User's guide of Proceessing Modflow 5.0

Processing Modflow 3
the three-dimensional advective-dispersive-reactive transport equation. MT3D is based on the
assumption that changes in the concentration field will not affect the flow field significantly. This
allows the user to construct and calibrate a flow model independently. After a flow simulation
is complete, MT3D simulates solute transport by using the calculated hydraulic heads and various
flow terms saved by MODFLOW. MT3D can be used to simulate changes in concentration of
single species miscible contaminants in groundwater considering advection, dispersion and some
simple chemical reactions. The chemical reactions included in the model are limited to
equilibrium-controlled linear or non-linear sorption and first-order irreversible decay or
biodegradation.
MT3DMS is a further development of MT3D. The abbreviation MS denotes the Multi-
Species structure for accommodating add-on reaction packages. MT3DMS includes three major
classes of transport solution techniques, i.e., the standard finite difference method; the particle
tracking based Eulerian-Lagrangian methods; and the higher-order finite-volume TVD method.
In addition to the explicit formulation of MT3D, MT3DMS includes an implicit iterative solver
based on generalized conjugate gradient (GCG) methods. If this solver is used, dispersion,
sink/source, and reaction terms are solved implicitly without any stability constraints.
The MOC3D transport model computes changes in concentration of a single dissolved
chemical constituent over time that are caused by advective transport, hydrodynamic dispersion
(including both mechanical dispersion and diffusion), mixing or dilution from fluid sources, and
mathematically simple chemical reactions, including decay and linear sorption represented by
a retardation factor. MOC3D uses the method of characteristics to solve the transport equation
on the basis of the hydraulic gradients computed with MODFLOW for a given time step. This
implementation of the method of characteristics uses particle tracking to represent advective
transport and explicit finite-difference methods to calculate the effects of other processes. For
improved efficiency, the user can apply MOC3D to a subgrid of the primary MODFLOW grid
that is used to solve the flow equation. However, the transport subgrid must have uniform grid
spacing along rows and columns. Using MODFLOW as a built-in function, MOC3D can be
modified to simulate density-driven flow and transport.
The purpose of PEST and UCODE is to assist in data interpretation and in model calibration.
If there are field or laboratory measurements, PEST and UCODE can adjust model parameters
and/or excitation data in order that the discrepancies between the pertinent model-generated
numbers and the corresponding measurements are reduced to a minimum. Both codes do this by
taking control of the model (MODFLOW) and running it as many times as is necessary in order
to determine this optimal set of parameters and/or excitations.
1. Introduction