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Cltologkal7eckafcslrrc Page 17 Revised Site Conceptual Model Report Project No.507.2 �.. April 3,2008 4.0 DISCUSSION AND CONCLUSIONS 4.1 Discussion ' The data indicate that a localized plume of contamination persists in vicinity of wells MW-4, ... MW-5 and MW-9. Previous field investigations in this location utilizing a five foot sampling interval supported a conclusion that the site possessed predominantly very fine grained clayey soils. But the advancement of three 36 foot continuously cored borings in 2006 revealed that w more sandy units are present than previously thought. The above photographs confirm this situation. �. The data from borings SB-7 through SB-9 confirm that shallow soils at depths of 0— 12 feet are clayey. This material would tend to absorb gasoline emanating from piping or dispenser leaks presumed to be located in the alley south of the Hesston Shop UST excavation. The a gasoline entrained in the soil would slowly move downward until it encountered the water table with associated dissolving to source the groundwater plume. The gasoline in the shallow clay soils has probably not been significantly affected by natural biodegradation �- given that the buildings and cement road surface prevents infiltration and the water table has historically been at 13— 1 S feet bgs and as such below the clayey units. �- But the three borings also revealed that from 12—30 bgs the soils are mostly sands,including silty or clayey sands. This situation allows the dissolved gasoline contaminant mass to move with groundwater and attenuate with distance away from the release point as evidenced in well MW-6 located approximately 55 feet down gradient of well MW-9. Recent advancement of two shallow 30 ft GeoProbe borings indicate the presence of clay and �- sandy clay, with a shallow clayey sand unit approximately 20 ft below ground surface. �- 4.2 Conclusions Based on our interpretation of the data collected over the course of this subsurface ` investigation, GTI has reached several conclusions. These conclusions are based on the premise that the data considered, although incomplete, are representative of actual site conditions. We acknowledge that there may be undiscovered conditions, which would upon their consideration,change our interpretation and thus our conclusions. Geological Technics Inc. makes the following conclusions. • The shallow clayey soils from surface to 12 feet contain a significant amount of gasoline mass that has not been affected by natural biodegradation processes due to the cement road surface and low water table. �. • The contamination entrained in the shallow clays is sourcing a groundwater plume that is decaying very slowly.