RE: PHREEQC questions

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

> I do have some more questions. First, I do not seem to have N(-3) in the
database that came with the program but have something defined for Amm
(which I'm guessing is ammonia), and after performing an equilibrium
reaction in which the final pE came out to be 9.3, I still had a lot of Amm
- so it appears I have some kinetically- modified, non equilibrium version
of PHREEQC. I downloaded it two days ago, perhaps the wrong version has
been
saved on the server?

Sounds like you downloaded Phreeqc for Windows, which was written by
Vincent Post. The default database for PfW has ammonia split out as a
separate element (Amm). This arrangement does not allow oxidation of
ammonia or ammonification, which makes sense in some environments.

PhreeqcI is a different GUI for phreeqc, which we have been developing. A
beta version is available at https://wwwbrr.cr.usgs.gov/projects/GWC_coupled/phreeqc/ and
http://water.usgs.gov/software. The final beta version should be put on the
web in the next few days, but the current version is reasonably complete.
PHREEQC (and phreeqci, and probably PfW) is released with other databases
that have N(-3) defined as part of the rest of the nitrogen system. With
this definition, equilibrium is calculated among N2, NO3, and NH3. This
allows N2 to be reduced to NH3, which may not be reasonable. You have to
pick the best representation for your problem. Probably the Amm version is
best, if you are in a sulfate reducing environment.

> Second, I've been trying to model my respirometry experiment in a manner
similar to example 7 from your manual. My input is pasted below. In my
experiments I saw that under sulfate reducing conditions, the pH rose to
8.7. The solution input is what I started with, and I believe that the gas
phase contains all the components that would possibly be involved. The
phase
I know the least about is the solid sediment material that I used as
innoculum.  Does the decreasing pH, as opposed to my observed increasing
pH,
infer that this contradiction is due to some occurence with the solid
phase?

I think the problem is the accumulation of sulfide. A lot of H2S is
generated, which induces a low pH. You probably have to get into the iron
system and make some FeS or pyrite with goethite or Fe(OH)3a. The source of
iron and the sink for sulfide will make a considerable difference in the
resulting pH. It will be more efficient if you use inverse modeling to
calculate the net reactions that can account for your final water
composition, rather than trial and error with forward modeling. Then you
can go back and make a forward model if needed.

David

David Parkhurst (dlpark@xxxxxxxx)
U.S. Geological Survey
Box 25046, MS 413
Denver Federal Center
Denver, CO 80225