Basic approaches and important models of chemical transport will be addressed briefly. Three major approaches can be used to modeling chemical transport:
1. Retardation-factor models, which incorporate a simple retardation factor derived from a linear- or linearized-distribution coefficient
2. Integrated models, in which all mass, momentum, and energy transfer equations, including those in which chemical reactions participate, are solved simultaneously for each time step in the evolution of the system
3. Two-step models, which first solve mass momentum and energy balances for each time step and then reequilibrate the chemistry using a distribution-of-species code.
Empirically determined retardation factors (either partition coefficients or breakthrough curve measurements, which are the change in solute concentration measured over time in laboratory or field experiments) have been widely used because of their inherent simplicity.162 Modeling of specific geochemical partition and transformation processes is not necessary if the retardation factor can be determined empirically.
The problems with linear-distribution coefficients apply equally to any retardation factor derived from them. Field measurements can be made but are expensive to obtain and highly site specific. Nevertheless, retardation factors provide some insight into organic chemical transport.
Integrated and two-step chemical-transport models incorporate distribution-of-species or reaction-progress codes into hydrologic transport codes. The few studies in which the two approaches have been tested using the same set of field data have agreed reasonably well; thus one approach does not have an obvious advantage over the other. The two-step approach tends to be computationally less intensive than the integrated approach but may have difficulty maintaining mass balance when rapid precipitation and dissolution occur.120
A number of models of both types have been described in the literature. Of the models, DYNAMIX would appear to have the greatest potential for use in simulating chemical transport in the deep-well environment because it incorporates the reaction-progress code PHREEQE, which can handle deep-well temperatures. PHREEQE, however, does not incorporate pressure equilibria.
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