Geophysical characterization of a nonaqueous-phase liquid–contaminated site

Geophysical methods have been adopted to characterize a site whose soils and groundwater had been contaminated by nonaqueous-phase liquid (NAPL); the hydrocarbons were found in both residual form and as a free phase above the water table. The geophysical survey was aimed at improving the knowledge of the hydrogeological setting of the site; we wanted to estimate the soil heterogeneity and the hydrogeological parameters (porosity, water content, and hydraulic conductivity). We also wanted to assess the effect of the water table fluctuations on the interaction between the groundwater and the residual NAPL. The geophysical investigation was conducted using geoelectrical, electromagnetic (frequency domain), and georadar methods; data calibration was conducted using borehole geochemical logs. The presence of residual NAPL in the silty and sandy material was confirmed by a remarkable increase of the attenuation in the georadar energy and by peak of induced polarization response. The hydrocarbons have been detected in concentrations of more than 1000 mg/kg (1000 ppm) at a depth of 5 m (15 ft) and in concentrations of 3000 mg/kg (3000 ppm) at a depth of 6 to 8 m (18–24 ft).