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27 June 1.996 AGE-NC Project No. 95-0118 Page 10 of 23 _� 6.2.5. DISSOLVED IRON CONCENTRATION IN GROUND WATER J Dissolved iron was not detected in the water sample collected from the site. Therefore, it is unlikely that precipitation of iron as a result of oxygen introduction into ground water will be a specific cause of clogging of the sparging point or pore space within the formation. The laboratory report (MAI Laboratory I. D. No. 62321) and chain-of-custody are included in Appendix H. 7.0. SOIL REMEDIATION ALTERNATIVES F;- 7.1. SOIL VAPOR EXTRACTION Soil vapor extraction(SVE)is a well-accepted method of treatment that has been used successfully at numerous sites and.has an excellent rate of success. The method works best on volatile hydrocarbons in permeable soil. 7.1.1. PRINCIPLES For most SVE systems, a vacuum blower is used to volatilize hydrocarbons in the vadose P'J (unsaturated) zone and draw the vapor into extraction wells installed and screened within the contaminant plume. The vapors are drawn from the wells and treated in one of several possible treatment units (internal combustion engines, carbon canisters, thermal oxidizers and catalytic oxidizers). Air injection wells may be installed outside the contaminant plume to increase air flow to the extraction unit, An internal-combustion,(IC) system draws the vapor stream into the carburetor system of an engine '_J and mixes the vapors with air before undergoing combustion. Exhaust gasses.(oxides of carbon and r=. hydrogen)are further broken down in a catalytic converter before being emitted to the atmosphere. '== 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 when in-situ hydrocarbon concentrations -:t approach the 40,000 ppmv required for peak performance of the engine. As concentrations decrease below 20,000 ppmv, 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. Hydrocarbon vapor is routed through activated carbon filters in a carbon-canister system, where `..� adsorption of hydrocarbons onto carbon takes place, removing the hydrocarbons from the vapor stream. This type of system works best for low concentrations of hydrocarbons, but become i ,_J