> 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
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