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Extraction Well Memorandum _ <br /> DDRW, Sharpe Site -3 17 December 1991 <br /> Page 2-15. One extraction well , NC-1, is planned for installation in the C Zone. <br /> This well was placed midway between MW-517C and MW-438C to capture the <br /> plume as it migrates northwesterly. Although the rationale for placement <br /> of this well is sound, Sharpe should consider moving the well slightly to <br /> the southeast more toward the center of the plume where the VOC <br /> concentrations may be higher. If possible this well could be combined <br /> with NB-4, depending on the lithology. <br /> Page 3-1 . Figure 3-1 presents the well specifications for the proposed extraction <br /> wells. The Figure depicts the extraction wells with ten foot screen <br /> lengths. The extraction wells should not be installed with standardized <br /> screen lengths but should be installed on the basis of the lithology and <br /> VOC concentrations from nearby monitor well clusters, with the objective <br /> to capture contaminated ground water from across the entire targeted zone. <br /> Page 3-4. The Work Plan indicates that the extraction wells will be installed using <br /> 0.020 inch wide slots and graded silica sand for the filter pack. The <br /> Plan does not state what type of filter pack. The potential problem with <br /> this size slot and accompanying filter pack (commonly Lone Star #3) is <br /> that the filter pack and slot size may be restrictive, that is, having a <br /> median grain size smaller than the surrounding sediments. If the filter <br /> pack and slots are restrictive, the well losses will be increased, causing <br /> the extraction well to be less efficient. <br /> The extraction wells should be designed for maximum efficiency like would <br /> be wanted with a water supply well , and as opposed to a monitor well where <br /> efficiency is usually not a design concern. Therefore, Sharpe needs to <br /> consider collecting sediment samples from the target zones for sieve <br /> analysis to properly size the filter pack and slot size. In order to <br /> optimize the performance of the extraction system, well efficiency should <br /> have been considered in the design of the North Balloon extraction wells. <br /> Well efficiency must be considered for the design of the extraction system <br /> in the Central Area where the off-site contamination is of much greater <br /> extent. <br /> In addition, the extraction wells are to be installed with a one-inch <br /> diameter sounding tube in the annular space. With this well construction, <br /> water levels from the annular space and inside the casing can be used to <br /> determine if well losses are excessive using the particular filter pack <br /> and slot size at a specific target zone of known transmissivity. <br /> Page 3-4. The Work Plan indicates that the filter pack will be placed a minimum of <br /> five feet above the screened interval and that a minimum of five feet of <br /> bentonite will be placed over the filter pack. In general , a minimum of <br /> five feet for the filter pack and bentonite is excessive whereas, two to <br /> three feet should suffice. <br /> Page 3-5. The Work Plan indicates that sand is to be added to the cement slurry. <br /> Generally, sand is added to a cement slurry for additional durability and <br /> strength. It is unknown why sand is proposed because additional <br /> durability and strength are not needed for sealing the annular space of <br /> monitor wells. The use of sand in a cement slurry implies that it will be <br /> obtained from a ready-mix plant rather than being prepared on-site. <br />