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Re: Phreeqci 2.6 charge balance question


No, I don't think the program is calculating incorrectly.  I think I didn't
fully understand how the Advection keyword worked.  Realistically the
phenomena I was concerned about (solid phases resulting from the initial
condition dissolving) was very small after one pore volume and probably not
worth worrying about.  If I ran a large number of steps I think I'd get what
I'm looking for, but I think just running a few pore volumes through is
close enough.

I did try running ten separate simulations representating ten steps through
the reactor, where at each step I saved the solution and used it on the next
one, each time using the same set of equilibrium phases and reaction.  I got
pretty much the same answer.  The bad thing about this way is, I guess if I
use the selected output keyword to write to a spreadsheet, I would have to
write to ten separate spreadsheet files to save the results for plotting.  I
don't suppose there's a way for results from different simulations to be
written to the same spreadsheet file is there?

Brian Park

----- Original Message -----
From: "David L Parkhurst" <dlpark@xxxxxxxx>
To: "Brian Park" <bpark@xxxxxxxxxx>
Sent: Thursday, March 13, 2003 1:07 PM
Subject: Re: Phreeqci 2.6 charge balance question

> >What happens is, as the water flows through, various metals are
> precipitated
> 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
> slightly
> which should not happen - they should either decrease or stay the same,
> 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
> to
> nonzero amounts, so that if conditions dictate they dissolve slightly.
> Right. If a phase precipitates during the simulation, it will redissolve
> 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
> 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
> 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
> David Parkhurst (dlpark@xxxxxxxx)
> U.S. Geological Survey
> Box 25046, MS 413
> Denver Federal Center
> Denver, CO 80225
> Project web page: http://wwwbrr.cr.usgs.gov/projects/GWC_coupled

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