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5.0 SUMMARY • Petroleum hydrocarbons appear to have been initially released to the subsurface from beneath the former product dispensers and product piping The fuel delivery and storage system at the site was replaced and upgraded • An unknown quantity of petroleum hydrocarbon impacted soil was excavated and removed from the site between October 1990 and February 1991 Approximately 315 tons of petroleum hydrocarbon impacted soil was excavated and removed from beneath the former product dispensers and piping trenches in January 2002 • Fine grained soils have predominately been encountered beneath the site to approximately 202 feet bgs, the maximum depth explored beneath the site The regional water table does not appear to have been encountered during subsurface investigations at the site Increased soil hardness was generally observed in the fine grained strata encountered below approximately 140 feet bgs, these soils are expected to have relatively low permeability • Two laterally continuous sand intervals, approximately 2 to 6 5 feet in thickness (sand zone 1) and 12 to 23 feet in thickness (sand zone 2) have consistently been encountered Sand zone 1 and sand zone 2 have been observed between approximately 48 to 64 feet bgs, and 68 to 104 feet bgs, respectively Approximately 10 to 18 feet of fine grained soils (predominately silt) appear to separate sand zone 1 and sand zone 2 • The strata beneath the site appear to dip to the southeast, approximately 5° from horizontal, at a strike of approximately N170E • Groundwater elevations in the shallow screened wells were measured between approximately 38 and 65 feet bgs at the time of the second quarter 2004 monitoring event This significant variation in the water level measurements suggests that groundwater in the shallow screened wells is perched and localized • Depth to groundwater was measured between approximately 93 and 107 feet bgs in the four wells screened in sand zone 2 south of Patterson Pass Road An east-southeast groundwater flow direction, at an average gradient of 0 035 ft/ft, was calculated using the groundwater elevations measured in these wells Groundwater flow in sand zone 2 appears to be generally perpendicular to the strike of the subsurface stratum, suggesting that groundwater movement in sand zone 2 appears to be significantly influenced by the dip of the subsurface stratum beneath the site Given the groundwater elevations observed in the four wells, and the close, upgradient position of the on-site water supply well relative to these wells, it appears likely that groundwater recharge into sand zone 2 has occurred from the on-site water supply well P 1Bp-Arco for URS161005Reports\a 6100-43sessment s uA_update doe Page 12 5 MA 7115