free phase of hydrocarbon in the vadose zone above the water tabIe. Because of the limited thickness of the layer embedded with the residual oil, the geoelectrical resistivity measurements were not effective for detecting this level. The resistivity measurements pointed out very low values in the upper zone of the aquifer; this effect can be associated to complex chemical reactions between the dissolved hydrocarbon phase and the aquifer system. These effects were investigated using high resolution geoelectrical tomography. Soil data resulting from several boreholes confirmed the reliability of the geophysical response. Geochemical and physical analysis of the soil and groundwater samples, combined with the geophysical investigation, allowed the definition of a reliable model ofthe polluted area.