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' 15 October 2002 AGE-NC Protect No 96-0232 ' Page 15 of 19 ' MTBE concentrations were reported from EW-4,EW-8,EW-10 and MW-8A between June 2002 and September 2002 sampling events • The highest off-site concentrations of MTBE were detected from from Rocky's Upper Zone well MW-11B at 79 gg/l, in First Zone water from Flag City Chevron monitoring well MW-7 at 22 gg/l, and in Three B's Truck Plaza monitoring well MW-4 at 56 gg/l(Table 6, Appendix F), decreasing MTBE concentrations were reported from all three off-site wells ' between June 2002 and September 2002 sampling events • On Isolated occurrences, low concentrations of TPH-d have been detected at various wells ' during previous quarterly monitoring events at Flag City Chevron During September 2002, low concentrations of TPH-d were detected from Secondary Aquifer Zone at MW-9B and from Rocky's Upper Zone at MW-11B AGE requested McCampbell Analytical to further ' review the TPH-d sample chromatograms from the September 2002 sampling event The laboratory indicated to AGE that the low TPH-d detections from MW-9 and MW-I 1 did not have typical diesel characteristics,and that the detected chromatogram peaks were probably ' the result of biological Interference lie 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 13 • In the vicinity of Flag City Chevron,the subsurface from surface grade to approximately 150 feet bsg is composed primarily of fine-grained soil with intervals of coarse-grained material (sand) The coarse grained material occurs in approximately five intervals above 150 feet bsg that are thought by AGE to form five informal hydrologic units as described above and in AGE's Final Remediation Plan, dated 31 August 2000 Generally,the informal hydrologic units are described as the First Zone from surface grade to 35 feet bsg,the Second Zone from 40 to 60 feet bsg, Rocky's Upper Zone from 65 to 75 feet bsg, the Secondary Aquifer Zone from 85 to 105 feet bsg, and the Primary Aquifer Zone from 130 to 150 feet bsg Ground water below the Primary Aquifer Zone is believed to be non-potable due to high total dissolved solids content • 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 entering the storm drain system, spilled fuel entering the subsurface under the dispenser(s), . and/or overfill or a slow release from the USTs or piping currently onsite Available data is ' suggestive of a release In the UST/dispenser area Advanced GeoEnvironmental,Inc