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Table 4 SITE CONCEPTUAL MODEL SUMMARY Revised July 22, 2004 Item Comment Source of Contamination Former gasoline and diesel UST's Chemicals of Concern Petroleum hydrocarbons, predominantly MTBE Active service station facility located in an urban neighbored Site Vicinity with asphalt and concrete cover Adjacent properties utilized for commercial and residential use Flat topography, nearest surface water is X1,000 feet UST's removed and compliance soil samples were collected Summary of Investigations in 1998 A total of 33 soil borings and 10 groundwater monitoring wells have been installed on- and off-site Silts and clays with interbedded sand strata to approximately 100 feet bgs Three of the sand strata are laterally continuous across study area Sand A is 3 to 10 feet thick, and is Subsurface Geology encountered between 5 and 8 feet bgs Sand B is 7 to 18 feet thick, and is encountered between 25 and 35 feet bgs Sand C is from 15 to greater than 25 feet thick, and is encountered between 50 and 65 feet bgs Sand strata do not appear to be stratigraphically connected Based on CPT investigation, all groundwater encountered beneath the site is part of the first water-bearing zone Calculated depths to water in the sandy strata were similar, Groundwater flow direction and gradient suggesting hydraulic communication In the monitoring wells, depth to water fluctuates from 6 to 10 feet bgs Predominant flow direction northeast and east at flat gradients (between 0 00083 and 0 0015 ft/ft) North and northwest flow directions observed sporadically Highest reported concentrations were 1,000 mg/Kg of TPHD Maximum Concentrations in Soil (6' bgs adjacent to UST's), 47 mg/Kg of TPHG, 0 88 mg/Kg of benzene, and 0 46 mg/Kg mg/Kgof MTBE 10 5', base of UST it Borings 320,000 Ag/L of TPHD (B-3), 2,700 µg/L of TPHG (B-4), 16 pg/L of benzene (GP-4), and 2,600 µg/l_ of MTBE (GP-8) Toluene, TAME, and methanol also identified in some of the CPT borings Maximum Concentrations in Groundwater Wells historical 120 pg/L of TPHD (MW-1 2112/00), 440 µg/L of TPHG (MW-1 2/12/00), 3 1 µg/l_ of benzene (MW-1 2/12/00), and 2,800 p.g/L of MTBE (MW-1 4/28/00) Wells current 190 µg/L of TPHD (MW-4), 470 gg/L of TPHG (MW-2), and 780 /L of MTBE MW-2 Benzene is ND Mass Remaining in Place Groundwater= approximately 0 6 lbs of MTBE Soil =ap roximately 5 5 lbs of MTBE Lateral extent of soil impact has been adequately Extent of Impact in Soil characterized Vertically, soil impact appears to extend to approximately 25 feet bgs Lateral extent of groundwater impact has been adequately Extent of Impact to Groundwater characterized Vertically, groundwater impact extends to approximately 40 feet bgs Potential Sensitive Receptors None identified within 2,000 feet of site Potential Exposure Pathways No complete exposure pathways or receptors identified at this site Feasibility Studies Dual-phase extraction feasibility testing was conducted in �. June 2004 Remedial Efforts None after UST removal Next Phase of Activity Additional soil and groundwater characterization does not appear to be warranted P Voe s Truck Plaza\TablcslSCM Summary-revised after CPT doe 57 KA 7-05