Lesson 7 - Representing Transport through the Sediment Layer and Unsaturated Zone

The pathway network for this model can be seen inside the Contaminant_Transport container.  In the previous Lesson, we looked at the Pond element.  In this Lesson, we are going to focus on the Sediment_Layer and Unsaturated_Zone elements (which are represented using Aquifer pathways).  Recall that Aquifer pathways are used to represent components in which concentration varies continuously over space, with the governing equation defined in terms of a concentration gradient. In this case, the concentration in the sediment layer varies continuously from the top of the sediment layer to the bottom.

The dialog for Sediment_Layer looks like this:

The geometry of the pathway is defined by a Length (the thickness of the sediment layer) and an Area (perpendicular to the flow direction).  This is simply the horizontal area of the pond, since flow is vertical through the sediment. The Infill Medium specifies that the pathway is filled with a porous medium (in this case Sediment). Due to its low permeability, the sediment layer is assumed to be saturated, so Fluid Saturation is set to 1.

The Dispersivity specifies the degree of longitudinal (mechanical) dispersion in the pathway.  This is set to a specified fraction of the pathway length.

The Number of Cells is not a physical parameter at all.  Rather, it is associated with how GoldSim computes transport through the pathway.  For Aquifer pathways, it is necessary to discretize the pathway into a series of well-mixed compartments. The default discretization is 10 (and in this case, this is sufficient).  We will discuss this discretization in detail in Unit 9.

The Inflows tab indicates that there is an inflow from the Pond Cell pathway (with a Flow Rate equal to Seepage_Rate):

Note that this inflow is displaying the same information displayed in the outflow for the Pond pathway discussed in the previous Lesson. That is, for every outflow from one pathway, there is a corresponding (and identical) inflow to another pathway.

The Outflows tab indicates that there is an outflow from the Sediment_Layer pathway (with a Flow Rate equal to Seepage_Rate) to the Unsaturated_Zone pathway:

Now let’s look at the Unsaturated_Zone pathway:

It is also represented using an Aquifer pathway, and is defined in the same way as the Sediment_Layer.  In fact, the Area is identical (it is the area of the bottom of the pond).  However, the Length (and hence the Dispersivity) is larger. In addition, the Fluid Saturation is not specified as 1 (the unsaturated zone, by definition, is assumed to only be partially saturated).  Due to our simplified flow model, however, we are assuming that this is constant.

Like the Sediment_Layer, the Unsaturated_Zone has one inflow (from the Sediment_Layer) and one outflow (to the underlying pathway named Saturated_Zone). The Flow Rate for both of these is equal to the Seepage_Rate.

Note that Flow Rate in the Inflows and Outflows tabs balance (in fact, is identical) for both the Sediment_Layer and the Unsaturated_Zone pathways.  As we will see in the next Lesson, however, this is not required. In particular, when defining an Aquifer pathway, oftentimes the outflow Flow Rate will be specified to be greater than the inflow Flow Rate.  This simply indicates that the mass is being diluted as it flows through the pathway (i.e., in addition to any contaminated inflows listed in the Inflow tab, “clean” water is assumed to also be entering the pathway).  Mass is being delayed and dispersed as it moves through the sediments and the unsaturated zone, but is not being diluted with clean water (all of the water entering these pathways is contaminated). We will discuss specifying and balancing material flows for pathways in details in subsequent Units.