Lesson 1 - Unit 7 Overview

It isn’t pollution that’s harming the environment. It’s the impurities in our air and water that are doing it.

Dan Quayle, former Vice President of the United States

In the previous Unit, we started to consider systems in which mass is being transported between pathways. In particular, we discussed the process of advection between well-mixed compartments. In this Unit, we will continue to discuss transport between well-mixed compartments (simulated in GoldSim using Cell pathways), but will discuss some more complex processes and how they can be simulated.

One of the biggest simplifications of the models we have seen so far is that the volumes and flow rates were constant.  Of course, in the real world, we would expect the volumes and flows associated with environmental compartments to dynamically change (e.g., responding to seasons, or simply daily fluctuations). We will spend several Lessons discussing how this can be represented in GoldSim.

As discussed in Unit 3, Lesson 5, in some cases at some locations in the system you are simulating, the dissolved chemicals may be present at high enough concentrations such that they precipitate out of solution (i.e., exist in both a dissolved state as well as a solid or liquid state). In these situations, the solute is still present in water, but its concentration is fixed at its solubility limit. This process can obviously have an enormous impact on mass transport, as it has the effect of limiting the dissolved concentration (and hence the advective flux) of contaminants through a system. For some applications, it may be necessary to model the various geochemical processes in great detail. GoldSim itself cannot carry out detailed geochemical calculations. You could, however, dynamically link GoldSim to a tool that specifically does so. In many cases, however, such an approach may not be necessary or appropriate at all. A simpler approach, such as specifying a solubility value for each species that you are modelling may be appropriate. We will discuss this approach in this Unit.

Advection is the transport of material (e.g., contaminants or other chemicals) via the bulk movement of the medium with which those materials are associated.  Typically, the medium of interest is water, and the materials that are advected are solutes that are dissolved in that water. However, advection does not need to involve only fluids such as water. Conceptually, chemicals can also be advected with solids.  The chemicals must first become associated with the solid in some way. Most commonly, they will be sorbed (partitioned) onto the solid, but they may also be precipitated out of solution (due to solubility constraints) onto the solid. Once they are associated with the solid, the mass can be advected in two different ways: 1) the solid itself can move (e.g., soil erosion); or 2) the solid can be suspended in water and be transported with the water.  We will spend several Lessons discussing these kinds of processes.

For most systems that you will simulate, advection will typically be the dominant transport process that you will model. As we will discuss in later Units, for many systems, the processes of diffusion and dispersion are also important. However, there may be some types of processes that transport and/or modify the mass in your system that cannot be easily described using just advection or diffusion. This is typically because they are relatively complex processes (e.g., animal burrowing, plant uptake, chemical treatment) that would be very difficult to fully describe and represent accurately using the actual physical and chemical processes controlling them, but can be readily approximated using a simplified approach.  GoldSim provides some specialized tools for representing these kinds of processes, and these will be discussed at the end of this Unit.

This Unit has three Exercises, and eight Examples that we will walk through together. It has a total of 12 Lessons (including this overview and a summary at the end).

Note: Starting with this Unit, the concepts discussed will become more complex and detailed. It is recommended that you work through these Units slowly, and carefully follow along with the Examples and Exercises. As a result, it is likely that they will take significantly longer to complete than earlier Units.