Ground Penetrating Radar Surveys Over An Alluvial Dnapl Site, Paducah Gaseous Diffusion Plant, Kentucky

Ground penetrating radar (GPR) surveys were used to map shallow sands and gravels which are<br>DNAPL migration pathways at the Paducah Gaseous Diffusion Plant in western Kentucky. The<br>sands and gravels occur as paleochannel deposits, at depths of 17-25 A, embedded in Pleistocene<br>lacustrine clays. More than 30 GPR profiles were completed over the Drop Test Area (DTA) to<br>map the top and base of the paleochannel deposits, and to assess their lateral continuity. A<br>bistatic radar system was used with antenna frequencies of 25 and 50 MHz. An average velocity<br>of 0.25 ft/ns for silty and clayey materials above the paleochannel deposits was established from<br>radar walkaway tests, profiles over culverts of known depth, and comparison of radar sections<br>with borings. In the south portion of the DTA, strong reflections corresponded to the water table<br>at approximately 9-10 ft, the top of the paleochannel deposits at approximately 18 ft, and to<br>gravel horizons within these deposits. The base of these deposits was not visible on the radar<br>sections. Depth estimates for the top of the paleochannel deposits (from 50 MHz records) were<br>accurate to within 2 ft across the southern portion of the DTA. Continuity of these sands and<br>gravels could not be assessed due to interference from air-wave reflections and lateral changes in<br>signal penetration depth. However, the sands and gravels appear to extend across the entire<br>southern portion of the DTA, at depths as shallow as 17 ft. Ringing, air-wave reflections and<br>diffractions from powerlines, vehicles, well casings, and metal equipment severly degraded GPR<br>profiles in the northern portion of the DTA; depths computed from reflection times (where<br>visible) were accurate to within 4 ft in this area. The paleochannel deposits are deeper to the north<br>and northeast where DNAPL has apparently pooled (DNAPL was not directly imaged by the<br>GPR, however). Existing hydrogeological models of the DTA will be revised with the newly<br>defined extent and depth of these sands and gravels.<br>Note: The submitted manuscript has been authored by a contractor of the U.S. Government<br>under contract No. DE-ACO584OR21400. Accordingly, the U.S. Government retains a<br>nonexclusive, royalty-free license to publish or reproduce the published form of this<br>contribution, or allows others to do so, for U.S. Government purposes.