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i30 April 2005 AGE-NC Project No 95-0141 Page 7 of 10 ' 4.0. SITE CONCEPTUAL MODEL The site is located on essentially flat land that is occupied by a heating and air conditioning company Site structures include, a wain building utilized for office space and storage, a sheet metal shop utilized to manufacture parts and a covered storage area housing air conditioning units,the majority of the ground surface is covered by asphalt or concrete (Figure 2) Based on data collected to date from onsite and offsite 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 ' Based on cone penetrometer test(CPT)log and soil borings,the soil stratigraphy generally consists of sand to silty sand interbedded with fine grained material From surface grade to 50 ft bsg the lithology is predominately fine grained with a discontinuous sandy layer from approximately 15 ft ' to 30 ft bsg From 50 ft to 105 ft bsg the lithology consists of alternating layers of sandy silt to sand and fine grained material The layers are laterally continuous and generally 15 to 20 ft thick, acting as aquifers and aquatards ' Based on the soil stratigraphy, supplemented with pore-pressure data from the CPT log, AGE g pp developed a hydrogeologic model consisting of six informal zones (Figure 11), a vadose (unsaturated) zone and five hydrogeologic units (HUs), in general, low pore pressures associated with the sandy HUs would suggest more rapid pore-pressure dissipation (e g , higher hydraulic Iconductivities) Site assessment activities to date identified the area where subsurface soil appears to be impacted by petroleum hydrocarbons, soil is impacted at the former UST location and appears to have migrated northeast towards MW-4 (Figures 12, 13 and 14) At the UST location, the highest concentrations of THP-9 were detected at shallow depths(less than 45 ft bsg) surrounding the UST ' location, but extend, in lower concentrations, to a depth of 65 feet bsg in the area of boring MW-6 Moderate to low concentrations of BTEX compounds were detected in soil samples collected from borings MW-1 through MW-4 and MW-6 at depths between 15 and 65 feet bsg ' The plume of impacted soil at the former UST release area has an estimated long dimension of 110 ft and short of dimension of 40 feet and a vertical depth of 70 ft bsg at MW-6 and 50 ft bsg at MW-4 ' (Figures 12, 13 and 14) Analytical results of soil samples are included in Tables 4 and 5 Between November 2000 and December 2004, sixteen ground water monitoring events were ' performed at the site to monitor dissolved petroleum hydrocarbon concentrations in ground water and to characterize ground water flow direction and gradient on the site Since initiation of the ground water monitoring program, ground water elevations have ranged from approximately 11 to ' 15 feet below mean sea level, and the ground water flow has been consistently towards the east to southeast at a low hydraulic gradients TPH-g and/or BTEX compounds have been detected in water ' Advanced GeoEnvironmentel,Inc