This relationship is described by an adsorption function which relates the adsorption to the concentration. An example of this relationship is shown in Figure 1.5. In general, the adsorption characteristic of a contaminant in a soil is represented by a function of S vs. C, where S is the mass of contaminant adsorbed per unit mass of soil particles, and C is the concentration of the contaminant in the porous medium. The chemical reactions that cause the partitioning are assumed to be instantaneous and reversible. Another important attenuation process in the case of a radioactive contaminant mass is radioactive decay.
Radioactive decay causes a loss of contaminant mass from the flow system.
However, unlike adsorption, the decayed mass is proportional to the travel time and is irreversible. Return to top . Advective Contaminant Transport As described above, contaminant transport in soils involves the processes of both advection and dispersion. Early in a contaminant transport analysis it is often useful to isolate the magnitude of purely advective transport without the extra data input and computational requirements of including dispersion. It is impossible to numerically solve the advection-dispersion equation when the dispersive component is small relative to the advective component because the numerical solution is unstable in these cases.