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Site Background Information. Vanco Truck&Auto Plaza Page 5 of 6 Hydrogeoiogic Unit 7 (HU7)- Extending from 82 feet to total depth of the boring(100 feet bsg), HU7 is composed primarily of stiff,fine-grained clayey 'Its,silty clays,and clay.A thin,cemented sand is encountered at approximately 94 feet to 95 feet bs .Hydraulic conductivity generally ranges between 1 x 10-5 and 1 x 10-15 cm/s for clays, silts, and sti , fine-grained soils. HU7 is characterized by varying pore pressures ranging between moderate and very high and is identified as a confining layer. i SITE CONCEPTUAL MODEL Based on the data collected to date from soil boring ins t llation and sampling,AGE has developed 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 c ap.In general,a clayey silt layer extending from surface grade to approximately 30 feet bsg unde lies the assessed portion of the property (vadose zone & HUI). Beneath the clayey silt layer is a fine-grained layer consisting of silty sand to fine sand(HU2),which is encountered from approxi ately 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 stifffine-grained layers act as confining layers,retarc ing the vertical migration of contaminants. The sandy layers act as migratory flow units, allowin contaminants to migrate vertically and laterally away from the source. During site assessment activities in July andAugust 004 and July 2005, saturated soil, or the capillary fringe,was first encountered at approximately 5 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 by rocarbon concentrations were detected in pilot soil borings MW-4 and MW-6 at depths of 15 feet and 35 feet bsg, respectively (TPH-d: 2,600 mg/kg and 2,500 mg/kg). Until the monitoring well installation conducted in July 2005,the maximum petroleum hydrocarbon concentration(110 g/kg TPH-d)was detected adjacent to the former piping sample Iocation P 13,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 (CPT 1-60) and 6.3 mg/kg (CPT 1-90); TPH d was detected at a concentration of 110 Advanced GeoEnvironmental,Inc.