Geophysical discovery of a new LNAPL plume at the former Wurtsmith AFB, Oscoda, Michigan

A Light nonaqueous phase liquid (LNAPL) ground water contaminant plume has been discovered by purely geophysical means at the former Wurtsmith Air Force Base (AFB) near Oscoda, Michigan. It is located near another plume called FT-02, which is a well-studied area undergoing natural bioremediation. The plume was discovered by ground penetrating radar (GPR) profiling while extending a long line from FT-02 to establish background variability around that plume. The new plume was apparent because of a high-conductivity ''shadow'' or GPR reflection attenuation observed below the conductive zone at the top of the aquifer, identical to the pattern observed at the FT-02 plume. Further GPR surveys were conducted by students of a Western Michigan University geophysics field course to outline the proximal part of the plume. The GPR survey was supplemented by an electromagnetic induction (ER-I) survey which showed a group of four cables crossing the area. Finally, a magnetometer survey was conducted to search for any buried steel objects which might have been missed by the EM survey. The results of the three geophysical surveys were then used by students of a University of Michigan field course to guide subsurface soil and fluid sampling, which verified the presence of residual LNAPL product and ground water with conductivities 2.5 to 3.3 times above background. The plume source is in the vicinity of a vaulted underground storage tank (UST) formerly used for the collection of waste solvents and fuels for subsequent use in the fire training exercises at FT-02. This newly discovered LNAPL plume, along with other ''mature'' plumes, fits the electrical model which predicts conductive ground water below the decomposing but electrically resistive LNAPLs. Finally, this is a fine example of the cooperative use of a dedicated research site for training by students of two different universities.