Re: question about PHREEQC

["David L Parkhurst" <dlpark@xxxxxxxx>]

> Here's the strange thing:
if I put the same data into PHREEQC (with the wateq4f database selected) as
were put into WATEQ, I will get allophane as supersaturated in the WATEQ
results, but allophane is not even listed in the PHREEQC phase results.

The wateq4f.dat database was translated, largely by hand, from the WATEQ4F
databases and program. I did not include allophane in the wateq4f.dat
database because the stoichiometry had "1-x" and I didn't know what to do
with it. I have just dug into WATEQ4F and found how the saturation index
was calculated and the following is a USER_PRINT Basic program that will
print out the corresponding values in PHREEQC (I think, you'll have to
check). To me, the approach is kind of squirrelly because the "x" is a
function of pH and for some pH values, the x could produce coefficients
greater than 1 and less than 0. Further, the equilibrium constant should
not be a function of pH; it is supposed to be constant at constant
temperature. If you are interested in allophane, you'll have to go back to
the original reference, as cited in the WATEQ4F manual to see why they
chose the forms of the equation that they used.

USER_PRINT
      -start
10 ph = -LA("H+")
20 c1 = 1.24 - .135 * ph
30 iap = (1 - c1) * LA("Al+3") + c1*LA("H4SiO4") + (3 - 3*c1)*ph
40 lkt_a = -5.7 + 1.68*pH
50 lkt_P = -5.4 + 1.52*pH
60 si_a = iap - lkt_a
80 si_P = iap - lkt_P
90 print "Allophane(a) SI: ", si_a
100 print "Allophane(P) SI: ", si_P
      -end


Paces, Tomas, 1973, Steady-state kinetics and equilibrium between ground
water and granitic rock: Geochemica et Cosmochimica Acta, v. 37,
p. 2541-2563.

> Some of the other mineral phase log IAP values are dead on, most are off
a
little and others are off more, with no clear pattern in the results (so I
think I am entering them all correctly).  I would expect that with the same
input and same databases, the log IAP values would be a little off, but
that the same mineral phases would appear in the output.  Is this an
incorrect assumption?

I tried to include all of the minerals, but it is possible I missed some,
but more likely I did not include those without a fixed stoichiometry. So I
am not surprised that some minerals are missing. As for differences in
calculated values, (1) it is possible that I corrected a few discrepancies
when I translated the data, (2)that the WATEQ4F data has changed some since
I did the translation, or (3) that the activity coefficients are different.
You can look at the detailed distribution of species between PHREEQC and
WATEQ to try to figure out how activities, molalities, and activity
coefficients differ. It is also possible that the aqueous species are not
identical, although I tried to be as consistent as possible.

You can always add new phases to PHREEQC with the PHASES data block. Phases
like allophane that have unusual stoichiometry or mass-action expressions
can be coded into USER_PRINT or USER_PUNCH data blocks.

If you find other major discrepancies, I'll take a look at it.

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



                                                                                                             
                      Caroline Loop                                                                          
                      <loop@xxxxxxxxxx         To:      dlpark@xxxxxxxx                                      
                      edu>                     cc:                                                           
In-Reply-To: <200205080105.VAA01699@xxxxxxxxxxxxxxxxxx>
                                               Subject: question about PHREEQC                               
                      05/07/02 07:02                                                                         
                      PM                                                                                     
                                                                                                             
                                                                                                             



Hello Dr. Parkhurst,

I'm not certain if you are still answering the questions about PHREEQC, and
don't expect that this question will have significance to anyone other than
myself, but I have read all the questions on the PhreeqcI webpage and have
yet another.

I have been using the 2.4.2 Alpha version of Phreeqc Interactive and
selecting the wateq4f database because I am interested in allophane, and
the phreeqc database does not include allophane.  Here's the strange thing:
if I put the same data into PHREEQC (with the wateq4f database selected) as
were put into WATEQ, I will get allophane as supersaturated in the WATEQ
results, but allophane is not even listed in the PHREEQC phase results.
Some of the other mineral phase log IAP values are dead on, most are off a
little and others are off more, with no clear pattern in the results (so I
think I am entering them all correctly).  I would expect that with the same
input and same databases, the log IAP values would be a little off, but
that the same mineral phases would appear in the output.  Is this an
incorrect assumption?

I understand that the easiest thing for me to do is to use WATEQ, but I
have never used it (I got the run output from someone else) and prefer the
comfort of PHREEQC.


Any help would be greatly appreciated,

Caroline