> In my reading it seems the software is not very good at modelling saline environments. I am in a rather interesting predicament, where one side of my the sudy area is fresh and the other is briny (up to 4x seawater). The Pitzer specific interaction approach for activity corrections is better at higher ionic strengths. PHREEQC uses an ion-association model, which is less reliable as the ionic strength increases. > The waters are Na and Cl dominated, and I am hoping phreecqc will encompass such a wide range in salinity, with the purpose of modelling solution pathways, aragonite-strontianite solid solution exchanges and some basic speciation calculations. The activity coefficients of PHREEQC are in part based on chloride salts of cations. Thus, it should work better at high ionic strengths of NaCl solutions than in other electrolytes. However, you probably should make some comparison calculations using the Pitzer approach. There are a number of programs that use this approach, EQ3/6, Geochemist's Workbench, SOLMINEQ, TEQUIL. PHRQPITZ is an old code of ours that is pretty difficult to use. If none of the other programs are options, then you could use it to see how much difference there is in the results of the two approaches. > Also, I have no direct pe data and have not assessed ammonium or any iron speciation data for my samples. Will phreecqc be limited by my lack of redox data ? Any tips on how to get around such a dilema ?? It does not sound as if you are doing much with redox, so the lack of a pe may not make much difference. For speciation, the pe (or a redox couple) is necessary if you have total concentration of a redox element and you need to distribute it among its redox states, for example total iron to be distributed between ferrous and ferric. The pe will then affect the saturation indexes of minerals that have Fe(II) or Fe(III) in their formulae. Even with measured Fe(II) and Fe(III) the pe will be used in the calculation of SI for some minerals that have unusual redox states, pyrite for instance. The pe only affects the initial speciation calculation. If you put redox elements in their proper state, for example, you can assume all of the dissolved iron is ferrous at pH 7, then again, you do not need a pe. Pe is probably overrated as an input parameter. Note that the Pitzer approach does not include redox reactions in any event. 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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