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Saturated Zone Model <br /> R = retardation factor [-] <br /> M = source term [mass flux)for chemical [g/d/m3] <br /> B = porosity(effective)of the saturated zone [cm3/cm3] <br /> The source term, M, is non-zero over a finite rectangular area at the water table and <br /> zero elsewhere When the model is run, the user must specify the length, width and <br /> thickness of the source The length of the source is in the direction of groundwater <br /> flow <br /> The aquifer is considered to be infinite in depth and width The source term may be a <br /> constant concentration specified for a certain duration ("pulse") or it may vary with <br /> time The length of the pulse is tantamount to specifying how long the source is <br /> active (i e , the duration between the spill event and the point in time when either the <br /> source is removed or is naturally depleted) <br /> B.2.1 Initial and Boundary Conditions, <br /> At the beginning of the simulation, the aquifer is assumed to have a concentration of <br /> zero everywhere The mass loading is assumed to occur uniformly over the volume of <br /> the source The model simulates dissolved phase transport only so the contaminant <br /> mass is assumed to be instantly dissolved and mixed uniformly over the source <br /> volume The concentration at a great distance away from the source is assumed to be <br /> zero for all times The advection-dispersion equation (13-1) is solved using Green's <br /> functions for the conditions described(Galya, 1987) <br /> x� y n tar f 4 ( Is ?t 1v r _as <br /> B.2 2 Retardation Coefficient? <br /> Retardation describes a contaminants movement relative to the bulk movement of <br /> groundwater flow The retardation factor,R, is estimated using <br /> R=1+ Pb FB Ka for organic chemicals (B-2a) <br /> i <br /> B-3 <br />