> 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). PHREEQC ignores concentrations less than about 1e-25, at least in some of the time. Concentrations less than these could cause problems, so what you did (setting to zero) probably makes sense for elements other than H and O. For the O2, I think you are best to set it, either to the correct concentration if greater than 0, or to a value between 1e-15 and 1e-20 if equal to zero. I think you will get some erroneous results if you are generating dissolved O2 by the specification of pe and adjusting pH. PHREEQC will calculate an O2 that corresponds (maybe millimoles to moles given your example pH 9, pe 12) and this O2 will then be in the system, which would then probably oxidize pyrite. However, the added O2 would be an artifact of the calculation (spontaneous generation of O2) causing mass-balance and other misleading results. 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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