J. T. Nolan

Geophysical and geochemical attenuated signatures associated with hydrocarbon contaminated site undergoing bioremediation

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.