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I <br /> t <br /> Ir <br /> Ip <br /> t. <br /> t <br /> :i <br /> i <br /> SOIL MODELING <br /> VHS Model <br /> g <br /> The Vertical and Horizontal Spread (VHS) modelwas selected to estimate the <br /> concentration of chemicals dissolved in soil solution just'iabove the water table for this <br /> site. The VHS model, described b Dominico and Palciauskas 1 <br /> y ( 982), is recommended by <br /> the U.S. Environmental Protection Agency (USEPA) 'as a conservative means of <br /> approximating the spreading of a contaminant in the two dimensions perpendicular to the <br /> direction of transport. This model estimates the concentration of a substance in the <br /> dissolved state at a given distance from the source by accounting for the three main <br /> processes that govern solute transport: advection, transverse and longitudinal dispersion, <br /> and adsorption. It is important to note that the VHS model assumes steady-state and <br /> Y" <br /> homogeneous conditions. <br /> The VHS model is described by the following equation: <br /> ii <br /> CL= Co erf Z erf X <br /> 2 DtL/Vc 4 DtL/Vc ` <br /> where CL= concentration of chemical at distance L from the source (pg/1) <br /> E� <br /> Co = concentration of chemical at the source <br /> I� <br /> Z = depth of the plume in the saturated zone beneath the source (meters) <br /> Dt = dispersion coefficient perpendicular to the flow path (meters per day <br /> [m/day]) <br /> L=distance from the source (meters) <br /> Vc = velocity of the dissolved chemical (m/day) <br /> X = width of plume in the saturated zone beneath the source (meters) <br /> IiI€ <br /> I! <br /> erf = The error function, which cannot be ;fevaluated analytically, was <br /> approximated consistent with I.C. Javandal, and others [19841}. <br /> r <br /> i <br /> I <br /> i$ <br /> J:15ECINMU0740010.1 B R-95Mjt:2 A-1 <br />