The stagnant dispersive zone is defined via the Stagnant Zone… button on the Pipe dialog. When you press this button, the following dialog is displayed:
Clicking the Use Stagnant Zone checkbox activates ("turns on") the zone. If this checkbox is cleared, the Stagnant Zone inputs are grayed out (and are not represented in the model).
The Stagnant Zone requires three inputs: the Stagnant zone infill medium, the Fraction of area which is stagnant, and the Transfer Rate into stagnant zone:
Stagnant zone infill medium: This is the porous medium in the stagnant zone. If defined, it must be a previously defined Solid medium (the stagnant zone need not contain an infill medium).
Fraction of area which is stagnant: This is the fraction of the pathway cross-sectional area occupied by the stagnant medium. It is dimensionless and must be greater than or equal to 0 and less than 1.
Transfer rate into the stagnant zone: This is the equilibrium exchange rate from the mobile zone to the stagnant zone, and has dimensions of inverse length. It is defined as the probability of a dissolved molecule in the mobile zone entering the stagnant zone, per unit length traveled in the mobile zone. It cannot be negative.