# Lesson 12 - Unit 6 Summary

In this Unit, we started to consider systems in which mass is being transported between pathways. In particular, we discussed the process of advection between well-mixed compartments. Advection is the transport of material via the bulk movement of the medium (typically water) in which those materials are dissolved or suspended.  Advection will typically be the dominant transport process in systems that you will model.

We began by describing how advective transport processes can be defined between Cell pathways by specifying outflows (and inflows) between the Cells.  Doing so creates a link between the two pathways referred to as a mass flux link (as it signifies that mass is being transported between the Cells).  There are different types of mass flux links that can be created.  The type of link we explored in this Unit is an advective mass flux link.

It is important to remember that the Contaminant Transport Module itself is used to model the movement of contaminant mass through the system.  When we create advective mass flux links, we need to specify media flow rates.  The pathways then solve equations based on specified media flow rates.  Pathways do not themselves solve for the media flow rates (these are inputs to the pathways). This means that you are required to create a flow model (using basic GoldSim elements such as Pools, Reservoirs and Expressions) that produces the media flow rates that can subsequently be used by the Contaminant Transport Module.

GoldSim does, however, provide some warning messages to ensure that you are careful in this regard.  For example, if the Inflows and the Outflows you specify for a Cell do not match, conceptually this could potentially indicate a flow imbalance.  In particular, if the Inflows to a Cell exceed the Outflows, we saw that GoldSim issues a warning.  Although there are possible ways that this could physically happen (e.g., evaporation), in most cases it would not be physically correct.  Yet it is likely that EVERY model you build will have at least one Cell like this.  Why?  Because at some point, you reach the boundary of your system beyond which you are no longer interested in tracking the mass. By definition, such a Cell will have Inflows but no Outflows. It is a sink for mass in the model.  We can avoid this warning very easily. In particular, if the first four characters of a Cell pathway are “sink” (e.g., Sink, Sink1) GoldSim will not generate any warning messages for that Cell.  This is useful in two ways: 1) it eliminates the warning message; and 2) it alerts anyone viewing the model that those Cells are sinks.

Note that GoldSim does not generate a warning if the Outflows exceed the Inflows. This is because such a case simply implies that clean Water is entering the Cell (which is not an unreasonable assumption for many pathways).

After working through a simple advection Exercise (and discussing the equations that GoldSim solves), we worked through another one in which we modeled partitioning.  This illustrated the important impact of this process on advective transport: partitioning slows the advective transport process, since the concentration being transported is reduced.

Next, we showed how you can specify boundary conditions (i.e., a mass input rate) for a pathway.  We also discussed how you could combine an initial condition with a boundary condition for a pathway.

With the exception of the Example models we looked at in Unit 4 to provide an overview of the Contaminant Transport Module, all of the other Examples and Exercises we looked at in the Course involved well-mixed tanks.  A well-mixed tank provides an excellent example of the kind of physical system that can be readily represented by a Cell.  Although you may indeed have a need to simulate an actual well-mixed tank using GoldSim, more often, of course, you will need to simulate a natural environmental compartment such as a pond, reservoir, or lake, or perhaps an atmospheric or soil compartment. Although these are not tanks, it is important to understand that it is often completely appropriate to treat such environmental compartments as well-mixed and model them using Cell pathways. In some cases, the entire compartment can be treated using a single Cell pathway, while in others, it may not be appropriate to treat an entire environmental compartment as well-mixed, and you will need to discretize the compartment using multiple Cells. We worked through an example to illustrate this concept.

Finally, we closed the Unit by discussing some of the numerical implementation details of mass flux links. In particular, we introduced and compared CoupledNormal and Previous-value mass flux links. An understanding of these link types will be required in order to properly model some kinds of systems that we will discuss in later Units.

This Unit discussed only one type of pathway, the Cell, which is used to represent well-mixed compartments. The next Unit will continue to focus solely on the Cell pathway, discussing how to represent a number of complex processes in well-mixed compartments.