Re: PHREEQC questions

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

> I have been conducting several respirometry experiments with sediments
from
a wetland site under alternative electron accepting conditions. My problem
is that upon termination of these separate experiments I did not acidify
the
soil solution to release any aqueous phase CO2 in the form of carbonate and
bicarbonate. If all of the experiments were aerobic this would not be such
a
bad thing because I've found these to be acidogenic, dropping the pH to a
relatively low value so that most of the CO2 is actually already released.
However, under sulfate reducing conditions pH was found to become as high
as
8.8 units... so that most of the CO2 is in the aqueous phase.

Did you measure the alkalinity of the pore water at the end of the
experiment? If so, the total dissolved inorganic carbon can be calculated
from pH, alkalinity, and major ions.

> I know the final pH, final CO2 in the headspace, initial water
composition
in terms of Mg and Ca (in case there may be precipitation of carbonates),
and I even know the buffering capacity of the sediments.

If you can assume the CO2 in the head space is in equilibrium with the pore
waters, you can calculate the total dissolved inorganic carbon. If it is
simply a speciation calculation (without additional mineral equilibration,
ion exchange, and others). You can use SOLUTION
and define carbon as follows

C(4)  1 CO2(g) xxx

where 1 is simply an initial guess for carbon concentration and xxx is log
of CO2 partial pressure in the head space.

> Can I PHREEQC to estimate the microbially evolved CO2 that would be in
the
solid and aqueous phases?

Probably. I'm not sure what question you are asking. It may be that you
simply want to estimate carbon concentrations, given pH, alkalinity, PCO2,
and/or other measured concentrations. PHREEQC can probably do this with a
SOLUTION data block. On the other hand, you may want to model the entire
reaction, from starting concentrations, estimates of amounts of
degradation, mineral equilibria, and gas phase evolution. PHREEQC can do
this problem as well, but requires additional problem definition, probably
SOLUTION, REACTION or KINETICS (and RATES), EQUILIBRIUM_PHASES, GAS_PHASE
(closed system, fixed volume?), maybe EXCHANGE, and SURFACE.

Question 2:

Can I use PHREEQC as a model similar to that of Van Capellen and Wang
(referenced in your manual) : a multi-component, 1D, continuity equation?
Do
you have any examples of how that is done?

Examples 11 through 14 in the manual are 1D transport problems. If you are
modeling organic degradation, the simplest method is to determine the
amount (REACTION) or rate (KINETICS and RATES) of CH2O to add to the
system. Thermodynamics will determine which electron acceptor is reduced.

David


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