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I <br /> Dt = at V <br /> where at = the transverse dispersivity of the aquifer(meters) <br /> V = the average vertical infiltration velocity (m/day) <br /> The transverse dispersivity is usually about 5 to 20 times smaller than the longitudinal <br /> dispersivity (Freeze and Cherry, 1979). For this evaluation, the transverse dispersivity <br /> was assumed to be one-tenth the longitudinal dispersivity. Because the value for <br /> longitudinal dispersivity can be approximated by one-tenth the travel distance (Y) (Pickens <br /> and Grisak, 1981), at will be calculated as 0.01 V. <br /> The average infiltration velocity (V) was estimated using the average precipitation rate <br /> (i.e., 14 inches per year) and the assumption that 2 percent'of this value will leak through <br /> the pavement, <br /> Velocity of Dissolved Chemical (Vc). The averagelinear velocity of a dissolved <br /> chemical is usually slower than the average linear velocity of the water in which it is <br /> dissolved because the chemical may be retarded by absorption onto aquifer material. The <br /> velocity of the chemical is described as the ratio of the average velocity of the infiltrating <br /> water (V) to the retardation coefficient (R): <br /> Vc = VJR <br /> The retardation coefficient was estimated using the equation from Freeze and Cherry <br /> (1979): <br /> R = 1+(p • Kd/n) <br /> where: p=bulk soil density (grams per cubic centimeter [g/cm3]) <br /> Kd= soil/water partition coefficient(ml/g) <br /> The bulk density for the soil in the unsaturated zone was assumed to be 1.6 g/cm3, a value <br /> typical of sands. The information from borings MW-1 and B-1 (EMCON, December <br /> 1994) shows the unsaturated zone beneath the former sump to be composed primarily of <br /> sands and silty sands. <br /> The results of the modeling are presented in Tables A-1 through A-4. <br /> Estimating the Concentration and Travel Time for Each Chemical <br /> The results of the modeling are presented in Tables A-1 k through A-4. Because the <br /> horizontal extent of the chemical impact (X) has not been determined at this site, a <br /> sensitivity study was conducted for this parameter by calculating CL using values of 1, 5, <br /> J:1SEC71PJ2120740010.1 BR-95Vjt:2 A-3 <br />