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f <br /> Contaminant Fate and Transport Model <br /> f <br /> AT123D <br /> E <br /> The background, assumptions, and input parameters required to simulate <br /> contaminant transport using AT123D are presented in the following text. The input <br /> parameters specific to the Stockton Service Center are also presented. Attachment D-1 to <br /> this Appendix D presents the model output in graphical and code formats. The graphical <br /> output consists of charts that plot the concentrations of individual chemicals versus time at <br /> the receptor point. The code format is the Fortran output that lists the input parameters <br /> and the individual chemical concentrations at 5-year increments over the simulation period. <br /> Background <br /> AT123D is a generalized analytical, transient, one-, two-, and three-dimensional code <br /> for estimating the transport of wastes in a groundwater..porous system. The model was <br /> developed by G.T. Yeh at Oak Ridge National Laboratory in 19$1 and is included in the <br /> United States Protection Agency's Graphical Exposure Modeling System. AT123D solves <br /> the differential mass balance equation that describes contaminant fate and transfer in the <br /> saturated zone and accounts for advective and dispersive effects. AT123D includes three- <br /> dimensional dispersion; one-dimensional uniform advection; linear, reversible equilibrium <br /> adsorption; and lumped first-order decay. The model allows input of the source as a point, <br /> line, area, or volume oriented in different ways along the x, y, and z directions. AT123D <br /> also allows specification of the mass loading as a constant mass release rate for the entire j <br /> f simulation, as a constant mass release rate for a specified period of time, and as an I <br /> Uinstantaneous release. <br /> LI Assumptions <br /> The assumptions underlying the AT123D model include the following: <br /> S The saturated zone is homogenous, isotropic, and of uniform geometry. <br /> • The water table is steady without any fluctuations. <br /> 0 The flow direction is uniform, one-dimensional, and steady state. <br /> L • Contaminant decay follows a lumped first order decay rate. <br /> 0 Contaminant adsorption follows linear adsorption. <br /> L <br /> • Concentrations in the liquid and solid phase of the aquifer are in equilibrium at all <br /> times. <br /> IU <br /> 96R7 Q- East Bay Municipal Utility District <br /> 40429700.fcr Final Closure Report <br />