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Mr.Nuel Henderson,Jr. • October 15,2002 ARCO Station 6100 Project No.U 6100 Work Plan Page 4 feet bgs, the total depth explored. Soil samples collected from 152 feet and 202 feet bgs in boring B-32/MW-1 were submitted for physical analysis. These samples were reported to have relatively low permeability values of 6.5 x 10-centimeters per second(cm/s) and 2.4 x 10-6 cm/s, respectively. vtv},,..;eo.� lw.zw.�•,.2 per^-' Hydrogeolofic Conditions Groundwater is encountered in both Sand 1 and Sand 2, and appears to be perched. In both sandy strata, meteoric groundwater appears to flow along the contact between the sand stratum and an underlying silty unit. The lateral extent of the perched water in Sand 1 appears adequately characterized. The extent of the perched water in Sand 2 has not been adequately characterized. The lack of water in any of the deep-screened wells, except MW-1, suggests a localized vertical flow of groundwater through the silt between Sand 1 and Sand 2. Vertical connection between Sand 1 and Sand 2 was discussed above. The perched groundwater in Sand 1 and Sand 2 appears to flow down dip (southeastward)under gravity. Only one downgradient potential receptor has been identified. One artesian well is situated approximately 2000 feet southeast (down dip) of the site (Figure 1). Extent of Hydrocarbon Impact Based on the analytical data collected during UST and product line replacement activities, the dispensers and product piping was likely the primary source of the release. It appears that the bulk of the hydrocarbon mass is between 10 and 45 feet bgs, with localized high concentrations encountered at depths ranging from 45 to 80 feet bgs. Adequate characterization has been achieved on-site. Characterization of the lateral migration off-site of petroleum hydrocarbons remains incomplete to the southeast. A majority of the petroleum hydrocarbons (TPHG and BTEX compounds) off- site were detected in boring B-32/MW-1,with the highest concentrations detected at 111 feet bgs in boring B-32/MW-1, at the base of Sand 2 (just below the sand/silt contact). Proposed Scope of Work Further characterization of hydrocarbon impact to Sand 2 in the vicinity of well MW-1 is required. Stratus proposes to drill four off-site soil borings (B-38 through B-42) to further characterize the �( subsurface lithologic conditions. Four groundwater monitoring wells (MW-10 through MW-13) will be installed adjacent to each of the soil borings to evaluate the lateral extent of perched groundwater and floating product. Well VW-15 is obstructed and needs to be properly destroyed. Sufficient monitoring points are installed in the immediate vicinity of well VW-15 that a replacement well is not warranted. Stratus d proposes to drill out well VW-15 and gout the boring to ground surface. P:\Bp-Arco for URS\6100\Work Plans\6100 workplan-l.doc