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locations of the former USTs, respectively. The highest concentration of benzene was detected in the soil sample collected from Bl at 60 feet bgs. Benzene was detected in soil samples at ' concentrations ranging from nondetectable to 19 mg/kg. The lateral extent of benzene in vadose zone soil is shown on Figures 9 and 10. ' MTBE has been detected in only one boring (B2). MTBE was detected in soil samples collected from the vadose zone of B2 from 31 feet bgs, 35 feet bgs, and 50.5 feet bgs at concentrations of 0.012 mg/kg, 0.024 mg/kg, and 0.01 mg/kg, respectively. The lateral extent of MTBE in vadose zone soil is shown on Figures 11 and 12. ' 4_2 Estimated Extent of Hydrocarbon-Impacted Groundwater The lateral extent of impacted groundwater has not been defined yet. Laboratory analytical ' results of groundwater samples collected from monitoring wells MW 1 through MW9 and the on- site domestic well on November 20, 2003, indicate petroleum hydrocarbons were present in groundwater samples collected from MWI, MW2, MW4, MW5, MW6, and MW9. Petroleum ' hydrocarbons were not detected in the groundwater samples collected from upgradient well MW3, downgradient well MW8, or the domestic well. The estimated lateral extents of groundwater impacted with TPHg, benzene, and MTBE are shown on Figures 13, 14, and 15, ' respectively. The highest concentrations of dissolved petroleum hydrocarbons are present in the vicinity of MW2. ' 5.0 DESIGN PLANS AND SPECIFICATIONS The proposed interim soil and groundwater remediation systems will consist of combined vapor extraction and air sparge treatment systems.The systems will function as interim remedial measures for on-site hydrocarbon impacted soil and groundwater. 5_1 Design Parameters 5.1.1 Vapor Extraction-Wellhead Vacuum,Flow Rate, and Radius of Influence Vapor extraction pilot testing performed at the site on September 29, 2003, indicated that a radius ' of influence (ROI) of up to 55 feet was achieved with air flow rates ranging from 53.5 to 78.9 cubic feet per minute (cfm) and observation wellhead vacuums ranging from 0.44 to 2.7 inches of water. During the long-term(approximately five hours)vapor extraction test conducted on VE3,the vapor ' extraction rate appeared to equilibrate at approximately 62 cfm with a vacuum of approximately 42 inches of water. The actual radius of influence for each vapor extraction well will vary depending on subsurface geologic conditions, the extent and distribution of hydrocarbon-impacted soil, the ' effect of concurrently operating the air sparge system, and site-specific operating conditions of the vapor extraction system (VES). The effective radius of influence should be evaluated following long-term operation of the VES. S:\environmental\25291\reports\irp.doc 4