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SNW" �.►` 4cologicat 7i"k tics/me. Page 5 Interim Remedial Action Plan Project No.723.2 March 7,2005 Tables 1 & 2 in Appendix A contain the soil and groundwater analytical data generated for the site. GTI has produced a new cross section diagram that incorporates the new extraction wells installed at the site for the dual phase pilot test. Figure 5: Cross Section C— C' presents the geology in the source area beneath the dispenser island with the boring's soil contaminant concentrations indicated on the section (well MW-3 is projected northeast onto the section). Figure 5 also shows groundwater concentrations from the 4`h quarter of 2004 next to the screened intervals of wells MW-2 and MW-3, and from July 2004 for EW-1. The groundwater elevation range from 1995 — 2004 is shown on the diagram to illustrate the interception of the various well screens. The soil contamination remaining beneath the former dispenser island will continue to source a significant groundwater plume as evident by the 243,500 ug/1 detected in well EW-1. The dual phase extraction pilot test performed on wells EW-1, VEW-1 and VEW-2 demonstrated that the source area was amenable to extraction technologies. Removing this contamination will cut off the migrating groundwater plume. 3.1.1 Extraction Wells EW-1,VEW-1 & VEW-2 Existing extraction wells EW-1, VEW-1 and VEW-2 were completed with extended screen lengths so the wells could be used for both vapor and groundwater extraction (see Figure 2 for well locations). The wells' construction details are included in the following table: Well No. Diam./TD Screen Slot Sand Pack Transition Grout Seal (in/ft) Interval (ft) (in) (ft) Seal (ft) (ft) EW-1 4"/54.5' 19.5 —54.5' 0.020 18—55' 15 - 18' 15'-surface VEW-1 4"/50' 20—50' 0.020 18—50' 15 - 18' 15'-surface VIEW-2 411/50' 20—50' 0.020 1 18 —50' 13.5 — 18' 1 13.5'-surface The pressure necessary for demonstrating vacuum influence is generally regarded as at least 0.1 inches of water ("How to Evaluate Alternative Cleanup Technologies for Underground Storage Tank Sites", USEPA, May, 1995). The influence detected in wells VEW-1, VEW-2 and MW-5 during the July 2004 pilot test extraction from EW-1 achieved a minimum of 0.3 inches water vacuum. Well MW-5 is located approximately 46 feet from EW-1 and this distance is used as a conservative estimate as the radius of influence. Based on the radius of influence of 46 feet from the extraction points of EW-1, VEW-1 and VEW-2, no additional wells are proposed for source area contaminant reduction. All three wells will be subject to dual phase extraction as part of this interim remedial action. Because groundwater depth was approximately 40 feet bgs during the 4`h quarter 2004 monitoring event the wells are expected to go dry quickly. This will expose up to 15 feet of submerged soils to the effects of high vacuum soil vapor extraction.