Hello, 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? Thanks, 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, 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 > to > 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 > > > > > 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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