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RWQCB Analytical testing of groundwater samples would include TPH/D, and BTEX All <br /> monitoring activities would be discontinued five years after the system is decommissioned This <br /> is assumed to occur in 13 years <br /> 5 3 6 Excavation to Background Concentration and Off-Site Disposal - 012tion 6a <br /> Option 6a consists of excavation of impacted soils and off-site disposal In the case of <br /> petroleum hydrocarbons, the assigned cleanup level would be "non-detect" Based on site <br /> investigation data, it is estimated that approximately 11,300 cubic yards (14,700 tons) of soil <br /> contain detectable concentrations of hydrocarbons Based on the spatial distribution of the <br /> impacts, it is estimated that the total excavated volume would be on the order of 50,000 cubic <br /> yards, assuming excavation to 35 feet bgs and no shonng Shoring could be expected to reduce <br /> the total excavated volume by allowing nearly vertical excavation walls, however, shoring below <br /> depths of about 15 to 20 feet tends to be cost-prohibitive for this type of project It is estimated <br /> that this excavation could be completed without further demolition of existing structures <br /> However, significant amounts of dust could be generated by haul trucks making 750 trips in and <br /> out of the site <br /> During 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 loaded onto trucks, sampled for waste characterization purposes, and <br /> hauled to an off-site disposal/recycling facility Three off-site disposal options have been <br /> considered, and the least costly of these (based on hauling, final disposal, and waste <br /> characterization costs) is expected to be off-site bioremediation and subsequent Class III landfill <br /> disposal at the Forward, Inc facility in Stockton, California <br /> Confirmation soil samples would be obtained from the sidewalls and bottom of the <br /> excavation for analytical testing to determine whether the cleanup level has been achieved Once <br /> excavation is complete, the clean soil would be systematically replaced in lifts and compacted <br /> CIean import material would be brought from off-site as needed until the entire excavation is <br /> filled <br /> Because essentially all of the impacted soil would be removed, groundwater monitoring <br /> and maintenance of the cap would not be needed and the site would be suitable for permanent <br /> closure <br /> sAC127 10 17 <br /> DAMES &MOORE <br />