> I have a a problem with phreeqc calculation of alkalinity in strongly acidified mine pit lake water. The main constituents in well oxygenated surface water are Fe(3)-1.55mM, Al-1.24 mM and S(6)-13mM. The pH is given with 2.7. Phreeqc is calculating an alkalinity of -5.88e-03 eq/L. In my opinion this cannot be correct. Because of the infinite low CO2 concentration the alkalinity (alk) is defined via alk=-[H+]-aci[Fe-species]-aci[Al-species]-[HSO4-], > isn´t it? I´m calculating -8.21e-03 eq/L. > What about your ideas to solve this problem? PHREEQC does just as you indicate, except that each aqueous species is assigned an alkalinity. Not all Fe+3 species have the same alkalinity, nor do all the Al+3 species. PHREEQC assigns an alkalinity to each of the master species (SOLUTION_MASTER_SPECIES data block). Alkalinities for the aqueous species are derived from the alkalinities assigned to the master species by using an equation that does not contain e- (probably can not have O2 or H2 either). For example: Fe+3 is a master species assigned an alkalinity of -2 in phreeqc.dat; H+ is -1; H2O 0. Using the eqn Fe+3 + H2O = FeOH+2 + H+, FeOH+2 has an alkalinity of -1. After you have the alkalinity assigned to each aqueous species, sum the alkalinity times the molality of each species to get the alkalinity that PHREEQC calculates. There is some ambiguity because of the pK of each hydrolysis species and the assignment of integral alkalinities to the master species; a species may not be completely titrated in an acidity titration. You may want to simulate a titration and look at the difference in the calculated alkalinities, which might give a more accurate value. In any event, there is some ambiguity in the definition of alkalinity, because it refers to an acid neutralizing capacity relative to a reference state (see Stumm and Morgan). David David Parkhurst (dlpark@xxxxxxxx) U.S. Geological Survey Box 25046, MS 413 Denver Federal Center Denver, CO 80225 Project web page: http://wwwbrr.cr.usgs.gov/projects/GWC_coupled
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