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Remedial Investigation Report <br /> 6_ 8 March 1991 <br /> DDRW, Sharpe <br /> FINDING <br /> 12. The most significant source of ground water VOC contamination in the North Balloon <br /> Area is lume from the <br /> extends offpo located lnear well cluster 439. hC <br /> at low concentrations tothefarthest dow gradiNorth Balloon <br /> ent well cluster <br /> (505) . <br /> RESPONSE <br /> 12. Finding is noted. However, the Figures in the Report (6.1-64 and 6.1-67b) do not <br /> support the finding that the TCE plume extends off-site to monitor well cluster <br /> 505. In addition, the most recent ground water quarterly monitoring report <br /> (October to December 1990) also indicates that the contaminant plume does not <br /> extend to this well cluster. This quarterly report also indicates that the TCE <br /> concentrations exceeds the Primary Drinking Water Standard (DWS) of 5 µg/l in <br /> monitor well 439-CD (7.9 Ag/l) . Therefore, the plume is deeper than is depicted <br /> on Figure 6.1-67b. <br /> FINDING <br /> 13. Fluctuations of large magnitude in contaminant concentrations in North Balloon <br /> Area wells are probably due to the presence of a small , high-concentration plume <br /> moving back and forth in response to varying pumpage from the SHAD potable wells. <br /> Concentrations in the most contaminated portion of this plume seem to be <br /> increasing. <br /> RESPONSE <br /> 13. In general , I concur with this finding. It is reasonable to assume that the <br /> contaminant concentrations may vary in response to fluctuations in the ground <br /> water flow system caused by pumpage from supply wells. However, the Report does <br /> not provide adequate data to support this conclusion. The estimates of pumpage <br /> were not presented in the Report for any of the major supply wells. Therefore, <br /> fluctuations in pumpage cannot be compared to changes in the contaminant <br /> concentrations. <br /> In addition, the finding does not attribute the changes in the contaminant <br /> concentrations (specifically TCE, since this was the only constituent for which <br /> graphics were prepared) to the pumpage of off-site agricultural supply wells. <br /> This pumpage occurs on a seasonal basis, thus the ground water flow direction and <br /> gradient may change seasonally. Because the ground water flow maps were <br /> constructed using monthly averaged ground water elevations from 1987 to 1989, it <br /> is impossible to assess potential seasonal fluctuations in the ground water flow <br /> regime and thus, the changes in contaminant concentrations. <br /> This finding does not attribute the increasing TCE concentrations to influences <br /> from infiltrating rainfall (as it is assumed to influence concentrations in other <br /> areas of the site) . The increase in concentrations deeper and further out from <br /> the source was not attributed to the influence from on- and off-site supply wells. <br /> FINDING <br /> 14. areas, especiiallysinrthe South lBallo n ocated Area,alhave ly n formerear diTCE sposal <br /> (source) <br /> thepastto <br />