ARCHIVED REPORTS_XR0011964 - Laserfiche WebLink

Home Browse Search

• 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 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 223 and 252 feet mean sea level (msl) 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 181 and 185 feet ms] in the deeper screened monitoring wells located south of Patterson Pass Road A review of Figure 7 shows approximately evenly spaced potentiometric surface lines, with a groundwater surface sloping towards the east-southeast at an average gradient of 0 035 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-12, MW-13, and MW-15, 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- 12, MW-13, and MW-15, is problematic to this explanation The groundwater elevation contours illustrated on Figure 7 depict groundwater flow in sand zone 2 that is nearly perpendicular to the N17°E strike calculated for the subsurface stratum, with groundwater flow towards the down-dip direction Based on this observation, groundwater movement in sand zone 2 appears to be significantly influenced by the dip of the subsurface stratum beneath the site 3.3 Extent of Petroleum Hydrocarbon Impact to Soil Stratus previously stated in the SCM that 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 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 boring B-321MW-1 at 111 feet bgs During this most recent phase of investigation, the highest P 1Bp-Arco for VRS%6100 cpons%arw6100 u ss"tlt xt>Lupdate do Page 8 5MATU5 a