Equilibrium Models in Hydro- and Geochemistry

In the last decades, a large variety of models and computer programs has been developed for applications in hydro- and geochemistry. From a thermodynamic point of view they solve the following task:

Given: concentrations of chemical elements1, temperature T (and pressure P)
Find: concentrations ci (or molar amounts ni) of chemical species in aqueous and other phases in the equilibrium state

In the calculations partial/local equilibrium is assumed, therefore they are termed “thermodynamic equilibrium models” (in contrast to kinetic models).

Classification Schema

Despite the wide variety of hydro- and geochemistry models we encounter today, all these approaches fall into two major categories:

two main approaches in hydrochemistry modeling

LMA. LMA is the most common approach and constitutes the basis of many programs: PhreeqC, Minteq2, Wateq4f3, EQ3/64, Chess/JChess5, the Geochemist’s Workbench6 and others. The program aqion, that relies on the numerical solver of PhreeqC, belongs to the LMA category as well.

GEM. The GEM approach is less common, but is becoming increasingly popular. GEM software: Solgasmix7, ChemSage8, FactSage9, Selektor10 and GEMS-PSI11.

Remarks & References

  1. They are called ‘master species’. 

  2. JD Allison, DS Brown, KJ Novo-Gradac: MINTEQA2/ProdefA2, A Geochemical Assessment Model for Environmental Systems, Version 3.0, User’s Manual, EPA/600/3-91/021, Mar 1991 

  3. JW Ball and DK Nordstrom: WATEQ4F – User’s manual with revised thermodynamic data base and test cases for calculating speciation of major, trace and redox elements in natural waters, USGS Open-File Report 90-129, 1991. 

  4. TJ Wolery, SA Daveler: EQ6, A Computer Program for Reaction Path Modeling of Aqueous Geochemical Systems: Theoretical Manual, User’s Guide, and Related Documentation (Version 7.0), Lawrence Livermore National Laboratory UCRL-MA-110662 PT IV, Oct 1992 

  5. Jan van der Lee, L DeWindt: CHESS Tutorial and Cookbook, User’s Manual, Ecole Nationale Superieure des Mines de Paris Report LHM/RD/02/13, Centre d’Informatique Geologique, Apr 2002 

  6. CM Bethke, S Yeakel: The Geochemist’s Workbench User’s Guides, Version 10.0. Aqueous Solutions LLC, Champaign, 2014 

  7. G Eriksson, Chem. Scr., 8, p 100-103, 1975 

  8. G Eriksson, K Hack: CHEMSAGE – A computer program for the calculation of complex chemical equilibria. Metallurgical Transactions B b21, 1013-1023, 1990 

  9. CW Bale, P Chartrand, SA Degterov, G Eriksson, K Hack, R Ben Mahfoud, J Melançon, AD Pelton, S Petersen: FactSage Thermochemical Software and Databases, published by Elsevier Science Ltd, Calphad, Vol.26, No.2, pp. 189-228, 2002 

  10. IK Karpov, KV Chudnenko, DA Kulik, OV Avchenko, VA Bychinskii: Minimization of Gibbs free energy in geochemical systems by convex programming, Geochem. International, 39, 1108-1119 2001 

  11. D Kulik, U Berner, E Curti: Modeling chemical equilibrium partitioning with the GEMS-PSI code. PSI Scientific Report 2003 / Vol IV, Nuclear Energy and Safety. Villigen, Paul Scherrer Institut: 109-122, 2004 

[last modified: 2023-11-10]