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05 May 1997 <br /> AGE-NC Project No 95-0173 , <br />' Page 20 of 29 <br />' 7 4 4 Estimated Costs <br /> It is likely that some additional assessment, including additional laboratory analysis of soil samples <br /> and/or confirmation soil sampling would be required to monitor and verify natural attenuation <br /> Furthermore, a long-term monitoring program perhaps greater than 25 years, would be initiated <br />' Generally, the cost for natural attenuation would range from $11,000 to $30,000 annually Quarterly <br /> monitoring and sampling generally costs between $5,000 to $15,000 annually, semi-annual soil <br /> probings to monitor passive bioremediation progress typically would average $5,000 to $10,000 <br />' annually, report preparation would generally cost between $1,000 to $5,000 annually <br />' 8 0. GROUND WATER REMEDIATION ALTERNATIVES <br /> 8 1 IN-SITU AIR SPARGING <br />' In-situ air sparging (IAS) utilizes air injection to volatilize dissolved hydrocarbons and strip <br /> . hydrocarbon vapor into the vadose zone, where it can be withdrawn by a SVE system Air sparging <br />' is commonly conducted concurrently with a SVE system <br /> t8 1 1 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 injection (sparging) wells that are screened only within the <br /> saturated zone (i e below ground water) A low capacity blower connected to the infection wells <br /> forces air under pressure into the saturated zone, producing air channels that strip dissolved <br /> contaminants from the ground water and rise to the soil-air-water interface Upon reaching the <br />' unsaturated zone, the volatilized hydrocarbons are drawn toward the extraction well by the <br /> subsurface air flow induced by the vacuum potential <br /> 8 1 2 Basic System Design <br /> 1 Infection wells are screened wrtivn the saturated zone and normally have limited screened intervals, <br /> tvpically five to ten 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 and the depth of the <br /> 1 <br /> Advanced CeoEw,ironmental,Inc <br /> 1 <br />