Problems and sources of error in geochemical modeling

Hydrochemical analyses should be as complete and correct as possible because they are the basic prerequisite of a reliable hydrogeochemical model. They represent the essential information and errors propagate from them to the final result. Fig. 32 to Fig. 34 show an example of the saturation index calculation for calcite and dolomite, of the C02 equilibrium partial pressure, and of the consequences an incomplete analysis may have. The following analysis is given (pH = 7.4, temp. = 8.1°C, conductivity = 418 pS/crn, concentrations in mg/L)  

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. 

Numerical dispersion or oscillation effects can occur as accidental source of error when using finite differences and finite element methods while modeling mass transport. Utilizing the criteria of numerical stability (Grid-Peclet number or Courant number) or the random walk procedure, these errors can be either reduced or even eliminated. 

However, the most common sources of different results are both based on the approach used for the calculation of the activity coefficient (chapter 1.1.2.6) and the thermodynamic data sets themselves (chapter 2.1.4), which provide the respective program with the fundamental geochemical information of each single species. The thermodynamic databases available partly use severely differing data with different solubility products, different species, minerals and reaction equations. Nordstrom et al (1979, 1990), Nordstrom Munoz (1994), Nordstrom (1996, 2004) discuss this inconsistency of thermodynamic datasets in detail. For some species, for which stability constants have been published, not even the existence of the respective species has been proved doubtless, as can been shown in the following example. 

Two surveys consider uranium species in the year 1992 (Grenthe et al. 1992 [NEA 92] and Fuger et al. 1992 [IAEA 92]) lead to quite different interpretations regarding some hexavalent uranium-hydroxo-species. These differences do influence considerably the species distribution of a measured total uranium concentration at neutral and basic pH values (Table 24). 

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