In addition to the inputs describing Aquifer geometry, Aquifers require four additional inputs that control the basic behavior or the Aquifer:
Dispersivity: This is the longitudinal dispersivity of the pathway. It has dimensions of length. The longitudinal dispersivity acts to spread or disperse species around the mean travel time as they are transported through the Aquifer. Different systems disperse solutes to different degrees. A good first approximation for one-dimensional transport through a relatively homogeneous aquifer might be that the dispersivity is 10% of the total length of the pathway. You must specify a dispersivity which is greater than or equal to zero.
Number of Cells: This is the number of temporary Cell elements that will be created internally to carry out the calculations. In order to avoid numerical dispersion, the number of Cells should be no less than the Length divided by twice the dispersivity. (If the Number of Cells is too small, GoldSim will write a warning to the Run Log). The maximum number of Cells is 100 (as there would be little benefit in using a number greater than this), and the minimum number of Cells is 4. If you specify a value outside of this range, GoldSim adjusts the number of Cells accordingly and writes a warning message to the Run Log. Note that this value must be constant (it cannot change with time).
Infill Medium: This is the (optional) porous medium that fills the (entire) pathway. It must be an existing Solid medium in the model. You can, however, specify that there is no porous medium filling the pathway (e.g., if you were simulating a channel or river) by leaving this field blank.
The porous medium affects the behavior of the Aquifer in two ways: 1) it increases the flow velocity (by reducing the effective flow area); and 2) it can act to retard any species which partition onto it.
Fluid Saturation: This is the degree of saturation of the Aquifer. It is dimensionless, and must be greater than 0 and less than or equal to 1. If you were simulating a saturated aquifer or a river, you would specify the fluid saturation as 1 (the default). If, however, you were simulating flow through an unsaturated porous medium (e.g., surface soils or a vadose zone), this would be specified as less than 1.
Decreasing the fluid saturation affects the behavior of the Aquifer in two ways: 1) it increases the flow velocity (by reducing the effective flow area); and 2) it increases the degree of retardation due to partitioning onto the porous infill (by reducing the effective volume of fluid present in the pathway relative to these solids).
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