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1 • • <br /> Ground Water FS Report -3- 16 September 1991 <br /> DDRW, Sharpe Site <br /> environmental fate analysis, such as the Designated Methodology, so that the <br /> cleanup levels for soils will be ground water protective. <br /> 3. The data in the Draft FS Report indicate that the extent of the VOC plumes have <br /> not been adequately defined. Listed below are the highest concentrations (in <br /> µg/1 ) of trichloroethylene (TCE) or trichloroethane (TCA) in the deepest wells <br /> in a cluster for the plumes of concern. These concentrations indicate that the <br /> vertical extent of contamination has not been completely defined in each of the <br /> plumes. <br /> Plume Monitor well Concentration (ug/1 ) Reference <br /> 1 418C 73 of TCE Page 1-16 <br /> 4&5 504D 78 of 1, 1,1-TCA Page 1-61 <br /> 4&5 509C 70 of TCE Page 1-55 <br /> 7&8 421C 52 of TCE Page 1-75 <br /> Sharpe must install additional monitor wells either as part of the final FS or <br /> prior to the final ground water remedial design. The additional monitor wells <br /> will be used to define the lateral and vertical extent of the VOC plumes and to <br /> verify the effectiveness of the extraction system. This was an outstanding issue <br /> in our review of the Final RI Report. <br /> EXTRACTIDN SYSTEM <br /> 4. The Draft FS Report states that South Balloon extraction system is adequate for <br /> complete plume capture (page 5-6) . However, in the DDRW, Sharpe letter dated <br /> 1 May 1991, Sharpe states that the leading edge of the plume in the B Zone in the <br /> South Balloon Area is outside of the capture zone. Therefore, based on the 1991 <br /> data previously presented by Sharpe, we do not concur with the statement in the <br /> Report that the extraction system in the South Balloon Area is adequate. <br /> The Draft FS Report states in Sections 4. 1 and 5.2 that the North Balloon Area, <br /> capture of a portion of the plume is not occurring due to construction flaws with <br /> several of the extraction wells. The Report also states that if these wells were <br /> properly constructed, adequate plume capture would be achieved. The current in- <br /> place systems at both the North and South Balloon Areas were designed only as <br /> interim systems. Because complete plume capture has not been demonstrated, <br /> particularly in the North Balloon Area, the remedial design must be site-wide and <br /> must incorporate expansion of the existing systems so that there will be complete <br /> plume capture in all of the three ground water treatment areas. <br /> 5. Sections 5.2 and 5.3 of the Draft FS Report discuss the extraction systems for the <br /> North and South Balloon Areas and the Central Area. We have previously commented <br /> on the report of the North Balloon extraction system, by letter dated 5 December <br /> 1991, and on the Focused FS Report by letter dated 13 March 1991 . We commented <br /> that the location of the wells, depending on the design of the wells, should be <br /> reconsidered. Of particular concern, is that proposed locations of the extraction <br /> wells, in the Central Area, have not been changed. It appears that DDRW, Sharpe <br /> did not respond to our concerns as discussed in these letters. <br /> 6. The Draft FS Report states that two extraction wells will be installed near MW- <br /> 505A and MW-407C to recover contaminated ground water (page 4-15) . Based on <br />