Leaching of Contaminants to Groundwater - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 833

advection-dispersion equations for both heat and contaminant transport. The
unsaturated soil hydraulic properties can be described using Van Genuchten
( 1980a ), Brooks and Corey ( 1964 ), Kosugi ( 1996 ), and Durner ( 1994 ) type ana-
lytical functions, or modified Van Genuchten type functions that produce a better
description of the hydraulic properties near saturation.
The HYDRUS-1D software package additionally includes modules for simulat-
ing carbon dioxide and major ion contaminant movement (Šimunek et al. 1996 ;
Šimunek and Suarez 1993 ). Also included is a small catalog of unsaturated soil
hydraulic properties (Carsel and Parish 1988 ), as well as pedotransfer functions
based on neural network predictions (Schaap et al. 2001 ).
18.5.2.2 Biogeochemical Transport Models
Significant efforts have been made also in coupling physical flow and transport
models with biogeochemical models to simulate increasingly more complex reac-
tions, such as surface complexation, precipitation/dissolution, cation exchange,
and/or (micro)biological reactions. Reviews of the development of hydrogeochemi-
cal transport models involving reactive multiple components are given by Mangold
and Tsang ( 1991 ), Lichtner ( 1996 ), Steefel and MacQuarrie ( 1996 ), Šimunek and
Valocchi ( 2002 ), and Bell and Binning ( 2004 ). Most modeling efforts involving mul-
ticomponent transport have thus far focused on the saturated zone, where changes in
the flow velocity, temperature and pH are often much more gradual and hence less
important than in the unsaturated zone. Consequently, most multicomponent trans-
port models assumed one- or two-dimensional steady-state saturated water flow with
a fixed value of the flow velocity, temperature and p H. Several multicomponent
transport models have been published also for variably-saturated flow problems.
These include DYNAMIX (Liu and Narasimhan 1989 ), HYDROGEOCHEM (Yeh
and Tripathi 1990 ), TOUGH-REACT (Pruess 1991 ), UNSATCHEM (Šimunek
and Suarez 1994 ;Šimunek et al. 1996 , 1997 ), FEHM (Zyvoloski et al. 1997 ),
MULTIFLO (Lichtner and Seth 1996 ), OS3D/GIMRT (Steefel and Yabusaki 1996 ),
HYDROBIOGEOCHEM (Yeh et al. 1998 ), FLOTRAN (Lichtner 2000 ), MIN3P
(Mayer et al . 2002 ), HP1 (Jacques and Šimunek 2005 ; Jacques et al. 2002 , 2008a ,
2008b ), and HYDRUS-1D (Šimunek et al. 2005 ).
Geochemical models can be divided into two major groups: those with spe-
cific chemistry and those characterized by more general chemistry (Šimunek and
Valocchi 2002 ). Models with specific chemistry are limited in the number of species
they can handle, while their application is restricted to problems having a prescribed
chemical system. They are, however, much easier to use and computationally
can be much more efficient than general models. Typical examples of models
with specified chemistry are those simulating the transport of major ions, such as
LEACHM (Wagenet and Hutson 1987 ), UNSATCHEM (Šimunek and Suarez 1994 ;
Šimunek et al. 1996 ), and HYDRUS-1D (Šimunek et al. 2005 ). Models with gen-
eralized chemistry (DYNAMIX, HYDROGEOCHEM, MULTIFLO, FLOTRAN,
OS3D/GIMRT, and HP1, all referenced above) provide users with much more
freedom in designing a particular chemical system; possible applications of these
models are also much wider.