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Q9 May 2003 • AGE-NC Project No 96-0232 Page 13 of 16 ' aquifer AGE had previously requested McCampbell Analytical to further review TPH-d sample chromatograms from MW-12A,the laboratory indicated to AGE that the Iow TPH-d detections from MW-12A did not have typical diesel characteristics, and that the detected ' chromatogram peaks were probably the result of biological interference ' 5.0. 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 11 ' 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 intervals were interpreted to represent five informal ' hydrologic units,as described above and in AGE's Final Remediation Plan,dated 31 August 2000 Generally, the informal hydrologic umts 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 a high content of total dissolved solids 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 Isuggestive of a release in the UST/dispenser area • Once released to the subsurface,the contaminants rapidly migrated to and impacted shallow ground water at 2 to 20 feet bsg, at which point the contaminants nugrated laterally • Impacted ground water in the First Zone mixed with ground water in the Second Zone and Rocky's Upper Zone during migration toward the west-southwest, there is no well-defined aquitard between the three zones The low-gradient ground water table was strongly influenced by pumping from the Rocky's IRestaurant Domestic Well(RDW-1),which induced westward migration ofFirst and Second Zones MTBE-impacted ground water from the Flag City Chevron UST/dispenser/storm drain area toward RDW-1, RDW-1 was abandoned in March 2002 I Advanced GeoEn"ronmental,Inc I