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depth of the screened intervals (5 feet bgs), the area of the site impacted by <br /> chemicals of concern will be paved with a 2-inch-thick layer of asphalt. <br /> To maximize the efficiency of soil vapor extraction over time, it is <br /> anticipated that the extraction wells will be operated in a pulsed manner. <br /> This pulsed operation will involve utilizing extraction wells as both soil <br /> vapor extraction points, and passive air induction points. Existing soil <br /> vapor monitoring points and ground water monitoring wells may also be <br /> utilized as passive air induction points. The procedures and criteria for <br /> pulsed operation of extraction wells and monitoring points is described in <br /> Section 6 -Operation and Monitoring of the Remedial Action. <br /> VOC Mass in Vadose Zone Soil and the Expected Mass Removal Rate <br /> The mass of VOCs (including both HVOCs and AVOCs) in the vadose <br /> zone plume at the site is estimated to be approximately 2,000 pounds (lbs). <br /> The estimated VOC mass removal rate is expected to be 2 to 2.5 lbs per <br /> day for a design soil vapor extraction flow rate of approximately 75 scfm. <br /> This mass removal rate is estimated to be adequate to achieve vadose zone <br /> soil cleanup in 2 to 3 years. Calculations for the total mass of VOCs in <br /> vadose zone soil at the site, and mass removal rate, are contained in <br /> Appendix A. <br /> To estimate the VOC concentrations in extracted soil vapor, soil analytical <br /> data was reviewed, and the Jury Model ("Behavior Assessment Model for <br /> Trace Organics in Soil," Jury est al; J. Environ. Qual., Vol. 12, no. 4, 1983) <br /> was applied to estimate partitioning of VOCs between the soil phase and <br /> the vapor phase. The Jury Model takes into account soil properties <br /> including: <br /> • Soil bulk density and porosity; <br /> • Soil organic content; and, <br /> • Soil water content and air content. <br /> To estimate partitioning of VOCs from soil to the vapor phase, the Jury <br /> model also examines the following properties of each VOC constituent in <br /> soil: <br /> • Molecular weight; <br /> • Soil/water distribution coefficient; and, <br /> 4-3 <br />