Re:

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

> I have a question concerning KINETICS:
> I want to model kinetic oxidation of both organic carbon and ammonium in
groundwater. For this, I created two monod kinetic rates for Orgc_ox and
AmmH_ox. To test the kinetics, I created an input file (see attach) with
orgc, amm and oxygen present. Results (output_phreeqc.xls) show that
nitrate is increasing when ammonium is oxidated (as I expected), but once
oxygen is depleted, my results show a decrease in nitrate that I can't
explain.

I think the loss of nitrate is caused by your kinetic reaction formulae in
AmmH_ox KINETIC reaction. YOu are removing Amm, but replacing it with NH4
(charge is not considered in these reactions). Effectively you are adding
an extra 1/2 H2, which acts as a reductant.

> Another thing is that when I set the concentration of orgc to a low value
(my
background water does not contain organic carbon) the model won't run and
tells
me that it has negative concentration for orgc (error message).
My question now is if there is a way of "telling" the program that there is
no
organic degradation when there's no orgc present.

In the file you sent me, there was a mismatch between the kinetic reaction
listed in KINETICS (Organic_C) and the rate defined in RATES (Orgc_ox). I'm
not sure why the program did not simply stop with an error message.

Try and see if these couple of changes give you more reasonable 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