Geochemical modeling programs

Fig. 27 Overview on hydro-geochemical modeling programs in chronological order...

Thermodynamic databases are the primarily source of information of all geochemical modeling programs. Basically, it is possible to create one s own thermodynamic dataset with almost any program. However, it is a considerable effort and requires great care. Normally one accesses already existing data sets. 

Geochemical modeling programs are built, for the most part, on the fundamental laws of thermodynamics and kinetics. In this chapter we introduce the subject of equilibrium thermodynamics. Kinetics is dealt with in Chapter 11. 

To calculate the activity coefficient of ions, virtually all geochemical modeling programs today use either a variation of the Debye-Hiickel equation or the Pitzer equations. Two variations of the Debye-Hiickel equation in common use are the Davies equation and the B-dot equation. 

Geochemical modeling programs need to have information about the total carbonate concentration (Equation (3.51)). 

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. 

Table 4.2. Functions of popular geochemical modeling programs.

The databases associated with geochemical modeling programs consist of a list of the basis species, and a list of secondary or auxiliary species, minerals, and gases, each described in terms of the basis species, and with the equilibrium constant of the reaction linking the secondary species or mineral to the basis species. 

Data on samples to be examined are given in a table which is read by the expert system program. The table includes sample identification, analytical data, and the results of simple calculations such as for charge balance and the sum of analyzed dissolved constituents. It also includes mineral saturation indices and data on the carbonate system calculated by the geochemical modeling program. 

The carbonate module in the present system compares values of the total dissolved carbonate concentrations from several sources to evaluate the internal consistency of data on the carbonate system. Three total carbonate concentrations may be available tdcl, the sum of the analyzed concentrations of dissolved CO2, bicarbonate and carbonate, tdc2, the analyzed total dissolved carbonate concentration, and tdc3, the value calculated from the analyzed pH and alkalinity by the geochemical modeling program. Differences among these three concentrations are used to decide the quality of the carbonate analytical data. 

Having produced a better pH value, a coupled expert system could call a geochemical modeling program and rerun it with the new pH. The newly modeled total dissolved carbonate values and carbonate mineral saturation indices could then be substituted for the old values in the water data set and the expert system rerun to test for improved consistency among the new water data. 

The present expert system program discovers possible errors and inconsistencies in a ground-water analysis, but it is restricted to water from carbonate aquifers and it cannot directly interact with geochemical modeling programs. The next step in its development will be to couple the carbonate module to a geochemical model to enable the expert system to adjust an analysis for consistency among the parameters of the carbonate system. The expert system will then be applicable in practice to adjust... 

Usually, alkalinity is actually the ultimate parameter that is subject to a procedure of genuinely chemical titration, in this regard as a proxy parameter for carbonate. A new spectropho-tometric method for the determination of alkalinity was proposed by Sarazin et al. (1999). Mostly, alkalinity will be, for reasons of simplification, set equal to the total carbonate concentration, although a number of other substances in pore water will contribute to the titration of alkalinity as well. Most geochemical model programs (cf Chap. 15) foresee the input of titrated alkalinity as an alternative to the input of carbonate. The model program will than calculate the proportion allo-catable to the different carbonate species. 

Table 15.1 This ocean water analysis carried out by Nordstrom et al. (1979) has been frequently used to test and compare various geochemical model programs.

Table 15.2 For an analysis of Table 15.1, this allocation of the aquatic species was calculated with the geochemical model program PHREEQC.

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