Hydrogeochemical modeling helps in understanding and predicting the difficult combination of chemical and mineral interrelated processes that control the fate of chemical species as well as the transport in aquatic environments. Hydrogeochemical models have developed in recent years to simulate the geochemical processes only (geochemical model) and to also simulate both geochemical processes and transport (i.e., flow and transport model). Geochemical models include GEMS, MINEQL+, MINTEQA2, and Visual MINTEQ, among others. Flow and transport models that have just become available in the very recent years are PHREEQC and PHAST models.
3.1.1 Geochemical Modeling
Geochemical models are mostly used to simulate chemical equilibrium with thermodynamic databases of the elements of environmental interest. Equilibrium models assume that all reactions have completed and are in equilibrium with one another. The models have common capabilities in calculating speciation, sorption, and precipitation of aquatic chemical components. The model capabilities are described as follows.
GEMS: GEM-Selektor (GEM-Selektor Geochemical Software) was developed by Paul Scherer Institute in Switzerland. It is an interactive thermodynamic modeling of heterogeneous aquatic geochemical systems. The model uses the method of Gibbs Energy Minimization to calculate the multi-speciation at the equilibrium. It has capabilities to calculate aqueous-solid solution equilibrium, adsorption and ion exchange. It includes a built-in (default) thermodynamic database of common aqueous elements. The model provides the option for extension and modification of its database by the modeler.
MINEQL+: It was first developed at MIT, by John Westall and Francois Morel. It has abilities to perform the calculation of equilibrium aqueous speciation, of dissolved and solid phases, adsorption and ion exchange, at low temperature (0-50 °C) and low to moderate ionic strength (< 0.5 M). The MINEQL+ version 3 is available for the DOS operating system and is free for use, while version 4.6 is available for the window operating system, but at a cost to the modeler (MINEQL+, Geochemical Software).
MINTEQA2: This program is sponsored by the US Environmental Protection Agency (USEPA). It is an equilibrium speciation model, alike other geochemical models. MINTEQA2 has capabilities to calculate dissolved and solid phases and adsorption. The thermodynamic database in MINTEQA2 is well developed, including common aqueous elements. MINTEQA2 is a free and robust geochemical model that has been used by researchers. It is available for the DOS operating system (MINTEQA2, Geochemical Software).
Visual MINTEQ: It is a development of MINTEQA2; it has been maintained by Jon Petter Gustafsson at KTH, Sweden, since 2000. It has capabilities to calculate
aqueous equilibrium reactions as MINTEQA2. The difference with MINTEQ is that Visual MINTEQ is available for the windows operating system (Visual MINTEQ).
3.1.2 Flow and Transport
PHREEQC and PHAST models are models that handle the kinetics of chemical reactions, reverse reactions and link the concentrations of species to simulations of 1- dimensional (PHREEQC) and 3-dimensional (PHAST) transport scenarios (U.S. Geological Survey, 1999 and 2010).
PHREEQC: It is a product of the US Geological Survey (USGS), developed by Parkhurst and Appelo in 1999. It is designed to perform low-temperature aqueous geochemical calculations. It is similar to other geochemical models having capabilities to calculate speciation, saturation index, batch reaction, surface complexion, adsorption and ion exchange at equilibrium. In addition, PHREEQC also has capabilities to simulate reversible reactions, kinetic reactions, with rate expressions defined by the modeler and one dimensional (1-D) transport simulations. Its databases contain those from MINTEQA2 and other USGS’s models (i.e., WATEQF4). All these capabilities make PHREEQC more complete and advanced model than others. The coupling of geochemical and transport processes in PHREEQC allows the studying the behavior of aqueous components under flow and transport conditions at sites of interest. The model can link the chemical equilibrium (i.e., batch-reaction) calculations to simulations of flow and transport under two types of boundaries and time conditions (i.e., flux or third-type boundary and the Dirichlet or first-type) (see Appendix A for details of PHREEQC model’s capabilities and limitations).
PHAST: It is an integrated computer program between Geochemical model (PHREEQC) and the flow model (HST3D). PHAST is designed to simulate multicomponent, reactive solute transport in 3-dimensional flow system. The calculations of flow and transport are based on a developed HST3D model (Parkhurst et al., 2010) while the geochemical reactions are simulated with the geochemical model PHREEQC (Parkhurst, 1995; Parkhurst and Appelo, 1999). The PHREEQC model is embedded in PHAST (see Appendix B for details of PHAST model’s capabilities and limitations). Current hydrogeochemical model capabilities are summarized in Table 1.
Table 1 The main features and capabilities of available hydrogeochemical models
A: Average-comparable to other models; G: Good-better than other models; L: Limited; na: not available
Models database Hg Expandable database
Saturation Index calculation Precipitation Calculation Sorption processes Inverse model Transport processes System GEMS A G G G A Na na Windows MINEQL A L G G A Na na DOS MINTEQA 2 A L G G G Na na DOS Visual MINTEQ A L G G A Na na Windows PHREEQC A G G G G G G Windows PHAST A G G G G G G DOS
All of the Hg databases available in PHREEQC and PHAST models are for speciation calculations of common inorganic Hg species, but they lack organic complexation, ion exchange and sorption stability constants.
PHREEQC and PHAST are thus the chosen models to support this research because of their various capabilities (Appelo and Postma, 2005; Halim et al., 2005; Parkhurst and Appelo, 1999; Tiruta-Barna, 2008). It is foreseen that, once the databases are enhanced, it will allow a more objective assessment and analysis of the fate of Hg, in flow and transport conditions, at the two applications test beds, namely, EFPC and ENP, where Hg is present and of concern.