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Temporal Geophysical Signatures Due to Contaminant Mass Remediation
Citation:
Vukenkeng, C., E. Atekwana, E. Atekwana, W. Sauck, AND D. D. WERKEMA. Temporal Geophysical Signatures Due to Contaminant Mass Remediation . GEOPHYSICS. Society of Exploration Geophysicists, 74(4):1-11, (2009).
Impact/Purpose:
Land Preservation and Restoration - by providing improved scientific knowledge and developing and applying more cost effective tools, models and methods to inform decisions on land restoration.
Description:
Geophysical surveys acquired over a ten year period are used to document changes in bulk electrical conductivity associated with the attenuation of hydrocarbon contaminants at the former fire training facility (FT-02) Wurtsmith Air Force base (WAFB), Oscoda, MI, USA. Initial investigations at the site in 1996 showed that known areas of contamination were characterized by higher bulk electrical conductivity, positive self potential (SP) anomalies, and attenuated ground penetrating radar (GPR) reflections. The geophysical signatures were attributed to the intrinsic bioremediation of the hydrocarbon contaminants, consistent with the conductive plume model. However, repeated geophysical surveys in 2003 and 2007 over the FT-02 site documented changes in the geophysical signatures. By 2007, the conductive anomaly over the plume had reverted to uncontaminated conductivities; the positive SP anomaly had become more negative and zones of attenuated GPR reflections showed increased signal strength. The significant changes in the geophysical signatures between 1996 and 2007 suggested that significant attenuation of the contaminant mass or complete remediation of the site had occurred over the past years. We attribute these changes to the installation of a soil vapor extraction system in 2001 which resulted in the removal of several thousand kilograms of hydrocarbons. We hypothesize that removal of the source reduced the amount of hydrocarbons available for microbial metabolism and therefore reduced the level of microbial activity. Our electrical resistivity (ER) results did not show a 3 m thick conductive plume associated with the intrinsic bioremediation of the dissolved phase plume in the saturated zone. We infer from forward modeling results that the apparent bulk conductivity of the plume in the saturated zone has to be at least 4-5 times greater than background conductivities for it to be imaged by electrical resistivity surveys. Therefore we conclude that the contaminant mass reduction by natural bioremediation or enhanced engineered (bio)remediation can be effectively imaged using geophysical surveys especially if the contamination exists in the vadose zone.