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July 13, 1993 • Page 2 Location of Petroleum Hydrocarbon Mass Using analytical results of soil concentration data collected to date, an iso-concentration plot was prepared to locate the area of lateral soil impact. The largest value of total petroleum hydrocarbons calculated as gasoline (THP-g) found in each boring was used in the preparation(Figure 2). PACIFIC recognizes the presence of substances resembling diesel in analytical data; however, SVE most effectively removes more volatile substances from the subsurface. It is expected that biodegradation, augmented by air flow from sparging and vapor extraction,will address compounds heavier than gasoline. As noted in PACIFIC's RAP, a majority of the hydrocarbon impact is located from 5 to 6.5 feet below grade. Mass Removal Strategy To recover vapors in the impacted area, horizontal piping will be placed 5 to 6 feet below grade. Placement of these pipes was designed such that the influence area would affect the impacted area. Figure 2 shows an SVE system layout which is based on achieving influence over the identified area. Data collected during the SVE test indicate vacuum extraction will generate an influence radius of approxi- mately 14 feet. Air will be delivered to the subsurface environment via sparging points located within the influenced area. Pressurized ambient air will be used as the sparge gas. Air spar ' gpomts will consist of 1 inch-diameter driven probes. Impact outside t e vapor extraction boundary will be affected by the local gradi- ent, caused by low pressure generated by vacuum extraction, diffusive transport mechanisms, and the influx of oxygen into the subsurface via air sparging and vacuum extraction. Extracted vapor will be treated using catalytic thermal oxida- tion. A process and instrumentation diagram is shown on Figure 3. In addition to the above, Monitoring Well U-6 will be converted to a vapor extrac- tion well. Though data did not indicate a significant hydrocarbon impact in the soils, this well is located where dissolved hydrocarbon concentrations have been greatest. The well will be used to induce air flow into the capillary fringe area and across the dissolved hydrocarbon plume. Soil and Groundwater Characterization Further delineation of petroleum hydrocarbons in soil and groundwater will be considered following the first remedial system performance evaluation. Based on Unocal's past experiences negotiating with Southern Pacific Transportation Company (SPTC) for encroachment, the SPTC agreements are generally unac- 3100105\WKPLMEMO