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RE: convergence problem

> As the problem occurs during a transport
simulation it seems the best if I used
the last option you proposed.
When writing the input for the
reaction step I will check if the O(0) conc
is 0 and then set it to a small value ... or
do you think it would be a problem if I generally
avoided O(0) to be 0 ?

> As far as I remember I previously fixed a problem
by replacing very small concentrations (eg 1e-35)
with zeros in the input file(s).

PHREEQC ignores concentrations less than about 1e-25, at least in some of
the time. Concentrations less than these could cause problems, so what you
did (setting to zero) probably makes sense for elements other than H and O.
For the O2, I think you are best to set it, either to the correct
concentration if greater than 0, or to a value between 1e-15 and 1e-20 if
equal to zero. I think you will get some erroneous results if you are
generating dissolved O2 by the specification of pe and adjusting pH.
PHREEQC will calculate an O2 that corresponds (maybe millimoles to moles
given your example pH 9, pe 12) and this O2 will then be in the system,
which would then probably oxidize pyrite. However, the added O2 would be an
artifact of the calculation (spontaneous generation of O2) causing
mass-balance and other misleading results.


David Parkhurst (dlpark@xxxxxxxx)
U.S. Geological Survey
Box 25046, MS 413
Denver Federal Center
Denver, CO 80225

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