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Site Background Information: Larry's Auto AGE-NC Project No. 95-0184 Page 5 of 8 HYDROGEOLOGIC MODEL A hydrogeologic unit(HU) is defined as a body of rock or sediment that has hydrogeologic characteristics (i.e. hydraulic conductivity, pore pressure, etc.) that make it distinct from surrounding bodies of rock or sediment, or a part of a body of rock that forms a distinct hydrologic unit with respect to the flow of ground water. A 'flow unit' may be defined as a hydrogeologic unit and is characterized by significantly greater hydraulic conductivity values than overlying and underlying units; a flow unit is capable of serving as a preferred migration route, and could potentially allow migration of large contaminant masses over great distances. Based on a previous AGE-conducted CPT investigation (Cone Penetrometer Testing Report - November 2001, 2003), AGE had presented a preliminary subsurface hydrogeologic model of the site consisting of fifteen informal zones,a vadose(unsaturated) zone and fourteen hydrologic units, developed primarily from one CPT lithologic log (CPT- 3). Incorporating the CPT-generated lithologic data from the six additional CPT borings of December 2004-January2005,AGE has refined the site hydrogeologic model, which now consists of seven informal zones, a vadose(unsaturated)zone and six hydrogeologic units (Figures 3 through 6); the fifteen previous hydrogeologic units are incorporated as subunits within the present hydrogeologic units (Table 1). The seven hydrogeologic units identified are as follows (depths are approximate and can vary by several feet across the site): Vadose Zone - Extending from surface grade to approximately 38 feet bsg, the vadose zone is composed primarily of clay, silt, silty clay, clayey silt, sandy silt, and silty sand. This zone is characterized by low to moderate pore pressures. HU 1, HU3 and HU5-These zones are composed primarily of clay, silt, silty clay and clayey silt, with generally minor, discontinuous lenses of silty sand, sandy silt, and sand. These units are characterized by moderate or fluctuating (between low to moderate with depth) pore pressures. HU2, HU4 and HU6-These zones consist primarily of sand, silty sand, sandy silt, gravelly sand and cemented sands,with lesser occurrences of clay, silty clay orclayey silt intervals. Pore pressures within these units are generally low. These units may be considered flow units. Advanced CeoEnvironmental,Inc