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4.0 DATA EVALUATION AND DISCUSSION 4_1 Petroleum Hydrocarbons in Soil Data obtained from the soil borings provides reasonable delineation of TPHg within vadose zone soil beneath the site. Based on the UST removal and soil boring analytical data, the vadose zone plume exists primarily beneath the former gasoline and diesel USTs, extending vertically to the groundwater surface. The vadose zone soil plume is generally bounded by borings MW-3, SB-2 and SB-3 with an associated plume volume of approximately 1,500 cubic yards. However, because the exact location of the central portion of the sol p um een identified, the maximum concentration of impacted soil and the existing mass of TPHg cannot be estimate On the basis that the central portion of the vadose zone plume is generally located between borings VW-1 and VW-2,the assumed plume boundaries are approximated on Figures 3 and 6. 4.2 Vapor Extraction Pilot Test Results of the VET indicated that an effective ROI of approximately 35 feet can be expected at an extraction flowrate of approximately 50 scfm and a vacuum of approximately 50 inches H2O. This ROI estimate is based on a minimum required vacuum of approximately 0.25 to 0.50 inches H2O necessary for effective vadose zone hydrocarbon volatilization. The vacuum responses observed in wells MW-1 and VW-1 (0.3 to 0.7 inches of H2O)are considered adequate for effective remediation. Based on an effective ROI of 35 feet at a per well flowrate of 50 scfm, using wells VW-1 and VW-2 as the vapor extraction wells should provide sufficient vacuum influence for the entire on-site vadose zone soil plume. A vapor sample collected during the VET was analyzed for TPHg and BTEX in order to evaluate the expected mass removal rate for a soil VES. Calculations indicate that at an extraction rate of 100 scfm, approximately 900 pounds per day (ppd) of TPHg could be extracted from wells VW-1 and VW-2 at the time of VES start-up. Based on the expected start-up TPHg vapor concentrations in excess of 100,000 mg/m3, a thermal oxidizer (therm-ox) will provide for the most economic treatment and atmospheric discharge of extracted soil gas. At VES start-up, the maximum daily extraction rate will be limited by the maximum TPHg concentration that the selected therm-ox can process. Typical maximum process concentrations range from approximately 7,500 ppm to 10,000 ppm(33,400 mg/m3 to 44,600 mg/m3)and are manufacturer specific. Because the expected TPHg concentration in soil gas at the time of VES start-up will exceed the maximum process capacity of a therm-ox, extracted soil gas will initially require atmospheric dilution to reduce the influent concentration to approximately 7,500 ppm to 10,000 ppm. Therefore, the total system process capacity for a 100 scfm VES (assuming therm-ox inlet TPHg concentrations of 33,000 mg/m3) is expected to be approximately 300 ppd. Project No.S8100-06-1 I -5- December 17,1996