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Ground Water FS Report -4- 16 September 1991 <br /> DDRW, Sharpe Site <br /> Figures 4.1-1 and 4. 1-3 (which are assumed to be reporting second quarter 1990 <br /> data) , the concentration of TCE at these wells is 0.0 mg/l (non detect) . In <br /> addition, the most recent quarterly monitoring report (second quarter 1991) , the <br /> concentrations of TCE in these wells are less than 0.5 ug/l for all VOCs and only <br /> 0.6 µg/l of TCE, respectively. These concentrations indicate that the aquifer <br /> is essentially uncontaminated in the areas of these wells. <br /> Therefore, the installation of two extraction wells near MW-505A and MW-407C would <br /> only exacerbate the extent of the VOC plumes and would not contribute to the <br /> effectiveness of the plume capture and ground water cleanup. Of particular <br /> concern, is the installation of an extraction well near MW-505A which would only <br /> pull the plume further off-site. I am opposed to the installation of extraction <br /> wells at these two locations. The extraction wells should be relocated to within <br /> the plume so that the zone of capture for the extraction well will include the <br /> leading edge of the plume. <br /> 7. The Draft FS Report indicates that the extraction system is common to all of the <br /> treatment alternatives except the No Action alternative. Diagrams of the <br /> extraction well field were presented, however diagrams of the extraction wells <br /> were not presented. Because of the importance of the extraction system, of both <br /> the well and field designs, the Draft FS Report should have included construction <br /> diagrams for the design of the extraction wells. These preliminary diagrams of <br /> the extraction wells are need as a prototype for the final remedial design and to <br /> specify the design criteria for the wells. <br /> 8. Section 5.3 discusses the design approach to be taken for the Central Area <br /> extraction system. As proposed in the Draft FS Report, extraction wells will be <br /> installed, aquifer tests performed on the wells followed by a decision to accept <br /> or reject the extraction well . Rejection of the well may include abandonment. <br /> I am opposed to the installation of a well only to abandon it soon after <br /> installation, due to the lack of adequate planning. I do not concur with Sharpe's <br /> approach to the design of the extraction wells and well field. I have previously <br /> discussed my concerns regarding this approach in our letter dated 13 March 1991. <br /> Rather than installing extraction wells using a "trial-and-error" method, which <br /> has not proved successful particularly in the North Balloon Area, Sharpe should <br /> consider a different conceptual approach to the design of the extraction wells. <br /> Below I have listed the sequential steps that should be taken in the design of the <br /> extraction wells and well field. <br /> a. Collect all existing data on hydrostratigraphy and aquifer parameters for the <br /> Central Area. Different sources, such as aquifer tests, specific capacity <br /> values, step draw-down tests, may be used to develop the parameters. <br /> Determine average aquifer parameters for the different water bearing zones in <br /> the Central Area. <br /> b. Develop and present a conceptual model for the Central Area on which the <br /> preliminary design should be based. The preliminary design should include <br /> estimates for well depths, screen interval location and length, filter pack <br /> and slot sizes. <br /> c. Use modeling to propose locations of extraction wells. Zones of capture for <br /> each well should overlap and should capture the leading edge of the plume in <br /> order to have complete plume capture. Verify proposed modeling locations by <br />