Re: Phreeqc Files: Using KINETICS block

["David L Parkhurst" <dlpark@xxxxxxxx>]

> 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