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Appendix B Advanced AGE Project No. 99-0559 Environmental Page 11 of 12 n.M 9 low pore pressure. If laterally continuous beyond CPT-6, CPT-8 and MW-5, this unit should be characterized as a flow unit. SITE CONCEPTUAL MODEL Based upon current data collected AGE has prepared the following site conceptual model: • The subsurface lithology is composed dominantly of silty fine sand, sandy silt, clayey silt to silty clay and clay. Significant sand intervals were identified at between 50 and 60 feet bsg, between 90 and 110 feet bsg and between 165 and 170 feet bsg. Based on the soil descriptions and CPT hydrologic information (pore pressure), AGE has identified six hydrologic units (HU1 through HU6). • A release of petroleum hydrocarbons (TPH-g) and benzene, toluene, ethyl benzene and xylenes (BTEX) compounds and fuel additives (MTBE) was documented to have occurred from UST fuel dispenser line, at the in-tank pumps. Additionally soil samples collected from the product delivery line trenches indicated significant leaks at the dispenser islands. The historical operational period for the site appears to have extended over 50 years as a fueling station, prior the issuance of an unauthorized release report (UAR). • Petroleum hydrocarbons were detected in the soil samples collected from below the former UST area, while the most significant concentrations of hydrocarbons and MTBE were detected below the UST dispenser area proper. Available data is suggestive of a release was most severe at the northern edge of the UST excavation and the release was most severe at the southern-most dispenser islands (PL-2, PL-4), at the center-north dispenser (PL-5) and even less between the dispenser islands. • Once released fuel washed over the USTs and out of the product lines into the subsurface, the contaminants migrated under the former UST area (CPT2) vertical depth of approximately 80 feet bsg, possible to the former water table level, where the variation concentration in soil samples suggest a smear zone between 90 feet and 150 feet bsg. The highest TPH-g and BTEX impact to soil in the upper vadose zone is present at monitoring well borings through the former UST area (MW2). At this point the TPH contaminants migrating down to the water table and west and south to monitoring well MW-3 and MW-4, where non-detect soil samples were collected above 50 feet bsg and impacted soil below 50 feet bsg. The aerial extent of the soil contamination in vadose zone is defined only to the west of the former UST area. • The petroleum hydrocarbons then dissolved into the groundwater at the former UST area and likely under the former dispenser island. The presents of free phase petroleum product at the former UST excavation indicated adsorbed concentrations of petroleum hydrocarbons beyond the sorbtive capacity of the soil at the water table, a submerged soil plume below the current water table level,