Hi Jean-Philippe, >I need to use as PHREEQC input some output from another geochemical code. The problem is that I can only use concentrations that are given in terms of a few individual aqueous species, i.e., Ca++ instead of Ca total or SO4-- instead of S. Is there a simple way to use that kind of data as an input apart from picking some larger number for total Ca... and iterate until Ca++..... concentrations in PHREEQC output matches my input? You can use a trick and fix the activities of Ca++ and SO4--. Below is a file the makes the activity of Ca+2 ion 1e-4 and the activity of SO4-2 1e-5, for example. Sounds like you need the molalities to be fixed, so this approach would differ by the activity coefficients in the solution. If you need to get closer, you would have to figure out the ionic strength and change the target for the activities to be the target molality times the estimated activity coefficient. David SOLUTION 1 Ca 1 Fix_Ca -4 # -4 is log of target activity for calcium S 1 Fix_SO4 -5 PHASES Fix_Ca Ca+2 = Ca+2 logk 0 Fix_SO4 SO4-2 = SO4-2 logk 0 END David Parkhurst (dlpark@xxxxxxxx) U.S. Geological Survey Box 25046, MS 413 Denver Federal Center Denver, CO 80225
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