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1 ' 15 October 2001 • AGE-NC Protect No 96-0232 ' Page 14 of 17 ' A First Zone water sample collected off-site from well MW-12A contained significant concentrations of TPH-d, TPH-mo and toluene, a Fust Zone water sample collected off-site from well MW-13A also contained significant concentrations of TPH-g, TPH-d, benzene, ' ethylbenzene, xylenes and 1,2-DCA ' 5.0. SITE CONCEPTUAL MODEL Based on the data developed 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 12 ' 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 lie suggestive of a release in the UST/dispenser area Once released to the subsurface the contaminants rapidly mi to and impacts p y ates gr pacts shallow ground water at 2 to 20 feet bsg, at which point the contaminants start migrating laterally • The subsurface from surface grade to approximately 150 feet bsg is composed primarily of I fine-grained soil with intervals coarse-grained material (sand) interbedded at intervals The coarse grained material occurs in approximately five intervals above 150 feet bsg that are thought by AGE to form 5 informal hydrologic units as described above and in AGE's Final Remediation Plan, dated 31 August 2000 Roughly, the informal hydrologic units, from surface grade to 150 feet bsg are 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 ° Impacted ground water in the First Zone mixed with ground water in the Second Zone during migration toward the west-southwest There is no well-defined aguitard between the two zones • The low-gradient ground water table is strongly influenced by pumping from the Rocky's Restaurant Domestic Well(RDW-1), inducing migration of MTBE-impacted ground water in the First and Second Zones from the UST/dispenser/stoma drain area on the Flag City Chevron site westward toward RDW-1 Advanced GeoEnvironmental,Inc. I