Chlorinated Solvent DNAPL Extent Characterization at the East Gate Disposal Yard, Ft. Lewis, Washington

The EGDY contains mixed solvent and petroleum hydrocarbon dense non-aqueous phase liquids (DNAPLs) disposed in trenches as drummed waste or directly as liquid waste. The DNAPLs are the source of a 13,000 feet long TCE plume that contaminates an upper, unconfined aquifer and a lower, confined aquifer to depths of 220 feet below ground surface (bgs). From 1997 to 2002 the EGDY DNAPLs were characterized in two phases using a variety of tools (e.g., historical aerial photographs, EM-61 geophysical survey, soil gas sampling, exploration trenching, direct-push multi-level groundwater sampling, a membrane interface probe, and analysis of continuous rotosonic cores). During the EGDY characterization, samples were analyzed in field laboratories or fixed labs with rapid turnaround times to expedite the on site decision making process. The data collection tools used and the number and location of data collection points for each tool were decided by a team of scientists and engineers as the investigation progressed based upon a continuously updated conceptual site model. The first phase of the characterization resulted in the location of past disposal areas and several hot spots where DNAPL may have been present in groundwater. During the second phase of the EGDY characterization, DNAPL data were collected to support the design of thermal treatment of the suspected DNAPL areas identified during the first phase. The final result of the EGDY characterization effort was the location of three large and several small volume solvent DNAPL sources within the EGDY. DNAPL was encountered at all three large volume DNAPL areas to a maximum depth of forty-six feet bgs. The areal extent of the three large DNAPL areas are 0.6, 1.2 and 0.4 acres with an estimated volume of 26,000, 52,000 and 13,000 gallons of mixed solvent/hydrocarbon DNAPL, respectively. Approximately 100,000 yd³ of NAPL contaminated soils will be thermally remediated by soil heating beginning in 2003.