>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: https://wwwbrr.cr.usgs.gov/projects/GWC_coupled
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