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As estimated for Option 6a, it is estimated that approximately 11,300 cubic yards of soil contain <br /> detectable concentrations of hydrocarbons, and it is estimated that the total excavated volume <br /> would be on the order of 50,000 cubic yards <br /> Dunng excavation, clean soils would be separated from impacted soils based on visual <br /> assessment and/or field screening and stockpiled on-site for later use as excavation backfill <br /> Impacted soil would be thermally treated on-site in a thermal desorption unit Treated soil <br /> would be penodically tested to document achievement of the cleanup goal, and used as <br /> excavation backfill Additionally, confirmation soil samples would be obtained from the <br /> sidewalls and bottom of the excavation Once excavation is complete, the treated and stockpiled <br /> soils would be systematically replaced in lifts and compacted <br /> Because essentially all of the hydrocarbon impacts would removed and destroyed, <br /> groundwater monitoring and maintenance of the pavement would not be needed and the site <br /> would be suitable for permanent closure <br /> 5 3 9 Excavation to 1,000 mg/kg gnd On-Site Thermal Treatment -_Option 7b <br /> Option 7b is considered comparable to Options 5 and 6b in terms of contaminant mass <br /> reduction, and consists of excavation of impacted soils and on-site thermal treatment with a final <br /> cleanup goal of 1,000 mg/kg As discussed above, approximately 4,900 cubic yards (6,400 <br /> tons) of impacted soil would need to be treated, and the total excavated volume would be on the <br /> order of 37,000 cubic yards, assuming excavation to 35 feet bgs and no shonng <br /> As with Option 7a, clean soils would be separated from impacted soils during excavation <br /> based on visual assessment Impacted soil would be treated on-site in a thermal desorption unit <br /> Treated soil would be tested periodically to document achievement of the cleanup goal for use <br /> as excavation backfill Additionally, confirmation soil samples would be obtained from the <br /> sidewalls and bottom of the excavation. Once excavation is complete, the treated and stockpiled <br /> soils would be systematically replaced in lifts and compacted. <br /> Option 7b also includes quarterly monitonng of groundwater elevations in on-site <br /> monitoring wells (including the new well to be constructed in 1994), annual groundwater <br /> sampling and analysis of the five wells, and preparation and submittal of annual monitonng <br /> SAc127 10 19 <br /> DAMES &MOORE <br />