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SIM PROBLEM ASSESSMENT REPORT/INTERIM REMEDIAL ACTION PLAN - 4- Former Beacon Station No. 494, 2185 East Fremont Street Stockton, CA--AMV Project No. 19005.07 7.1.2 In .�Situ Soil Vapor Extraction In situ soil vapor extraction utilizes vapor extraction wells to remove volatile hydrocarbons from the soil matrix. A vacuum applied to the extraction wells results in the removal of soil vapors from the subsurface, accompanied by volatilization of petroleum constituents out of the soil matrix. Depending on the concentration levels of hydrocarbons entrained in the extracted vapor, the extracted soil vapors are either discharged directly to the atmosphere or treated before discharge to the atmosphere. The treatment would destroy hydrocarbons in accordance with the ' local regulatory agency air discharge requirements. Extraction rates vary with the consistency, moisture content, and grain size of the soil horizon. In addition to the extraction benefit, soil ' venting can also be effective at promoting biologic breakdown of petroleum hydrocarbon compounds contained in soil and ground water by the introduction of additional oxygen into the subsurface. ' The soil vapor extraction test performed at the site (Section 6.2) indicated the feasibility of inducing adequate airflow in the subsurface to cause recovery of petroleum hydrocarbon constituents via soil venting. The available data indicates that, in particular, extraction from VW-1a and VW-2a could efficiently remediate the soils currently containing -the highest concentrations of petroleum constituents. ' In anticipation of utilization of vapor extraction techniques to achieve remediation objectives Ultramar has already installed underground piping to convey extracted soil vapors from ' extraction wells, vapor probes, and monitoring wells, to an aboveground treatment system. In addition, Ultramar moved an oxidation unit to the site for connection to the underground piping, and treatment of extracted soil vapors. 1 7.2 Removal of Dissolved Petroleum Hydrocarbons from Ground Water ' Strategies considered for interim remediation of ground water containing dissolved petroleum hydrocarbons include: ' • Ground Water Pumping and Aboveground Treatment • In Situ Bioremediation • Vapor Extraction ' 0 Vapor Extraction with Air Sparging Because of possible migration of ground water containing dissolved benzene beyond site ' boundaries, this analysis assumes passive remediation is an unacceptable alternative.