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19 December 2003 AGE-NC Project No. 95-0135 Page 3 of 7 On 27 and 28 September 2001, three soil borings were advanced at the site. Pilot borings MW-2, MW-3 and MW-4 were completed as ground water monitoring wells. Soil in the area of borings MW-2, MW-3 and MW-4 is not impacted by petroleum hydrocarbons or fuel oxygenating compounds. The lateral and vertical extent of impacted soil has been defined on the site. 2.4. SITE CONCEPTUAL MODEL Based on the data collected to date from soil borings and monitoring well installations, AGE has developed a site conceptual model for the release,migration and distribution of the contaminants in the subsurface as described below: The shallow stratigraphy is composed of an upper layer (Layer 1) of between 15 and 20 feet of yellowish to brownish green, generally unconsolidated, fine to medium sand, underlain by an approximately 15-foot thick clayey silt layer(Layer 2);northwards,Layer 2 appears to transition into a sandy silt or silty sand with clay, and beneath former USTs No. 1 and 2, a discontinuous lense of green to olive gray fine sand was identified near the top of Layer 2. The third layer (Layer 3), at below 35 feet bsg,appears to be an olive green to brown,unconsolidated coarse to pebbly sand;this layer was defined on the basis of the two deepest soil borings(MW-6 and B-1)located at the former Q USTs' No. 1 and 2, respectively (Figure 2). Geologic cross sections are provided in Figures 3A—Gr'o"� t and 4A. Since initiation of ground water monitoring in March 2001, ground water elevations have ranged from approximately 26 to 44 feet below mean sea level. The ground water table underlying the site is generally flat, with a low gradient of less than 0.001 ft/ft (Figure 7). --- �Jo 1�71*0 u r4- 1 't n cl vd Releases of petroleum hydrocarbons occurred from the on-site USTs. Once released to the subsurface,the contaminants migrated through the sandy Layer 1 to a vertical depth of approximately 20 feet bsg, at which point the contaminants encountered the less porous clayey silt of Layer 2 and began to migrate laterally as well as vertically; the direction of lateral migration was dominantly towards the northwest, in the preferred ground water flow direction. Significant petroleum-hydrocarbon impact to soil in the vadose zone is present in borings B-2-20.25, B-4-20, MW-2-25 and MW-4-20, in the area of the former USTs, and off-site well MW-9-20 (Table 2). TPH-g,TPH-d and TPH-mo are present at maximum concentrations of 670 mg/kg(B-2- N a 20.25), 150 mg/kg(MW-9-20)and 120 mg/kg(B-2-20.25),respectively.Most of the contamination , appeared to reside at the top of the clay layer, at approximately 20 feet bsg. Advanced GeoEnvironmental,Inc.