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Re: phreeqc-alkalinity




> 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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