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L <br /> ` Former Beacon Station No. 12474 March 18,2002 <br /> Stockton, California Problem Assessment Report/Corrective Action Plan <br /> The majority of mass and concentration reduction of petroleum hydrocarbons is the result of <br /> biodegradation in which naturally occurring microorganisms promote attenuation of <br /> petroleum hydrocarbons in soil and groundwater by consumption and destruction of <br /> petroleum hydrocarbons. Biodegradation reduces the mass of petroleum hydrocarbons by <br /> microbial metabalization resulting in the production of carbon dioxide and water. <br /> 8.1.2 Soil Vapor Extraction <br /> Soil vapor extraction (SVE) utilizes soil vapor extraction wells to remove volatilized organic <br /> vapors from the subsurface. The air streams Orawn from SVE wells contain petroleum <br /> hydrocarbons that may be either treated or discharged directly to the atmosphere, depending <br /> on local air pollution control district requirements. Rates of vapor extraction vary with <br /> moisture content, permeability, and consistency of subsurface soils. The soil types that are <br /> present in the vadose zone beneath this site (silt and sand) are moderately conducive to <br /> vapor extraction techniques. Additionally, the movement of air through the subsurface <br /> • stimulates naturally occurring hydrocarbon-consuming microbes in the subsurface. The <br /> SVE system would be operated until concentrations of extracted vapors decline to <br /> asymptotic levels indicating the SVES was no longer cost effective in removing gasoline <br /> hydrocarbons from the subsurface. <br /> 8.1.3 Excavation and Disposal <br /> This remedial alternative involves the removal and disposal of all soils containing petroleum <br /> hydrocarbons above acceptable concentrations. Excavation would involve removal, <br /> disposal, and replacement of structures and surface materials consisting of asphalt and <br /> concrete. Excavated soils would be manifested and transported by a licensed contractor to a <br /> Class I or II Iandfill, or possibly for thermal destruction. Clean imported material would be <br /> used to backfill the excavation. <br /> • <br /> 474PARCAP3-02.doc 14 <br /> Project No. 1474.23 HORIZON ENVIRONMENTAL INC. <br />