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10 June 1999 <br /> S AGE-NC Project No 97-0290 <br /> Page 17 of 26 <br /> 7 4 4 Estimated Costs <br /> It is likelythat some additional assessment, including additional laboratory analysis of soil samples <br /> g <br /> and/or confirmation soil sampling would be required to monitor and verify natural attenuation <br /> Furthermore, a long-term monitoring program, lasting perhaps more than 25 years, would be <br /> 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 />' Four remedial methods for addressing hydrocarbon-impacted ground water are considered for <br /> application at the subject site, in-situ air sparging, bioremediation, pump and treat and natural <br /> attenuation <br />' 81 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' hydrocarbon vapor into the vadose zone, where it can be withdrawn by an SVE system Air sparging <br /> is commonly conducted concurrently with an SVE system <br />' 811 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 injection 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 vadose zone Upon reaching the unsaturated <br /> zone,the volatilized hydrocarbons are drawn toward the extraction well by the subsurface soil gas <br /> flow induced by the vacuum potential <br /> 1 <br />' Advanced GeoEn\Ironmental,Inc <br />