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To effectively delineate the spatial distribution of oil contaminant plumes, geophysical methods indirectly measure the physical properties of the subsurface and can provide spatial information and images on a large scale, as opposed to traditional direct methods such as borehole drilling, sampling, and chemical analysis, which are time-consuming and costly. However, delineating geophysical responses from non-aqueous phase liquids (NAPLs) contaminated sites is not straightforward due to inconsistent responses from biodegraded oil contaminants. Additionally, the presence of clay materials can complicate the interpretation of geophysical data in NAPL-contaminated sites. In this study, we present a case study of a multi-geophysical investigation, including seismic refraction, ground-penetrating radar (GPR), electrical resistivity tomography (ERT), and complex resistivity (CR), to delineate NAPL contamination in a clay-rich site. To reduce ambiguity in discriminating between oil contaminants and clay layers, we suggest constructing a 3D geological model that incorporates borehole data and geophysical data. Based on the 3D geological model, conductive zones generally correspond to high concentrations of hydrocarbons in the unsaturated zone, but it is difficult to distinguish contaminated areas from saturated soil. The IP response rapidly decreased to close to zero in several expected highly contaminated zones, which differs from the clay soil with high IP values. Finally, we compare the expected contaminated area from geophysical data and soil sampling data and discuss how geophysical interpretation can be improved in NAPL-contaminated sites.

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