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Addendum to Interim Corrective Action Plan Frontier Transportation, Inc. April 14, 2006 Page 5 sands as logged at this depth, the non aqueous phase gasoline occupies an area extending . at thickness 5 ' , depth of discovery to depth of the predominant subsurface retarding layer at 20' bgs. The lateral extent of the free phase gasoline contamination is unknown but was visually identified as extending at attenuated thickness into the west and north wall of the excavation. The mass of gasoline in the source are is estimated as follows: 1 . Area of tested free phase layer, 30' by 30 '= 900 sq. ft. 2 . Estimated thickness of layered non-aqueous phase gasoline 5 'x 900 sq. ft.— 4,500 cu. ft. Free product occupies approximately one-third of this volume in source i area = 1 ,500 cu. ft. I 3 . (1 ,500 cu. ft. gasoline) x (1 gallon/0. 13368 cu. ft. gasoline) = 11 ,220 gallons I- This unique light non-aqueous phase gasoline contamination is layered beneath the potentiometric groundwater table measured at 13 ' bgs. Excavation of the confined free phase gasoline which presently has been identified as migration to floating product on monitoring wells MW-2 and MW-21 will require removal of soil beneath the water table. The source area has been previously excavated and backfilled with clean soil. However, contaminated soil excavation had to be terminated before completion due to !� water seepage from the sidewalls. Groundwater was continued rising within the ( -I excavation pit and the sidewalls at the bottom of the pit were unstable soil within the 00 1 6 i� v s ` aquifer zone was caving into the rising groundwater in the pit. BEA agrees that excavation should be attempted again in the source area at the site. The source area is bounded to the east by a sidewalk and to the west by a pre- fabricated steel building on a concrete foundation. Consideration of the fine-grained soil types allows for excavation without shoring or pile driving. However, in order to ensure the safety and stability during excavation, the soil will be removed in two stages, hoL4 C� , ; First, a main pit area will be excavated which will leave a border of undisturbed soil 7' to 8' to both the east, bordered by the street, and the west, where the mechanic lbuilding is located. An on-site laboratory will be present during excavation to confirm completion of contaminant removal. Soil removed will be analyzed using the on-site y1 € laboratory. The excavation will be advanced until contamination levels of the removed - J soil demonstrate that a significant portion of the source contamination has been removed. Clean soil from the pit will be stored on-site in a separate pile. Along with clean backfill li - material for replacement of removed contaminated soil, the clean soil removed from the excavation pit will be used to fill in the pit. J 5