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�k r! CLEARWATER G R O U P Environmental Services CQntamnnant type The contaminants of concern found to be in the dissolved phase below the site are of gasoline origin, which are TPHg, BTEX compounds, and MTBE In the latest quarterly groundwater monitoring event (May 30, 2001) TPHg concentrations ranged from <50 gg/L (MW-2, MW-4) to 15,000 gg/L (MW-5B) Benzene concentrations ranged from <0 50 gg/L (MW-2, MW-4) to 2,600 gg/L (MW-5B) MTBE concentrations ranged from<5 0 gg/L(MW-2,MW-3,MW4,MW-7) to 42 gg/L (MW-1) It must be noted that MTBE was detected by EPA Method 8020 in the groundwater sample collected from monitoring wells an May 30, 2001 Previously, MTBE has been confirmed as "non-detectable" by EPA Method 8260B in all monitoring wells on the site 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 lyes within boreholes SB-3,MW-6, MW-7, and MW4 Concentrations of TPHg found within the plume range from 160 mg/kg(SB-6/MW-3 at 20 feet bgs)to 1,000 mg/kg (MW- 5B at 25 feet bgs) Concentrations of benzene ranged from below the detection limit (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 appears to be adequately delineated Figure 4 shows the aerial extent of sorbed-phase hydrocarbons below the site Distribution f contaminants in Uoundwater The hydrocarbon plume associated with the site extends downgradient from the source approximately 250 feet to the southeast and has been thoroughly delineated laterally and downgradient with MW-7 and MW-8, and vertically with a discrete water sample collected at 36 feet bgs in the former UST location (SB-7(936') 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 downgradient There is,however, an existing let fuel pipeline located under South Fresno Avenue which has possibly contributed to the subsurface contamination The jet fuel pipeline is located approximately 12 feet off the east curb, under the road approximately 4 feet bgs Unfortunately the Is City of Stockton would not allow Clearwater to drill in the road to accurately assess this possible ZBI78C Workplan for remedial feasibility testing 4 August 2001