> QUESTION 1: would you recommend software to use at higher salinities? EQ3/6 or something else? You need a program that includes the Pitzer formulation for activities to run at higher ionic strengths. EQ3/6, Geochemists Workbench, and TEQUIL (though I have never successfully downloaded this code) include or are based on the Pitzer formulation. Note that as far as I know, the Pitzer formulation does not include any redox reactions, aluminum, or silica. > Example 1 of PHREEQC determines the speciation of seawater. The data input corresponds to standard water quality for seawater. I have always assumed that this 'standard' WQ for seawater refers to the common elements in their dissolved state. In this instance calcium at 412.3 ppm would be assumed to be nearly all in ionic form. Yet the results of the simulation suggest that about 10% of the calcium is present as CaSO4. While nucleation of CaSO4 can be such that initially it is dissolved I would assume that here CaSO4 implies solids that are precipitated. > QUESTION 2: Is this correct? OR am I missing something? You are missing something. The CaSO4 in the distribution of species is a dissolved ion pair or complex. All species listed in the distribution of species are dissolved and concentrations defined in SOLUTION refer to total dissolved concentrations (free ion + complexes). Saturation indices show the saturation state of the solution relative to minerals. Mineral equilibria are defined with EQUILIBRIUM_PHASES; KINETICS can be used to define kinetic reaction of minerals. 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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