> Am I correct that as in the mass action equation only the H+ activity has an exponent of 2, Phreeqc should provide similar results to Waite et al.? I think you are right. The mole-fraction activity for surface species would not affect the mass-action equation in this case. > I set it up as follows but I run into problems that the total H balance of the system is distorted. I would be glad if you have any suggestions. I don't have a copy of Waite and others close at hand and I have pretty much forgotten their paper. How is the total H distorted? Thinking about what phreeqc does, there is a mole balance equation for H and =Fe sites. I think the sum of the species in the calculated distribution of species must conserve both H and sites, so I don't know where the error would be. What are you comparing to? If you are comparing to calculations by Waite, I wonder if there is an error in their total H condition when they use the funny mass-action equation. I am not sure if they used that mass-action for a good reason or just by necessity because they didn't use a more reasonable activity convention for bidentate sorbed species. Let me know a little more about the total H balance being "distorted" and I'll try to give you a better answer. 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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