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' remediation will take between 12 and 30 months. If only one inoculation event is performed, 50 to 75 gallons of inoculant will be utilized. Up to 200 gallons may be used if a second inoculation takes place, or if it is necessary to inoculate MW-1. 6.1.2.2 Vapor Extraction ' Vapor extraction is a well-accepted epted method of treatment that has been used successfully at a large number of sites. The method works best on volatile hydrocarbons in permeable soil. 6.1.2.2.1 Principles In systems of this type, a vacuum blower is used to volatilize hydrocarbons and draw the vapors into extraction wells screened within the contaminant plume. The vapors are drawn ' from the wells and treated in one of several possible treatment units (internal combustion engines, thermal oxidizers, and catalytic oxidizers). ' In an internal-combustion system, vapors are drawn into an engine and mixed with air before undergoing combustion. Exhaust gasses (oxides of carbon and nitrogen) are further broken down in a catalytic converter before being expelled to the atmosphere. Geological ' Audit has various-location permit to operate}such a system in San Joaquin County. These systems require supplemental fuel, such as propane or natural gas, which can become expensive as vapor concentrations decrease below levels that are normally required for combustion. Typically, internal combustion engines work effectively where in-situ hydrocarbon concentrations approach the 40,000 ppm required for peak performance of the engine. As concentrations decrease below 20,000 ppm, supplemental fuel requirements ' become burdensome. Another disadvantage of these systems is the noise generated by the engine; however, this can be mitigated by placing the unit inside an enclosure, such as a building. ' In a carbon-canister system,vapors are drawn into activated carbon filters,where the organic P g carbon serves to filter the hydrocarbons and remove them from the vapor stream. These ' systems work best for low concentrations of hydrocarbons, but become expensive when large amounts of hydrocarbon vapors necessitate frequent canister replacement and disposal. ' In thermal oxidation systems, hydrocarbons are destroyed by open-flame combustion. Natural gas is commonly used as supplemental fuel, heating the extracted vapors to a combustion temperature of approximately 1400°F. Relative to internal combustion systems, these systems may be slightly more expensive to purchase and install. However, they can operate at higher air flow rates, and remediate sites at a faster rate. They can be used when ' hydrocarbon concentrations exceed 20,000 ppm, but they are more effective at lower = concentrations, such as at the Art & Artist's site. They also operate at much lower noise -- levels than internal combustion systems. ' Catalytic oxidation units provide another option for treating vadose-zone contamination, caw ,i A,,W S.M-,Ln 29 ABA PAPn-2E-YQj A7