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15 June 1999 AGE-NC Project No 98-0450 Page 17 of 27 5.0. GROUND WATER REMEDIAL ALTERNATIVES Various alternatives for remediation of the on-site petroleum impacted ground water are available including in-situ air sparging, soil vapor extraction, in-situ bioremediation, and natural attenuation ' Discussion of technology principles, feasibility and effectiveness, duration, and estimated cost are j presented for each of the remedial alternatives I I 5 1 IN-SITU AIR SPARGING In-situ air sparging (IAS) is a remedial technology that reduces concentrations of volatile constituents of petroleum products in the saturated zone that are adsorbed to soil and dissolved in ground water This technology involves the injection of air into the subsurface saturated zone, facilitating a phase transfer of hydrocarbons from a dissolved state to a vapor state The injected air is then vented through the unsaturated zone and typically extracted by SVE technology(Section 4 2) 511 Principles IAS induces the transfer of hydrocarbons from ground water to soil gas in the vadose zone by ' creating 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 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 toward the extraction well by the 1 subsurface air flow induced by the vacuum potential Injection wells are screened within the saturated zone and normally have limited screened intervals, ' typically up to five feet Since most of the air escapes from the upper part of the interval where head piessure is the least, little is gained by increasing the length of the screen interval 512 Requirements - 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 Unless injection wells are properly located,channeling . of air flow streams may occur and contamination pockets may be bypassed This problem is more common where fine-grained strata having low permeability are interbedded with coarse-grained ' Ads arced CeoEn ironmental,Inc