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27 December 1996 <br /> AGE-NC Project No 95-0143 <br />' Page 12 of 19 <br /> 6•0. GROUND WATER REMEDIATION ALTERNATIVES <br />' 6 1 IN-SITU S U 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 hydrocarbon vapor into the vadose zone, where it can be withdrawn by the vapor extraction system <br />' Air sparging is commonly conducted concurrently with vapor extraction <br /> 611 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 is <br /> surrounded by specifically designed air infection (sparging) wells that are screened only within the <br /> saturated zone (i e below ground water) A low capacity blower connected to the injection wells <br /> forces air under pressure into the saturated zone producing air channels that strip dissolved <br /> 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 /> 6 12 BASIC SYSTEM DESIGN <br /> Infection wells are screened within the saturated zone and normally have limited screened intervals. <br /> typically five to ten 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 infection 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 interbedded with 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 infection points, screened at a depth of <br /> approximately 30 to 35 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 /> I <br />