I think the problem probably did not converge when you got the strange X moles. I think there is a problem with your problem definition. Right now, you are adding .01 mol of NaAcetate per day because of the REACTION definition. I think you probably want to add the NaAcetate once. To do this: USE solution 1 REACTION ... SAVE solution 1 END Follow this set of instructions with the kinetics reaction and do not "USE reaction 1". The formula for the kinetic reaction needs to be charge balanced. It should probably be: -formula CH3COOH -1 HAcetate 1 given a negative moles of reaction calculated in the RATES definition. PHREEQC ignores any charge given in the -formula fields. Think of the formula in terms of addition or removal of elements from solution, not aqueous species. Finally, the check for removal of Acetate in the rate definition has the wrong sign. Here is an alternative definition. 20 if -moles > tot("Acetate") then moles = -tot("Acetate") #avoid 'overdegradation' (more degradation then Acetate present) 25 if abs(moles) < 1e-11 then moles = 0 This still slows down a bit toward the end of the year, but it seems to work. A revised file is attached below. David (See attached file: Phrqc6.pqi) 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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