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Mr.Nuel Henderson,Jr. • October 15,2002 ARCO Station 6100 Project No.U 6100 Work Plan Page 3 115 feet bgs. Results of laboratory analyses on soil samples collected from the boring indicate that petroleum hydrocarbons are predominantly present at depths between approximately 105 to 125 feet bgs. Liquid-phase hydrocarbons have been observed in monitoring well MW-1 since September 2000. A free-phase product skimmer was installed in the well to remove the product. The skimmer was removed from the well in January 2001, and product has been recovered by manual bailing on a weekly basis. Product thickness has ranged from 0.39 to 1.89 feet. Depth to water in well MW-1 has consistently been measured at approximately 106 feet bgs. Delta conducted an additional onsite site assessment in August and September 2001. Delta oversaw the advancement of six soil borings (B-33 through B-38) to depths ranging from approximately 135 to 140 feet bgs, and the installation of eight wells (MW-2 through MW-9). A report documenting the findings of the investigation was submitted on February 28, 2002. The report concluded that the petroleum hydrocarbon impact to soil decreased significantly below approximately 65 feet onsite, and that the vertical extent of petroleum hydrocarbon impact to soil was adequately assessed onsite. ARCO upgraded the product lines and product dispensers in January and February 2002. Lateral conveyance piping for future use in soil vapor extraction was connected to wells VW-1, VW-3, VW-4, VW-6, VW-7, VW-8, VW-11 through VW-14, VW-16, and MW-2 through MW-9 at the time of the site renovation. The conveyance piping was stubbed up at the northwest corner of the property to be manifolded for connection to an SVE system at a later date. RATIONALE Geologic Conditions An upper silty unit, underlain by a sandy zone, underlain in turn by a lower silty unit, comprises the lithology in the first 200 feet below the site. The upper silty zone contains strata of relatively transmissive, sandy sediments, approximately 3 to 5 feet thick, that appear to form tabular- shaped lenses of limited lateral extent. Minor amounts of gravel-sized clasts were observed in some of the sandy strata. The sandy zone comprises two laterally continuous sandy strata (Sand 1 above and Sand 2 below) with an intervening stratum of silt. Sands between approximately 80 and 90 feet bgs appear consistently cemented. The two sandy strata exhibit a fairly consistent lateral continuity, with an attitude of N17°E, dipping 5° from horizontal toward the east. It should be noted, however, that as observed in the overlying silty unit, a fair degree of heterogeneity exits within the subsurface, and it is likely there is some vertical connection between Sand 1 and Sand 2 through the intervening silt stratum. The lower silty zone consists predominantly of silt. Except for a 6-inch thick sand stratum in boring B-35 (at 111 feet bgs) and an 18-inch thick sand stratum in boring B-37 (at 117 feet bgs), sand was not encountered in the lower silty zone. The lower silty zone extends to at least 202 P:\Bp-Arco for URS\6100\Work Plans\6100 workplan-l.doc