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moving, as described, explain how spill constituents will often <br /> be observed in ground water in ratios that differ as a function <br /> of distance from the spill (Hinchee and Reisinger 1987 ) . <br /> In general, the following ordering of migration rates is _ <br /> observed. <br /> Most Mobile Least Mobile <br /> Benzene > Toluene > Xylenes, Ethylbenzene <br /> To illustrate the point, ground-water data from a similar site <br /> ( Site X) with a known source is shown below (concentrations in <br /> mg/L) : <br /> Benzene to <br /> Xylenes <br /> and _ <br /> Xylenes/ Ethyl- <br /> Ethyl- benzene <br /> Benzene Toluene benzene Ratio <br /> Site X <br /> Ground water 20 ft <br /> downgradient of _ <br /> the source 0. 3 0 . 35 1 . 3 0 . 23 <br /> Site X <br /> Ground water 200 ft <br /> downgradient of <br /> the source 0 .41 0.88 0 . 11 3. 7 <br /> Site X <br /> Ground water 300 ft <br /> downgradient of <br /> the source 0. 42 ND 0 . 06 7 . 0 <br /> As shown, ground water in close proximity to product or product- <br /> contaminated soils will typically exhibit higher relative levels <br /> of the heavier, more hydrophobic gasoline constituents. This _ <br /> will result in something of a chemical gradient, with a relative <br /> increase in the more-mobile compounds with increasing distance <br /> from the source and a relative decrease in the less-mobile <br /> compounds with increasing distance from the source. The ratio of <br /> 32 <br />