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07 November 2004 AGE-NC Project No 96-0232 Page 3 of 11 at the Flag City Chevron may no longer be the most advantageous or cost-effective remediation method Therefore, the following section will evaluate four additional remediation methods that could be utilized at the site for addressing the remaining MTBE-impacted ground water at the site The methods include in-situ air sparge (IAS), in-situ ozone sparge (IOS), bioremediation and natural attenuation 41 IN-SITU AIR SPARGING IAS utilizes air injection to volatilize dissolved hydrocarbons (i e MTBE) and strip volatile hydrocarbons from ground water and transfer the hydrocarbons into the vadose zone in a vapor state, where they can be recovered by soil vapor extraction(SVE) Air sparging is commonly conducted concurrently with an SVE system 41I Principles IAS induces the transfer of hydrocarbons from a dissolved state in ground water to soil gas in the vadose zone by creating an intense concentration gradient between the two regimes Commonly, a SVE well is surrounded by specifically designed air injection(sparging)wells that are screened only within the saturated zone (i e below ground water) A low capacity air compressor is typically utilized to connect to the injection well(s) to force air under pressure into the saturated zone, producing air bubbles 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 toward an SVE well by the subsurface air flow induced by the vacuum potential 4 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 The IAS system must be combined with vapor extraction to contain vapor stripped from the ground water The 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 Advanced GeoEnvironmental,Inc