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supply well has been measured at approximately 40 to 45 feet bgs Based on this information, the water supply well appears to be screened within a confined regional water supply aquifer The source of the groundwater observed in the monitoring and vapor extraction wells beneath the site cannot be positively determined at this time In the previous SCM, Stratus identified percolation of meteoric water into the subsurface, leakage from on-site water pipes, and recharge from the on-site water supply well as possible sources of the encountered groundwater Stratus reported in the SCM that the water encountered in the shallow-screened wells was likely meteoric water The basis of this argument was a general increase in water levels in the shallow screened wells during the winter and spring months, and decrease in water levels during the summer and autumn months Groundwater elevations in the shallow screened wells were measured between approximately 254 and 223 feet mean sea level (msl) at the time of the second quarter 2006 monitoring event (excluding wells connected to the vapor extraction system) This significant variation in the water level measurements suggests that groundwater in the shallow screened wells is perched and localized Groundwater elevation contours, and groundwater elevations, for the shallow-screened wells at the time of the second quarter 2006 monitoring event are presented in Figure 7 Groundwater elevations were measured between approximately 178 and 186 feet msl in the deeper screened monitoring wells during the second quarter 2006 A review of Figure 8 shows relatively evenly spaced potentiometric surface lines, with a groundwater surface sloping . towards the southeast at an average gradient of 0 032 ft/ft Based on this observation, an upgradient source of groundwater recharge, providing relatively constant head pressure, appears to be supplying groundwater to sand zone 2 Given the groundwater elevations observed in wells MW-1, MW-10, MW-12, and MW-15 through MW-18, 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 However, the consistent absence of groundwater in deeply screened wells MW-3 and MW-7, which are situated between the on-site water supply well and off-site wells MW-1, MW-10, MW-12, and MW-15 through MW-18, is problematic to this explanation 3.3 Extent of Petroleum Hydrocarbon Impact to Soil Petroleum hydrocarbons appear to have been released to the subsurface beneath the former product dispensers, product piping, and former UST complex 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 appears to be adequately characterized on-site SVE has removed significant petroleum hydrocarbon mass from the on- site area Petroleum hydrocarbons (predominately GRO and BTEX) have also impacted soil south and southeast of the site The highest off-site concentrations of GRO (4,400 milligrams per kilogram [mg/Kg]) and benzene (22 3 mg/Kg) were historically reported for a soil sample collected from K IBp Arco for URS161001Repoos16100 assessment rpt&scm update doc Page 9 OTRATUJ