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4. A plume appears to exist in the area between Monitoring Wells 441A and 442A. Low <br />levels of TCE contamination observed in Well 442 B indicate that this plume (if it <br />exists) may have a downward component. However, there are no C zone wells in the <br />region of this potential plume. Pumping at Extraction Well A-2 would increase <br />spreading of this plume, and would help move contaminants downgradient away from <br />the probable source (in the vicinity of the hazardous waste storage facility), but <br />would not capture the plume. How can SHAD be confident that Extraction Well A-2 <br />is not worsening the problem in this area? Is Well PW4A in Figure 11.0-1 correctly <br />located or should it be shown further to the south? <br />5. A monitoring well (PW4A) is proposed downgradient of a soil gas peak area located <br />near the site of a buried helicopter (Grid Location E-60) according to Figure 11.0-1. <br />Is this well correctly plotted? Why isn't this well located nearer to the source, as <br />indicated by soil gas data? <br />6. A large area in the central part of the base, <br />shown <br />in Figure 6.1.5-2 as <br />two <br />connected plumes, should be further <br />evaluated in <br />terms <br />of hydraulic gradients <br />and <br />probable source areas. There does <br />not appear <br />'to be <br />a basis for relating <br />the <br />contaminants in Monitoring Well 422A <br />which may <br />have an <br />origin in the vicinity <br />of <br />the hazardous waste storage area (see <br />preceding <br />comment <br />#4), to contamination <br />in <br />Monitoring Well 407A, which appears <br />to be located <br />near <br />to its potential source west <br />of the runway. <br />There appear to be at least two separate source areas for the contaminants observed <br />in Wells 442A and 443A. The bulk of the contamination appears to originate in the <br />vicinity of Buildings 404 and 408, while additional source areas appear to be located <br />to the south, southwest, and west of these buildings. <br />It appears that the contamination in the area surrounding Buildings 404 and 408 is <br />confined to the A zone. However, there are no existing or proposed wells down - <br />gradient of these source areas to determine the areal or vertical extent of these <br />plumes (proposed Wells PW6A and B are probably too far downgradient to monitor the <br />downgradient extent of the Building 404/408 plume). SHAD should address these data <br />gaps. <br />U <br />