# Lesson 11 - Unit 3 Summary

In this Unit we provided an overview of the overall philosophy and approach to environmental modeling embodied in GoldSim and the Contaminant Transport Module. We took a somewhat circuitous route (describing the various processes GoldSim can simulate, how those processes are represented mathematically, why uncertainty is such an important issue, and the overall GoldSim modeling philosophy).  However, covering this basic material was necessary in order for you to fully understand why GoldSim is designed the way it is. That is, in order for you to build effective contaminant transport models in GoldSim, it is necessary for you to fully understand (and “buy into”) the philosophy on which GoldSim is based. This provides the fundamental underpinnings for understanding the material in the remaining Units.

We started the Unit by providing by some introductory material on the key decisions required before you start to build a contaminant transport model: In particular, we discussed the importance of the following:

• Defining your objectives (why are you building the model) in terms of specific quantitative questions you are trying to answer;
• Carefully define the boundaries of the system; and
• Determining the processes you will need to include in your model, along with the level of detail with which the processes should be represented.

Next, we briefly introduced the physical and chemical processes controlling mass transport, described in general terms how these are represented mathematically in equations, and how the equations are solved in GoldSim:

• Advection (transport via the bulk movement of a medium, typically a fluid) will likely be the dominant physical transport process in systems that you model.
• Diffusion (molecular and eddy) and mechanical dispersion act to disperse mass as it is transported.
• When modeling a chemical process, it is important to differentiate between whether we are representing the process from a standpoint of kinetics or equilibrium. Kinetics describe the rate at which a process takes place.  Equilibrium describes the system in terms of the final expected outcome. Equilibrium models of a chemical process are appropriate if the process is rapid compared to other processes of interest. GoldSim can represent both.
• Chemical processes that can influence mass transport include reactions, sorption, volatilization and solubility constraints.
• In some cases, there may be processes that can’t easily be described using the physical and chemical processes described above. GoldSim provides several specialized mass transport mechanisms to represent these processes.
• We described two different ways we might represent parts of an environmental system mathematically. In one, we represented the system as a series of well-mixed compartments. This would be appropriate, for example, when dealing with a system of well-mixed tanks, or perhaps a stratified lake. In the other, we described a system (a one-dimensional “tube”) in which the concentration varies continuously over the component, and the governing equation is defined in terms of a concentration gradient. This would be appropriate, for example, when dealing with a system such as an aquifer. In GoldSim, we treat and solve these equations using different approaches.

We then discussed a critical issue associated with contaminant transport modeling that is often overlooked or ignored, the problem of uncertainty:

• There are very large uncertainties (in some cases, several orders of magnitude) in many of the parameters, processes and events associated with contaminant transport models.
• Explicitly representing these uncertainties (by carrying out probabilistic simulations) is critical.

This then led directly into a discussion of the modeling philosophy embodied in GoldSim and the Contaminant Transport Module:

• The GoldSim philosophy revolves around the idea that the complexity and detail that you include in your model should be consistent with the amount of uncertainty in the system.
• Realistically acknowledging and representing uncertainty has important implications for how you should build and structure your models. In particular, GoldSim encourages a “top-down” approach to model building.
• In a top-down model, as opposed to representing all processes with great detail from the outset, details are added only when justified (e.g., if additional data are available, and if simulation results indicate that performance is sensitive to a process that is currently represented in a simplified manner).
• In a top-down modeling approach, models are built iteratively. That is, they evolve over time to a level of complexity that is appropriate given the modeling objectives, information about the system, and the uncertainty in that information.
• GoldSim contaminant transport models tend to have a relatively low level of spatial resolution. They focus on integrating and coupling all system components and explicitly representing uncertainty.

Finally, we provided a very broad overview of how contaminant transport models are built in GoldSim: