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Site Background,Information- Vanco Truck & Auto Plaza Page 5 of,6 Hydrogeologic Unitt6 (HU6)- HU6 extends from 70 feet-to 82 feetbsg and is composed of silty.sand to sand type soils. Hydraulic conductiv ities for'HU6 generally range between 1 x10-2 and 5.0 cm/s. HU6 is characterized by very low pore pressures, and is classified as a flow unit. Hydrogeologic 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 silts, silty clays, and clay. A thin, cemented sand is encountered-at approximately 94,feet'to 95 feet bsg;,Hydraulic conductivity generally ranges between 1 x 10-6 and 1 X 1 T15 cm/s for.-clays, silts, and stiff, fine-grained soils.- HU7 is characterized by varyng pore pressures ranging between moderate and very high and is identified as a corif ning layer: SITE CONCEPTUAL MODEL Based,on the data collected to date from soil boring installation 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 flatland that lis dev loped as a truck and auto plaza. The majority of-the ground surface is covered by an sphalt-or-concrete cap. In general, a clayey silt layer extending from surface grade to a proximately 30 feet bsg underlies the assessed portion of-the property (vadose zone & U1): Beneath the clayey silt layer is a fine-grained layer-consisting of-silty sand to fine s nd (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 alternatin layers of stiff, fine-grained soils and sandy soils. The stiff fine rained layers act as confining layers, retarding the vertical migration of contaminants. The sandy layers z ct- as migratory flow units, allowing contaminants to migrate vertically and laterally aw 3y from the source. During site assessment activities in July and August 2004and July 2005, saturated soil, or the capillary fringe,was first encountered at-appr ximately 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 tw areas where soil is impacted by concentrations of petroleum hydrocarbons. The maximum petroleum hydrocarbon concentrations were detected in pilot soil borings M -4 and MW-6 at depths of 15 feet and 35 feet bsg, respectively(TPH-d: 2,600 mg/kg and ,500 mg/kg): Until the monitoring well installation conducted in July 2005, the maximum petroleum hydrocarbon concentration Advanced GenEnvirownentai,Inc.