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D�Q CLEAKWATER ' G R O U P Environmental Services IGeolojzy and Hydrogeology The subsurface is generally comprised of silty clay with subsidiary clayey sand and occasional sandy lenses Depth to groundwater has historically ranged from approximately 4 71 feet bgs (MW-2 on March 9, 2000) to 11 96 ' feet bgs (MW-3 on September 29, 1996) Groundwater elevation measurements within the last 14 months have indicated groundwater flows generally towards the southeast at gradients ranging from 0 005 ft/ft based on May 30, ' 2001 gauging data, to 0 018 fflft,based on February 13, 2001 gauging data Contarrunant Type_ ' The contaminants of concern found dissolved in the groundwater below the site are of gasoline origin, and include TPHg (total petroleum hydrocarbons as gasoline), BTEX compounds (benzene, toluene, ethylbenzene, and xylenes), ' and MTBE(methyl-tertiary-butyl ether) ' TPHg concentrations from the latest sampling event on February 14, 2002, ranged from<50 µg/1(MW-2 and MW- 4) to 14,000 µg/1 (MW-5B) Benzene concentrations ranged from<0 5 µg11 (MW-2, MW-3, MW-4) to 1,900 µg11 (MW-5B) MTBE has been detected previously using EPA Method 8020, however confirmation analysis by EPA lieMethod 8260 indicate that this compound has only been detected in the February 14, 2002 sampling event in MW-1 at a concentration of 1 1 µg/l(Figure 4) Distribution of Sorbed-phase Contaminants Review of cumulative soil sample analytical results from the site reveals significant concentrations of hydrocarbons ' in the sorbed-phase at a depth from 20 to 25 feet below the surface Laterally, the sorbed-phase hydrocarbon plume is delineated by boreholes SB-3, MW-6, MW-7, and MW-4 Concentrations of TPHg found within the plume range ' from 160 mg/kg (SB-61MW-3 at 20 feet bgs) to 1,000 mg/kg (MW-5B at 25 feet bgs) Concentrations of benzene ranged from below the detection limn (less than 0 5 mg/kg in SB-6)to 6 mg/kg(SB-1 at 25 feet bgs) At this point, ' sorbed-phase contaminants appear to be adequately delineated Figure 3 shows the aerial extent of sorbed-phase hydrocarbons below the site ' Distribution of Contaminants in Groundwater The hydrocarbon plume associated with the site extends down-gradient from the source approximately 250 feet to ' the southeast and has been thoroughly delineated laterally and down-gradient by MW-7 and MW-8, and vertically with a discrete water sample collected at 36 feet bgs in the former UST location(SB-7@36') ' The presence of high concentrations of dissolved hydrocarbons down-gradient from the former USTs (MW-5B) with lower levels found closer to the former USTs (MW-1) suggests that the core of the plume has migrated down- IZB 178C Feas Test Report 2 May 9, 2002