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' in coarse-grained aquifer material such as that found at this site For this reason, <br /> molecular diffusion will not be considered in this evaluation For any given aquifer, <br /> mechanical dispersion is a function of groundwater velocity Dispersion perpendicular to <br /> the flow path is defined as follows <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 average linear 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 = V /R <br /> The retardation coefficient was estimated using the equation from Freeze and Cherry <br /> (1979) <br /> R = I+(p Kd/n) <br /> The bulk density for the sora in the unsaturated zone was assumed to be 16 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 /> 1 The results of the modeling are presented in Table A-1 <br /> The time to reach groundwater was estimated using Vc, calculated above, and the <br /> following relationship <br /> Vc = LIT <br /> where T = travel time (days) <br /> T was converted to years be dividing by 365 days/year, and is presented in Table A-1 <br /> 1 J ISECriPJ2120740010 IBR-9511Jt 2 D-3 <br />