Leaching of Contaminants to Groundwater - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 832

While earlier models solved the governing flow and transport equations for rel-
atively simplified system-independent boundary conditions (i.e., specified pressure
heads or fluxes, and free drainage), the more recent models can cope with much
more complex system-dependent boundary conditions evaluating surface flow and
energy balances and accounting for the simultaneous movement of water, vapor,
and heat. Examples are DAISY (Hansen et al. 1990 ), TOUGH2 (Pruess 1991 ),
SHAW (Flerchinger et al. 1996 ), SWAP (Van Dam et al. 1997 ), HYDRUS-1D
(Šimunek et al. 1998a , 2005 ), UNSATH (Fayer 2000 ), and COUP (Jansson and
Karlberg 2001 ). Several models now also account for the extremely nonlinear pro-
cesses associated with the freezing and thawing cycle (e.g., DAISY, SHAW, and
COUP).
Contaminant transport models have also become more sophisticated in terms of
the type and complexity of processes that can be simulated. Transport models are
no longer being limited to contaminants undergoing relatively simple chemical reac-
tions such as linear sorption and first-order decay, but now consider also a variety of
nonlinear sorption and exchange processes, physical and chemical nonequilibrium
transport, volatilization, gas diffusion, colloid attachment/ detachment, decay chain
reactions, and many other processes (e.g., the HYDRUS-1D, -2D, and (2D/3D)
codes of Šimunek et al. ( 1999b , 2005 , 2006c ), or MODFLOW-SURFACT of
HydroGeoLogic, Inc. ( 1996 )). For example, the general formulation of the transport
equations in the HYDRUS codes permit simulations of non-adsorbing or linearly
sorbing contaminants, in addition to a variety of other contaminants, such a viruses
(Schijven and Šimunek 2002 ), colloids (Bradford et al. 2002 ), cadmium (Seuntjens
et al. 2001 ), and hormones (Casey et al. 2003 , 2004 ), or contaminants involved
in the sequential biodegradation of chlorinated aliphatic hydrocarbons (Casey and
Šimunek 2001 ; Schaerlaekens et al. 1999 ).
Much effort has been directed also toward improving models for purposes of sim-
ulating nonequilibrium and/or preferential flow. Examples are the TOUGH codes
(Pruess 1991 , 2004 ), MACRO (Jarvis et al. 1994 ), and HYDRUS-1D (Šimunek
and Van Genuchten 2008 ). These models typically assume the presence of dual-
porosity and dual-permeability regions, with different fluxes possible in the two
regions. Example applications of these dual-porosity and dual-permeability models
are given by Mallants et al. ( 1997 ), Šimunek et al. ( 2001 ), Haws et al. ( 2005 ), Köhne
et al. ( 2004 , 2006 , 2009a , b ), and Pot et al. ( 2005 ), among many others.
As an example of available vadose zone flow and transport models, we briefly
discuss here the HYDRUS software packages of Šimunek et al. ( 1999b , 2005 , 2008 ).
The HYDRUS Software Packages
HYDRUS-1D (Šimunek et al. 2005 ), HYDRUS-2D (Šimunek et al. 1999b ), and
HYDRUS (2D/3D) (Šimunek et al. 2006c ) are software packages ( http://www.pc-
progress.com/en/Default.aspx ) that simulate the one- and two-dimensional move-
ment of water, heat, and multiple contaminants in variably saturated porous
media, respectively. Both programs use finite elements to numerically solve
the Richards equation for saturated-unsaturated water flow and Fickian-based