Lesson 8 - Representing Transport through the Saturated Zone

Note: In this Lesson, we continue  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.

The pathway network for this model can be seen inside the Contaminant_Transport container.  In the previous Lesson, we looked at the Sediment_Layer and Unsaturated_Zone elements.  In this Lesson, we are going to focus on the Saturated_Zone element (which, like the Sediment_Layer and Unsaturated_Zone, is also represented using an Aquifer pathway). 

The dialog for the Saturated_Zone Aquifer element looks like this:

In order to explain the various inputs, let’s take another look at the conceptual model schematic:

The geometry of the pathway is defined by a Length. The Length represents the distance from the upstream edge of the vertical column representing the unsaturated zone pathway (right at the point where clean groundwater enters the saturated zone pathway) to the point where it discharges into the stream.

An Aquifer pathway can be though of as a one-dimensional “pipe” or “column”.  By default, mass enters at the upstream end and leaves at the downstream end.  For the Sediment_Layer and the Unsaturated_Zone, this was in fact an accurate description of how mass entered (and left) each pathway.  However, that is not the case for the Saturated_Zone.  This is because the mass from the Unsaturated_Zone does not actually enter the Saturated_Zone at its upstream end.  Rather, the mass input from above is spread out over a particular length (equal to the side parallel to the flow direction).  We represent this in GoldSim by defining a Source Zone Length. This spreads the mass input out over that length of the Aquifer pathway (the default Source Zone Length of zero would input the mass at the upstream end).

The specified pathway Area is the area perpendicular to the flow direction (in this case of the horizontal groundwater).  Recall from Lesson 4 that we described the flow rate though the groundwater pathway as follows:

The Area must be the same area we used to define the plume area when we defined that flow rate: 

As can be seen, the Plume Area in this equation is defined as

Pond_Width*Plume_Thickness

This is identical to the Area we have defined for the pathway itself.

What we are assuming here is that the plume has a fixed width (the width of the column of contaminated water entering from the unsaturated zone above, which is also the width of the pond perpendicular to the saturated zone flow direction) and a fixed thickness or mixing depth (the distance that this contaminated water mixes downward into the flowing groundwater). In reality, a plume does not have a shape like this.  Due to transverse and vertical dispersion along the horizontal flow path, the effective area will tend to grow with distance (as more clean water is mixed in). In this model, however, this area is arbitrary and will not substantially affect the final result of interest (the peak concentration in the stream).  This is because we are assuming that the entire plume (regardless of its size) is captured by the stream.  The plume area does indeed impact the computed concentration in the plume itself (e.g., at the end of the pathway where it discharges to the stream), but we are not interested in that value in this model. All we really are concerned with is the rate at which the contaminant mass is discharging into the stream, and this is not impacted at all by the plume area (the plume area simply linearly impacts the concentration at the end of the pathway, not the mass discharge rate).

The Infill Medium specifies that the pathway is filled with a porous medium (in this case Sand), and because this is pathway is saturated the Fluid Saturation is set to 1. The Dispersivity and Number of Cells are defined as we did for the other two Aquifer pathways.

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

Note that this inflow is displaying the same information displayed in the outflow for the Unsaturated_Zone 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 Saturated_Zone pathway (with a Flow Rate equal to the Plume_Flow_Rate) to the pathway representing the Stream:

Note that Flow Rate in the Inflows and Outflows tabs do not balance (while they did for both the Sediment_Layer and the Unsaturated_Zone pathways).  This simply indicates that the contaminant mass is being diluted as it flows through the saturated zone (i.e., in addition to any contaminated inflow from the unsaturated zone listed in the Inflows tab, “clean” is assumed to also be entering the pathway).  The contaminant mass is not only being delayed and dispersed as it moves through the saturated zone, but is also being diluted with clean water. We will discuss specifying and balancing media flows for pathways in detail in subsequent Units.