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Ground Water FS Report -5- 16 September 1991 <br /> DDRW, Sharpe Site <br /> using current water quality data to determine if well locations are optimal . <br /> More than one extraction well (or well cluster) may be needed in the areas of <br /> highest contaminant concentration. <br /> d. Install pilot holes in the areas in which the extraction wells are proposed <br /> to confirm the stratigraphy and to collect lithologic samples for design of <br /> the filter pack and slot sizes. <br /> e. Modify the well and field designs based on the field data. <br /> f. Install the extraction wells using the modified well design. <br /> g. Perform pump and aquifer tests on the extraction well to determine well yield <br /> and additional hydraulic information on the aquifer. <br /> h. The final design or "fine tuning" of the extraction well field should be <br /> based on modeling and on actual field data collected during the initial <br /> period of the extraction well field performance. <br /> 9. Figure 5.3-3 of the Draft FS Report presents the proposed extraction well <br /> locations for the C Zone in the Plumes 4&5 area. As depicted on the figure, an <br /> extraction well is proposed to the south of MW-524C and northwest of MW-508C. <br /> This extraction well has apparently been proposed based on a plume which contains <br /> greater then 5 µg/l but less than 50 ug/l of TCE. I do not concur with the <br /> proposal and am opposed to the installation of an extraction well at this location <br /> for the following reasons. <br /> a. There are no data to support the presence of a plume at this location. <br /> Figure 1.3-25 (page 1-55) of the Draft FS Report shows that MW-524C and MW- <br /> 508C as having 0.0 µg/l (non-detect) of TCE in the well samples. <br /> b. Installation of an extraction well at this location may serve to only <br /> exacerbate the extent of the plume as the area of the proposed extraction <br /> well is off-site and is uncontaminated. <br /> 10. I am concerned that the extraction, systems for the North and South Balloon Areas <br /> have been installed and the Central Area extraction is being planned using data <br /> that has not been evaluated to determine seasonal effects in the hydraulic system. <br /> It is unknown whether the North and South Balloon extraction systems are being <br /> operated to counteract the seasonal variations which are known to occur due to <br /> off-site agricultural pumpage. As a seasonal data analysis has only begun this <br /> year, it may be determined that the currently employed extraction well pumping <br /> cycles may need to be changed to optimize the effectiveness of the system. <br /> I am also concerned that once the current drought ends and precipitation and <br /> pumping patterns return to "normal ", that there may be variations in the ground <br /> water flow directions and ultimately the plume migration. Based on these two <br /> factors, the final remedial design for the North and South Balloon Areas and the <br /> Central Area may need to be changed from what is presently conceptualized in the <br /> RI/FS process. <br /> 11 . It appears that the existing data from the South Balloon Area extraction system <br /> is not being used. Because this system has been in-place since 1987 and because <br />