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PHAST question

Hi Dr. Parkhurst,
I have a questions about PHAST.
The question relates to the choice of a time-step when simulating an ion exchange problem.
I have included the input file of the problem and a graph for Cl- concentrations vs distance (the results are similar for Ca, Na and K).
I'm comparing the results with MIN3P and I get a good match with a time step of 0.5 days.
I don't know why, when using a small time step, I get oscillations.  The smaller the time step the larger the oscillations. I would have thought, due to the Courant conditions, the results would be more precise with a smaller time steps.
If you could solve this for me, or give me an indication of how best to choose a time step when setting up a new problem, I would appreciate it.
Best regards,

Attachment: ionx.chem.dat
Description: Binary data

Attachment: ionx.trans.dat
Description: Binary data

Attachment: phast-timestep.wmf
Description: Binary data

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