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 Wurtsmi th Air Force Base (AFB) near Oscoda, Michigan. It is located near anot her plume called FT-02, which is a well-studied area undergoing natura l bioremediation. The plume was discovered by ground penetrating radar (GPR) profiling while extending a long line from FT-02 to establish b ackground variability around that plume. The new plume was apparent be cause of a high-conductivity ''shadow'' or GPR reflection attenuation observed below the conductive zone at the top of the aquifer, identica l to the pattern observed at the FT-02 plume. Further GPR surveys were conducted by students of a Western Michigan University geophysics fie ld course to outline the proximal part of the plume. The GPR survey wa s supplemented by an electromagnetic induction (ER-I) survey which sho wed a group of four cables crossing the area. Finally, a magnetometer survey was conducted to search for any buried steel objects which migh t have been missed by the EM survey. The results of the three geophysi cal surveys were then used by students of a University of Michigan fie ld course to guide subsurface soil and fluid sampling, which verified the presence of residual LNAPL product and ground water with conductiv ities 2.5 to 3.3 times above background. The plume source is in the vi cinity of a vaulted underground storage tank (UST) formerly used for t he collection of waste solvents and fuels for subsequent use in the fi re training exercises at FT-02. This newly discovered LNAPL plume, alo ng with other ''mature'' plumes, fits the electrical model which predi cts conductive ground water below the decomposing but electrically res istive LNAPLs. Finally, this is a fine example of the cooperative use of a dedicated research site for training by students of two different universities.