Department of Geosciences
North Dakota State University
AQUEOUS GEOCHEMICAL MODELING
FREQUENTLY ASKED QUESTIONS (FAQ)
A: Yes, yes, yes! This web version is just a front end to the UNIX version, called PHREEQC, available at the USGS web site.
A: This web-based version is very simplified - the standalone versions are quite powerful, and you might consider installing one of them.
A: Use the log of the partial pressure. For atmosphere, the partial pressure of CO2 is approximately 350 ppm, which is 3.5x10-4. So the log of 3.5x10-4 is -3.46.
A: Also take the log of partial pressure of O2 in bars. For atmosphere, this value is approximately (log 0.21) = -0.678.
A: (from David Parkhurst) "The program only allows equivalents for the definition of alkalinity, no other element concentrations can be input as equivalents, it defaults back to mmol/L for everything but alkalinity. You'll have to convert to mmol/L for calcium and other non-monovalent elements. Note calcium molality should be .825 mmol/L to give 1.65 meq/L."
A: As a result of the way web-phreeq constructs the input files for phreeqc, phreeqc first equilibrates the solution at your chosen pH, then it reacts the solution with atmosphere (or a stable mineral). During this reaction phase, pH is changed to maintain charge balance. For pH truly to be fixed, a reaction such as a titration with HCl or NaOH (or with an appropriate mineral) would have to be inserted into the input file. Since web-phreeq is designed as a tool for teaching basic modeling, I suggest running a standalone version for this type of problem. Note: keeping pH fixed with no reaction (simple speciation) still works in web-phreeq.
A: Two places: The PHREEQC user's guide is a great place to start. It's also available in PDF form.
Also, David Parkhurst of the USGS maintains a great Frequently Asked Questions for PHREEQC page and a Mail Archives page.
A: Please email me at email@example.com. I invite comments and questions.
Acknowledgments:WEB-PHREEQ is part of an instructional module for Geochemistry 428/628 at North Dakota State University. Thanks to Sandy Sprafka for obtaining funding for this project, and to Andrew Yahin (v. 1) and Tim Mooney (v. 2) for the coding. Many thanks to David Parkhurst and his crew for developing PHREEQC. This web implementation © B.Saini-Eidukat, Andrew Yahin, and Tim Mooney.