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13 July 1992 (GeoAudit CV 55-59 E2-1.32) Page 7 of 8 Both methods will require additional sampling in order to meet regulatory and/or disposal facility requirements and would require proper certification. Tile preferred on-site treatment method would be bioremediation. The site is underlain by bedded sand and silt. The upper sand bed is tine grained and moderately weathered and oxidized. It is separated from a lower, less weathered and coarser-grained sand bed by a thin, slightly consolidated silt bed. A thicker silt bed underlies the lower sand bed at a depth of 25 feet. Eleven soil borings have been drilled, and the lateral and vertical extent of soil contamination have been delineated. At its maximum extent, the contaminant plume extends 10-15 feet beyond the walls of the vault. Tpand no c ntam nat ion r was detected both below aadepal hof and vertically, 23 feet. The present depth to groundwater is estimated to be in excess of 60 feet, implying that approximately 40 feet of uncontaminated soil are present between the contaminant plume and groundwater. Groundwater levels have dropped several feet in recent years, but in 1988 the depth to groundwater still exceeded 50 feet. Should the water level return to its former position in the future, more than 25 feet of uncontaminated soil would still separate the contamination from groundwater. The upper several feet of this "buffer zone" consist of coarse-grained and permeable sand, but at least one silt bed that is more than 5 feet thick is prevent beneath tlis sand bed. We believe that the buffer zone of uncontaminated soil will prevent the contamination from leaching downward and contaminating groundwater. Heating oil is a high-hydrocarbon, low-viscosity is relatively immobile and non-volatile. In petroleum product that addition, the entire area around the heating oil vault is surfacer with concrete, preventing infiltration of rainwater and reducing the potential for leaching of contaminants. we therefore recommend allowing the contamination to remain in place. Neither removing nor treating the contamination are feasible alternatives. Access is limited around the vault area, making it impossible to use a backhoe or excavator to remove the contaminated soil without first demolishing the men's restroom and the cooling tower equipment on its roof. Due to the heavy nature of the and would contaminants,.....-in-situ---treatment--would-_-be.._diff.icult., _...._ _ -..___._ _. require the installation of bio-remedial wells in a heavily trafficked area. In addition, in-situ bioremediation would be of limited effectiveness in the consolidated silt bed, where the highest TPH levels are present.