# Lesson 1 - Unit 5 Overview

Environmental science has learned that chemical behavior in [a] complex assembly of media is not a random process like leaves blowing in the wind.  The chemicals behave in accordance with the laws of nature which dictate chemical partitioning tendencies and rates of transport and transformation.

Donald Mackay

In earlier Units, we provided a review of the basic contaminant transport processes modeled by GoldSim, described in general terms how these processes are represented mathematically, and discussed the overall philosophy and approach to environmental modeling embodied in the software.  In the previous Unit, we provided an overview of what a contaminant transport model looks like in GoldSim and showed the kinds of results it can produce. Starting with this Unit, we are ready to discuss in detail how you can use the Contaminant Transport Module to build models.

We are going to begin our discussion of the details of the GoldSim Contaminant Transport Module by starting with a very simple example (that we will build together): a model of a single well-mixed compartment (e.g., a tank). Lesson 2 describes this system.

The fundamental building blocks on which all GoldSim contaminant transport models are based are 1) the species which are to be simulated; 2) the environmental media in which these species are transported and stored; and 3) pathways that perform the transport calculations. In GoldSim, the first two are collectively referred to as material. GoldSim provides three specialized material elements: Species, Fluids and Solids. Fluids and Solids are referred to as media. Specification of these material elements is the first step in representing an environmental system within GoldSim. Lessons 3 through 6 describe the use of these elements.

Transport pathway elements are the key objects used to build contaminant transport models in GoldSim (and these are the elements that actually solve the equations we discussed in Unit 3). Transport pathways represent physical components through which contaminant species can move and/or be stored, such as aquifers, lakes, sediments, surface soil compartments, and the atmosphere. You define the properties of the pathways, such as their geometry and which environmental media (e.g., water, soil, air) they contain. All pathways contain at least one environmental medium.

In Unit 3, we described two different ways for representing parts of an environmental system mathematically.  In one, we represented the system as a series of well-mixed compartments.  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.  In GoldSim, we treat and solve these equations using different types of transport pathways (the most important being Cells, Aquifers and Pipes).

This second part of this Unit (as well as all of the next three Units) describe how Cell pathways can be used to represent part of a system as a well-mixed compartment. We will describe how media are assigned to Cells, how mass is added, the types of outputs they produce, and how the equations they represent are solved.

As we shall see, in this Unit we will be considering only a single Cell pathway. Although the Example and Exercise we will discuss involve multiple species and environmental media, we will not yet discuss the transport of mass into or out of the Cell.  In that respect, these simple examples are not really transport models at all (as nothing is actually being transported)!  As such, they may not seem to be particularly interesting.  But be patient. We need to start with a very simple system in order to introduce and describe some fundamental features. We will build on this in subsequent Units.

In this Unit we will build a model together, and afterward will then work through a separate Exercise to review what we have learned. It has a total of 12 Lessons (including this overview and a summary at the end).