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locations of these wells were selected on the basis of kriging analysis <br />of monitoring well data and are intended for plume definition. Well <br />420 A has shown higher levels of PCE than of TCE although the TCE <br />concentration in well 420 B was generally greater than that of PCE. <br />Wells 413 A, B, and C generally mirror .the results from wells 420 A <br />and B with TCE higher in the B zone than the A zone, and PCE <br />generally higher than TCE. <br />■ One well cluster (PW -12 ABC) is to be located offsite, upgradient and <br />south of well cluster 504, where very high levels of TCE were <br />detected in the C zone but not in the B or D zones. The purpose of <br />this well, which was sited on the basis of kriging- analysis, is for <br />plume definition. <br />Tetra Tech proposes the following additional recommendations regarding groundwater <br />characterization: <br />31. Extraction wells should be sampled quarterly, along with monitoring wells. <br />32. Water levels should be measured monthly in all monitoring and extraction wells. <br />33. The effects on contaminant transport of SHAD domestic well pumping rates should be <br />determined. For example, water levels in adjacent wells could be continuously <br />recorded to evaluate the impacts of this pumping on performance of the North <br />Balloon extraction system and on downward migration of contaminants. <br />12 <br />