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MOM... . The contaminant masses are likely biased high for several reasons. First, samples were likely biased to contaminant hot spots. Second, the soil volumes used in the calculations include the excavation ramp area and soil above the tanks that was most likely clean. 5.2 REL%MIATION In 1986 contaminated soil was remediated by excavating to a depth of 31 feet bgs. This method of remediation represents the "best available technology" because over half of the total mass of contaminants were removed from the environment over a short period of time. All of the rcontamination was not removed because of the great depth to which the excavation would have to have extended to and because the lower permeability layer at approximately 30 to 32 feet bgs was believed to be sufficient to retard vertical migration of hydrocarbons (Weiss Associates). 5.3 GROUNDWATER CONTAMINATION Groundwater analytical results are presented on Table 3. In 10 years of sampling no petroleum hydrocarbons have been detected in groundwater. It is highly unlikely that contaminants will impact groundwater at the site for several reasons: 1) The predominantly asphalt surface prevents surface water infiltrating the vadose zone and transporting contamination down to the water table; 2) The contaminant source, i e the gasoline UST, and a large portion of the contaminant mass have been removed; 3) The residual soil contamination will degrade over time, and 4) Contaminants to samples collected below 31 feet bgs were detected at very low concentrations compared to concentrations in samples collected above 31 feet bgs (see Table 2) 6.0 WELL ABANDONMENT AND SOIL DISPOSAL 6.1 WELL ABANDONMENT The monitor wells located at 200 N. Beckman will be destroyed following the guidelines established in the California Department of Water Resources Bulletin 74-90, California Well Standards The well destruction will be conducted under the direction of a California Registered Geologist All field related activities will be documented in a log book or data forms. The SJCPHS will be notified prior to commencement of any on-site work and a well destruction permit will be obtained The following procedures will be used to destroy the wells LtiPROJEC-rWANJOQNCt=LTREWVC LOO CLO 8 1 M■y 1997