Environmental Engineering Reference
In-Depth Information
•
the conventional advection-dispersion equation, ADE;
•
the chemical and physical nonequilibrium ADEs;
•
a stochastic stream tube model based upon the local-scale equilibrium or
nonequilibrium ADE.
These three types of models all consider linear adsorption, and include zero- and
first-order decay/source terms.
18.4.2.2 Multi-Dimensional Models
Some of the more popular multi-dimensional analytical transport models have been
AT123D (Yeh
1981
), 3DADE (Leij and Bradford
1994
), N3DADE (Leij and Toride
1997
), and MYGRT (Ungs et al.
1998
). These programs provide analytical solutions
to transport problems in two- and three-dimensional domains. 3DADE also includes
parameter estimation capabilities.
A large number of analytical models for one-, two-, and three-dimensional
contaminant transport problems were incorporated into the public domain soft-
ware package STANMOD (STudio of ANalytical MODels) (Šimunek et al.
1999a
)(
http://www.hydrus2d.com
). This Windows-based computer software pack-
age includes not only programs for equilibrium advective-dispersive transport such
as the CFITM code of Van Genuchten (
1980b
) for one-dimensional transport and
3DADE (Leij and Bradford
1994
) for three-dimensional problems, but also pro-
grams for more complex problems. For example, STANMOD also incorporates the
CFITIM (Van Genuchten
1981
) and N3DADE (Leij and Toride
1997
) programs for
nonequilibrium transport (i.e., the two-region mobile-immobile model for physical
nonequilibrium and the two-site sorption model for chemical nonequilibrium) in
one and multiple dimensions, respectively. A more recent version of STANMOD
includes additionally the screening model of Jury et al. (
1983a
) for transport and
volatilization of soil-applied organic contaminants.
18.5 Numerical Models
18.5.1 Numerical Approaches
Although analytical and semi-analytical solutions are still popularly used for solving
many relatively simple problems, the ever-increasing power of personal computers
and the development of more accurate and stable numerical solution techniques have
led to the much wider use of
numerical models
over the past ten years. Numerical
methods in general are superior to analytical methods in terms of their ability to
solve much more realistic problems (Šimunek
2005
). They allow users to design
complicated geometries that reflect complex natural pedological and hydrological
conditions, control parameters in space and time, prescribe more realistic initial
and boundary conditions, and permit the implementation of nonlinear constitutive
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