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La ` "Cjeolog,LTechnies inc. %--w Former Moresco Property Page 7 Site Characterization Report Project No.251.2 16. October 11, 1996 �• 3.4 Soil Gas & Geophysical Testing Several lithologic horizons were tested using soil gas collection procedures with the Lo SimuiProbe tool. Analyses of soil gas samples shows that a vapor plume extends beyond the bounds of the liquid/adsorbed phase chemical contaminant zone. This information was incorporated into the mass balance calculations. The data is summarized in Table 2. Representative geologic horizons were tested for selected geophysical parameters. These parameters were mutually agreed upon by SJCPHSEHD and GTI personnel and are summarized in Table 2. The purpose of these data is to provide site specific data for L, computer modeling efforts to simulate the fate and transport of these chemicals, the potential for natural attenuation process to occur at this site, and the risk that these chemicals pose to public health and the environment. 4.0 RECEPTOR SURVEY +N Ingestion Well construction logs for the domestic well should be available through the State of California Department of Water Resources. If vadose zone contamination was to impact groundwater computer modeling can predict if detectable concentration of contaminants will impact the domestic well located approximately 200 feet former the point of release. Inhalation The soil vapor flux rate to the atmosphere can also be evaluated using risk models designed for volatile emissions to the air from contaminated soil. La Dermal Contact The stockpiled soil has been removed and the excavation has been backfilled with clean soil. It is not expected that the subsurface contaminated soils will come into contact with humans in its present location. Should land use change this may need to be reevaluated. 5.0 CONCLUSIONS AND RECOMMENDATIONS 5.1 Conclusions La The data obtained during this investigation suggest that the release of petroleum hydrocarbons sourced from a former leaking UST has caused a significant impact to the subsurface soil environment near the former leaking UST. Laboratory evidence indicates that at least a fraction of fuel that was released contained the fuel oxygenate compound MTBE. The distribution of the center of high concentration of gasoline suggest that the mass of chemicals is migrating vertically and may impact groundwater in the future. The presence of +— MTBE may be a significant factor in contributing to the mobility of these chemicals. In addition, the soil profile is dominated by sand down to 45 feet and then a mixture of silts, sands and gravel below that. The area around the former tank is in close proximity to LW irrigated landscaping. This will cause a hydraulic head that will drive the chemicals deeper in the sediments.