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1 <br /> 6.2.2.1.4 Duration <br /> If groundwater remediation is initiated concurrent with soil remediation and the radius of <br /> influence of each well reaches 15 feet in 3 months, the area between the tank cavity and <br /> MW-3 can be remediated within 3 months.Assuming that expansion of the remediated area <br />' will slow after that (see section 6.1.2.6), an additional 3 months or more will probably be <br /> required to treat the area between MW-1 and MW-4, unless an additional inoculation well <br /> is installed between these wells. A large untreated area will remain in the alley, between <br /> MW-5 and RW-1. Due to access problems, installing an inoculation well in this area is <br /> difficult. Full remediation of the site may be accomplished within 24 to 36 months. <br /> If treatment of groundwater is postponed until after results of soil remediation have been <br /> obtained, full remediation may be delayed 6 to 12 months. <br />' 6.2.2.2 Air Sparging <br /> A fourth alternative for remediation of groundwater utilizes air injection to volatilize <br /> dissolved hydrocarbons and force hydrocarbon vapors into the vadose zone, where they can <br /> be withdrawn by vapor extraction systems. Air sparging is commonly conducted concurrently <br />' with vapor extraction. <br /> 6.2.2.2.1 Principles <br /> Air sparging involves the transfer of hydrocarbons from water to air by creating a strong <br /> Y <br /> concentration gradient from groundwater to air. The vapor extraction wells are surrounded <br /> ' by specifically designed injection wells that are screened only within the saturated zone. <br /> Blowers connected to the injection wells force air under pressure into the saturated zone, <br /> producing air bubbles that strip dissolved contaminants from the groundwater and rise to <br /> ' the air-water interface. Upon reaching the unsaturated zone, the vaporized hydrocarbons are <br /> drawn into the extraction wells. <br /> ' Injection wells are screened within the saturated zone, and normally have short screened <br /> intervals (less than 5 feet), since most of the air escapes from the upper part of the interval <br /> where head pressure is the least. Little is gained by increasing the length of the screen <br /> interval. <br /> ' Unless injection wells are properly located, channeling of air-flow streams may occur and <br /> contamination pockets may be bypassed. This problem is more common where fine-grained <br /> strata with low permeability are interbedded with coarse-grained strata in the saturated zone. <br /> ' Air flow is retarded within the fine-grained strata, and hydrocarbon recovery is poor. <br /> The radius of influence of a sparging well is dependent on site geology, the depth of the <br /> ' sparging well, and the depth of the screened interval. Relatively large radii can be attainted <br /> in homogeneous sandy aquifers, like that at the Art & Artists site. <br /> C-1.8i-1 Audit Scrv�­.I., �� <br /> A&A PAAl1-28-94x.4'1 <br />