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� w fl Site Background Information: Stallworth Auto Sales LI Page 7 of 9 HYDROGEOLOGIC MODEL Hydrogeologic or hydrostratigraphic units (HUs) are 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 (Sanders, 1998), or apart of a body of rock that forms a distinct hydrologic unit with respect to the flow of groundwater (Maxey, 1964). 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 the CPT results and comparison with soil descriptions from borings previously advanced on-site, AGE has refined the previous subsurface hydrogeologic model presented in the AGE- prepared Cone Penetrometer Testing Report, dated 10 December 2004. The model is comprised of eleven informal hydrogeologic units : a vadose (unsaturated) zone and ten hydrogeologic units; the j model presented is interpretational and may be modified as additional data is collected. The eleven vertical zones identified are as follows (depths are approximate and can vary by several feet across the site): Vadose Zone - Extending from surface grade to 31 feet bsg, the average depth to the ground water table. The vadose zone is predominately coarse grained (sand and silty sand) from surface grade to 10 feet bsg, with fine material (silt and clay) and some coarser grained materials inferred from 10 feet bsg to the surface of the water table. i Hydrogeologic Unit 1 (HUl ) - At the top of the current saturated zone, HUl extends from 31 feet to between 49 feet and 55 feet bsg and is comprised primarily of silt and clay with discontinuous sand lenses and is characterized on the CPT logs by low to moderate pore pressures. Hydrogeologic Unit 2 (HU2) - extends from between 49 feet and 55 feet to 60 feet bsg. HU2 is characterized by silty sand interbedded with thinner silt and clay layers with moderately low to moderate pore pressures. Hydrogeologic Unit 3 (HU3) - extends from 60 feet to 63 feet bsg; HU3 is a sand to silty sand layer characterized by very low pore pressure. Hydrogeologic Unit 4 (HU4) - extends from 63 feet to 127 feet bsg; 11U4 consists primarily of silts and clays with some discontinuous layers of sand to sandy silt. The unit is characterized by low to moderate pore pressure. Hydrogeologic Unit 5 (HU5) - extends from 127 feet to 131 feet bsg; HU5 consists of sand to silty sand, characterized by very low pore pressure. Advanced GeoEnvironmental, Inc.