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. 15 July 2002 AGE -NC Protect No 96-0232 Page 16 of 21 • Due to the low ground water gradient at the site, small variations in ground water measurements or changes of recharge/discharge of the local ground water could greatly modify the inferred ground water flow direction • Increasing ground water elevations have been reported at the site between September 2001 and the March 2002 monitoring events, however, a decrease in ground water elevation was reported between March and June 2002 Hydrographs depicting recorded ground water elevations at wells MW -2, MW -6A and MW -6B vs time are provided in Appendix I • The highest on-site concentrations of dissolved MTBE were detected in the samples collected from First Zone extraction well EW -4 at 29 µg/1 and from Second Zone monitoring well MW -8A at 250 µg/l, low concentrations of on -sate MTBE were detected in water samples from First Zone extraction wells EW -5, EW -6, EW -8 and EW -10, and from Second Zone monitoring well MW -9A (Table 6, Figures 16,17 and 18) • The highest off-site concentrations of MTBE were detected from from Rocky's Upper Zone well MW -I IB at 120 ,ug/l, in First Zone water from Flag City Chevron monitoring well MW -7 at 49 4g11, and in Three B's Truck Plaza monitoring well MW -4 at 90 µg/1(Table 6, . Appendix F) • On isolated occurrences, low concentrations of TPH-d have been detected at vanous wells during previous quarterly monitoring events at Flag City Chevron Low concentrations of TPH-d were detected from First Zone at MW- 1OA, MW -12A and MW -13A, from Second Zone at MW -12B and MW -13B, from Upper Rocky's Zone at MW -10C, from secondary aquifer at MW -9B, MW -1 OD, MW -11 C, MW -12C and MW -13C, and from Primary Aquifer at MW -9C, MW -11D, MW -12D and MW -13D during the June 2002 monitoring event Therefore, AGE requested McCampbell Analytical to further review the TPH-d sample chromatograms from during the June 2002 sampling event The laboratory indicated to AGE that the low TPH-d detections from MW -9, MW -10 and MW -11 did not have typical diesel characteristics, and that the detected chromatogram peaks were probably the result of biological interference 6.0. SITE CONCEPTUAL MODEL Based on the data collected to date, AGE has developed a site conceptual model for the release, migration and distribution of the contaminants in the subsurface as described below The model is diagrammatically illustrated in Figure 21 • A release or releases of petroleum hydrocarbons and MTBE occurred from an unidentified source(s) on the site Possible sources, all in the same general area, include impacted soil left in place at the time of the UST replacements, wash -down water from the dispenser area Advanced GeoEnvironenental, 1—