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l 30 July 2004 ,i AGE-NC Project No 96-0232 Page 14 of 16 _ (Figure 8), offsite, the plume is most prevalent in the First Zone, with decreasing concentrations observed in the Second Zone hydrogeologic units Based on this current data,and based on previous ground water elevation data collected during the Flag City Chevron investigation (i e induced ground water depression in the areas of ground water extraction), it would appear that this third MTBE-impacted ground water plume could explain the recent phenomena of MTBE-impact being detected at Flag City Chevron well MW-8 following nine consecutive quarters of being non detect Laboratory results from the second quarter 2004 analysis of water samples from the pump-and-treat system continues to indicate that,in general,the pump-and-treat rernediation system influent MTBE concentrations were very low but stable,with slightly decreasing concentrations AGE estimates the arithmetic average concentration of MTBE in ground water extracted between 16 March 2004 and 15 June 2004 at 17 µg/1 During this time, the volume of water processed through the ground water pump and treat system was recorded at 1,212,292 gallons, yielding 0 18 pounds of MTBE (equivalent to 80 grams) from the plume of impacted ground water(Appendix D) Previously(March 2003), AGE had observed a`spike' in MTBE concentrations at extraction wells EW-4,EW-5,EW-7 through EW-10,which may have been related to the failure of some of the site's UST primary product Iines and the UST sump to pass the annual `Line and Leak Detector'test in January 2003 As a result of the `leaking lines', the Flag City Chevron upgraded the entire UST system(i a replacement of fuel dispensers and pans,product piping, secondary containment piping and UST sumps)during May 2003 Between May 2003 and June 2004, the MTBE dissolved plume appeared to have generally stabilized, however, slight to moderate increases were still observed in the March and June 2004 water samples collected at extraction wells EW-7, EW-9 and EW-10 (located near the fuel dispenser island area) Analytical results from the June 2004 quarterly ground water monitoring event indicate that the highest concentrations of on-site MTBE occur in the First Zone sample collected from EW-7 at 79 }tg/l and in the Second Zone sample collected from MW-8A at 570 µg/1 (Figures 8 and 9, respectively) During the last five consecutive quarterly ground water monitoring events, MTBE concentrations observed from MW-8A have fluctuated between 290 µg/1 and 570 µg/1 6.0 RECOMMENDATIONS Based upon data collected from the site, AGE recommends o Continued quarterly ground water monitoring and sampling of monitoring wells MW-1 through MW-18,future quarterly groundwater monitoring events should be coordinated with the Three B's Truck Plaza investigation i Advanced GeoEnvironmental,Inc