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11 February 2003 . . <br /> AGE-NC Project No. 48-0520 <br /> ' Page 15 of 21 <br /> ' extraction/treatment of approximately 100,000 gallons of contaminated ground water. Costs for the <br /> entire treatment project would likely be'between $100,000 and $300,000. <br /> 7.2. IN-SITU AIR SPARGING <br /> In-situ air sparging (IAS). utilizes air injection to volatilize dissolved hydrocarbons and strip <br /> hydrocarbon vapor into the vadose zone, where it can be withdrawn by a vapor extraction system. <br /> Air sparging_is commonly conducted concurrently with soil vapor extraction. <br /> 7.2.1. Principles <br /> IAS induces the transfer of hydrocarbons from ground water to soil gas in the vadose zone by <br /> creating a strong concentration gradient between the two regimes. Commonly, an extraction well <br /> ' is surrounded by specifically designed air injection (sparging)wells that are screened only within <br /> the saturated zone(i.e.below ground water).A low capacity blower connected to the injection wells <br /> . forces air under pressure,t into the saturated zone producing air channels that strip dissolved contaminants from the ground water and rise to the soil-air-water interface_ Upon reaching the <br /> unsaturated zone,the volatilized hydrocarbons are drawn into the extraction well by the subsurface <br /> air flow induced by the vacuum potential. <br /> ' 7.2.2. Basic System Design <br /> Injection wells are screened within the saturated zone and normally have limited screened intervals, <br /> typically five feet. Since most of the air escapes from the.upper part of the.interval where head <br /> pressure is the least, little is gained by increasing the length of the screen interval. <br /> ' The.radius of influence of a sparging- well is dependent on site lithology and the depth of the, <br /> sparging well. Sandy soil types tend to produce a larger radius of,influence; clayey soil types tend <br /> to produce a more limited radius of influence. <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 strata <br /> with low permeability are interbeddedwith coarse-grained strata in the saturated zone. Air flow is <br /> retarded.within fine-grained strata, and hydrocarbon recovery.is poor. <br />' Use of this technology would require the installation of air injection points, screened at a depth of <br /> • approximately 20 to 25,feet bsg.The IAS system must be combined with vapor extraction to contain <br /> vapor stripped from the ground water. The installation of extraction wells would also be required. <br /> Advanced GeoEnvironmental,Inc. <br /> 1 <br />