Integrated Geophysical Characterization of the Trecate Oil-Spill Site Through Surface ERT, FDEM, and Borehole GPR

This study investigates the spatial distribution and long-term evolution of hydrocarbon contamination at the Trecate oil-spill site (Italy) through an integrated hydrogeophysical approach. The site, affected by a major oil spill in 1994, represents a complex environment where geological heterogeneity and biodegradation processes strongly influence subsurface properties. To address these challenges, three complementary geophysical methods were employed: Electrical Resistivity Tomography (ERT), borehole Ground Penetrating Radar (GPR) in Zero Offset Profiling (ZOP) configuration, and Frequency Domain Electromagnetics (FDEM) using the CMD-DUO system. ERT surveys provided high-resolution images of subsurface resistivity, revealing significant reductions in the vadose and smear zones associated with contamination and biodegradation processes. Borehole GPR measurements allowed the investigation of electromagnetic wave propagation between wells, highlighting variations in dielectric properties related to water content and contamination; comparison with previous studies indicates that these conditions have remained largely unchanged over the past decade. FDEM data enabled rapid and non-invasive mapping of conductivity variations over a wider area, capturing spatial trends consistent with ERT observations despite lower resolution. The combined interpretation of these datasets demonstrates the complementary strengths of the applied methods, balancing accuracy, spatial coverage, and operational efficiency. Overall, the results confirm the persistence of contamination-related anomalies and highlight the effectiveness of integrated geophysical approaches for the characterization and monitoring of hydrocarbon-impacted sites.