Lesson 6 - Other Processes Controlling Mass Transport
In the majority of cases, you will be able to describe mass transport in terms of the physical and chemical processes we have described in the previous three Lessons (advection, dispersion, diffusion, reactions, sorption, volatilization, solubility).
In some cases, however, there may be processes that don’t fit neatly into one of these categories. Of course, they still follow physical and chemical laws (no magic is involved!), but the mathematical representations of the processes we described above may not be appropriate to describe them.
One such example is settling of particles in a water column. If particles are continuously added to the system, the chemical of interest sorbs onto them, and they subsequently settle out of solution. Depending on the system (e.g., is resuspension possible?), for practical purposes, this may effectively remove mass from the system (the water column). Settling (and resuspension) is a complex process. How would we account for this using the processes described above? Is it advection? You could potentially treat it that way (advection of the solid downward), but would that be correct? Or should we treat the particles themselves in the same manner as we treat dissolved mass and model the settling as a first-order loss mechanism? Or do we treat it as a generic process that simply removes mass from the water column at a specified rate that is proportional to the concentration?
What if you were trying to simulate a complex treatment process (e.g., if your model included a treatment plant)? Do we try to model all of the complex treatment processes explicitly? Or can we perhaps simply assume that the process removes a specified percentage of each species?
What about transport mediated by plants and animals? Plants, for example, have preferential uptake for certain metals in their roots. These metals may then be transported from the roots all the way to the ground surface and into the leaves. Animals (e.g., rodents or insects) may move bulk soil (containing your contaminant of interest) in solid, water or perhaps air phases upward (or downward) in the soil column. How would we represent these processes?
One transport process that is unique to long-term radioactive waste management models is that of unintentional human intrusion. That is, it is possible that somebody may unintentionally intrude into a facility (e.g., drill into an underground repository), bringing waste material directly to the ground surface. How do we represent this process?
You may need to occasionally represent one or more of these kinds of transport processes when you are building a model. To handle them, you may use some combination of the physical and chemical processes we described above (e.g., the effect of burrowing animals could perhaps be represented by advection of the soil and its associated water). As we will see in later Units, GoldSim also provides several specialized mass transport mechanisms (that are not physically-based) that allow you, for example, to simply move mass at a specified rate from one location to another, and this may allow you to represent these somewhat unusual and complex processes in your models.