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A Geochemical code is a computer program using equations to solve the equilibrium problem using chemical thermodynamics. The codes calculate the speciation of aqueous solutions and specified instantaneous reactions or time-dependent reactions.

Solution of equilibrium problem
The majority of the codes uses the law of mass action for calculating the state of equilibrium. The codes bring along thermodynamic databases, which incorporate equilibrium constants (KEq) for the specified mineral, aquatic and gaseous reactions.

Another set of codes uses the method of the Gibbs Energy Minimization (GEM). Geochemical codes including this approach are Gem-Selektor and FactSage (ChemApp). The advantage of this approach is, that the Gibbs Energy is calculated at every temperature-pressure (T-P) condition. A major disadvantage is that i.e. FactSage does not incorporate activity calculation ($$\ \gamma_i = 1 $$).

Activity calculation
The geochemical codes use the ion-dissociation and ion-association models to calculate activity coefficients ($$\ \gamma \, $$i) of aquatic species. Equations calculating activity coefficients following the theory of ion-dissociation are i.e. the Davies and the different types of the Debye-Hückel equation. For example, PHREEQC uses the different activity equations dynamically depending on the available database parameters. The different activity models are restricted to different ranges of ionc strength.

A different approach calculating activity coefficients following the theory of ion-association is the framework of Pitzer equations. The Pitzer equations include interaction parameters for representing the ionic interactions. The range of validity with respect to ionic strength is much higher compared to the equations following the ion-dissociation theory. However, this activity model does not include data for the aquatic species Al and Si. Most of the interaction parameters are only valid at 25°C. Thereby, simulation of reaction processes of feldspars is not possible, which are of high importance for most hydrogeochemical problems.

The Harvie-Möller-Weare model (HMW) is also based on the Pitzer framework. The HMW model incorporates the components Na, K, Mg, Ca, H, Cl, SO4, OH, HCO3-CO3, CO2 and H2O.

The Specific ion interaction theory (SIT theory) uses interaction coefficients to estimate activity coefficients at high concentrations. The integration coefficients are derived from equilibrium coefficients. The SIT theory calculates acceptable results up to ionic strength of 3 mol/kgw.

The mineral solubility model of Harvie and Weare (1980) is extended to the eight component system, Na-K-Mg-Ca-H-Cl-SO4-OH-HCO3-CO3-CO2-H2O at 25°C to high concentrations. The model is based on the semi-empirical equations of Pitzer(1973) and co-workers for the thermodynamics of aqueous electrolyte solutions. The model is parameterized using many of the available isopiestic, electromotive force, and solubility data available for many of the subsystems. The predictive abilities of the model are demonstrated by comparison to experimental data in systems more complex than those used in parameterization. The essential features of a chemical model for aqueous electrolyte solutions and the relationship between pH and the equilibrium properties of a solution are discussed.

List of geochemical codes
More information on geochemical codes can be found in Steefel et al. (2015), especially on the codes PHREEQC, HPx, PHT3D, OpenGeoSys (OGS), HYTEC, ORCHESTRA, TOUGHREACT, eSTOMP, HYDROGEOCHEM, CrunchFlow, MIN3P, and PFLOTRAN.

Special Issue in Applied Geochemistry (2015), Volume 55, Pages 1-198 (April 2015)

Geochemical Speciation Codes and Databases

Edited by Dmitrii A Kulik, Wolfgang Hummel, Johannes Lützenkirchen and Grégory Lefèvre

http://www.sciencedirect.com/science/journal/08832927/55
 * HYDROGEOCHEM
 * PHREEQC
 * SOLMINEQ.88, GAMSPATH.99
 * TOUGHREACT{Cite book|url=https://publications.lbl.gov/islandora/object/ir%3A123402/datastream/PDF/view |title= TOUGHREACT user's guide: A simulation program for non-isothermal multiphase reactive geochemical transport in variably saturated geologic media. Report LBNL-55460|last=Xu|first=T.|publisher=Lawrence Berkeley National Laboratory, Berkeley, California|year=2011|isbn=|location=|pages=}}

Xu, T., E.L. Sonnenthal, N. Spycher and K. Pruess, 2004
 * Visual MINTEQ
 * WATEQ4F

Weblinks

 * List of codes by V. Brendler (Helmholtz Zentrum Dresden Rossendorf)
 * List of Water Resources Geochemical Software by US Geological Survey