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6 • 11 April 2003 AGE-NC Project No 01-0865 Page 6 of 9 Beneath the peat layer is a 30 to 40-foot fine-grained layer consisting of clay, clayey silt and silty clay,with laterally discontinuous sand layers at the top western and eastern edges of the site(Unit 2) Underlying Unit 2 is a laterally continuous sand layer(Unit 3), which comprises the deepest layer identified at the site Geologic cross sections are provided as Figures 3 and 4 Since initiation of ground water monitoring in September 1997, ground water elevations in the flat land have ranged from approximately 18 5 feet to 12 5 feet below mean sea level(MSL) The ground water elevation on the levee ranged from 3 to 5 feet above MSL (Table 3) through March 2001, when the monitoring well (MW-1) on the levee was damaged Ground water flow has consistently been east-southeast, at relatively steep gradients in the immediate area of the levee and low to very low gradients east of the existing ASTs (e g , Figure 5) i Site assessment activities to date defined two areas where soil is impacted by high concentrations of petroleum hydrocarbons The highest petroleum-hydrocarbon concentrations,up to 4,800 mg/kg TPH-g, were detected beneath the former USTs at the time of their removal Petroleum hydrocarbons, up to 4,000 mg/kg, were detected at between 5 and 10 feet bsg in pilot soil boring MW-6 approximately 5 feet west of the ASTs(Figure 2) Only the fuel additive MTBE was detected at higher concentrations at the MW-6 location Based on AGE interpretation of the available data, the initial release(s) of petroleum hydrocarbons occurred from the former USTs prior to their removal in 1991 Once released to the subsurface, the contaminants migrated through the apparently sandy levee material to a vertical depth of approximately 20 feet bsg, at which point the contaminants encountered the less permeable peat (Unit 1) and began to migrate laterally as well as vertically, lateral migration was dominantly towards the east, in the direction of ground water flow Currently, the source of the petroleum hydrocarbons impacting soil in the area of well MW-6 is equivocal The high concentrations of petroleum hydrocarbons at well MW-6, in proximity to the ASTs, may be due to a release(s) from the ASTs, a spill during fuel transfer, or because the petroleum hydrocarbons represent the down-gradient extent of the UST petroleum hydrocarbon plume The reasons for attributing a release(s)from the ASTs would be the proximity of well MW-6 to the ASTs, the high MTBE concentrations detected in the soil to greater than 15 feet bsg at well MW-6 and the high MTBE concentrations in ground water surrounding the ASTs Alternatively,the petroleum hydrocarbon impact to soil at the AST location may have originated from the USTs' release(s),which migrated within the uppermost peat layer down-gradient to the area of well MW-6, subsequent modifications to the levee and parking lot in 1997 resulted in removal of soil, including the uppermost peat,from a portion of the east side of the levee for a retaining wall According to Mr Smith, a strong odor was detected in soil removed during site modification, and several barrels of possibly contaminated soil are still stored at the site The low MTBE concentrations in the former UST area maybe an effect of the elevated location of the former USTs Meteoric waters and seepage from the Little Connection Slough,with a tidal influence,may have flushed MTBE from the shallow Advanced GeoEnvironmental,Inc