MT3DMS is a new version of the Modular 3-D Transport model, where MS denotes the Multi-Species structure for accommodating add-on reaction packages. MT3DMS has a comprehensive set of options and capabilities for simulating advection, dispersion/diffusion, and chemical reactions of contaminants in groundwater flow systems under general hydrogeological conditions. The key features of MT3DMS are summarized below.

MT3DMS is unique in that it includes three major classes of transport solution techniques in a single code, i.e., the standard finite difference method; the particle-tracking-based Eulerian-Lagrangian methods; and the higher-order finite-volume TVD method. Since no single numerical technique has been shown to be effective for all transport conditions, the combination of these solution techniques, each having its own strengths and limitations, is believed to offer the best approach for solving the most wide-ranging transport problems with desired efficiency and accuracy.

In addition to the explicit formulation of the original MT3D code, MT3DMS includes an implicit formulation that is solved with an efficient and versatile solver. The iterative solver is based on generalized conjugate gradient (GCG) methods with three preconditioning options and the Lanczos/ORTHOMIN acceleration scheme for non-symmetrical matrices. If the GCG solver is selected, dispersion, sink/source, and reaction terms are solved implicitly without any stability constraints. For the advection term, the user has the option to select any of the solution schemes available, including the standard finite-difference method, the particle tracking based Eulerian-Lagrangian methods, and the third-order TVD method. The finite-difference method can be fully implicit without any stability constraint to limit transport step sizes, but the particle tracking based Eulerian-Lagrangian methods and the third-order TVD method still have time step constraints associated with particle tracking and TVD methodology. If the GCG solver is not selected, the explicit formulation is automatically used in MT3DMS with the usual stability constraints. The explicit formulation is efficient for solving advection-dominated problems in which the transport stepsizes are restricted by accuracy considerations. It is also useful when the implicit solver requires a large number of iterations to converge or when the computer system does not have enough memory to use the implicit solver.

MT3DMS is implemented with an optional, dual-domain formulation for modeling mass transport. With this formulation, the porous medium is regarded as consisting of two distinct domains, a mobile domain where transport is predominately by advection and an immobile domain where transport is predominately by molecular diffusion. Instead of a single "effective" porosity for each model cell, two porosities, one for the mobile domain and the other for the immobile domain, are used to characterize the porous medium. The exchange between the mobile and immobile domains is specified by a mass transfer coefficient. The dual-domain advective-diffusive model may be more appropriate for modeling transport in fractured media or extremely heterogeneous porous media than the single porosity advective-dispersive model, provided that the porosities and mass transfer coefficients can be properly characterized.

MT3DMS retains the same modular structure of the original MT3D code, similar to that implemented in the U.S. Geological Survey modular three-dimensional finite-difference groundwater flow model, MODFLOW, (McDonald and Harbaugh, 1988; Harbaugh and McDonald, 1996). The modular structure of the transport model makes it possible to simulate advection, dispersion/diffusion, source/sink mixing, and chemical reactions separately without reserving computer memory space for unused options; furthermore new packages involving other transport processes and reactions can be added to the model readily without having to modify the existing code.

As in the original MT3D code, MT3DMS is developed 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 affect the flow field significantly. After a flow model is developed and calibrated, the information needed by the transport model can be saved in disk files which are then retrieved by the transport model. Since most potential users of a transport model are likely to have been familiar with one or more flow models, MT3DMS provides an opportunity to simulate contaminant transport without having to learn a new flow model or to modify an existing flow model to fit the transport model. In addition, separate flow simulation and calibration outside the transport model can result in substantial savings in computer memory. The model structure also saves execution time when many transport runs are required while the flow solution remains the same. Although this report describes only the use of MT3DMS in conjunction with MODFLOW, MT3DMS can be linked to any other block-centered finite-difference flow model in a simple and straightforward fashion.

MT3DMS can be used to simulate changes in concentrations of miscible contaminants in groundwater considering advection, dispersion, diffusion and some basic chemical reactions, with various types of boundary conditions and external sources or sinks. The chemical reactions included in the model are equilibrium-controlled or rate-limited linear or non-linear sorption, and first-order irreversible or reversible kinetic reactions. It should be noted that the basic chemical reaction package included in MT3DMS is intended for single-species systems. An add-on reaction package such as RT3D (Clement, 1997) or SEAM3D (Widdowson and Waddill, 1997) must be used to model more sophisticated multi-species reactions. MT3DMS can accommodate very general spatial discretization schemes and transport boundary conditions, including: 1) confined, unconfined or variably confined/unconfined aquifer layers; 2) inclined model layers and variable cell thickness within the same layer; 3) specified concentration or mass flux boundaries; and 4) the solute transport effects of external hydraulic sources and sinks such as wells, drains, rivers, areal recharge and evapotranspiration.