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• 15 July 2002 <br />AGE -NC Project No 96-0232 <br />Page 19 of 21 <br />Ground water elevations versus ground water monitoring dates (hydrograph) were plotted for <br />monitoring wells MW -2, MW -6A and MW -6B, generally, all three plots depict an overall trend of <br />decreasing ground water elevation from March 2000 to December 2001 and in June 2002, however <br />ground water elevations did show slight increases during the May 2000, December 2000, March <br />2001, December 2001 and the March 2002 monitoring events (Appendix 1) <br />Figures 19 and 20 depict the approximate extent of MTBE -impacted ground water beneath the site <br />in cross sectional views C -C' and D -D', Figures 16, 17 and 18 depict the estimated extent dissolved <br />MTBE -impact in plan view These figures depict Flag City Chevron wells EW -4, EW -5, EW -6, <br />EW -8, EW -10, MW -7, MW -8A, MW -9A, MW -IIA, MW -11B, MW -11D and Three B's Truck <br />Plaza wells MW -1, MW -2, MW -4 and MW -6 to be located within dissolved MTBE -impacted <br />plumes The vertical extent of impacted ground water on-site appears to he between 12 feet bsg <br />(estimated depth to ground water) and 50 feet bsg in the area of MW -1 and MW -9 (Figures 19 and <br />20) Impacted ground water in the area at 40 to 50 feet bsg in MW -9 is underlain by two thin fine- <br />grained units (Problem Assessment Report, dated 19 June 2000) that appear to have retarded the <br />downward migration of the MTBE Low concentrations of MTBE were previously detected in the <br />Primary Aquifer (MW -9C), but have been non -detect in water samples for three consecutive <br />sampling events <br />Going off-site toward the west, the plume may extend to as much as 75 feet bsg approaching the area <br />of MW -11 B, near the former Rocky's Restaurant domestic well (RDW -1) where the underlying fine- <br />grained sedimentary units may thin and pinch out, low concentrations of MTBE have been detected <br />in Primary Aquifer (MW -11D), screened at between 130 and 150 feet bsg Further westward from <br />MW -11, the plume of water impacted by MTBE appears to extend beyond Three B's Truck Plaza <br />well MW -4, with a vertical extent from possibly 12 feet bsg to possibly 75 feet bsg The full <br />westward extent of the Flag City Chevron plume is uncertain at this time Contaminants in Fust Zone <br />ground water at MW -13 include TPH-d and TPH-mo which suggest that the contaminants may be <br />from a source other than the Flag City plume It is not known if the suspected plumes are co - <br />mingled in the area between Three B's Truck Plaza wells MW -1 through MW -6 and Flag City <br />Chevron wells MW -7, MW -11, MW -12 and MW -13, AGE has modeled them as separate plumes <br />(Figures 16 and 17) <br />Relative to the inferred source area near the Flag City Chevron UST area (EW -1), the Flag City <br />Chevron plume appears to extend approximately 200 feet to the west at depths as great as 75 feet bsg <br />(Figures 16 and 18), approximately 20 feet to the south at depths as great as 30 feet bsg (Figure 8), <br />and approximately 80 feet to the east at depths as great as 30 feet bsg (Figures 16 and 18) The <br />northern extent of the Flag City plume is uncertain at the present time because of the high <br />concentrations of MTBE (up to 250 µg/l) detected in northernmost Flag City Chevron well MW -8 <br />during the June 2002 monitoring event <br />Advanced GeoEnvironmental, Inc. <br />