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contamination as the dissolved plume usually impacts <br /> water supply wells or groundwater discharge points <br /> (streams, spring&). The dissolved phase typically <br /> increases with time as the free product and adsorbed. - <br /> phases continues to leach contaminants into the 7r6und- <br /> M. water system. _�_.. . <br /> The vapor phase of contamination results from the <br /> volatile portions of the released product coming out of <br /> solution and concentrating in the subsurface. The <br /> magnitude of the vapor plume depends on the volume . <br /> Of loss, type of contaminants, and the site geology. <br /> " Impacts from the vapor phase depend on the pressure <br /> differential between the subsurface and the below <br /> ground structures (i.e. , basements, foundations, <br /> _ underground utilities). This means that vapors tend <br /> ` to migrate from high pressure areas to low pressure . <br /> areas. Depending on the time of year and constuction <br /> t of belowround or at <br /> g grade structures, the subsurface <br /> may be high `pressure zone and basements or foundation <br /> areas may become the low pressure , :ones. This phase <br /> needs to be addressed in every investigation as the <br /> '•= exposure to the public may be the greatest as vapors <br /> i� tend to migrate further and faster than the other three <br /> s phases of contamination. <br /> With this cognizance, when approaching an aquifer contamina- <br /> tion evaluation and clean up, it is necessary to both be aware of <br /> the various phases an knowledgeable in the amount of impact they <br /> contribute. See Figure 6 for schematic of four phases of <br /> contaminants. <br /> It <br /> 11 <br />