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The ratios of benzene concentrations in the water to xylene and <br /> ethylbenzene increase with time and distance from the source of <br /> .. contamination, because of the greater relative solubility of <br /> benzene. Thus, benzene will become relatively enriched in a <br /> downgradient direct as less soluble compounds are retarded <br /> ( Hinchee and Reisinger 1987 ) . In the investigated case the ratio <br /> of benzene to xylene and ethylbenzene in the well MW8 is 0. 02 <br /> (Table 2 ) , and in the well MW10 is 2. 06 ( i.e. , 100 times ) , lower, <br /> suggesting that fuel release occurred upgradient, west of the <br /> Exxon site. Figure 5 shows ratios of benzene to xylene and <br /> ethylbenzene found in the area of investigation. <br /> Ground-water flow and contaminant dispersion from monitoring well <br /> MW8 to monitoring well MW6 were modeled using RESSQ and the fol- <br /> lowing defined and assumed parameters: <br /> ` flow velocity = 6 . 10 m/yr. <br /> direction of flow = 450 <br /> thickness of aquifer = 6 m <br /> porosity = 30 percent <br /> initial aquifer concentration = 0 . 0 mg/L <br /> default injection concentration = 1 . 0 mg/L <br /> adsorption capacity of rocks = 0 . 0 percent <br /> injection rate = 0. 05 m3/hour = 0 . 2 gals/min <br /> pumping rate = 0. 05 m3/hour = 0 . 2 gals/min <br /> r <br /> The results of the model suggest that in 7 . 5 years the contam- <br /> inant plume originating at MW8 would be detected at MW6 at 1 . 4 <br /> percent of its original concentration. In 12 years, the contam- <br /> inant plume would be detected at MW6 at 50 percent of its origi- <br /> nal concentration. These results suggest that an upgradient <br /> source could contribute a significant amount to the contamination <br /> detected at the site. <br /> e20/73942 9 <br />