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George's Service Remedial Action Work Plan Project No.425.2 June 30,2011 6. RECOMMENDED CORRECTIVE ACTION PLAN The recommended corrective action alternative is presented below. This alternative was selected based on an evaluation of the results of the 2009 SVE pilot test and our own experience with UST site remediation. In addition, GTI consulted the USEPA's "How to Evaluate Alternative Cleanup Technologies for Underground Storage Tank Sites", May, 1995. Based on the investigations conducted at the site to date, GTI recommends applying low vacuum soil vapor extraction combined with air sparging to address contamination in the soil and groundwater at the George's Service site, utilizing the two wells installed during the backfill of the soil excavation in 2002 for vapor extraction, and installing two new air sparge wells. 6.1. Air Sparging Air sparging is the process of injecting air directly into groundwater. Air sparging remediates groundwater by volatilizing contaminants and enhancing biodegradation. At higher rates of sparging, the contaminants are removed from the groundwater by physical contact with the air (i.e., stripping) and are carried up into the vadose zone (i.e., unsaturated soil). As the contaminants move into the soil, a soil vapor extraction system can be used to remove vapors. At lower sparging rates, the addition of oxygen to contaminated groundwater and soils enhances biodegradation of contaminants in and above the water table, as it acts as a nutrient for bacteria. Biodegradation is also very important result of higher rates of sparging, but mass transfer may be the dominant process until concentrations are reduced. GTI proposes to utilize air sparging to address the groundwater plume. The results of the ozone sparging pilot test from 2005 show that ozone sparging is an effective groundwater remedial alternative. However, due to the presence of hexavalent chromium in the groundwater, ozone was not a feasible remedial alternative. Air sparging will work, potentially at a slower rate which is not expected to mobilize metals. As shown in the Remedial Improvement Plans, GTI proposes three air sparging points be advanced in the former UST over-excavation backfill area to accelerate the remediation of groundwater. AS-1 is proposed to be located in the gravel backfill area, southwest of the most contaminated groundwater monitoring well MW-2. The location of AS-I was selected based on the existing location of VEW-1, with the expectation that pulsing between the sparging well and the vapor extraction well may be an effective method of remediation. AS- [ is expected to target the sand unit that extends to the north at approximately 22 feet bgs. The second air sparging well AS-2 is proposed to be located west of AS-1, in the center of the gravel backfill area. Based on the expected good hydraulic conductivity between VEW-I and VEW-2, it is anticipated that the central location of AS-2 will allow for pulsing between GeologicalTecknfesInc. 15 4252 RAP.docx