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I 5. 1.2 Vapor Extraction ' A technologically more advanced approach is vapor extraction. In this method, wells are installed in the impacted soil and a blower draws the volatile fraction of the contaminants through the soil ' into recovery wells. Vapor from the extraction wells is drawn through a combustion system and is ignited. Regulated by a fuel control computer, natural gas is added as necessary to maintain the proper air/fuel ratio for maximum combustion. The exhaust from the ' combustion process is passed through a three-way catalytic converter for further oxidation. ' Vapor extraction is one of the more common and approved in-situ methods for soil remediation. Vapor extraction works most effectively in permeable sandy soils. Most of the soil beneath the ' Geweke site consists of fine to medium-grained, channelized sand deposits that would be amenable to vapor extraction. The interbedded finer-grained overbank deposits create inhomogeneities in the aquifer, however, and would impede vapor flow. The degree to which these low-permeability layers might reduce the effectiveness of the vapor system has not been determined. Vapor extraction systems are less disruptive to normal site operations than is excavation, but excessive noise generated by the combustion system is a problem at some sites where customer visits are frequent. Noise problems could probably be mitigated, however, by housing the unit in a temporary building or shed. In addition, a few days of construction will be required to manifold the four vadose-zone wells (VW-1 through VW-4) that were drilled in May 1993 . Vapor extraction costs vary greatly and are time-dependent. The I time required depends on the stratigraphy and permeability of the contaminated zone and the number and location of extraction wells in the system. In most cases, it is difficult at the outset to estimate the length of time required, and no estimate can be given I at this time. Costs can become excessive toward the end of the remediation effort, when contaminant levels decrease and increasing amounts of supplemental fuels may be required to maintain vapor ' destruction efficiency. Vapor extraction units typically cost $50, 000 to $75, 000, with installation costs totaling an additional $10, 000 to $20 , 000. Units must be monitored and maintained on at least a weekly basis. Monthly monitoring and maintenance can cost between $2 , 000 to $5 , 000 per month. For a 36-month treatment period, the monitoring and maintenance costs could total $180, 000. These costs do not include yearly or biannual progress reporting or confirmatory drilling. 5. 1. 3 In-situ Bioremediation The third method of soil remediation that has been considered is bioremediation. Acceptance of this technique by the regulatory 7