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i I 20 March 1996 AGE-NC Project No 95-0130 Page 16 of 23 i7.0. REMEDIATION OF HYDROCARBON-IMPACTED GROUND WATER I7 1 IN-SITU AIR SPARGING I711 PRINCIPLES In-situ air sparging (IAS) utilizes air injection below the water table to volatilize dissolved hydrocarbons in ground water and force hydrocarbon vapors into the vadose zone, where they can be withdrawn by a vapor extraction system Air sparging is commonly conducted concurrently with vapor extraction IAS induces the transfer of hydrocarbons from ground water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 wells that are screened only within the saturated zone (i a below the ground water table) 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 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 short screened intervals, perhaps 5 to 10 feet in length 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 screened interval The radius of influence of a sparguig 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 with low permeability are interbedded with coarse-grained, permeable strata in the saturated zone Air flow is retarded within the fine-grained strata, and hydrocarbon recovery typically is poor E Use of this technology would require the installation of between 15 and 20 injection points at the TFS site, screened at a depth of approximately 65 to 70 feet bsg It is likely that up to 10 sparging points will be required off-site, north and east of the property Considering the relatively clay-rich soil at the site, we estimate that these wells could create an inverted cone of influence of approximately 15 to 25 feet in diameter IAS must be combined with vapor extraction to contain vapor stripped from the