I'm a new user of PHREEQC and I had been carrying around a couple questions that your e-mail stimulated. Here goes: > As I (and my students) read the manual, there are two possible boundary conditions for the end of the domain: 1) constant concentration, or 2) constant flux (3rd type). My student was unable to implement the zero derivative condition. >My experience with interpreting column experiments has led me to adopt either a zero-derivative or semi-infinite condition for the exit (domain end). Certainly, a constant concentration or constant flux is not applicable to most common experimental conditions. It seems like it would be straightforward to implement the other conditions. Tony Appelo (appt@xxxxxxxxx) knows the boundary conditions part much better than I, so I'd recommend writing to him if you are still having problems. He uses Phreeqc largely to interpret column experiments. You should use flux boundary conditions at both ends of the column in your simulation. It is not a constant flux of solute, but a constant flux of fluid. Solute flux depends on the depends on the inflow concentration and concentration gradient at the inlet, and the concentration and concentration gradient at the outflow cell. There is also a -correct_disp option to make a correction to the dispersivity in the final cells that is applicable to column experiments, though I don't think the effect is very large. David David Parkhurst (dlpark@xxxxxxxx) U.S. Geological Survey Box 25046, MS 413 Denver Federal Center Denver, CO 80225
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