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,f <br /> SOIL MODELING <br /> VHS Model <br /> The Vertical and Horizontal Spread (VHS) model was selected to estimate the <br /> concentration of chemicals dissolved in soil solution dust above the water table for this <br /> site The VHS model, described by Dormnico and Palciauskas (1982), 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 /> 1 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 /> homogeneous conditions <br /> The VHS model is described by the following equation <br /> Ci = Co erf erf X <br /> 12,f5t L Vc I 47 Dt L Vc <br /> Lwhere CL= concentration of chemical at distance L from the source (micrograms per <br /> liter [µg/1]) <br /> Co = concentration of chemical dissolved in the soil solution at the source <br /> (µtil) <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 /> erf = The error function, which cannot be evaluated analytically, was <br /> approximated consistent with I C Javandal, and others [1984]) <br />' J 1SECT\PJ2120740010 1BR-95VJt 2 D-I <br />