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Monitoring Well Installation and VET Report—Former Unocal Station#2859,Stockton,CA July 30,2001 Summary of Vapor Extraction Test The VE test was performed for a total of 39 hours An IC engine was used to extract vapors from wells VW-1,VW-2,and VW-3 System flow rates ranged from 8 to 14 scfm Based on the data from the VE pilot test, it appears that the maximum radius of influence from any vapor extraction well is less than 45 feet at test conditions Air samples were collected from the influent and effluent vapor streams Petroleum hydrocarbon concentrations detected in the influent vapor stream were up to 311 ppmv of TPHg, and up to 2 2 ppmv of benzene No detectable concentrations of MtBE were reported The only well where significant concentrations of petroleum hydrocarbons were removed was well VW-3 Based on the VE monitoring data,no significant amount of TPHg or benzene were removed during the pilot test. It is estimated that less than one pound per day of petroleum hydrocarbons could be extracted Vapor extraction would have little or no effect at this site CONCLUSIONS The following site conceptual model is based on data and observations from field investigations performed at the site- 1 The source of dissolved hydrocarbons detected beneath the site was eliminated in 1988 with the removal of the USTs and excavation of approximately 1,350 cubic yards of impacted soil. Vapor extraction tests conducted at the site through 1996 removed approximately 2,750 pounds of residual hydrocarbons from soil beneath the site 2 Two groundwater zones have been identified beneath the site In Zone A, which is based on wells screened at various intervals between approximately 40 to 60 feet bgs, groundwater flow has been consistently to the northeast In Zone B, which is based on newly installed wells and existing well NP-1 that are screened between 85 to 90 feet bgs, groundwater flow is to the southeast, but this is based on only one monitoring event Additional monitoring events will be needed to confirm groundwater flow in Zone B The dtflFerent groundwater flow directions suggests that an aquitard may exist between the two zones Beneath the southern half of the site, soil from 60 to 90 feet bgs was saturated, but beneath the northern half of the site and downgradtent, interbedded damp to moist silt and clay horizons exist in this interval. These silt and clay horizons could be an aquitard inhibiting the downward migration of hydrocarbons In the southern half of the site, these local horizons are missing and may explain the presence of hydrocarbons in Zone B in that area 3 The lateral extent of the dissolved hydrocarbon plume in Zone A is delineated to the west, south, and east In Zone A, impact appears limited to an area along the eastern boundary of the site, where the highest concentrations detected are in wells MW-10 (130,000 ppb TPHg) and MW-2 (62,000 ppb T`PHg) Hydrocarbon concentrations decrease to the north and south of those wells Concentrations in Zone A drop significantly from the site toward downgradient well MW-17 (75 ppb TPHg), located on the east site of Pacific Avenue across from the site 24007204-1 7