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REMEDIATION TESTING AND DESIGN March 29 , 1993 <br /> DISCUSSION <br /> The results of the LSCS showed that the shallow subsurface soils <br /> around the wormer USTs are not significantly -impacted witft petrcleun- <br /> hydrocarbons ThiE indicates that the gasoline likely migrated <br /> vertically downward to more permeable cones prior to migrating <br /> 1 laterally in the "a" and "b" cores ani down to the water table . The <br /> greatest so-i1 gas concentrations occur in the "b" :one , possible due <br /> to residual gasoline remaining after the water table dropped during <br /> the drought. The "b" ::one -is clearly the prime target of vacuum <br /> extraction remediation <br /> The VEFT clearly showed that vacuum extraction is an Effective <br /> remediat-ion technique at the site for removing gasoline from <br /> unsaturated soils , the capillary fringe and the water table surface <br /> Vacuum influence measured in the existing wells indicated strong <br /> subsurface flow characteristics . Significant vacuum -influence near <br /> the water table appears to be a local feature due to low permeability <br /> soils in the vicinity of the groundwater level Vacuum -influence and <br /> flow characteristics observed in the shallow sods through tine vapor <br /> probes showed almost no detectable vacsum -influence. Vacuum influence <br /> in the shallow soils -nay -increase durrng longer term vacuum extraction <br /> as the clays and silts dry out. Strong vacuum influence was noted <br /> within the backfilled UST pit There must be some route of flow <br /> between the "a" zone and the backfilled pit that somehow bypasses the <br /> shallow soils <br /> Based on the results of the flow step test , it is estimated that a <br /> total vapor flow rate of 250 scfm from the 8 vacuum extract-ion wells <br /> can be easily achieved under a moderate vacuum load of 10 inches of <br /> mercury. Given the high TPHg and BTEX levels , thermal destruction is <br /> recommended. The ideal vacuum extraction system would consist of a <br /> 250 scfm package with dual thermal andIcatalytic/ox-idat-ion (CAT/UX) <br /> destruction capabilities The dual mode system configured for thermal <br /> destruct-ion can handle the initial high concentrations Once the <br /> -influent levels drop below 5 ,000 ppm (molar concentration) , the system <br /> can be switched to CAT/OX mode to reduce fuel costs <br /> If you have any quest-ions or comments regarding this report, please <br /> contact me at ( 408 ) 427-0668 or Thomas Armstrong at ( 408 ) 458- 1612 <br /> Sincerely, <br /> REMEDIATION TESTING AND DESIGN <br /> Howard E Whitney, R G. <br /> Principal Geclog-ist <br /> k J i <br /> 5 <br />