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1 ' Site Background Information Vanco Truck& Auto Plaza Page 5 of 6 1 a site conceptual model for the release, migration, and distribution of the contaminants in the ' subsurface as described below The site surface is essentially flat land that is developed as a truck and auto plaza The majority of ' the ground surface is covered by an asphalt or concrete cap In general, a clayey silt layer extending from surface grade to approximately 30 feet bsg underlies the assessed portion of the property (vadose zone & HU1) Beneath the clayey silt layer is a fine-grained layer consisting of silty sand ' to fine sand (HU2), which is encountered from approximately 30 feet to 38 feet bsg HU2 has the characteristics of an aquifer, or the water table ' Beneath the silty sand to sand layer are alternating layers of stiff, fine-grained soils and sandy soils The stiff fine-grained layers act as confining layers,retarding the vertical migration of contaminants ' The sandy layers act as migratory flow units, allowing contaminants to migrate vertically and laterally away from the source ' During site assessment activities in July and August 2004, saturated soil,or the capillary fringe,was first encountered at approximately 25 feet to 30 feet bsg Based on information from quarterly monitoring events performed by AGE, ground water flow is generally toward the northeast However, this may be greatly modified by local recharge and discharge patterns Site assessment activities to date defined two areas where soil is impacted by concentrations of petroleum hydrocarbons The maximum petroleum hydrocarbon concentration(I 10 mg/kg TPH-d) was detected adjacent to the former piping sample location P13, at 15 feet bsg in soil probe boring B3 Petroleum hydrocarbons were also detected at a concentration of 12 mg/kg in the soil sample ' collected from boring B7 at 30 feet bsg, adjacent to piping sample P7 Soil samples collected at 60 feet and 90 feet bsg from boring CPT-1 were impacted by petroleum hydrocarbons TPH-g was detected at concentrations of 2 1 mg/kg(CPT1-60) and 6 3 mg/kg(CPT1-90), TPH-d was detected at a concentration of 110 mg/kg (CPT 1-90) Site assessment activities to date defined an area where ground water is impacted by high concentrations of petroleum hydrocarbons The highest petroleum-hydrocarbon concentrations, up to 2,200,000 µg/1 TPH-d,were detected in grab ground water samples adjacent to the former piping sample location P13 Petroleum hydrocarbon concentrations as high as 190,000 µg/1 were detected in grab ground water samples dust north of dispenser D1 MTBE was detected in minor concentrationslust north of dispenser sample D7 Groundwater was not impacted below 3 5 feet bsg Ground water samples collected from the ground water monitoring wells defined the dissolved plume at the northern edge of the dispenser islands Based.on AGE's interpretation of the available data,the initial release(s)of petroleum hydrocarbons occurred from the former dispensers and associated piping prior to their removal in 2003 Once released to the subsurface, the contaminants migrated through and/or adhered to the clayey silt Advanced GcoEnvlronmcntai,Inc