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and percolation from the pit area. By contrast , well p7 <br /> represents a calcium, magnesium, bi -carbonate chloride water type <br /> which we suspect is typical of the general ground water in the <br /> upper aquifers. <br /> The water-quality data were also plotted on a trilinear plot <br /> basis , to better address the relative percentaqe impact of pond <br /> j seepage at well 5. As shown in Figure 2 , the wells all cluster <br /> ( relatively close together, and at a plotting position remote from <br /> the plotting position of the storm pond (shown by the symbol SP). <br /> This appears to indicate that there has been relatively little <br /> pond- water infiltration and subsequent modification of ground- <br /> water types. <br /> GeochemicaI attenuation has played a role in the migration of <br /> contaminants associated with percolation from the retort pit and <br /> the storm pond. Such attenuation is now well understood, as is <br /> discussed in the attached Appendix. It is known that many metals <br /> are attenuated by the ground water environment , while other <br /> materials are generally conservative in the ground water. <br /> Hexavalant chromium is one of these more conservative <br /> contaminants, whereas tri valant chromium, arsenic and copper are <br /> not mobile in the ground water environment. Any evaluation of <br /> the ground water data associated with the Marley facility must be <br /> conducted in cognizance of such geochemical attenuation. <br /> �- As stated above, it appears that the existing data are sufficient <br /> Lto confirm the existence of a ground water contamination <br /> 12 <br />