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Temporal Geophysical and Geochemical Signatures Due to Contaminant Source Reduction at Wurtsmith Airforce Base in Oscoda Michigan, USA

Che-Alota, Vukenkeng
Geophysical investigations have documented characteristic changes (e.g., higher bulk electrical conductivity, positive self potential (SP), attenuated ground penetrating radar (GPR) reflections) associated with hydrocarbon biodegradation in field experiments. These characteristic geophysical signatures result from biogeochemical transformations of the bulk properties of the contaminated relative to uncontaminated media. In this study, we present the results of surface geophysical surveys acquired in 1996, 2003, and 2007 that document changes in geophysical signatures associated with removal of hydrocarbon mass in the contaminated zone. Initial investigations in 1996 showed that relative to background, the contaminated area was characterized by higher bulk electrical conductivity, positive SP anomaly, and attenuated GPR reflections. Repeated surveys in 2003 and 2007 over the plume showed that in 2007, the bulk electrical conductivity had reverted to near background conditions, the positive SP anomaly became more negative, and the zone of attenuated GPR reflections showed increased signal strength. The removal of hydrocarbon mass in the vadose zone over the plume by a soil vapor extraction system installed in 2001 was primarily responsible for the changing geophysical responses. Although chemical data from groundwater showed a 3 m thick conductive plume in 2007, the plume was not imaged by electrical resistivity. Forward modeling suggests that the apparent bulk electrical conductivity of the saturated zone plume has to be 4-5 times higher than background values to be imaged by electrical resistivity. We conclude that hydrocarbon contaminant mass reduction by natural or engineered (bio)remediation can be effectively imaged by temporal geophysical surveys.