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Question about Phreeqc speciation calculation

Dear Dr. Parkhurst:

  I am Li Yang, a graduate student in the department of Geological Sciences at 
the University of Alabama.I and My advisor, Dr, Donahoe, are currently working 
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 given 
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 solution 
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 the 
database to choose the minerals to be listed for a given solution? Is there a 
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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