Computer codes GEOCHEM

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. 

Jackson, K. J. and T. J. Wolery, 1985, Extension of the EQ3/6 computer codes to geochemical modeling of brines. Materials Research Society Symposium Proceedings 44, 507-514. 

Kulik, D. A. 2002. Minimising uncertainty induced by temperature extrapolations of thermodynamic data A pragmatic view on the integration of thermodynamic databases into geochemical computer codes. Proceedings of the Workshop on The Use of Thermodynamic Databases in Performance Assessment , 29-30 May 2001, Barcelona, Spain. Organisation for Economic Cooperation and Development OECD, Paris, France, 125-137. 

Surface complexation model A computer code or geochemical model that provides an explanation and attempts to predict the partitioning of a chemical species between the surface of an adsorbent and the associated solvent. The models consider a number of factors, including pH and ionic strength (see (Langmuir, 1997), 369-395 for details compare with charge distribution multisite complexation model). 

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) ... 

A computer code is obviously not a model. A computer code that incorporates a geochemical model is one of several possible tools for interpreting water-rock interactions in low-temperature geochemistry. The computer codes in common use and examples of their application will be the main focus of this chapter. It is unfortunate that one commonly finds, in the literature, reference to the MINTEQ model or the PHREEQE model or the EQ3/6 model when these are not models but computer codes. Some of the models used by these codes are the same so that a different code name does not necessarily mean a different model is being used. 

Bradbury, M. H., and Baeyens, B. (1994) Sorption by Cation Exchange Incorporation of a Cation Exchange Model into Geochemical Computer Codes. Bericht No. 94-07, Paul Scherrer Institute, Wiirenlingen. 

Selecting the least components (also called master species) is one of the fundamental and essential input decisions made in geochemical computer codes such as PHREEQE (Parkhurst et al. 1990), WATEQF (Ball and Nordstrom 1991), and M1NTEQA2 (Allison et al. 1991), for example. 

In studies of the state of saturation of minerals in natural waters and in most of the geochemical computer codes, the saturation index (SI) is used. The index is defined as 5/= log,o((2/ eq). so that 5/ = 0 at equilibrium (at saturation) of the mineral with the solution. The saturation index and AG, are related through SI = AG,./(2.3026 RT). If the reaction is written with the mineral as the reactant, then when SI and AG, are both negative, the mineral is undersaturated and so will tend to dissolve. When both are positive, the mineral is supersaturated and will tend to precipitate from solution. 

Calculate the volume-corrected titration curve for the titration of 0.01 N HCl with 0.01 N NaOH between pH 2 and 12 using a geochemical computer code and compare the result to the corresponding curve in Fig. 5.5. This problem can be solved using the mixing option in SOLMINEQ.88 (Kharaka et al. 1988). The titration is simulated by mixing different fractions of solution 1 (pH = 12.0, Na" = 0.01 mol/kg) with solution 2 (pH = 2.0, Cr = 0.01 mol/kg). In the output Cg = Na". Computed results are given here. 

So far we have used phase diagrams to visualize clay mineral stabilities and phase relations involving the clays in natural waters. Given the complex chemistries of mixed-layer clays in particular, geochemical computer codes offer a more rigorous way to evaluate their stabilities. The thermodynamic data bases of most of these codes list stability constants for a variety of clay minerals which, except for kaolinite, are usually of nonideal composition. Most of these stability constants have been obtained from solubility measurements and are of mixed reliability. It is appropriate to... 

Debraal, J. D., and Y. K. Kharaka. 1989. SOUNPVT A Computer code to create and modify input files for the geochemical program SOLMINEQ.. Open-Kle Report 89-616. Menlo Park, CA U.S. Geol. Survey. 

The NEA Data Bank maintains a library of computer programs in various areas. This includes geochemical codes such as PHREEQE, EQ3/6, MINEQL, MINTEQ and PHRQPITZ, in which chemical thermodynamic data like those presented in this book are required as the basic input data. These computer codes can be obtained on request from the NEA Data Bank. 

The most widely used geochemical modeling programs consist of a computer code plus a related file of data called a database. The database contains thermodynamic and kinetic parameters. The code uses the thermodynamic and kinetic parameters in the database and concentrations or other constraints as input, and produces results that describe a geochemical model for a particular chemical system. 

The historical evolution of chemical codes has not been documented nor will it be attempted here, yet several publications are available that describe the variety of codes from which to choose (2-4). One distinct trend of this past decade has been the cessation of the rapid increase in the number of models being developed this has been replaced with a more focused effort to document d improve existing models. A small number of these codes have become the mainstay for geochemical applications. An abbreviated pedigree of computer codes is illustrated in Figure 1. The initial conference on this subject (2) was convened during the early phase of model development, and... 

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. 

PHRQPITZ A program adapted from the computer code PHREEQE which makes geochemical calculations in brines and other electrolyte solutions at high concentrations, using the Pitzer virial coefficient approach (see paper, this volume). 

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. 

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