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Appendix B AdvancedGeo AGE Project No. 99-0559 Environmental Page 9 of 12 n. IN • The vertical extent of dissolved TPH-g, benzene and MTBE does not extend to below 130 feet bsg. • The solvent TCE was detected in HU2 and HU4 in the off-site borings CPT-5 and CPT-8 (Figure 2), at a maximum concentration of 1,700 pg/I. The TCE appears to originate from an off-site source, since TCE was not detected in CPT borings (e.g., CPT-4 and CPT-6) closer to the site. Computer generated CPT soil descriptions augmented with previously obtained soil sample and CPT soil descriptions were utilized to construct the vertical and lateral geologic model; laboratory analysis of groundwater samples was utilized to assess the hydrocarbon-impact on the dissolved media. SITE STRATIGRAPHY Based on the soil descriptions from CPT and previous soil boring logs, interpretive cross sections were constructed to depict the general hydrogeologic framework beneath the site. The site vadose zone currently extends from the ground surface to 65 feet bsg (water table), and is composed of silt, clayey silt, clay, and `stiff fine grained' soil, and a gravelly, fine to coarse sand layer between 35 feet and 50 feet. Below the vadose zone, the lithology is composed dominantly of silty fine sand, sandy silt, clayey silt to silty clay and clay. Significant sand intervals were identified at between 90 and 110 feet bsg over the site, a thin discontinuous sand layer was encountered at 150 feet bsg and between 165 and 170 feet bsg. Fine grained, silty clay and clayey silt layers were observed at depths between the surface and 30 to 35 feet bsg, 80 and 90 feet bsg, 105 to 150 feet bsg. Variable lithology including silty fine sand, sandy silt and clayey silt to silty clay were observed at all other depths. HYDROGEOLOGICAL MODEL Hydrological units may be defined as significant vertical intervals of sediments having significant lateral extent and having similar hydrologic characteristics (i.e. hydraulic conductivity, transmissivity, etc.) over its vertical and horizontal extent. A `flow unit' may be defined as a hydrologic 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. AGE had developed a site hydrogeologic model comprised of six informal zones, a vadose (unsaturated) zone and five hydrological units. However, based on the CPT results and comparison with soil descriptions from soil borings previously advanced on the site, AGE presents a revised hydrologic model as follows (depths are approximate and can vary by several feet across the site):