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Re: Phreeqci 2.6 charge balance question

>What happens is, as the water flows through, various metals are
as they should (Cu, Zn, Cd as sulfides, Al as hydroxide).  But eventually
towards the end of the reactor the Cu/Zn/Cd concentrations increase
which should not happen - they should either decrease or stay the same, not
increase.  I have the equlibrium phase amounts set to zero, so that they
should not dissolve but can precipitate if supersaturated.  It seems like
during the advection sequence, the phase amounts for those metals get set
nonzero amounts, so that if conditions dictate they dissolve slightly.

Right. If a phase precipitates during the simulation, it will redissolve if
conditions become undersaturated. I'm not sure why you think that the
concentrations should not increase. Do you think the program is making an
incorrect calculation? or that the program is calculating correctly, but
you want to eliminate this effect in some way? My guess is that the
calculations are done correctly, but perhaps there is an effect that you
have not expected.

>Is there a way to get around that with the advection keyword, to have each
shift contact equilibrium phases with zero amounts?  What I'm trying to do
is predict water quality change as it flows through the reactor.

Unfortunately, there is a dissolve_only option, but not a precipitate_only
option. There is no easy way to do what you want. You could introduce
kinetic reactions that precipitate the phases, but do not dissolve. This
should work, but there can be problems if you try to make the rates fast to
simulate equilibrium. You can also redefine the EQUILIBRIUM_PHASES in the
column, but at the extreme you would only run ADVECTION for one step at a
time, redefine EQUILIBRIUM_PHASES, and then run another ADVECTION for
another step.


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

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