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Geological Tecfuucs 1= • Page 2 ' George's Service Project No. 425.3 February 5,2002 ' 1.1 Contaminant Distribution/Mass Estimate ' Mass estimate calculations performed as part of the Soil Excavation Report showed that approximately 11,600 pounds of gasoline were present in the soil at the site prior to the excavation. The majority of the contamination at the site is present in the upper 15-feet of ' soil. The soil excavation removed approximately 4,900 pounds of gasoline from the site, approximately 42%. Table 1 in Appendix A shows the calculations performed. The Soil Excavation Report prepared on January 28, 2002 contains a full discussion of the calculations. 2.0 BIO-VENTING/VAPOR EXTRACTION Operation of a small bio-venting/vapor extraction system in the excavation backfill is proposed at the site for the following reasons: Active removal of contaminants that will be sorbed to the backfill material during the seasonal groundwater fluctuation. As groundwater fluctuates at the site, as much as 4- feet, petroleum hydrocarbons in the groundwater will sorb onto the backfill materials in the excavation. These contaminants can then be removed by vapor extraction. • Removal of vapor phase contaminants from the excavation backfill and sidewalls. ' Petroleum vapors will continue to seep into the excavation backfill from the surrounding soil and groundwater. The vapor extraction system will create a low pressure area in the excavation aiding in the volatilization and remove these contaminants helping to reduce the remaining contaminant mass. Introduce more oxygen into the subsurface to aid in natural attenuation. The continual sweep of air through the excavation will provide oxygen needed for biological activity and will prevent anoxic conditions, low biological activity, from occurring. 2.1 Vapor Extraction Wells ' During the backfill of the excavation, GTI and Del Tech installed two 4-inch diameter drainpipes in the excavation pit at the west and east ends, VEW-1 and VEW-2, respectively. The drainpipes contain rows of 3/8-inch diameter holes from approximately 1-foot bgs to a ' depth of 10-feet bgs. Pea gravel was placed around the drainpipes. No additional vapor extraction wells (VEW) are proposed at this time. Figure 2 is a site map showing the locations of the two vapor extraction wells. 2.2 Piping&Manifold System It is proposed that the vapor extraction treatment system (VETS), blower and carbon units, be placed inside an enclosure along the eastside of the building. This will protect the equipment from vehicle damage and is located adjacent to the power supply for the ' building. PVC piping will be connected from VEW-2 to the VETS enclosure via an underground trenching and piping system. The pipe will be run underground to prevent damage and reduce the likelihood of someone tripping. VEW-1 will be modified to allow it to be used