# Lesson 1 - Unit 12 Overview

Remember that all models are wrong; the practical question is how wrong do they have to be to not be useful.

George Box

As we noted in the first Unit, the purpose of this Course was to teach you the fundamental concepts associated with contaminant transport modeling using GoldSim. And if you took the time to work through the Examples and Exercises, we should have accomplished that.

These Examples and Exercises, while covering the fundamental concepts, were nevertheless relatively simple models.  In this final Unit, we will attempt to “pull it all together”. That is, we will discuss how we can build upon the simple fundamental concepts we discussed in earlier Units in order to build larger, more complex models.

To do this, we will discuss a variety of topics associated with building models of complex systems. In particular, we will discuss the following:

• Dealing with Uncertainty. We discussed this topic briefly in Unit 3.  In this Unit, we will reiterate the key points made in that earlier Unit, as well as introduce some additional concepts.
• Building Complex, Multi-Dimensional Models. In the Examples and Exercises we have seen in previous Units, the dimensionality of the system was quite simple (typically, zero or one-dimensional).  Of course, real-world systems are three-dimensional.  We will briefly discuss how we can build multi-dimensional models using simple zero and one-dimensional components.
• Representing Complex Flow Systems. The flow systems driving the advective transport in the models we have discussed thus far have been simple (e.g., typically steady-state).  We will discuss how more complex (e.g., transient) flow systems can be represented.
• Representing Spatial Variability.  In most complex systems, there will be spatial variability in parameters such as partition coefficients, solubilities, and even reaction rates.  We will discuss the tools that GoldSim provides to represent this.
• Representing Complex Geochemistry. In previous units we discussed a simple way in which we could impose solubility constraints on species. Although such an approach may be appropriate in some situations, in others it will not be, and a more rigorous approach is required.  We will discuss how this can be done.
• Instantaneously Changing Mass in Pathways Using Discrete Events. We discussed in detail how mass can be moved between pathways via advection and diffusion (as well as several specialized mass transfer processes).  Sometimes, however, you may want to “instantaneously” change the amount of mass in a pathway (e.g., to simulate inadvertent human intrusion into a waste site).  We will discuss how this can be done.
• Strategies for Managing Large, Complex Models. For large and more complex models GoldSim provides some tools to help you manage these models (to improve computational efficiency).  We will discuss these features.
• Advanced Contaminant Transport Elements.  We will discuss several advanced CT elements that we have not discussed in previous Units.

Some of these topics involve only conceptual discussions (e.g., dealing with uncertainty), some involve advanced features that we have not yet discussed (e.g., advanced contaminant transport elements), and some involve both (e.g., representing spatial variability).

We will close the Course by discussing your next steps, such as resources you can take advantage of to start to become an expert modeler.

This Unit has 10 Lessons (including this overview and a summary at the end). Since much of the discussion is conceptual, it does not have any Exercises, but it does have several Examples that are used to illustrate some of the features and approaches we will discuss.