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1 <br /> Vadose Zone Model <br /> ' R = retardation factor(defined in Equation A-31) [-] <br /> ' The x-axis is assumed to be aligned with the direction of fluid flow, in the vadose <br /> zone that direction is vertically downward The model considers dispersion in the <br /> downward direction only (longitudinal dispersion) The advection-dispersion <br /> equation is used to solve for aqueous-phase concentration with depth below the <br /> source This concentration at the water table will be used with the infiltration rate to <br /> ' estimate mass loading to groundwater <br /> ' A note on nomenclature In this appendix the vanable, C(C,,, C,,, or CT), will always <br /> refer to the concentration of the individual chemical being modeled (not the TPH <br /> mixture) If the concentration of TPH is being referenced, the variable CjpH will be <br /> ' used The same applies to all chemical properties For example, Def-refers to the <br /> chemical-specific diffusion coefficient, MW refers to the chemical-specific molecular <br /> weight, and MWTry refers to the molecular weight of the TPH mixture <br /> A.3.1 Initial and Boundary Conditions <br /> Below the source it is assumed that concentrations are zero at time=0 <br /> 1 <br /> C. (x,0) � 0 (A-2) <br /> The Ieachate concentration leaving the source zone is assumed to decay exponentially <br /> with time <br /> ' <br /> C.(0,t) = Cwo e-ft (A-3) <br /> ' where <br /> C,„ = dissolved phase concentration of chemical in the source <br /> ' at the beginning of the simulation [mg/L] <br /> _ <br /> f3, source depletion term [-] <br /> A-5 <br />