Courses: The GoldSim Contaminant Transport Module:

Unit 5 - Building a Simple Model of a Well-Mixed Environmental Compartment

Lesson 12 - Unit 5 Summary

In this Unit we built two simple models of a single well-mixed compartment (e.g., a tank). We used these to explore the use of the Species, Reference Fluid, Solid and Cell elements. The key points that were discussed are as follows:

  • The fundamental building blocks on which all GoldSim contaminant transport models are based are 1) the species which are to be simulated; and 2) the environmental media in which these species are transported and stored. In GoldSim, these are collectively referred to as materials. GoldSim provides three specialized elements: SpeciesFluids and Solids. Specification of these material elements is the first step in representing an environmental system within GoldSim.
  • In a new model, the Reference Fluid (Water) and the Species element are contained in a Container named Material. This Container is actually only created as a convenient way to organize these (and well as other Fluids and Solids you may create. If you wanted to, you could move these elements out of the Material Container to some other location, and you could then delete the Container itself. However, you cannot delete either the Species or Reference Fluid element.
  • The Species element is used to add, delete, and edit species.  It is used to define a list of species that you wish to simulate, and specify their properties (in particular, those relating to the possible decay of the species). Unlike any other element in GoldSim, you cannot rename the Species element.
  • There are two sets of Array Labels that are automatically added when the Contaminant Transport Module is activated (they would not be present if you deactivate the module).  These are named Elements and Species.
  • The items in the Species Array Label set are the names of the species defined in the Species element.  The Elements Array Label set is created based on the names of the chemical elements that are assigned to the species when you define their properties.  If a species is not defined as an isotope, the chemical element name is the same as the species name. But if a species is defined as an isotope, this will not necessarily be the case. In our example, none of the species are isotopes, so the Species and Elements Array Label Sets are identical.
  • Many of the inputs to other Contaminant Transport Module elements (e.g., partition coefficients and solubilities in environmental media, boundary conditions in pathways) require that you enter a vector of Species (i.e., a vector with an item for each species) or a vector of Elements (i.e., a vector with an item for each chemical element). These two sets allow us to create these.
  • 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.  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).
  • 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 in our example was 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.
  • The primary inputs that must be defined for a Cell pathway are the quantity of each Medium present in the pathway. 
  • Cell pathways (like all pathways) also provide a mechanism for you to provide an initial contaminant mass in the pathway for each species (and as we will see in later Units, you can also provide a mass rate).
  • 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. 
  • For Solid media in a Cell, you can specify whether the Solid is suspended (in the Fluid listed directly above it).
  • Cells output (for each species) the total mass in the Cell, as well as the concentration in each medium. If any Solids are suspended, GoldSim computes a total (effective) and a dissolved concentration for the fluid in which it is suspended.
  • GoldSim solves the governing equations for the Cell by representing them using a finite difference approximation. You can control the precision of the result (by adjusting how GoldSim subdivides the timestep when solving the equations using the approximation).

The Example (and the Exercise) we discussed in this Unit were very simple in two important ways.  First, they were not really transport models at all, since no mass was moving (only decaying).  Obviously, this does not make for very interesting problems. However, it was necessary to start with a very simple system in order to introduce and describe the basic use of Species, Fluids, Solids (and Cells).  In the next Unit, we will start to consider systems in which mass is being transported between pathways.

Secondly, the system we simulated was a tank (as opposed to a natural environmental compartment such as a lake or soil).  The concept of a well-mixed tank serves as a good example to understand how Cells work (and we will use this again).  But as we progress through the Course (in fact, starting in the next Unit) we will consider representing natural environmental compartments using Cells.