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I 16 site and documented soil contamination at the soil/water interface, it is questionable if this alternative would meet regulatory criteria for the protection of groundwater quality I 6.2 Soil Excavation Excavation of the shallow soil contamination and subsequent on-site aeration or off-site disposal to a certified waste disposal facility would remove the potential continuing source of groundwater contamination It is estimated approximately 300 to 350 cubic yards of residual hydrocarbon contaminated soil remain in the vicinity of the former tank pit On-site aeration/bioremediation, if possible, would be preferred to off-site disposal because of the 1 expense and continued long term liability associated with disposal of contaminated material at an off-site facility Confirmation soil sampling should be conducted on the walls and floor of ' the excavation to assure successful excavation of impacted soils However, if as anticipated, standing water is observed in the pit upon completion of the excavation a groundwater sample should be collected for laboratory analysis in substitution of the floor soil sample The recent rise in the water table (to approximately 10 feet below grade) may render excavation infeasible at this time due to the required depth of excavation It may be necessary to postpone excavation activities until water levels drop to historical levels (14-15 feet below grade) 6.3 In-Situ Soil o Venting/Vacuum Extraction In some circumstances in-situ remediation of soils can be accomplished by installation of a soil venting/vacuum extraction system This would entail the completion of shallow vapor extraction wells within the area of soil contamination A vacuum pump would be used to apply a negative pressure head to drive volatile hydrocarbon constituents to the vapor wells These wells would ' be plumbed to a carbon filter unit to adsorb the contaminants or some other off gas treatment technology prior to being emitted to the atmosphere Tight soil types such as the silts and clays at this site are not as conducive to venting as sandy soils Soil venting is not considered cost effective for this site In terms of protection of public health and water quality, each of the alternatives are essentially equivalent since the contamination has been demonstrated to be of limited extent and ' PAR-0593 FNL 5