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Addendum to Interim Corrective Action Plan r Frontier Transportation, Inc. April 14, 2006 Page 6 If pit samples identify elevated contamination on the eastern and western walls of the excavation pit, a second stage of excavation will be implemented. Trenched - - measuring from 18" up to 24" in width will be excavated to 20' bgs to remove contaminated soil that is closer to the sidewalk and steel building. The fine-grained soils ff 1 identified in the subsurface at the site should support trenching to the required depth of 15 ' bgs to 20' bgs, depending upon the soil sample results. Should caving begin during this part of the excavation, the trenches will be backfilled with clean backfill material. i1 BEA proposes that the impacted soil be excavated and removed by ramping and excavation to 1 ' above the groundwater table at 13 ' bgs and removal of the free phase gasoline and soil to 20 ' bgs by a tracked backhoe or by a clamshell apparatus. Since the water-bearing horizon extends vertically from 13 ' to 20' bgs that soil horizon is principally saturated sand, caving is anticipated and removal by clamshell may be recommended. r D. Aquifer Testing and Groundwater Extraction BEA proposes aquifer testing at the site to facilitate implementation of a groundwater pumping and treatment system that will attempt to contain continuing down ` gradient and potential off-site migration of the dissolved plume. BEA proposes that groundwater pumping be performed in two separate events in two groundwater areas : one f l at the down-gradient area of the dissolved plume, near MW-9, to act as containment of further migration, the second pumping to be performed north of the apparent source at MW-8, near recently identified free floating product in MW-2 and MW-21 . MW-8 was chosen based upon its proximity to the source area and wells with floating free product. AQUIFER TEST SPECIFICATIONS lIn addition, BEA proposes that groundwater pump and treatment feasibility testing be completed by aquifer testing. Two additional groundwater wells will be =1 e> t nl' d advanced to 20 ' below grade surface (bgs) as confirmation wells for each of the two pumping wells. Shallow subsurface dewatering will be identified at 10' and 20 ' intervals from existing wells MW-8 and MW-9. See Figure 2 for the locations of the additional i wells for the aquifer tests at MW-8 and MW-9. In addition, selected soil samples from each well will be collected within the saturated zone of the perched groundwater to obtain physical parameters from this zone. J These soil samples will be analyzed for permeability, moisture content, dry density, specific gravity, porosity, and total organic carbon (TOC) using ASTM D-2434 and Walkely-Black method for TOC. 6 i