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i trace gravel, with porosities at between 34.5% and 30.6%, low organic content (0.4 to 0.6%), and ispecific gravity values at between 2.67 and 2.72.The samples 135-20 and B5-30 were described as ' brown sandy clay and a gray clay (silty), with porosities at between 46% and 47.7%, low organic content (1.2 to 1.4%), and a specific gravity of 2.72. 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 tine 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 four deepest soil borings(MW6,B 1,B5 and B6) located at the ' former USTs' No. 1 and 2 and down gradient off-site to the north-west. Additional figures and tables can be found in the AGE-prepared Corrective Action Plan, dated 11 February 2003 (CAP). Since initiation of ground water monitoring in December 1999,groundwater elevations have ranged from approximately 13 to 19 feet above mean sea level. The ground water table underlying the site is generally in the form of a trough-like depression, with the base of the trough plunging northwest ' a low gradient of less than 0.01 ft/ft. 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 permeable clayey silt of Layer 2. The contaminates began to migrate laterally as well as vertically in Layer 2; the direction of lateral ' migration was dominantly towards the northwest,in the preferred ground water flow direction.The lateral and vertical extent of soil and ground water contamination has been defined. ' Significant petroleum-hydrocarbon impact to soil in the vadose zone is present in borings B2-20.25, 134-20,MW2-25 and MW4-20, in the area of the former USTs, and off-site well MW9-20.TPH-g, ' TPH-d and TPH-mo are present at maximum concentrations of 670 mg/kg(B2-20.25), 150 mg/kg (MW-9-20)and 120 mg/kg(B-2-20.25), respectively. Most of the contamination appears to reside at the top of the clay layer, at approximately 20 feet bsg. Advanced GeoEovironmental,Inc.