3D Resistivity Mapping of Aged Hydrocarbon-Contaminated Site

Resistivity and GPR surveys were conducted to characterize a former gas station site contaminated with fuel hydrocarbons. The results of geophysical investigations, incorporated with soil boring data provided the shallow geologic structure of (1) upper soil unit of mostly boulders and rock fragments, (2) lower soil unit of sediments and/or residual soils weathered from the bedrock, and (3) granite bedrock. The water table of the site is located in the lower soil unit, and the elevation of water table varies with topography from approximately 1.5 to 3 meters in depth. 3-D inversion of resistivity data have shown that the aged hydrocarbon-contaminated soils show low resistivity anomalies attributed to the biodegradation of petroleum hydrocarbons, and that the fuel hydrocarbons leaked from the USTs and fuel lines at the station have migrated downgradient over the water table toward the streamlet, and that most of petroleum-impacted zone lies near the water table in the smear zone within the lower soil unit. The study results also demonstrated that the geophysical methods, as a non-invasive environmental sounding technique, can be a very useful tool for characterization of the contamination sites.