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i <br /> December 30, 1994 • • - <br /> Page 5 <br /> capital outlay and a relatively short operation period (2 years). Alternative <br /> 2 requires a less intensive initial capital outlay but a greater relative <br /> operation period (5 years). Alternative 3 is associated with substantial <br /> initial capital outlay, and a relatively short operation period (1 year). The <br /> most economical alternative was Alternative 1, the least economical <br /> alternative was Alternative 2, and the economic variability was 27 percent. <br /> Considering the data presented, PACIFIC recommends application of Alternative 1. This <br /> recommendation is based on the probability that the implementation of Alternative 3 will <br /> provide little incremental benefit over the implementation of Alternative 1, while requiring a <br /> greater capital influx. Alternative 2 was discounted based on the extended remedial life span <br /> and resulting increase in cost. Primary elements of the recommended alternative are detailed <br /> below. <br /> Implementation <br /> Implementation of Alternative I consists of three major components: (1) groundwater <br /> monitoring (2) soil vapor extraction and treatment, and (3) air sparging. Groundwater <br /> monitoring is not discussed here because a program has been established. SVE and air <br /> sparging are addressed in the sections below. <br /> In order to recover soil vapor in the impacted area, 18 soil vapor extraction wells will be <br /> placed throughout the impact area and screened through the vadose zone to the piezometric <br /> surface. Placement of these wells will be designed s6ch that the influence area will affect the <br /> impacted area. Data collected during the SVE test indicate vacuum extraction will generate <br /> an influence radius of approximately 14 feet. <br /> In order to promote volatilization of petroleum hydrocarbons in the saturated zone and to <br /> increase the availability of oxygen for biodegradation, air will be delivered to the saturated <br /> zone via sparge points located within the influence area. Pressurized ambient air will be used <br /> as the sparge gas. Air sparging points will consist of 2 inch-diameter driven probes. Impact <br /> outside the vapor extraction boundary will be affected by the local gradient, caused by low <br /> pressure generated by vacuum extraction, diffusive transport mechanisms, and the influx of <br /> oxygen into the subsurface via air sparging and vacuum extraction. <br /> Extracted soil vapor will be treated using either an internal combustion engine, <br /> catalytic/thermal oxidation, or vapor-phase carbon adsorption, depending on the initial <br /> petroleum hydrocarbon concentrations. An.Authority to Construct and a Permit to Operate. <br /> from the San Joaquin Valley Unified Air Pollution Control District has previously been <br /> obtained for this site, but will require modifications. <br /> SVE and Air Sparging Termination <br /> As SVE and air sparging proceed, hydrocarbon concentrations in soil vapor decay. At a <br /> constant extraction flow rate, concentration decay,may be attributed to the decrease in <br /> 3 100011 b/wkpin <br />