Computer codes PHREEQE

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). 

The saturation states of these waters with respect to a range of minerals were calculated using the computer code PHREEQE. This approach indicated that the groundwaters are approximately in equilibrium with respect to quartz, calcite, kaolinite, fluorite, ferrosilite, ferric hydroxide and siderite (Table 2). The water/rock interactions deduced from the SEM-EDS analyses and the theoretical calculations using PHREEQE, are summarized in Table 3. 

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. 

In another example, five test cases were computed by PHREEQE and EQ3/6 and the same thermodynamic database was run for each program (INTERA, 1983) to test for any code differences. The five examples were speciation of seawater with major ions, speciation of seawater with complete analysis, dissolution of microchne in dilute HCl, reduction of hematite and calcite by titration with methane, and dedolomitization with gypsum dissolution and increasing temperature. The results were nearly identical for each test case. Test cases need to become standard practice when using geochemical codes so that the results will have better credibility. A comparison of code computations with experimental data on activity coefficients and mineral solubilities over a range of conditions also will improve credibility (Nordstrom, 1994). 

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. 

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. 

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. 

Falck, W.E. (1991) Multisite binding equilibria and speciation codes incorporation of the electrostatic interaction approach into PHREEQE. Comput. Geosci., 17, 1219-1234. 

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