A Light Non-Aqueous Phase Liquid (LNAPL) ground water contaminant plume has been<br>discovered by purely geophysical means at the former Wurtsmith AFB near Oscoda, Michigan.<br>The plume was discovered by Ground Penetrating Radar (GPR) while extending a long line<br>horn another contaminated area (FT-02) to establish background variability around that plume.<br>The new plume was apparent because of a high-conductivity “shadow” or GPR reflection<br>attenuation, identical to the pattern observed at the FT-02 plume. The GPR survey was<br>supplemented by an electromagnetic (EM-31) survey which revealed a group of cables<br>crossing the area, but no anomalous regions associated with the presence of the plume. A<br>magnetometer survey was conducted to search for any buried steel objects which might not<br>have been detected by the EM survey. The results of these initial geophysical surveys were<br>used by the National Center for Integrated Bioremediation and Development (NCIBRD) staff<br>to guide subsurface soil and fluid sampling, which verified the presence of residual LNAPL<br>product and ground water with conductivities 2.5 to 3.3 times above background. Subsequent<br>surveys were conducted, including mise-a-la-masse (MALM), dipole-dipole resistivity/induced<br>polarization (IP) and vertical resistivity probe (VRP) measurements. The MALM survey<br>shows marked elongation along the plume axis, while the resistivity/IP showed two anomalous<br>regions coincident with buried utilities. In addition, the VRP revealed the presence of a<br>conductive zone at the top of the aquifer. This newly discovered LNAPL plume, along with<br>other “mature” plumes, fits an electrical model which predicts conductive ground water below<br>the decomposing LNAPLs.


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