To illustrate the use of matrix diffusion zones, let's consider a simple example. This particular example file, Pipe2_MatrixDiffusion.gsm, can be found in the Contaminant Transport Examples folder in your GoldSim directory (accessed by selecting File | Open Example... from the main menu).
In this example, we wish to simulate a tracer test in which we inject two tracers into a single fracture in a rock block. The fracture is 2 m long, 0.5 m wide and has an aperture of 1 mm. Water flows through the fracture (which is saturated) at a rate of 2E-5 m3/day. The dispersivity (in the absence of matrix diffusion) for the fracture is assumed to be 0.2 m. 1 mg of species A and B are injected into the fracture. A 10 cm thick matrix diffusion zone (with a porosity of 0.01) exists in the rock matrix on either side of the fracture. Species A, however, can not diffuse into the rock (it is too large).
To simulate this system in GoldSim, you would do the following:
1. Define the two species and define their properties (particularly the relative diffusivities);
2. Define two media (Water and Rock) and specify their properties;
3. Define a single Pipe to represent the fracture;
4. Specify the properties of the Pipe;
5. Define a sink Cell and create an advective mass flux link from the Pipe to the Cell.
6. Specify the initial condition for the Pipe; and
7. Specify the simulation settings (i.e., duration and timestep) and run the model.
The output of this simulation, in the form of time histories of the mass rate of species A and B exiting the fracture, is shown below:
Note that species B, which can diffuse into the rock matrix, has a long "tail" indicative of the matrix diffusion process. Species A, which was not allowed to diffuse into the matrix, is unaffected (and controlled only by the advective-dispersive process).