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r � �. until, the excavation was ready for backfilling with clean soil, because of the lack of on-site storage If the impacted soil zone extends off-site to the east and west, the demolition of neighboring structures might be necessary to have access to the impacted soils in these off-site locations Assuming the boundaries of the excavation are contained within the site, the estimate costs for excavation of approximately 7,655 cubic yards of both clean and impacted soil, transportation, analytical characterization and disposal of 1,300 cubic yards of impacted soil at an off-site, Class III landfill facility and backfilling, compacting and resurfacing of the excavation ranges from j $700,000 to $900,000 5.2 In-Situ Soil Vapor Extraction Gasoline vapors in unsaturated soils are found in various stages of equilibrium Portions of the f chemicals are adsorbed on the soil particles, in the water phase, and in air that is present in soil void space Extraction of ani from the soil void space removes that fraction of the chemical in the vapor state As this process continues, more of the chemical vaporizes from the solid and liquid phases With continued vapor extraction, the concentration of the chemical in the soil can potentially be reduced to levels below the selected cleanup criteria The main components of a soil vapor extraction system (SVE) consist of extraction wells screened in the vadose zone, a blower for extracting the gas from the soil, and (in some cases) a treatment unit that removes the extracted vapors from the air stream prior to discharge to the atmosphere The gas treatment unit could consist of granular activated carbon (GAC) to adsorb the vapors or a thermal/catalytic oxidation system to destroy the vapors in the air stream 5 2 1 Extraction Techniques Vapor extraction can be performed by using either multiple wells or a system of gravel-filled trenches The number of wells needed to extract vapors is a function of the amount of air that can be removed per well The range of vacuum influence each well can create is commonly assumed to be circular with the well in the center Coarse-grained soils will require relatively few vapor extraction wells, while fine-grained soils will require substantially more wells installed on a much tighter grid Vapor extraction wells can be designed and spaced to improve the performance of the system For the Site, multiple wells with an appropnately-engineered filter pack would optimize the zone of capture and would be most suitable for the site conditions. 5 2 2 Vapor Treatment Technologies Extracted soil vapors are typically treated either by GAC or by thermal oxidation. GAC is a widely used treatment technology which removes vapor via adsorption of the constituents on to the carbon, which is a highly adsorptive medium Once its adsorptive capacity is exhausted, the carbon is removed and replaced The spent carbon usually can be regenerated and recycled. GAC is a proven treatment technology for a variety of chemical constituents, and is most cost . effective with low concentrations of hydrophobic compounds. I i 101ca3 16