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- <br /> above detection limits, and even in this sample, concentrations — <br /> were orders of magnitude below product saturation. Ground-water <br /> concentrations, while more significant than those in the soils, <br /> are still, even in MW6, the most contaminated well, below levels <br /> found in contact with free product. This is an indication that <br /> free product gasoline never existed on this site, and that the <br /> source of most of the groundwater contamination may be off-site. <br /> Examination of chemical data can provide insights into the trans- <br /> port mechanisms operating at a site. Ground-water transport <br /> results in relative enrichment in the more soluble, less hydro- <br /> phobic hydrocarbon compounds (those on the right hand side of <br /> Figure 3 ) as a function of distance from a spill. Thus, contam- <br /> inated ground water samples collected at a distance hydraulically <br /> downgradient of a bulk free product phase will typically contain — <br /> greater relative concentrations of these less hydrophobic, more <br /> soluble compounds than a sample collected more immediately down- <br /> gradient of the free product. <br /> The process can be described mathematically as follows, beginning <br /> with the one-dimensional advective dispersive ground-water trans- <br /> port equation of Bear ( 1972 ) : <br /> aC = D 32C - vaC + p/n aS <br /> at ax2 ax at ( 1 ) <br /> where <br /> C = concentration of compound in water <br /> t = time <br /> D = dispersion coefficient — <br /> v = ground-water velocity <br /> x = distance of travel — <br /> p = density <br /> n = porosity — <br /> S = concentration of compound absorbed to soil. <br /> 30 <br />