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e a Mr. Marty Hartzell Page 8 of 10 • The rate of removal of gasoline constituents using soil vapor extraction would be limited by the rate of diffusion of contaminants from the contaminated zone into the moving vapor stream, since vapor flows around but not through the zone of contamination. • The location of the site in a residential setting will limit the hours of operation, which will further extend the necessary operational period of a full-scale system. • The heterogeneous nature of the soil, for which vapor-phase diffusion is a limiting factor, suggests that removing gasoline constituents in soil to non-detectable levels by implementing soil vapor extraction is not attainable. • The laterally continuous silt layer present from a depth of approximately 38 to 42 feet is relatively impermeable and virtually no contamination has been detected below this depth. • A laterally continuous clay layer between the depths of approximately 48 to 54 feet is presumably an additional barrier to downward vertical migration of contaminants to the water table, which occurs at a depth of approximately 60-61 feet. • Groundwater samples collected from borings SBI and SB4 contained no detectable concentrations of gasoline constituents. RECOMMENDATIONS FOR FINAL REMEDIATION PLAN Based upon the conclusions presented above, Ground Zero recommends adopting a site cap/natural attenuation approach for the Final Remediation Plan for the site. This alternative would involve no active remediation of gasoline hydrocarbons in soil beneath the site. Naturally occurring microbes which degrade gasoline hydrocarbons over time would be utilized to gradually reduce residual gasoline hydrocarbons in soil, and the site would be capped with sealed asphalt to limit percolation of surface water that could drive downward vertical migration of residual soil contamination. The laterally continuous silt layer present from a depth of approximately 38 to 42 feet is relatively impermeable and virtually no contamination has been detected below this depth. A laterally continuous clay layer between the depths of approximately 48 to 54 feet would act as an additional barrier to vertical migration of contaminants to the water table, which occurs at a depth of approximately 60-61 feet. Based upon the above information, drinking water supplies are not at risk from residual soil contamination remaining at the site. According to City of Escalon personnel, the nearest water supply well is located approximately 1,200 feet south (generally upgradient) of the subject site. A water supply well has reportedly been recently installed approximately 3,000 feet northwest (downgradient) of the subject site. Proposed Remediation Activities Activities under this plan would include: 0.%Dea OROUNDZMgm lMRepaMIVLrIRTT.doc