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x s " <br /> 4. <br /> 4. RISKS ASSOCIATED WITH CONDITIONS AT THE SITE AND <br /> WI'1'1I' HE' PROPOSED REAII,DIAI,AurERNATIVES <br /> 4.1 RISK ASSOCIATED WITIIS ITF CONDITIONS <br /> The risk associated with the surface conditions ai the site is minimal. As Table I shows,the soils <br /> that have been impacted by petroleum hydrocarbons are a minimum of 16 feet below ground <br /> surface. Fifteen to 20 feet of impermeable silty/clay sediments Ile above the impacted soils, <br /> making diffusion of hydrocarbon vapors through th,surface soils very slow. Operation of the <br /> proposed soil vapor extraction system will reverse the upward migration of hydrocarbon vapors, <br /> forcing them back down into tlne subsurface, through the vapor extraction wells to the treatment <br /> systern. <br /> The risk posed by the glume of dist,alved hydrocarbons is minimal at this time. To date only <br /> shallow low-quality groundwater that is not used as a local water supply has been impacted by <br /> petroleum hydrocarbons,and there are no downgradient water wells within two miles of the site. <br /> 4.2 RISK ASSOCIATED WITH THE'PROPOSED REMEDIAL <br /> AI;fERNATIVES <br /> The risk associated with the proposed remedial :illernatives is also min;nial. Any free-phase <br /> gasoline recovered from the recovery wells will be separated from the water containing the dis- <br /> solved hydrocarbons and field in a temporary storage tank. The recovered groundwater will be <br /> treated with an air stripper, follow,-(l by an activated carbon polish. The treated water will con- <br /> tain no detectable concentrations of hydrocarbons,making the risk s!ssaciated with discharging it <br /> to the sanitary sewer negligible. The effluent vapors from the air stripper and the vapors from <br /> the extraction wells will be treated with a catalytic oxidizer. This treatment system,as mandated <br /> by the San Joaquin County Air Pollution Control District,will have a minimum destruction effi- <br /> ciency of 95 percent. The risk posed by the treatment system will be much less than that posed <br /> by the op^,rating service station located across Pershing Avenue. All treatment equipment used <br /> at the site will be secondarily contained and equipped with safety shut-offs to shut the entire <br /> extraction and treatment system clown in the event of a malfunction. <br /> r <br /> ` S <br /> i s <br /> armaz,. Q <br /> i <br />