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0 0 <br /> Remedial Investigation Report -5- 8 March 1991 <br /> DDRW, Sharpe <br /> It also appears that the northeastern edge of Plume 1 and the southwestern edge <br /> of Plume 2 may have migrated into each other, assuming that Plume 2 represents a <br /> separate source area. However, there are no monitor wells between the leading <br /> edges of these plumes to confirm or deny that these are separate plumes at this <br /> location. In addition, the Report states that no waste disposal has been reported <br /> and that no soil contamination has been detected in the area of Plume 2. <br /> FINDING <br /> 10. Vertical migration of TCE was observed to have occurred at SHAD in the areas of <br /> known contaminant sources. Horizontal movement in the upper three water-bearing <br /> zones is evident. Downward movement of contaminants appears to be limited by <br /> unfavorable hydraulic gradients under natural conditions. Distribution of <br /> contaminants in the lower zones of the aquifer system is primarily influenced by <br /> on-site and off-site pumping of water supply wells. <br /> RESPONSE <br /> 10. I concur with portions of this finding, however this finding is unclear. I concur <br /> that the distribution of the contaminants in the lower zones is influenced by the <br /> water supply well pumpage. However, it is unclear what "unfavorable hydraulic <br /> gradients under natural conditions" means. The term natural conditions" is <br /> confusing because on- and off-site pumpage, which appears to influence the <br /> downward migration of contaminants, is not a natural condition but rather is an <br /> induced hydraulic effect on the system. If the implication is that upward <br /> vertical gradients in some areas of the site limit the downward migration of <br /> contaminants, then this would not be an unfavorable condition. <br /> Based on the hydraulic head difference distributions in well clusters, as shown <br /> on Figures 5.4-29 to 5.4-40, there appears to be a strong downward vertical <br /> gradient between the upper and lower water bearing zones year round, across most <br /> of the site. These figures reveal that downward vertical gradients are increased <br /> during seasonal periods when agricultural pumpage occurs. These figures also <br /> indicate that even without the influence of this pumpage, the downward migration <br /> of contaminants would still occur under the existing ground water flow conditions. <br /> Therefore, it is unclear why the downward movement of contaminants is limited. <br /> FINDING <br /> 11. Although TCE is the dominant VOC at SHAD, one well cluster (413) exhibits higher <br /> concentrations of another analyte (TCLEE) . <br /> RESPONSE <br /> 11. Finding is noted. However, the text provides little discussion on the reasons for <br /> the elevated concentration of PCE in monitor well 413. It is known that TCE is <br /> a degradation product of PCE. The ratios of the concentrations of TCE to other <br /> VOC degradation products is not discussed in text. These concentration ratios may <br /> be used to estimate the relative ages of the contaminant plumes. <br /> In addition, the industry standard acronym for tetrachloroethene is PCE which is <br /> not used in the Report. The use of TCLEE is confusing. <br />