Lesson 7 - Representing the Tank Using a Cell Pathway

Now that we have defined the Species and the media for this model, we can define the single transport pathway required. Transport pathway elements are the key objects used to build contaminant transport models in GoldSim. 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 one or more environmental media.

GoldSim provides a number of different types of pathway elements. The tank will be represented by a Cell pathway. Cell pathways are used to represent well-mixed compartments. Although a well-mixed compartment may seem like quite a simple component, we will see that such components, when properly linked together, can represent quite complex systems.  As such, the Cell pathway is perhaps the most important and commonly used transport pathway.

Go to the top-level Container of the model, and insert a Cell pathway now:

The Cell dialog looks like this:

Let’s start by renaming the Cell (e.g., you can call it “Tank”).

The primary inputs that must be defined for a Cell pathway are the quantity of each Medium present in the pathway. The Reference Fluid (in this case, Water) is automatically present in a Cell when it is first created. In the Amount column, enter the name of the Data element defining the volume of Water.

Next, we want to specify that we also have Sand in the Cell.  To do this, press the Add Medium button, browse into the Material Container in the Insert dialog, and select Sand. After doing so, in the Amount column, enter the name of the Data element defining the mass of Sand. The dialog should look like this:

Next we need to put some contaminant mass into the Cell. This is specified in the Cell Inventory section of the dialog. As we will see in later Units, the drop-list provides several ways to do this (e.g., as an input rate or initial value). In this example, we are assuming that the Tank starts with an initial amount of mass of the two species added at the beginning of the simulation, so we will stick with the default setting (“Initial Inventory”). Enter the name of the Data element defining the mass of species. The dialog should then look like this:

The last thing we need to do is instruct the Cell to save some results (by default, GoldSim does not save any results in transport pathways since for some models these results can be quite large). Instead, you must specify which results you wish to save.  

You will note that next to each Medium row there are three checkboxes (F, H and S):

If you hold the cursor over these headings, they will display what they represent:

• F instructs GoldSim to save the Final Value of the concentration for that Medium. Check this for both Media.
• H instructs GoldSim to save the Time History of the concentration for that Medium. Check this for both Media.
• S (which is only available for Solids)specifies that the Solid is suspended in the Fluid. In our case, we are assuming that the Solid is indeed suspended, so we can check this box also.

Finally, at the bottom of the dialog (in the Save Masses in Pathway section), check the boxes for Final Values and Time History. Now the dialog should look like this:

Close the dialog, and hold your cursor over the Cell element.  When you do so, two ports (small boxes) will appear on either side of the element. The box on the right is the output port. You should know from the Basic Course that this is how you can access all of the outputs for an element. Left-click on the output port now:

The outputs from a Cell always include 1) the mass of each species in the Cell; and 2) the concentration of each species in each Medium in the Cell. In this example, what you will see is that the Cell has four different outputs that we will be able to view once we run the model.  All four outputs are vectors of Species:

• Mass_in_Pathway: This is the total amount of mass (of each species) in the Cell.
• Concentration_in_Water: This is the total (effective) concentration (of each species) in Water.  Because there is a suspended solid present, this includes the mass associated with the suspended solid.  It has dimensions of mass/volume.
• Concentration_in_Sand: This is the total concentration (of each species) in/on the Sand particles themselves. It has dimensions of mass of species/mass of sand.
• Dissolved_Conc_in_Water: This is the dissolved concentration (of each species) in Water.  It does not include the mass associated with the suspended solid, and only includes the mass dissolved in the Water itself. It has dimensions of mass/volume. Note that if the Solid was not suspended, this output would not appear.

We have two different Water concentrations here because we have specified that the Sand is suspended in the Water. To understand these two water concentrations (and why they are important), consider taking a sample of water from the tank. If after taking the sample, we filtered the water in some way (such that all of the suspended Sand was removed), and then analyzed the sample for the concentration, we would observe only the dissolved concentration (Dissolved_Conc_in_Water).  If, on the other hand, we did not filter the water before analyzing the sample, we would observe the total effective concentration (Concentration_in_Water). As we will see in a later Unit, this becomes particularly important if the water is moving (the contaminant mass is being advected). This is because in such as case, the rate of advection is a function of not the dissolved concentration (Dissolved_Conc_in_Water), but the total effective concentration (Concentration_in_Water), since the suspended solids would also be advected with the water.

As noted, all of these results are vectors of Species.  You can see this by expanding any of the four outputs:

Finally, you will note that there is an influence from the Material Container to the Cell:

If you hold your cursor over the influuence, you will notice that it actually represents three different links: one from Water, one from Sand and one from the Species element. The Water and Sand are directly referenced by the Cell.  The Species are indirectly referenced (all pathways are a function of the Species element) since, by definition, all the pathways solve the equations for all species in the model.

Make sure you save your model. In the next Lesson we will run it and look at some results.