Previous geophysical investigations (1996, 2003, and 2004) conducted at the decommissioned Wurtsmith Air Force Base former Fire Training Cell (FT-02) showed a clearly defined high conductivity anomaly associated with hydrocarbon contaminants in the vadose zone and ground water near the source area. The source of the geophysical anomaly was attributed to biogeochemical modifications of the contaminated zone by intrinsic bioremediation. During previous surveys, ground penetrating radar (GPR) data showed a zone of attenuated GPR reflections extending from the vadose zone to below the water table. Self potential (SP) data defined a positive anomaly coincident with the hydrochemically defined plume, while electrical resistivity data showed anomalously high conductivity within the zone of impact. In 2007, another integrated geophysical study was conducted at of the site. GPR, SP, electrical resistivity, and induced polarization survey were conducted with expectations of obtaining similar results as the past surveys. However, preliminary assessment of the data shows a marked decrease in groundwater electrical conductivity and SP response over the plume. GPR data showed the attenuated signals, but the zone of attenuation was only observed below the water table. We attributed the attenuation of the observed geophysical anomalies to ongoing soil vapor extraction initiated in 2003. Significant removal of the contaminant mass by the vapor extraction system has altered the subsurface biogeochemical conditions and these changes were reflected in the 2007 geophysical data. The results show that the biological and physical attenuation of the contaminant plume is detectable with geophysical methods.
Figure 1 shows the FT-02 contaminated site in the Wurtsmith Air Force Base which was decommissioned in 1993. The site was utilized by the US Air Force for biweekly fire training for a period of 24 years. Typical activities included combustion of several thousand liters of jet fuel and other hydrocarbon fuels. Installation of a concrete fire containment basin with an oil-water separator in 1982 help reduced the amount of fuel sipping into the ground. However, an unknown quantity of the fuel had already infiltrated into the subsurface. Previous groundwater analysis showed elevated total petroleum hydrocarbons (BTEX) and electrical conductivities of up to ~650 mg/l and 1100 ?S/cm, respectively. A contaminant LNAPL plume (see Figure 1) has been defined by electromagnetics, GPR and resistivity and from chemical analyses of groundwater (Bermejo et al., 1997; Sauck et al., 1998; Bradford et al., 2004)
The Wurtsmith Air Force base is located on an 8 km wide sandy plain that is part of the Oscoda Lake Plain (Bermejo et al., 1997). The Airbase is underlain by glacial deposits which consist primarily of sands and clays that are approximately 69 m thick. Underlying the glacial deposit is a Mississippian sandstone and shale bedrock formation that dips southwest into the Michigan Basin. A thick clay unit that exceeds 38.1 m in thickness separates the bedrock from the principle aquifer. The shallow subsurface stratigraphy is uniform and consists of well sorted fine to medium sands that grade downward from fine to coarse (Robbins et al., 1995).
