MT3D is a 3D solute transport model for simulation of advection, dispersion, and chemical reactions of dissolved constituents in ground-water systems. The model uses a modular structure similar to that implemented in MODFLOW. The modular structure makes it possible to independently simulate advection, dispersion, sink/source mixing, and chemical reactions without reserving computer memory space for unused options. MT3D uses a mixed Eulerian-Lagrangian approach to solve the three-dimensional advective-dispersive-reactive equation, in three basic options: the method of characteristics (MOC), the modified method of characteristics (MMOC), and a hybrid of these two methods (HMOC). This approach combines the strength of the MOC for eliminating numerical dispersion with the computational efficiency of the MMOC. The availability of both MOC and MMOC options, and their selective use based on an automatic adaptive procedure under the HMOC option, make MT3D uniquely suitable for a wide range of field problems. MT3D is intended for use with any block-centered finite-difference flow model such as MODFLOW and is based on the assumption that changes in the concentration field will not measurably affect the flow field. This allows the user to independently construct and calibrate a flow model. MT3D retrieves the hydraulic heads and the various flow and source/sink terms saved by the flow model, automatically incorporating the specified hydrologic boundary conditions. Although the MT3D documentation describes the use of MT3D in conjunction with MODFLOW, the transport model can be linked to any other block-centered finite-difference flow model. MT3D can be used to simulate changes in concentration of single-species miscible contaminants in ground-water considering advection, dispersion, and some simple chemical reactions. The chemical reactions included in the model are equilibrium-controlled linear or non-linear sorption and first-order irreversible decay or biodegradation.
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