Relation of mobile cells and stagnant cells

[Sidol]

Dear Dr. Parkhurst!

When I would like to model transport in dual porosity media (for example: fluid flow along a fracture in a medium) I have to define stagnant cells to model interaction between mobile and stagnant zone. In the Finite Differences for the Stagnant Zone subsection of the manual (USER’S GUIDE TO PHREEQC, Equations on which the program is based) you mention a volume factor (fbc) which value depends on the volume of mobille zone (Vm) and the volume of boundary cell (Vbc) which contact with the mobile zone. In the example 13C you defined 5 stagnant cells to every one mobile cells.
My questions are: 1. How do you determine the number of stagnant cells to one mobile cell?; 2. The volume of one stagnant cell (Vbc) is equivalent to one mobile cell (Vm)?

Thank You for Your help in advance.

[dlparkhurst]

The TRANSPORT option -stagnant gives the number of stagnant cells connected to mobile cells.

The TRANSPORT algorithm is not my strength, but here goes. Example 13 is the best resource. The stagnant cells connected to cell 1 are 1 + ncells + 1, 1 + 2*ncells + 1, ...

The volume of water in the cell is determined by the definitions of SOLUTION 1 + ncells +1, etc. The MIX definitions specify fractions of these solutions that mix to account for diffusion.