SITE INFORMATION AND CORRESPONDENCE_1992

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Antonia K.J. Vorster -2- 16 April 1992 Camilla Williams 3. Removal rates of TCE from soils were calculated assuming an additive effect from all vapor extraction wells. This assumption is probably an over simplification and does not consider the effects of zones of stagnation from competing vapor extraction wells. 4. Based on the analytical results from the vapor extraction well air samples, concentrations of TCE above 1,000,000 p g/m' were detected prior to treatment. These elevated concentrations may indicate the presence of neat liquid (Nonaqueous phase liquids or NAPLs) in the soil column. If this is the case, the estimated mass of VOCs in the soil column may be underestimated and the length of time for soil remediation may increase significantly over original estimates. 5. The report failed to discuss impacts associated with shallow ground water containing volatile organic compounds (VOCs) on the operation of the SVE system. The impact of contaminated ground water on the SVE program needs to be investigated further. If shallow ground water contains VOCs, the concentration of chemicals in the ground water must be considered. This will probably impact the design of the soil remediation system because the total mass of contaminants in the vadose zone and ground water is difficult to estimate and the time it will take to remediate the vadose zone is difficult to predict. Also, there are additional design considerations if ground water is being extracted through a pump and treat system in conjunction with SVE because the vertical extent of the vadose zone needing remediation will increase as the water table drops. 6. During remediation or future pilot studies, I recommend that Sharpe monitor vapor flow, vapor pressure, vapor chemical concentration and composition, ground water elevation, and in-situ soil gas vapor concentration and composition. These parameters must be monitored so that the SVE system design can be optimized. These data also are needed during operation of the SVE system to allow the operator to determine if or when the system operation (flow rates, pulsing, etc.) should be varied or when ultimate vadose zone cleanup levels are close to being met. 7. ESE performed an analysis of offgas treatment alternatives and recommended thermal oxidation based mainly on costs. In my opinion, the estimated costs for carbon adsorption and thermal oxidation ($260,000 and $222,785 per year, respectively) are essentially the same. Therefore, I recommend carbon adsorption because solvents can be recycled. Also, carbon adsorption does not produce air emissions such as HCI and CO2, that are produced during the thermal oxidation process. DOUBLE RING INFILTROMETER (DRI) TESTS The DRI tests were conducted in accordance with the work outlined in the original work plan. ESE concluded that although soil flushing will remove TCE in the soil column, it is not as efficient nor as responsive as vapor extraction for the in-situ remediation of contaminated soils. ESE ruled out the use of soil flushing as an in- situ remediation technology at Sharpe.