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Historical Background & Site Conceptual Model g p AGE-NC Project No 01-0827 Page 5 of 9 • Hydrogeologic Unit 1 -generally between surface grade and 20 feet bsg locally less, consists primarily of laterally continuous, less permeable, fine grained, silty clay and clayey silt • Hydrogeologic Unit 2-Located between 20 and 30 feet bsg,consists primarily of permeable to semi-permeable, fine to medium sand and silty sand with limited minor gravels cut and fills, representing the first ground water currently observed at a depth of 25 feet bsg • Hydrogeologic Unit 3 - Located between 30 and 60 feet bsg, consists primarily of less permeable, fine grained, silty clay and clayey silt zone, some limited sand zones of limited lateral continuity (only at well MW-2) • Hydrogeologic Unit 4 Located between 60 feet and 65 feet bsg, consists primarily of permeable to semi-permeable,fine to medium sand with limited minor fine grained material, the depth interval of well MW-6 The hydrostratigraphic model presented is interpretational and will be modified as more data is collected Based upon ground water monitoring well information obtained from the site, the depth to ground water at the site has ranged from 45 feet bsg, during late 1992,up to the most shallow depth to water of 20 feet bsg in mid 1998 The low ground water table coincides with the operation of the former UST system The water table currently occurs at a depth of approximately 25 feet bsg i 4 0. PETROLEUM HYDROCARBON DISTRIBUTION IN SOIL I Between the first UST removal in April 1987 and February 2005,eighteen soil borings(B 1 through B17 and OZ1) and seven soil excavation samples were analyzed as part of an ongoing subsurface petroleum hydrocarbon investigation • To date, a total of 43 soil samples at depths ranging from 2 feet to 46 5 feet bsg have been submitted for laboratory analysis of petroleum hydrocarbons, TPH-g, TPH-d and BTEX compounds were detected in 20 of the soil samples analyzed,analysis for fuel additives were not generally included when the soil samples were analyzed The greatest concentrations of petroleum hydrocarbon-impacted soil were encountered in soil borings B 10 and B 16,which were completed as soil vapor extraction wells, with TPH-g concentrations as high as 5,100 mg/kg (B 16 at 15 feet), and BTEX compounds at concentrations as high as 11 mg/kg benzene,40 mg/kg toluene, 18 mg/kg ethylbenzene and 110 mg/kg xylenes(B1 at 25 and 35 feet) The boring B10 trace was established through the former UST excavation area • Soil boring B9 (VW-1)had low detections of TPH-g and BTEX compounds,however, soil sample collected from boring B16 (VW-4), located further east from the UST excavation Advanced GeoEnwonmental,Inc