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March 18, 1993 <br /> Page 9 <br /> Considering the data presented and other pertinent information, PACIFIC <br /> recommends application of Alternative 1. The recommended alternative is <br /> detailed below. <br /> The Recommended Alternative <br /> The recommended alternative (Alternative 1) consists of three major components: <br /> soil vapor extraction and treatment, completion of residual plume delineation and <br /> groundwater monitoring. Soil vapor extraction and treatment and residual plume <br /> delineation are discussed in the sections below. Groundwater monitoring is not <br /> discussed because a program has previously been established. <br /> Soil Vapor Extraction and Treatment <br /> Removal of petroleum hydrocarbons will be accomplished by circulating ambient <br /> air through the subsurface environment. Air flow will promote volatilization of <br /> liquid-phase, dissolved, and adsorbed petroleum hydrocarbons. Vapor-phase <br /> hydrocarbons will be removed with the air stream. Extracted petroleum bydrocar- <br /> bons will be reduced or recovered as part of soil vapor treatment before atmo- <br /> spheric discharge. <br /> Extracted soil vapor will be treated using either an internal combustion engine, <br /> catalytic thermal oxidation, or vapor-phase carbon adsorption. Treatment method <br /> specification will be based upon initial concentration and modeled concentration <br /> decay. If soil vapor concentrations are relatively high,use of an internal <br /> combustion engine will probably be recommended. If vapor concentrations are <br /> moderate,use of catalytic thermal oxidation will be suggested. Vapor-phase <br /> carbon adsorption would be recommended for low concentrations. In some <br /> instances,it may be economically prudent to begin with an internal combustion <br /> engine or with catalytic thermal oxidation and finish with vapor-phase carbon <br /> adsorption. An Authority to Construct and Permit to Operate from the governing <br /> Air Pollution Control District will be necessary for use of either treatment <br /> technology. <br /> Termination of soil vapor extraction will be considered after soil vapor hydrocar- <br /> bon concentration drops to 10 percent of the maximum concentration exhibited <br /> after sparging has been initiated. This guideline is specified because it is the point <br /> where further operation is not incrementally beneficial. <br /> 3100104/1RAP <br />