Lesson 5 - Defining the Species and Environmental Media

Three Contaminant Transport Module elements can be thought of as highly specialized Data elements that are used to define the basic physical and chemical properties of the materials in the system: Species, Fluids and Solids. In this model, these can be found in the Material Container:

The Species element is used to define a list of the contaminant species that you want to simulate:

In this model, we have only one species defined (named X).  No Half-Life is defined (indicating that it does not decay). There is a (default) molecular Weight defined (of 1 g/mol).  In this simple model, this is not used at all (we will discuss in subsequent Units when and how this is utilized).  In this simple model, the Species element is simply used to define the species we wish to simulate (and in this case, there is only one).

The dialog above is what you will see if you are using the CT Module.  If you are using the RT Module, the dialog for the Species element is more complex (offering a number of additional options and features required when modeling radionuclides):

We will discuss the Species element in more detail in the next Unit, but will not discuss the various options available only when using the RT Module until Unit 10.

This model has three media elements: a Fluid and two Solids.  All models will have at least one Fluid (referred to as the Reference Fluid, and this is typically Water):

The properties associated with Water are diffusion coefficients and solubilities. In this simple model, we are actually not using these properties. The diffusion coefficient is essentially ignored (transport is advectively-dominated).  Solubilities are assumed to be unlimited (if you were to click the Edit… button next to Solubilities, you would see that the default value for the solubility is negative, which, as we will discuss in a later Unit, is used to indicate an infinite solubility).

The Solids represent the porous media through which contaminant transport takes place (through the sediment layer, the unsaturated zone, and the aquifer).  Their properties are important for this model.  The Sediment dialog looks like this (the Sand dialog is similar):

The Dry Density and Porosity directly impact transport through these porous media (the mathematics describing their impact will be described in detail in subsequent Units). The Tortuosity is only used for diffusive transport, and hence is not used in this model.  We noted that the contaminant is sorbed onto both the Sediment and the Sand.  We can see that a partition coefficient has been specified for X by pressing the Edit… button next to Partition Coefficients:

Now that we have seen how the various materials have been defined, we will start to examine the actual pathways used to create the contaminant transport model in the next Lesson.