Environmental Engineering Reference
In-Depth Information
subsoil. The reactions between the chemical species in the porewater and also with the
reactive soil particle surfaces discussed in the previous sections and chapters constitute
the basic platform. Because individual chemical species have the ability to participate in
several types of reactions, the equations to describe the various equilibrium reactions can
become complicated, particularly since one needs to be assured that all the reactions are
captured.
Geochemical modeling provides a useful means for handling the many kinds of cal-
culations required to solve the various equilibrium reactions. Speciic requirements are
a robust thermodynamic database and simultaneous solution of the thermodynamic and
mass balance equations. Appelo and Postma (1993) provide a comprehensive treatment of
the various processes and reactions, together with a user guide for the geochemical model
PHREEQE developed by Parkhurst et al. (1980). As with many of the popular models, the
model is an aqueous model based upon ion pairing and includes elements and both aque-
ous species and mineral phases (fractions).
Other available models include the commonly used MINTEQ (Felmy et al., 1984)
and the more recent MINTEQA2 version 4.03 released in 2006 that includes PROFEFA2
(Allison et al., 1991; EPA, 2014), a preprocessing package for developing input iles,
GEOCHEM (Sposito and Mattigod, 1980), HYDROGEOCHEM (Yeh and Tripathi, 1990),
and WATEQF (Plummer et al., 1976). GEOCHEM-EZ is a multifunctional chemical
speciation program, designed to replace GEOCHEM-PC, which can only be used on
DOS consoles. HYDROGEOCHEM 2 is the newer version of HYDROGEOCHEM V1.0
(Yeh and Tripathi, 1990). The modiication includes replacement of the EQMOD chemi-
cal equilibrium subroutines by a mixed chemical Kinetic and Equilibrium Model
(KEMOD) to deal with species whose concentrations are controlled by either thermo-
dynamics or kinetics. HYDROGEOCHEM 2 is a coupled model of hydrological trans-
port and geochemical reaction in saturated-unsaturated media. The newer version of
the chemical speciation program WATEQF is WATEQ4F, which is maintained by the
Chemical Modeling and Thermodynamic Data Evaluation Project of the USGS and
mainly applicable for large numbers of water analyses (Ball and Nordstrom, 2001).
PHREEQC version 3, which is now available (USGS, 1998) works with various models
such as the Lawrence Livermore National Laboratory model and WATEQ4F), a Pitzer spe-
ciic ion interaction aqueous model, and the SIT (Speciic ion Interaction Theory) aqueous
model. The PHREEQC model can perform
1. Speciation and saturation index calculations
2. Batch reaction and one-dimensional (1D) transport calculations
3. Inverse modeling
By and large, most of the geochemical codes assume instantaneous equilibrium, i.e.,
kinetic reactions are not included in the calculations. In part, this is because reactions
such as oxidation-reduction, precipitation-dissolution, substitution-hydrolysis, and to
some extent, speciation-complexation, can be relatively slow. To overcome this, some of
the models have been able to provide analyses that point toward possible trends and inal
equilibria. The code EQ3/6 (Delaney et al
.
, 1986) does, however, provide for consideration
of dissolution-precipitation reactions. Transformations, however, are essentially not han-
dled by most of the codes, although more models are including them.
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