Cell Pathway Example #2: Advective Flux Links

It is worthwhile to examine a simple example of an advective flux link between Cells. This particular example file, Cell2_Advection.gsm, can be found in the Contaminant Transport Examples folder in your GoldSim directory (accessed by selecting File | Open Example... from the main menu).

Suppose that 10 g of chemical A and B is released into a small pond. The pond contains two media Materials (such as water, sand, clay, air) that constitute (are contained within) transport pathways. GoldSim provides two types of elements for defining media: Fluids and Solids.: Water and Sediments. Neither species The chemical (or non-chemical, such as bacterial or viral) constituents that are stored and transported through an environmental system in a contaminant transport model. In GoldSim, the Species element defines all of the contaminant species being simulated (and their properties). decays. A sorbs onto the Sediments. This sorption can be quantified by specifying a partition coefficients The ratio of the species’ concentration in a medium to its concentration in the Reference Fluid at equilibrium. Partition coefficients are inputs to Solid and Fluid elements. for A (equal to 4 m3/kg). B does not sorb. Because the pond is shallow, it is assumed that it is well-mixed, and the Water and Sediments are instantaneously at equilibrium with respect to partitioning of the species.

Finally, assume that Water is pumped from the pond at a rate of 50 m3/yr. We assume that none of the Sediments leave the pond with the Water. Moreover, clean Water is pumped into the pond at the same rate (so the pond volume stays constant).

You wish to predict the concentration of the two species in the Water and Sediments in the pond as a function of time.

To simulate this system in GoldSim, you would do the following:

  1. Define two species (A and B);
  2. Define two media (Water and Sediments) and specify their properties;
  3. Define a Cell pathway A transport pathway element that is mathematically equivalent to a finite difference node. Cells are commonly applied to simulate discrete compartments in an environmental system (such as ponds, lakes, shallow soil compartments, or the atmosphere). to represent the pond;
  4. Define a second Cell pathway to represent the location to which the pond discharges;
  5. Specify the quantities of Water and Sediment present in the pond Cell (the amount in the downstream Cell is unimportant as this just represents a sink);
  6. Specify an initial mass of species in the pond Cell;
  7. Create an advective flux link from the Pond to the downstream Cell and specify the quantity of Water flowing; and
  8. Specify the simulation settings (i.e., duration and timesteps), and run the model.

The output of this simulation, in the form of time histories of the concentrations of species A and B in the Water and Sediments in the Pond, is shown below:

Note that B decreases at a much faster rate than A. This is because B is not sorbed onto the Sediments at all (the concentration of B in Sediments is 0). Hence, it flushes from the system much faster. Because A is sorbed onto the Sediments (and only Water is flushed from the pond, not the Sediments), it takes much longer to leave the system. This is made more apparent by looking at the total mass of each species in the Cell (as opposed to the concentrations in Water and Sediments):

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