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TreadvvelORollo <br />' (up to 8,100 to 14,000 mg/kg at 10 to 11 feet bgs) This indicates that a source of contamination <br /> continues to exist in this area Previous soil sampling indicated that TPHg concentrations up to <br />' 9,000 mg/kg extend to 25 feet bgs in this area This is likely residual contamination present <br /> because the groundwater table was deeper than 25 feet bgs prior to the UST removal <br /> Boring logs indicate that free product may be present in soil and groundwater within the former <br /> UST location extending to at least 16B-8 to the west and MW-3 to the east Significant, but <br /> lower TPHg concentrations are present in soil (at the groundwater interface) and/or groundwater <br /> 1n borings 16-BIO to the south, and in boring 16B-7B and well EW-4 to the north TPHg and <br /> benzene concentrations in groundwater drop off beyond this area to borings 16B-9 and 16B-11, <br /> but appear to be capillary fringe related <br /> 6.1 Proposed Remedial Method <br /> Previous in-situ remedial systems have not removal the residual contamination around the former <br /> UST location We propose to excavate the contaminated soil in this area as shown on Figure 3 <br /> 1 The proposed excavation is 30' long by 28' wide by 22' deep The proposed excavation would <br /> be extended to approximately 4-feet below the groundwater table in order to remove <br /> ' contaminated groundwater, and contaminated saturated soil During excavation backfilling, <br /> oxygen release compound (ORC) would be mixed into the backfill below the groundwater table <br /> to assist with enhanced bioremediation of groundwater downgradjent of the excavation <br /> The excavation dimensions were determined after considering the previous and current soil and <br /> groundwater analytical data and the geologic and hydrogeologic data gathered during our recent <br /> investigations, as described below Excavation contractors have stated that shoring will be <br /> required due to the depth of the excavation into saturated sands <br /> Groundwater was measured ata approximately 18 feet <br /> pp y bgs in all 4 of the wells in the excavation <br /> footprint (MW-3, EW-3, EW-4 and VW-5) Similar stabilized groundwater depths were <br /> measured in our recent borings at Area 16 <br /> 9 <br /> 25971014 DGD 7 April 2004 <br />