Hi, thanks a lot for your quick reply. As the problem occurs during a transport simulation it seems the best if I used the last option you proposed. When writing the input for the reaction step I will check if the O(0) conc is 0 and then set it to a small value ... or do you think it would be a problem if I generally avoided O(0) to be 0 ? As far as I remember I previously fixed a problem by replacing very small concentrations (eg 1e-35) with zeros in the input file(s). Cheers Henning -----Original Message----- From: David L Parkhurst To: Henning.Prommer@xxxxxxxx Sent: 8/30/2002 11:08 PM In-Reply-To: <OF27BACA59.8AD84324-ON87256C25.00523F9E@xxxxxxxx> Subject: Re: convergence problem > I have come across a PHREEQC convergence problem for which I couldn't find a cure. I am not sure, maybe there is something wrong with my input or my definitions in the database. Could you, if you have the time, have a quick look at it ? I have attached the input file and the database file that I used. I had this type of problem before after I included Ferrihydrite or Goethite as equ. mineral (allowed precipitation) in an anaerobic, pyritic aquifer in which aerorobic water was injected. Pretty subtle problem, but I think it is caused by fixing the pe but letting the pH vary. You are right at the edge of water stability and the pH must increase in this problem to produce charge balance. The result is, in a sense, stepping over the stability of water boundary. The real effect is that the dissolved oxygen concentration is calculated to be large. This in turn affects the activity of water, and the numerical solution gets unstable. Maybe the numerical solution could be better, but the real problem is that you are calculating a higher pH than input with a fixed pe. Several things will alleviate the problem: (1) if you do not charge balance you should have no problem, (2) if you charge balance on anything but pH and pe, (3) you set the pe by a redox couple, or (4) define the concentration of O(0). The last may be the easiest, if you set O(0) to a small number, say 1e-18 things will probably work ok. 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
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