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Ij y <br /> . 15 November 1995 <br /> AGE-NC Project No 95-0142 . <br />' Page 13`of 19 <br />' will be between $90,000 and $175,000 <br /> 7.0. GROUND WATER REMEDIATION ALTERNATIVES <br /> 7 1 IN-SITU AIR SPARGING <br />' In-situ air sparging (IAS) utilizes air injection to volatilize dissolved hydrocarbons and strip <br /> p <br /> hydrocarbon vapors into the vadose zone, where they can be withdrawn by the vapor extraction <br />' system Air sparging is commonly conducted concurrently with vapor extraction <br />' 711 PRINCIPLES <br />' IAS induces the transfer of hydrocarbons from ground water to soil gas in the vadose zone by <br /> creating a strong concentration gradient between the two regimes Commonly, an extraction well is <br /> surrounded by specifically designed air,mjection wells that are screened only within the saturated zone <br /> (i a below ground water) A low capacity blower connected to the injection wells forces air under <br /> pressure into the saturated zone producing air channels that strip dissolved contaminants from the <br /> ground water and rise to the air-water interface Upon reaching the unsaturated zone, the volatilized <br />' hydrocarbons are drawn into the extraction well by the subsurface air flow induced by the vacuum <br /> potential <br />' 7 12 BASIC SYSTEM DESIGN <br />' Injection wells are screened within the saturated zone and normally have short screened intervals, <br /> perhaps 5 to 10 feet Since most of the air escapes from the upper part of the interval where head <br /> pressure is the least, little is gained by increasing the length of the screen interval <br />' The radius of influence of a sparging well is dependent on site lithology, the depth of the sparging <br /> well and the length of the screened interval Sandy soils tend to produce a larger radius of influence, <br />' clayey soils tend to produce a more limited radius of influence <br /> Unless injection wells are properly'located, channeling of air flow streams ,may occur and <br />' contamination pockets may be bypassed This problem is more common where fine-grained strata <br /> with low permeability are interbedded with coarse-grained strata in the saturated zone Air flow is <br />' retarded within the fine-grained strata, and hydrocarbon recovery is poor <br /> 'i None of the present wells at the site are suitable for air sparging Use of this technology would <br />' require the installation. of 9 or 10 IAS injection points, screened at depths between 45 and 50 feet <br /> I <br />