Geochemical models computer-based

The most common approach used by geochemical modeling codes to describe the water-gas-rock-interaction in aquatic systems is the ion dissociation theory outlined briefly in chapter 1.1.2.6.1. However, reliable results can only be expected up to ionic strengths between 0.5 and 1 mol/L. If the ionic strength is exceeding this level, the ion interaction theory (e.g. PITZER equations, chapter 1.1.2.6.2) may solve the problem and computer codes have to be based on this theory. The species distribution can be calculated from thermodynamic data sets using two different approaches (chapter 2.1.4) ... 

Ball J. W. and Nordstrom D. K. (1987) WATEQ4F—a Personal Computer Fortran Translation of the Geochemical Model WATEQ2 with Revised Data Base. US Geol. Surv. Open-File Report, 50-87. 

Computer-based modeling on geochemical principles, using either the equilibrium constants or Gibbs free-energy values. Both approaches are subject to the conditions of equilibrium and mass balance. 

Throughout the present review the SIT is used for ionic strength corrections. However, numerous computer codes for geochemical model calculations, in particular for calculations in concentrated chloride solutions, are based on the ion interaction equations of Pitzer [1991PIT]. Pitzer parameters reported in the literature to calculate activity coefficients for the Th" ion in chloride solutions are briefly discussed and summarised in Table VI-2. 

Using basic chemical theory, Sposito (1986) and Morgan (1987) have outlined the principles for elucidating metal speciation in the natmal environment. Sposito and Mattigod (1980) and Sposito and Coves (1988) have developed computer-based geochemical modeling approaches specifically for soils. 

In recent years, several computer-based models have become widely available and are used successfully, e.g. SOILCHEM (the updated version of GEOCHEM Sposito and Coves, 1988), HYDRAQL (Papelis et al, 1988), ECOSAT (Keizer, 1991), and MINTEQA2 (Allison et al, 1991). 

The expression of chemical fate can be computerized using a code to perform the computations and predict the results when inputs simulating conditions of interest are provided. Two critical aspects of the use of computer codes for predicting geochemical fate are the verification and validation of the models on which the codes are based. 

A USGS model for computing the chemical evolution of the aqueous phase as the result of specified geochemical reactions. Based on an ion-pairing aqueous model,... 

Based on the idea of HS as a macromolecular polymer, some chemical model structures have been proposed combining geochemical, wet-chemical, biochemical, spectroscopic, thermal, agricultural, and ecological data with analytical pyrolysis and modern computational chemistry. An example is shown in Schulten and Leinweber... 

PHREEQE A geochemical computer code based on PC (w/PHRQ- the ion-pairing model which calculates INl T and pH, redox potential and mass transfer. 

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