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Re: Question about Phreeqc speciation calculation

For the most part, the selection of minerals is straightforward--if calcium
is in the mineral, then you will only get a saturation index if calcium is
in the analysis.

It can be a little tricky with redox elements. I'll just give an example
with FeS (Mackinawite). If S is defined in the solution, it will be
distributed between S(6) (sulfate) and S(-2) (sulfide) and there will be a
saturation index calculated for FeS. If S(-2) is defined in the solution,
there will be a saturation index calculated for FeS, however, if only S(6)
is defined for the solution, there will be no saturation index calculated
for FeS. Whether a saturation index is calculated for a mineral with a
redox active element depends on the redox state of the element in the
mineral. If individual redox states are defined in the solution composition
then saturation index for a mineral will be calculated if there is data for
the appropriate redox states in the solution (it also depends on the
chemical equation used to define the mineral in the PHASES data block).


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

                      <yang003@xxxxxxxx        To:       "David L Parkhurst" <dlpark@xxxxxxxx>                              
                      edu>                     cc:                                                                          
In-Reply-To: <3DE30A30@xxxxxxxxxxx>
                                               Subject:  Question about Phreeqc speciation calculation                      
                      11/25/02 10:15 PM                                                                                     

Dear Dr. Parkhurst:

  I am Li Yang, a graduate student in the department of Geological Sciences
the University of Alabama.I and My advisor, Dr, Donahoe, are currently
on a project studying the leachability and sequestering mechanisms of heavy

metals from power plant ash disposal facilities. We use your geochemical
modeling code PHREEQC to set up some simulations for pore water speciation
calculation according to our analytical data. In the model output, we get a

list of minerals and their saturation indexes (S.I) with respect to the
pore water solution. What puzzles us is that for a series of pore water of
similar chemical composition collected at different depth, we get different

list of minerals for each pore water. In the user's manual, it is said that

all minerals with their saturation indexes appropriate for the given
will be listed in the output file. We are interested in what caused such a
difference in the simulation output results. How does the program search
database to choose the minerals to be listed for a given solution? Is there
criteria that the program will follow to calculate the S.I of this mineral
over another for a given solution?

  Thanks for your time! We are looking forward to your reply!

Li Yang

Li Yang
Department of Geological Sciences
University of Alabama

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