# Lesson 6 - Representing the Pond

Note: In this Lesson, we are continuing to explore the example file named Example1_ContaminatedPond.gsm.  It can be found in the “Examples” subfolder of the “Contaminant Transport Course” folder you should have downloaded and unzipped to your Desktop.

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.  You define the properties of the pathways, such as their geometry and which environmental media (e.g., water, sand, sediment) they contain. All pathways contain one or more environmental media.

GoldSim provides a number of different types of pathway elements.  This particular model uses two types of pathways: the Cell and the Aquifer:

In Unit 3, we described two different types of components in a mass transport model: those that can be represented as well-mixed compartments, and those in which concentration varies continuously over space, with the governing equation defined in terms of a concentration gradient.  Cell pathways are used to model the former, whereas Aquifer pathways are used to model the latter. You create an environmental system by defining a network of transport pathways.  The pathway network for this model can be seen inside the Contaminant_Transport container, as shown below:

In this Lesson, we are going to focus on the Pond (which is represented using a Cell pathway).  The dialog looks like this:

A Cell Pathway represents a well-mixed compartment. Hence, to define it we need three things:

• The amount of fluids and solids it contains;
• The flow rates for advective inflows and outflows of mass; and
• Any applied boundary and/or initial conditions.

As can be seen, the Pond contains only water (the sediments are modeled using a separate pathway).  The volume of water is computed as the pond area multiplied by the pond depth (and is assumed constant).

We noted when describing the conceptual model that the pond had two inflows (clean rainfall and runoff and a brief addition from the pipeline), and two outflows (evaporation and seepage). If you select that Inflows tab you will see it is empty, while the Outflows tab has only the seepage:

So what is happening here?  As we will discuss in detail in Unit 6, the inflows and outflows that we add in these tabs only include those flows that are transporting contaminant mass. The rainfall, runoff and evaporation (flows that we actually did not even bother to calculate as we assumed the pond volume remained constant) move only “clean” water.  That is, they do not add or remove contaminant mass from the pond. Hence, they are not included in these tabs. The seepage rate is included in the Outflows tab because it removes contaminant mass from the pond. The Outflow indicates that Water is moving (and hence advecting contaminants) from the Pond pathway to the Sediment_Layer pathway, and the volumetric Flow Rate of the Water is equal to Seepage_Rate (which we described in Lesson 4).

The pipeline does indeed add mass to the pathway, so why is it not included in the Inflows tab? The inflows that are included in the Inflows tab include only advective material flows from other pathways.  Any other contaminant mass inflows represent external forcing functions (i.e., boundary conditions).  Such contaminant mass inflows are specified in the Cell Inventory section of Cell pathway.  In this case, it is specified as an Input Rate equal to Pipeline_Mass_Discharge.  This element (a Time Series element) can be found in the Inputs Container.  Go there now, run the model (press F5), right-click on the element and select “Time History Result…”:

As can be seen, the pipeline discharges contaminant mass for 100 days at a steady rate of 250 g/day starting on 1 January 2021. This results in a total of 25kg (250g/day * 100 days) being added to the pond.

Close the result display and press F4 (to return to Edit Mode). Navigate back to the Contaminant_Transport Container.  In the next Lesson we will explore how the sediments are being modeled.