Re: New database, new versions of PHREEQC and PhreeqcI

["David L Parkhurst" <dlpark@xxxxxxxx>]

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