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' 16 September 2004 AGE-NC Project No 97-0249 ' Page 7 of 8 ' occurrence was reported at Flag City Chevron well MW-12A, approximately 200 feet southeast of the site • The most intense on-site MTBE-impacted ground water was reported from Hydrologic ' Unit 1 in the in the area of Three B's Truck Plaza well MW-8A at a concentration of 130 µg/1 and from Flag City Chevron well MW-16 at a concentration of 53 µg/l, from Hydrologic Unit 2 in the area of Flag City Chevron well MW-17 at a concentration of 87 µg/1 ' (Figure 12), and from Hydrologic Unit 3 in the area of Flag City Chevron wells MW-11B and MW-18 at concentrations as high as 130 µg/l (Appendix A) On-site MTBE-impacted ground water from Hydrologic Units 1 and 2 are depicted in Figures 11 and 12,respectively • The highest off-site concentrations of dissolved MTBE were detected in monitoring wells toward the east and northeast, in water samples collected from Flag City Chevron wells ' EW-7 (79 µg11, Hydrologic Unit 1), MW-8A(570 µg/1,Hydrologic Unit 2), and MW-19A (4,800 µg/l, Hydrologic Unit 1) Off-site MTBE-impacted ground water from Hydrologic Units 1 and 2 are depicted in Figures 11 and 12, respectively ' • Low concentrations of TBA and 1,2-DCA have been detected from wells MW-1, MW-2, MW-3, MW-5, MW-6, MW-8A, MW-10A and MW-11A, suggesting that only a minor TBA-impact and 1,2-DCA-impact to ground water in Hydrologic Unit 1 is present at the site C The lateral and vertical extent of the dissolved petroleum hydrocarbon plume is not known toward the northeast of Three B's Truck Plaza well MW-2, toward the south of Three B's Truck Plaza well MW-6 and toward the south of Flag City Chevron wells MW-16/17118 (Figures 9, 10, 11 and 12) 1 5.0. RECOMMENDATIONS ' Based upon the results of the second quarter 2004 investigation, AGE recommends 1 • Continued quarterly ground water monitoring of wells MW-1 through MW-12, the next quarterly ground water monitoring event is tentatively scheduled for 14 and 15 September ' 2004, • Continue to conduct future ground water monitoring events concurrently with the Flag City Chevron site (6421 Capitol Avenue), and • Additional ground water monitoring wells(screened in Hydrogeologic Unit 1)are warranted in the area northeast of Three B's Truck Plaza well MW-3, south-southeast of Three B's Truck Plaza well MW-6 and southwest of MW-16/17/18, installation of the additional ground water monitoring wells should aid in the preparation of a site conceptual model Advanced GeoEnvi ron mental,Inc