Lesson 6 - Defining the Environmental Media
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. All pathways contain one or more environmental media. You define the general properties of each medium (e.g., its density) as well as the properties of each species in each medium (e.g., solubilities and partition coefficients).
You can define two types of media in GoldSim: Fluids and Solids. All models contain a special type of Fluid called a Reference Fluid. The Reference Fluid provides a basis for defining partition coefficients between media for the various species in the model (we will talk about this more below). All models must have a Reference Fluid, and it cannot be deleted. For convenience, the Reference Fluid "Water" is automatically present in the Material Container when you create a new model (since this is the most common Reference Fluid).
Let’s go to the Material Container now, and double-click on the Water Reference Fluid:
You will note that although you can’t delete the Reference Fluid, you can rename it. Why can this be renamed? In almost all environmental models, the Reference Fluid will indeed be water, but, for example, if you were simulating the fate and transport of a contaminant within an organism (such as a fish) the Reference Fluid might be blood.
In this example model, we don’t need to make any changes at all to the Water Reference Fluid element. The first three input fields pertain to quantifying how the species diffuse through the Fluid. In this example, we are not simulating diffusion, so we can leave these at their default values. We will discuss modeling diffusion in Unit 8. The final input is for specifying the solubility of the various species in the Fluid. Again, in this example, we are not simulating solubilities, so we can leave these at their default values (which assumes that the species are infinitely soluble). We will discuss modeling solubility limits in Unit 7.
One final note on the Reference Fluid element: Like the Species element, you will almost always only have single Reference Fluid element in a GoldSim model. In fact, you cannot insert or copy a Reference Fluid element. There are, however, special cases where you can create additional Reference Fluid elements (via a process called cloning) in the same model (i.e., in situations where the Fluid properties, such as solubility, of the species being simulated vary spatially in a system). We will discuss this special case in Unit 12.
Recall in Lesson 2 that we said that the tank also contained a fine-grained sand. To represent this, we need to insert a Solid element (and we will do this in the Material Container):
When you do so, the Solid dialog will be displayed:
Let’s begin by renaming this to Sand (and perhaps providing an appropriate Description). The first input is a physical property of the Solid (the Dry Density). As it turns out, in this particular model, this input is not actually used (so that we could keep it at its default value). However, for many models (in particular, those that use Aquifer or Pipe pathways to simulate transport through a porous medium) this is an important input, so it is good to get in the habit of entering it. You should have previously created a Data element for this input, so enter it now. When you are done, the Solid should look something like this:
Like the Dry Density, the next two inputs (Porosity and Tortuosity) are not actually used in this simple example model (they are only used when to simulating transport through a porous medium). In this case, for simplicity, we will just keep the default values.
The next input (Partition Coefficients) is important and we need to enter it here. Partition coefficients need to be entered for every species. That is, we will enter them as a vector of Species.
Note: If your list of species included isotopes of the same element, you would need to specify Partition Coefficients for every element (rather than species). This is because they are actually an elemental property, as opposed to a species property. That is, they would need to be entered as a vector of Elements. If you are not simulating isotopes (and we are not in this example), however, you need not be concerned with this, and they can be entered as a vector of Species. We will discuss entering these as a vector of Elements (when simulating isotopes) in Unit 10.
By default, the vector is defined locally, which means that you can edit it directly from this dialog (by pressing the Edit… button to the right of the input field). If you do that, the following dialog will be displayed:
This allows you to enter values directly here. However, you will rarely want to do that (and we don’t want to do that here). This is because it is much better to create a single element (a vector) containing all of the values, as opposed to entering each separately here. So close this dialog (press OK), and in the main Solid dialog, press the Clear button. Answer Yes to the prompt, and this will clear the local vector, and make the edit field available to you for entering a link:
This allows us to add a link to a vector containing the partition coefficients for all of the species. You should have previously created a Data element for this input, so enter it now. When you are done, the Solid should look something like this:
After you do this you will note that there is an influence line from Water to Sand:
This indicates that Sand is a function of Water (i.e., Water affects Sand in some way). This might seem odd since the Sand does not explicitly reference the Water element as an input. However, the Sand element implicitly references Water. Why is this the case? Sand, Like all Solids, has partition coefficients. A partition coefficient is defined as the ratio of the species’ concentration in the medium to its concentration in the Reference Fluid (Water) at equilibrium. As a result, the partition coefficients for Sand are inherently a function of Water (since they are specifically defined relative to Water). Therefore, the Sand element itself is a function of the Water element.
Note: By default, the color of an influence in GoldSim is a function of the type of information it is carrying. Influences representing values are black and influences representing other kinds of information (e.g., conditions or discrete events) are other colors (e.g., green or red). Influences carrying information from Media (and the Species) elements are brown.
Note: Partition coefficients are one of the properties that are most likely to be highly uncertain in most models. Moreover, the partition coefficients of various species are likely to be correlated to some degree. We will discuss this very important topic in Unit 12.
You can close the Sand element (and save the model). We have added and edited all of the media elements that we need for this example model (the Reference Fluid Water and the Solid Sand).
Note: Although this example model only has one Fluid (the Reference Fluid Water) and one Solid, a model can have multiple Solids, as well as Fluids other than the Reference Fluid (e.g., air). We will discuss this later in the Unit.