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. 09 May 2002 AGE-NC Project No 96-0232 Page 14 of 18 • Primary Aquifer Zone ground water flow direction was inferred to be bi-directional,from the north and the south towards an east-west-trending depression of the water table in the area of MW-9C and MW-11D, at approximate gradients of 0 001 ft/ft or less (Figure 6) • 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 Hydrographs depicting recorded ground water elevations at wells MW-2, MW-6A and MW-6B vs time are in Appendix J • The highest on-site concentrations of dissolved MTBE were detected in the samples collected from First Zone extraction well EW-4 at 100 µg/l(Figure 9)and from Second Zone monitoring well MW-8A at 700 µg/l, low concentrations of on-site MTBE were detected in water samples from First Zone monitoring wells MW-1, MW-2 and extraction wells EW-5, EW-6, EW-8, EW-9, EW-10, and from Second Zone monitoring well MW-9A (Table 6, Figures 7, 8 and 9) . The highest off-site concentrations of MTBE were detected from Second Zone well MW-1 lA at 94 µg11, from Rocky's Upper Zone well MW-11B at 150 µg/1, in First Zone water from Flag City Chevron monitoring well MW-7 at 41 µg/1, and in Three B's Truck Plaza monitoring well MW-4 at 83 µg/l (Table 6, Figures 7 and 8) On isolated occurrences, low concentrations of TPH-d have been detected at various wells during previous quarterly monitoring events at Flag City Chevron In a letter dated 28 February 2002, the RWQCB requested that AGE discuss the December 2001 detection of TPH-d from sentinel well MW-10C Furthermore, low concentrations of TPH-d were detected from first zone at MW-10A, from secondary aquifer at MW-1 OD and from primary aquifer at MW-l0E during the March 2002 monitoring event Therefore, AGE requested McCampbell Analytical to further review the TPH-d sample chromatograms from MW-10 during the December 2001 and March 2002 sampling events The laboratory indicated to AGE that the low TPH-d detections from MW-1 OA,MW-1 OC,MW-1 OD and MW-1 OE did not have typical diesel characteristics, and that the detected chromatogram peaks were probably the result of biological interference (Appendix 1) 7.0. 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 Figures 12 Advanced Geo Environmental,Inc