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1 v ' 30 October 2003 AGE-NC Project No 96-0232 ' Page 14 of 15 replacement of fuel dispensers and pans, product piping, secondary containment piping and UST ' sumps)during May 2003 Since then,the MTBE dissolved plume appears to have stabilized,MTBE concentrations in extraction well samples have showed steady declines since the peak in ' March 2003 Analytical results from the September 2003 quarterly ground water monitoring event indicates that the highest concentrations of on-site MTBE occur in First Zone ground water samples EW-5 and ' EW-8 at 24 gg/l (Figure 8), and in the Second Zone sample collected from MW-8A at'420 µg11 (Figure 9) The MTBE concentration reported in sample MW-8A is a significant decrease from the ' March 2003 concentration of 900 µg/1, however, an increasing or decreasing trend of MTBE concentrations has not been established ' The vertical extent of MTBE-impacted ground water at the Flag City Chevron site is interpreted to be at between 25 feet bsg and 45 feet bsg(Figure 10) Relative to the inferred source area near the Flag City Chevron UST area (EW-1), the Flag City Chevron plume may extend approximately ' 200 feet to the west at depths as great as 75 feet bsg(MW-11B), the maximum westward extent of the Flag City Chevron plume may be in the area of the former Rocky's Restaurant domestic drinking water well (RDW-1), where the plume appeared to be co-mingled with the Three B's Truck Plaza plume Figures S, 9 and 10 depict Flag City Chevron wells EW-2 through EW-9, MW-8, MW-9, MW-ll and Three B's Truck Plaza wells MW-1,MW-2,MW-4,MW-6 and MW-8 through MW-12 to be located within two, dissolved MTBE-impacted plumes that converge near the vicinity of the former Rocky's Restaurant domestic well RDW-1 The Flag City Chevron plume appears to extend less than 130 feet to the south (based on previous detection of MTBE at MW-6B) at depths as great as 48 feet bsg, and at least 60 feet to the east (between EW-9 and MW-2) at depths as great as 35 feet bsg(Figure 8) The northern extent of the Flag Cityplume is uncertain at the present time because of the high concentrations of MTBE (up to 900 µg/1) detected in northernmost Flag City Chevron well MW-8 during the previous seven quarterly ground water monitoring events The highest concentrations of off-site MTBE occur in the area between the Three B's Truck Plaza and Rocky's Restaurant, the highest concentrations in the First Zone Aquifer are in the vicinity of Three B's Truck Plaza wells MW-8A and MW-4,whereas the highest concentrations in the Rocky's Upper Zone Aquifer are in the vicinity of Flag City Chevron wells MW-11B and MW-18 Between December 2002 (fourth quarter 2002)and September 2003 (third quarter 2003), substantial MTBE decreases were observed in First Zone well MW-16, Second Zone wells MW-11 A and MW-17,and Rocky's Upper Zone wells MW-15 and MW-18 at the Rocky's Restaurant site, suggesting lateral attenuation of the 3 B's Truck Plaza and Flag City Chevron MTBE plumes Ile It is not clear at this time why MTBE was detected at MW-8A over the past seven quarters, at concentrations which fluctuated between 4 7 µg/1 and 900 gg/l, after non-detectable concentrations Advanced GeoEnviron mental,Inc- 1