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31 January 2013 AGE-NC Project No. 02-0926 Page 15 of 21 The site conditions, which include buried (below water table) adsorbed soil impact, fine grained and coarse grained lithologies in both the smear and saturated zones and the current water table depth make DPE a low to moderately feasible remedial alternative for the site. Based on duration, cost and site lithology at the site, DPE does not appear to be the most cost-effective strategy to remediate the remaining adsorbed and dissolved impact at the site. 9.3. IN-SITU AIR SPARGING In-situ air 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air injection to volatilize dissolved hydrocarbons and strip hydrocarbon vapor into the vadose zone, where it can be withdrawn by a SVE system.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. 9.3.1. Principles IAS induces the transfer of hydrocarbons from groundwater to soil vapor 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 (or air compressor) 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 subsurface air flow induced by the vacuum potential. 9.3.2. 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 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. Injection wells should be properly located to prevent channeling of air flow streams. This problem is generally more common where fine-grained strata having low permeability are interbedded with coarse-grained strata in the saturated zone. This condition is not Advanced GeoEnvironmental, Inc.