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Site Background Information: West Clay Properties Page 8 of 10 SITE CONCEPTUAL MODEL The model presented is interpretational and will be modified as more data is collected. Depths are approximate can vary by several feet across the site. Hydrogeologic units are depicted in cross onal views A- '. ased on the data developed to date, including all soil borings, monitoring soundings, AGE has developed a site subsurface model for the release, migration and distribution of the varying contaminants in the subsurface as described below: • The subsurface from site grade to approximately 170 feet bsg is composed primarily of fine- grained soil with a coarse-grained interval of variable thickness and lateral distribution.The major coarse-grained sand intervals occurring between 30 and 40 feet below surface grade (bsg), a laterally continuous coarse-grained sand interval at approximately 75 feet to 80 feet bsg and again at a depth 100 feet bsg and a deep laterally continuous coarse-grained sand interval at approximately 180 feet bsg.A laterally discontinuous sand unit extends from 135 feet to 140 feet bsg in the UST No. 3 area. The remaining stratigraphy of the site is comprised of silt with clay inter-channel fill areas. A laterally continuous clay is present at 65 feet to 70 feet bsg across most of the UST No. 3 area. These characteristics allowed construction of a site hydrogeologic model as described above with the water table near approximately 20 feet bsg. Southern UST Area • A release of petroleum hydrocarbons(TPH-g,TPH-d)and benzene,toluene,ethyl benzene and xylene (BTEX) compounds with low concentrations of 1,2-DCA occurred from dispenser line releases near the northern excavation (UST No. 3 area). The southern UST removal samples were non-detected for chemicals of concern. Additionally, shallow soil samples collected near well MW-2,MW-7 were non-detect for chemicals of concern.TPH-g was reported at a concentration as great as 2,100 mg/kg in the soil sample collected from the northern UST removal samples; TPH-g was also reported at 320 mg/kg in a soil sample collected at 20 feet bsg in boring B10. The highest TPH-g and BTEX impact to soil in the upper vadose zone was removed through excavation at the UST No. 3 area; it does not appear that any residual vadose zone hydrocarbons are present in this area.The aerial extent of the soil contamination in soil appears defined to the near the former UST No. 3 area. • Available data is suggestive of a release was most significant at the northern edge of the former UST No. 3 area. Once the petroleum hydrocarbons were released to the subsurface, the contaminants migrated north under well MW-7 and vertically to a depth of approximately 60 feet bsg near well MW-2.The"deep"migration of hydrocarbons indicates a historically depressed water table depth of near 35 feet bsg. • Based on ten years of ground water monitoring for TPH-g and BTEX compounds near the UST No. 3 area,the impacts to ground water appear to be stable in the areas of wells MW-1, Advanced GeoEnvironmentai,Inc.