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. • The source of the groundwater observed in the shallow and deeply screened wells beneath the site can not be positively determined at this time Percolation of meteoric water into the subsurface, leakage from on-site water pipes, and recharge from the on-site water supply well appear to be possible sources of the groundwater • The lateral extent of free product thickness appears to be adequately assessed on-site and off-site • The bulk of the petroleum hydrocarbon mass appears to be situated above approximately 45 feet bgs on-site, with localized high concentrations encountered on-site between approximately 45 and 80 feet bgs The vertical and lateral extent of subsurface petroleum hydrocarbon impact to soil appears to be adequately characterized on-site • Increased residual petroleum hydrocarbon concentrations were generally observed for soil samples collected near sand zone 1 and sand zone 2 in borings B-39 through B-42, suggesting that these stratum may be providing a preferential pathway for subsurface movement of the petroleum hydrocarbon impact • Soil analytical data collected from borings B-33 through B-42 and B-46 appears to show that the vertical extent of subsurface petroleum hydrocarbon impact does not extend more than approximately 10 feet deeper than the base of sand zone 2 Based on this observation, the fine grained soils situated immediately beneath sand zone 2 appear to be . of sufficiently low permeability to impede additional vertical migration of the petroleum hydrocarbons at these depths • Petroleum hydrocarbons were only reported in one soil sample (benzene, boring B-46, 106 feet bgs) collected during the November 2005 soil boring investigation Given this observation, the vertical and lateral extent of petroleum hydrocarbon impact to soil appears to be adequately assessed • Free product has historically been reported at off-site deeply screened wells MW-1 and MW-15 Relatively high concentrations of TPHG and benzene are consistently observed at off-site deeply screened wells MW-10 and MW-12 • Benzene was detected in the samples collected from newly installed wells MW-16 (2 4 µg/L) and MW-18 (8 8 µg/L) GRO and MTBE concentrations were reported below laboratory detection limits for the well MW-16 through MW-18 samples Given these observations, the existing well network appears adequate for monitoring off-site hydrocarbon impact to groundwater at this time • High concentrations of GRO and BTEX continue to be removed from the subsurface by the SVE remediation system As of March 1, 2006, an estimated 77,507 pounds of GRO have been removed from the subsurface by SVE K 16p Arco for URSW00aeports\5100 assessment rpt&scm update doc Page 14 STRATUS