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i i18 February 1997 • AGE-NC Project No 95-0104 iPage 14 of 22 i7.0 GROUND WATER REMEDIATION ALTERNATIVES 7 l IN-SITU AIR SPARGING iIra-sau a.r sparging (IAS) utilizes air injection to volatilize dissolved hydrocarbons and strip hydrocarbon vapor into the vadose zone, where it can be withdrawn by a SVE system Air sparging is commonly conducted concurrently with a SVE system I711 PRINCIPLES IAS induces the transfer of hydrocarbons from ground water to soil gas in the vadose zone by Icreating a strong concentration gradient between the two regimes Commonly, an extraction well is surrounded by specifically designed air injection (sparging) wells that are screened only within the I saturated zone (i e below ground water) A low capacity blower connected to the injection wells forces air under pressure 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 • unsaturated zone, the volatilized hydrocarbons are drawn into the extraction well by the subsurface air flow induced by the vacuum potential 7 1 2 BASIC SYSTEM DESIGN Injection wells are screened within the saturated zone and normally have limited screened intervals, typically five to ten feet Since most of the air escapes from the upper part of the interval where head pressure is the least, little is gained by increasing the length of the screen interval IThe radius of influence of a sparging well is dependent on site lithology and the depth of the sparging well Sandy soil types tend to produce a larger radius of influence, clayey soil types tend to produce a more limited radius of influence Unless injection wells are properly located, channeling of air flow streams may occur and rcontamination pockets may be bypassed This problem is more common where fine-grained strata with low permeability are interbedded with coarse-grained strata in the saturated zone Air flow is retarded within fine-grained strata, and hydrocarbon recovery is poor Use of this technology would require the installation of air injection points, and screened intervals I located approximately five feet below the water The IAS system must be combined with vapor extraction to contain vapor stripped from the ground water The installation of extraction wells would • also be required I