Hydrogeochemical Modeling Programs

The most frequently used models are MINTEQA2 (Allison et al. 1991),WATEQ4F (Ball Nordstrom 1991), PHREEQC (PHREEQE) (Parkhurst Appelo 1999, Parkhurst 1995 Parkhurst et al. 1980) and EQ 3/6 (Wolery 1992a and 1992b). 

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Assumptions made in hydrogeochemical modeling programs complicate the transferability to natural systems, e.g. assuming thermodynamic equilibrium. This assumption is often not true especially for redox reactions being dominated by kinetics and catalyzed by microorganisms, and precipitation of certain minerals. Both processes can maintain disequilibria over a long time period. 

Hydrogeochemical models are dependent on the quality of the chemical analyses, the boundary conditions presumed by the program, theoretical concepts (e.g. calculation of activity coefficients) and the thermodynamic data. Therefore it is vital to check the results critically. For that, a basic knowledge about chemical and thermodynamic processes is required and will be outlined briefly in the following chapters on hydrogeochemical equilibrium (chapter 1.1), kinetics (chapter 1.2), and transport (chapter 1.3). Chapter 2 gives an overview on standard... 

Modeling hydrogeochemical processes requires a detailed and accurate water analysis, as well as thermodynamic and kinetic data as input. Thermodynamic data, such as complex formation constants and solubility products, are often provided as data sets within the respective programs. However, the description of surface-controlled reactions (sorption, cation exchange, surface complexation) and kinetically controlled reactions requires additional input data. 

Rehability of final data depends, first, on the correct selection of thermodynamic and chemical laws and their use in hydrogeochemical mathematical models, and second, on the selecting means of its solving and reliability of utilized values. For this reason at the evaluation of final result quality by means of software programs are identified two levels of revision validation and verification. They have the same objective but differ in properties, rules and restrictions. Their violation is considered an error. 

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