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Mr. Maurice Benson <br /> January 13, 2010 <br /> Page 2 <br /> footprints to best remediate and monitor the system's effectiveness. The VEWs will be <br /> installed using pneumatic fracturing techniques designed to create preferential <br /> pathways in the unsaturated zone soils to enhance the effectiveness of the SVE <br /> system. To create the fractures in each of the five boreholes, compressed nitrogen will <br /> be injected using a directional fracturing tool that creates fractures 90 degrees from <br /> vertical. The fracture networks are planned to be initiated in the subsurface at three <br /> depths, resulting in planned fractures at 10, 12, and 14 feet below ground surface (bgs) <br /> for a total of 60 fractures. <br /> Planned construction activities include equipment mobilization, installation of <br /> conveyance supplies (i.e. water and electrical power), equipment installation, <br /> connection of SVE wells to aboveground manifold, pluming of above-grade vapor piping <br /> to vacuum blower, installation of control panel and synchronizing instrumentation to <br /> vacuum blower, and the mobilization of the trailer to the control location. <br /> Background <br /> In 1 Q02, TCE concentrations in MW437C located in the Central Area increased from <br /> 150 pg/L to 650 pg/L, indicating that a persistent source remained up-gradient in the <br /> vadose zone. Follow-up fieldwork efforts detected and delineated the vadose zone <br /> contamination located at the end of a former runway, previously closed as a No-Further <br /> Action site, named P-5A. Analytical data showed site P-5A groundwater contamination <br /> as high as 1,600 pg/L at 45 feet bgs and TCE in soil gas in excess of 8,300 parts per <br /> billion volume (ppbv). Characterization data confirmed that former site P-5A is a <br /> persistent source of TCE contamination to groundwater with approximately 270 pounds <br /> of contaminant mass estimated to remain in source area soil and groundwater. <br /> DTSC has reviewed the report and has the following comments: <br /> General Comment <br /> 1.) The pneumatic fracturing techniques for the SVE system used at DDJC-Tracy <br /> Area 1 Building 237 provided favorable results in efficiency and mass removal by <br /> reducing the necessary optimization efforts in order to reach the cleanup goals <br /> and are anticipated to have similar successful results at DDJC-Sharpe. <br />