David You probably don't remember me, but I attended the Geochem Modeling class in 1992 taught by you and Messrs. Plummer and Glynn, and I have swapped emails with you occasionally regarding PHREEQE/PHREEQC questions. I also used to work with John Mahoney at HCI, whom you no doubt remember and perhaps have nightmares about. I was wondering if you would have time to help me with a modeling problem. I have been trying to solve this myself but alas I realize that I must drop back into punt formation. My work with the Colorado Geological Survey involves collecting water samples and taking field measurements. I decided that, to get Redox information, rather than measure field Eh that I would measure Dissolved Oxygen in the field, as I remembered that WATEQ has the capability to calculate pe from DO data using the Sato equation. To save time, I was hoping to just enter the formula into a spreadsheet. The formula I got from the PHREEQC manual ( which I notice is commented out) is: Kw = (0.25O2(aq) + H+ + e-)/(0.5H2O) where Kw = -11.385 Questions: 1) Is "O2" equal to the molar concentration of O2 calculated from the Dissolved Oxygen in mg/L? 2) Is "H+ " equal to pH or the activity of hydrogen ion, or something else? 3) Is "H2O" equal to the activity of water (in which case it can be assumed to be 1.0)? 4) Is "e-" equal to pe? I have a field DO of 5.0 mg/L at a pH of 7.42. Running this through WATEQF gives a pe of 3.014, but calculating manually I get something different. Can you shed any light on this mystery? Thanks ! David A. Bird Geochemist Colorado Geological Survey Department of Natural Resources 1313 Sherman Street Denver, CO 80203 Tel: 303-894-2169 Fax: 303-894-2174 email: david.bird@xxxxxxxxxxx http://geosurvey.state.co.us/
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