Many times in geochemical modeling we want to understand not only what reactions proceed in an open chemical system, but where they take place (e.g., Steefel et al., 2005). In a problem of groundwater contamination, for example, we may wish to know not only the extent to which a contaminant might sorb, precipitate, or degrade, but how far it will migrate before doing so.
To this point, we have discussed how to model the reactions occurring within a single representative volume, as shown in Figure 2.1. Such configurations are sometimes called lumped parameter models, because the properties of the entire system are represented by a single set of values. There is one pH for the entire volume, one ionic strength, a single mass for each chemical component, and so on. In studying reaction within flowing groundwater, in contrast, we may want to build a distributed parameter model, a model in which the properties vary across the system.
To construct models of this sort, we combine reaction analysis with transport modeling, the description of the movement of chemical species within flowing groundwater, as discussed in the previous chapter (Chapter 20). The combination is known as reactive transport modeling, or, in contaminant hydrology, fate and transport modeling.
A reactive transport model, as the name implies, is reaction modeling implemented within a transport simulation. It may be thought of as a reaction model distributed over a groundwater flow. In other words, we seek to trace the chemical reactions that occur at each point in space, accounting for the movement of reactants to that point, and reaction products away from it.