Numerical modeling of arsenic mobility - In-Situ Remediation of Arsenic-Contaminated Sites - page 36

fact that its open-source code is well documented. Other frequently used codes include HST3D
(Kipp, 1986), FEMWATER (Yeh et al. , 1992) and FEFLOW (Diersch, 1997) ( Table 2.1 ).
Based on these (underlying) groundwater flow simulators, advective-dispersive transport codes
such as MT3DMS (Zheng and Wang, 1999), FEFLOW (Diersch, 1997), or FEMWATER (Yeh
et al. , 1992) allow for the simulation of multi-dimensional solute transport behavior due to
physical transport processes (i.e., advection, dispersion, diffusion), often also in conjunction with
simplified representations of reactive processes such as linear and non-linear sorption and decay.
The application of such codes to arsenic problems can sometimes be justified, for example in
aquifers where geochemical conditions remain more or less stable in space and time and thus the
variations in arsenic transport characteristics are less pronounced. However, these solute transport
codes do not account for coupling and interactions between various dissolved species and reactive
mineral phases, which often strongly impact arsenic mobility.
Simulation of these more complex reactive transport systems can be achieved through multi-
component reactive transport simulators. Such codes often couple a geochemical model (such
as PHREEQC) with a solute transport simulator to model changes in water composition due to
both physical transport and geochemical reactions as it travels through the subsurface. Significant
advances in computational power, numerical solution techniques and parallelization of codes have
allowed routine simulation of increasingly complex groundwater quality problems in heteroge-
neous aquifer systems where flow, transport and reactive processes need and can be considered
simultaneously.

Table 2.1. Widely used geochemical, flow, solute transport and reactive transport modeling tools.

Code Website Summary

A: Geochemical models
PHREEQC http://wwwbrr.cr.usgs.

gov/projects/GWC_
coupled/phreeqc/

PHREEQC (version 3) is designed to perform a wide
variety of aqueous geochemical calculations. PHREEQC
implements several types of aqueous models: two
ion-association aqueous models (the Lawrence Livermore
National Laboratory model and WATEQ4F), a Pitzer
specific-ion-interaction aqueous model, and the SIT
(Specific ion Interaction Theory) aqueous model. Using
any of these aqueous models, PHREEQC has capabilities
for (1) speciation and saturation-index calculations;
(2) batch-reaction and one-dimensional (1D) transport
calculations with reversible and irreversible reactions,
which include aqueous, mineral, gas, solid-solution,
surface-complexation, and ion-exchange equilibria,
and specified mole transfers of reactants, kinetically
controlled reactions, mixing of solutions, and pressure and
temperature changes; and (3) inverse modeling.

Geochemists
Workbench

http://www.gwb.com/ Geochemist's Workbench is an integrated set of interactive

software tools for solving a range of problems in aqueous
geochemistry including balancing chemical reactions,
speciation calculations, calculating stability diagrams and
the equilibrium states of natural waters, tracing reaction
processes, modeling reactive transport in one and two
dimensions, and plotting the results of these
calculations. Heat transfer by advection and conduction,
mineral precipitation and dissolution, complexation
and dissociation, sorption and desorption, redox
transformations and reactions catalyzed by microbial
activity can be simulated.

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