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q Former Beacon Station No 12419 March 18,2002 Stockton California Problem Assessment Report/Corrective Action Plan The majority of mass and concentration reduction of petroleum hydrocarbons is the result of biodegradation in which naturally occurring microorganisms promote attenuation of petroleum hydrocarbons in soil and groundwater by consumption and destruction of petroleum hydrocarbons Biodegradation reduces the mass of petroleum hydrocarbons by microbial metabalization resulting in the production of carbon dioxide and water 10.1.2 Soil Vapor Extraction Soil vapor extraction (SVE) utilizes soil vapor extraction wells to remove volatilized organic vapors from the subsurface The air streams drawn from SVE wells contain petroleum hydrocarbons that may be either treated or discharged directly to the atmosphere, depending on local air pollution control district requirements Rates of vapor extraction vary with moisture content, permeability, and consistency of subsurface soils The soil types that are present in the vadose zone beneath this site (silt and sand) are moderately conducive to vapor extraction techniques Additionally, the movement of air through the subsurface stimulates naturally occurring hydrocarbon-consuming microbes in the subsurface The SVE system would be operated until vapor concentrations decline to asymptotic levels indicating a reduction of volatile petroleum hydrocarbon concentrations in the vadose zone, the SVE system was no longer effective in removing the gasoline hydrocarbons from the subsurface, or until operation of the SVE system no longer is cost-effective for the extracted vapor concentrations 10.1.3 Excavation and Disposal This remedial alternative involves the removal and disposal of all soils containing petroleum hydrocarbons above acceptable concentrations Excavation would involve removal, disposal, and replacement of UST systems, and site surface material consisting of asphalt and concrete Excavated soils would be manifested and transported by a Iicensed contractor to a Class I or II landfill, or possibly for thermal destruction Clean imported material would be used to backfill the excavation 419PARCAP3-02 doc 20 Project No 1419 23 HORIZON ENVIRONMENTAL INC.