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CONESTOGA-ROVERS &ASSOCIATES February 17,2010 Reference No.241661 -2- • Shell well MW-10 is located offsite,400 feet northwest of MW-6,across Highway 205. • Groundwater flow direction beneath the site is primarily north to northwesterly. • The surface topography is relatively flat with a slight downward slope to the north. • The groundwater gradient beneath the site ranges from 0.006 to 0.01. • Periodic groundwater extraction was used as an interim remedial method at this site for approximately 3 years. This involved extracting a total of 38,812 gallons of groundwater from wells MW-1, MW-2,and MW-6. • Historically,MW-6 has contained the highest MTBE concentration at the Shell site. • The highest concentration of MTBE in MW-6 was 4,200 micrograms per liter (µg/L). • The highest concentration of MTBE in MW-10 was 760 gg/L. • Figure 1 depicts the highest MTBE concentrations in MW-6 and MW-10 and shows the distance between the two wells. • Assuming the MTBE in MW-10 originates from the Shell site,the difference (reduction) in maximum MTBE concentration between MW-6 and MW-10 represents a concentration decline of one order of magnitude over a distance of approximately 400 feet. s Adjacent Site (7-Eleven) Data We reviewed the environmental case data for the 7-Eleven site located immediately south of the Shell site. The 7-Eleven site is located at 2360 West Grant Line Road in Tracy,California (Figure 2). During 2003,high concentrations of MTBE and tertiary butyl alcohol(TBA)were discovered in groundwater samples from the underground fuel storage tank backfill wells at this site. The pertinent facts regarding the 7-Eleven site are listed below. • The 7-Eleven site is located south of the Shell site,across West Grant Line Road. • No remediation was ever performed at this site. • Historically,well MW-5 has contained the highest MTBE concentrations at the 7-Eleven site. • The highest concentration of MTBE in MW-5 was 120,000µg/L. • Shell well MW-4 is located 90 feet downgradient of MW-5. • Well MW-9 (7-Eleven's) is located 75 feet downgradient of MW-5. • Groundwater flow direction beneath the 7-Eleven site is primarily northerly. • The highest concentration of MTBE in MW4 was 150 gg/L. • The highest concentration of MTBE in MW-9 was 590 gg/L. • Figure 2 depicts the highest MTBE concentrations in wells MW-5,MW-4,and MW-9 and shows how far downgradient MW4 and MW-9 are from MW-5. • The MTBE concentration reduction observed for the 7-Eleven plume represents a concentration decline of three orders of magnitude over a distance of approximately 90 feet (or less)_ Worldwide Engineering,Environmental,Construction,and IT Services