> We ran into the following problem trying to use the KINETICS block: One of our students was working on a final project, her input files are below (the simplest one first). She was trying to model an exponential growth, and was running into the problem where PHREEQC was taking a long time to do this calculation (it shouldn't take too long to do this?) and her values were maxing out at whatever value she chose for the "formula" for the organic matter, e.g., "phyto" and "12" in her first file. > We weren't sure if this was a coding problem, i.e., PHREEQC just doesn't like the big numbers or if we just set up the problem wrong. Any input you have would be very helpful. I think you are close, but you've got a couple mistakes. First, phreeqc uses seconds for the time unit, so the -steps in KINETICS should be in seconds. Also, because the TIME in RATES programs is seconds, you should convert the rate constant to /sec in the Basic program.. I think you should probably have a coefficient of 1 for Phyto in -formula, not 12. The concentration you are calculating (making the changes in time) is C0*exp(.93*12*day), seems like C0*exp(.93*day) is what you want. Another point, the reason Phyto maxed out at 12 is that by default, there is 1 mol of the kinetic reactant (explicit definition is with -m in KINETICS). As the reaction proceeds, the moles that are calculated for the reaction for the time interval are subtracted to get the amount of the kinetic reactant that remains. Once there is no reactant left, the reaction stops. The net effect is that, given the default, you can only add 1*12 mol of Phyto to the solution before the kinetic reactant is depleted. Given exponential growth, you need some sort of limitation in growth. You could use the -m to limit it, or more physically, there is probably a limitation in substrate availability or nutrients plus a death rate that keeps growth in check, which it looks like you are leading to in the other input files. David David Parkhurst (dlpark@xxxxxxxx) U.S. Geological Survey Box 25046, MS 413 Denver Federal Center Denver, CO 80225
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