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r+ 1 <br /> I <br /> During vapor extraction,vapor in the interstices between soil particles is induced fo flow in I <br /> response to pressure gradients created by the vacuum applied to the extraction wells. <br /> Residual hydrocarbons adsorbed on the soil particles are volatilized and entrained in the <br /> soil vapor, which is drawn into the extraction wells. The circulation of air in the subsurface <br /> ry , <br /> also enhances natural microbial degradation of the less-volatile hydrocarbons. <br /> The advan'ages of a vapor-extraction system at this site are that excavation is not required, <br /> disruption to site activities is minimized,and the method is usually less expensive than deep <br /> excavation. Because the scii underlying the site consists dominantly.:,f clay and silt, which <br /> generally have low permeability to air,closely spaced extraction wells may be m-cessary and <br /> extraction rates may be low. However, the permeability of the sediments may be <br /> Prohibitively low and vapor-extraction may not be applicable at the site. <br /> In Situ Bioremediation <br /> Hydrocarbon compounds are degradable by naturally occurring microorganisms if an <br /> adequate supply of oxygen and nutrients is available. The process,however,is generally too <br /> slow to prevent migration of hydrocarbons into ground water, The process can be <br /> accelerated by providing oxygen and nut,:cnts to the naturally occurring microorganisms in <br /> the affected soil. In addition, proprietary strains of microorganisms can be introduced into <br /> the soil to induce bioremediation. O.Vgen and nutrients are generally supplied to the <br /> microorganisms by circulating water; an alternative method utilizes a forced-air circulation <br /> system similar to a vapor-extraction ss•stem. Because the sediments underlying the site are <br /> u <br /> Apty/ied 47&oSystel <br />