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yUJOUD-J�4 UUn-tel <br /> 4.3.5.4.14 Concentration contours have also been plotted on all of the <br /> cross sections (Figures 3.4-2 through 3.4-6) drawn for DDRW Tracy. This <br /> information permits an evaluation of the three-dimensional distribution of <br /> contamination. The contours on these cross sections follow the maximum <br /> concentration of either TCE or PCE. Thus, if the concentration of TCE in <br /> one area exceeds that of PCE, the contours are constructed with the TCE <br /> data, while if the PCE concentrations exceed that of TCE, the contours are <br /> drawn based on the PCE data. This display is consistent with the fact that <br /> elevated levels of either contaminant will be subject to-remediation. <br /> Several assumptions and simplifications have been made in constructing the <br /> concentration contour maps, and these are discussed below: <br /> 4.3.5.4.14.1 First, unlike the contours on the plan view figures <br /> (Figures 4.3-1 through 4.3-6) , which show contaminant distribution <br /> within each lithologic unit, the contours on the cross sections have <br /> been drawn without regard to lithology. It is probable that lithology <br /> does influence concentrations, but;in the absence of data from many <br /> more locations, it is not possible to ;evaluate the detailed effect of <br /> lithology. As a result,-_)t was considered preferable to ignore the <br /> lithology during construction of the contours. These contours should, <br /> therefore, be considered to provide a general picture of the <br /> distribution of contaminants and the general direction in which they <br /> are moving, rather than detailed information of contaminant <br /> concentrations int hesubsurface at DDRW Tracy. <br /> 4.3.5.4.14.2. Most of the information on contaminant distribution from <br /> the subsurface -at DDRW Tracy comes from wells that are screened below <br /> the finer-grained unit which is present in the upper 20 feet at DDRW <br /> Tracy. As a result, there is only limited knowledge of the <br /> distribution of contaminants in the near surface. Contamination <br /> contours in the near surface have been drawn assuming, in the absence <br /> of information to the contrary, that contamination extends to the <br /> ground surface only within the contaminant source areas. Outside of <br /> the contaminant source areas, the top of contamination is assumed to be <br /> at approximately the top of the Upper Horizon. <br /> 4-184a <br />