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Appendix A - Historical Background River City Petroleum and Connell Motor Lift Truck Page 11 of 14 'I SITE CONCEPTUAL MODEL Based on the data developed to date, including all soil borings,monitoring wells installed and CPT 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 240 feet bsg is composed primarily of fine-grained soil with a coarse-grained interval of variable thickness and semi-consistent lateral distribution A major coarse-grained sand and gravel interval in the vadose zone occurs between 20 and 35 feet bsg, a laterally continuous coarse-grained sand interval occurs at approximately 75 to 85 feet bsg and approximately 150 feet bsg,a vertically and laterally significant sand/gravel unit begins at 165 to 185 feet bsg, and is present across the site to a depth of 240 feet bsg A laterally discontinuous, yet significantly thick sand unit extends from 100 feet to 110 feet bsg • Overall high pore-pressure was documented in the CPT sounding of the coarse-grained sand below the water table The remaining stratigraphy of the site is comprised of silt with clay inter-channel fill areas A laterally continuous clay, locally silt at CPT9, unit is present beginning at 120 feet bsg across most of the distribution of the contaminate plumes DISTRIBUTION OF STODDARD SOLVENT WITH BTEX • A release of stoddard solvents occurred from an unidentified source on the western portion of the site The most possible source is a set of dry well storm drains Available data is suggestive of a spill or discharge at the western edge of the building in the area of borings P26,P27, CPT32 and various wells including the extraction well EW-1 The highest impact to soil in the upper vadose zone is present at borings P26, P27, P14, P25 and EW-1 Once released to the subsurface, the contaminants migrated to a vertical depth of approximately 40 feet bsg, at which point the contaminants migrated locally down to a depth of 70 or 80 feet bsg Minor concentrations of 1,2-DCA and tetrachloroethene were detected at the depths of 45 feet to 65 feet bsg, within the Stoddard plume area west of the building The highest impact to soil near the water table and below the water table to a depth of 70 feet bsg occurs in the area of boring EW-1,P27 to 55 feet and borings P26 and P32 to a depth of 70 feet bsg • Total petroleum hydrocarbons quantified as stoddard solvent (TPH-ss) were detected in various ground water samples at maximum concentrations of 20,000 gg/l TPH-ss(MW-16), with moderate concentrations of BTEX at 34 gg/1 benzene, 59 gg/1 toluene, 120 gg/1 ethylbenzene and 560 gg/l xylene, respectively - The vertical and the lateral extent of TPH stoddard has been defined by sample collected Advanced GcoEnvironmental,Inc