>I ran this file through PHREEQC to get a CO2 number. I wanted to plot the water on a Phase diagram which uses pCO2 , to find out if We are in the calcite or the dolomite field. at 25 deg the file was not converging, then I realized that the field temperature was 15 degs C. It runs at 15 and at every temperature up to and including 20. At 21 degrees it no longer converges. I thought this was curious. I am wondering if increasing the allowable iterations will allow convergence. Or do I recalculate the phase diagram for 15 deg C? Hi Jorie, The problem you are running into is related to alkalinity. At pH 12.3, there is a lot of OH- alkalinity, nearly equal to the alkalinity that you input (903 mg/L). As you raise the temperature, the OH- plus smaller contributions from Al, Fe, NH3, and OH- complexes is greater than the input alkalinity. The program is trying to find the right amount of carbon for the given alkalinity and by the time you get to 21 degrees, there can be no carbon contribution to the alkalinity with the given pH and total alkalinity. The program just fails. I'm not sure what the best way to go about this is. If you enter TIC [C(4)] instead of alkalinity, you will not have a problem, but you may not know TIC. You could set it by calcite equilibrium or fix the pCO2, but that may not be appropriate either. Anyway, the problem is that the alkalinity and the pH become incompatible at higher T. You can write or give me a call if you want to talk about it. Finally, you can look directly at saturation indices for calcite and dolomite to see whether they should form, rather than going through some of the assumptions needed for the activity-activity diagram. David David Parkhurst (dlpark@xxxxxxxx) U.S. Geological Survey Box 25046, MS 413 Denver Federal Center Denver, CO 80225
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