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_ .Moe to create negative pressures for vapor removal. A process control system is used to monitor and adjust the air deliveryand removal equipment. Additional aboveground equipment may also include a knockout pot to separate liquids from the vapor stream and a vapor treatment system. Once the extracted mass ` removal rate has reached an asymptotic level,the AS/SVE system can be run intermittently to be more cost effective. r SVE is implemented by applying a vacuum to the vadose zone to remove vapor phase contaminants from the soil above the groundwater table. AS is used in conjunction with SVE to inject air into the �- contaminated groundwater and extract vapor in the unsaturated vadose zone above the aquifer using 2-inch diameter AS wells installed into the aquifer. This technology also promotes biodegradation in saturated and unsaturated soils by increasing subsurface oxygen concentrations. Air is bubbled into the aquifer and travels both horizontally and vertically,removing contaminants byvolatilization. Volatilized contaminants would then be removed by SVE. L 4.2.1.2 Evaluation In situ AS/SVE techniques have been shown to be effective for treating volatile contaminants in groundwater and have been implemented successfully at many sites. However,this technology is relatively difficult to implement as it would require afield pilot studyto obtain the information necessary to design and configure the system,along with the installation ofan estimated 10 AS wells and,assuming a 50-foot radius ofinfluence,three SVE wells. Approximately 600 feet ofAS piping and 350 feet of SVE piping would be ` required and 13 vaults would be used for well head completion. Installation ofwells would require closing local streets, and trenches for underground piping would need to be constructed and backfilled. In addition,there are several utilities in the area that would require working around or relocating. The treatment unit,including piping,connections,compressors,vacuum pumps,and air monitoring equipment, would need to be placed in the vicinity ofthe intersection of Sutter Street and Washington Street,a highly visible and busy downtown comer. This technologyrequires significant operation and maintenance(O&M) costs; however, since the groundwater is treated in place, there is no need for disposal of treated groundwater. The effectiveness ofthis technology is also questionable as the soil in the contaminated zone consists of silt and clay(Lithologic Zone 4)overlain by sand and gravel(Lithologic Zone 3). AS/SVE technologies depend on the air permeability ofthe soil. SVE applied in the upper,more permeable zone would have much greater effectiveness. Finally,this technology is most effective when soil and groundwater concentrations are high and is much less effective when contaminants reach an asymptotic state,as they currently have at the site. The return,or amount of contaminants recovered,per dollar spent is not as favorable as it would be at more contaminated sites. WESTON therefore recommends that the in situ AS/SVE technology not be considered further. 10