First, do you realize that you can not have equilibrium with silica, gibbsite, kaolinite, kspar, and kmica simultaneously? The phase rule only allows 3 of the phases to be in equilibrium simultaneously. With a substantial amount of silica and gibbsite, I would not be surprised if you reach a steady state where the chemistry does not change until you have dissolved the silica or gibbsite. You are generating a lot of reactions even in the absence of your infilling solution. Second, the PHREEQC is not good at integrating the rate expressions you have chosen. It does not handle stiff equations very well under some conditions. Because the integration is explicit, the rate for k-mica does not account for the change in composition from the rate for kaolinite (for example). This can necessitate small internal time steps and it is possible that the method will not converge. I would suggest using SI in place of SR in your rates. I think this will make the rates smoother. SR can change many orders of magnitude with a small change in concentration. Finally, I would use a short column until you are happy with the chemistry. 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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