RE: PHREEQC questions

["LaRiviere, Daniel" <DLaRiviere@xxxxxxxxxxxxxxxx>]

David,

Thanks for your prompt reply. I have continued to read the manual and try
some examples - this program is really neat! Thanks for your efforts.

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?

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?


Sincerely,
Daniel LaRiviere
 

Title		Respirometry Experiment - Decompose organic matter

Solution 1	Synthetic San Jac Solution
	
	temp 		25.0
	units		ppm
	water		0.3  #0.3 kg of water
#cations
	Na		786
	K		49
	Mg		181
	Ca		101
	Amm		53.8	as AmmH+	

#anions
	Cl		1594
	S(6)		433
	P		16.75		
	C(4)		254 		as HCO3

Gas_Phase 1
	-fixed_pressure
	-volume 		0.3 
	CO2(g)		0.0
	N2(g)			1.0
	H2S(g)		0.0
	
Reaction 1
	CH2O(NH3)0.07	1.0
1.0	2.0	3.0	5.0	10.0	mmol

Selected_Output 
	-file			organic1.sel
	-reset		false
	-simulation	true
	-pH			true
	-reaction		true
	-si			CO2(g) N2(g) CH4(g)
	-totals		C(4) C(-4) S(6) S(-2) 
	-gases		CO2(g) CH4(g)	
	

End




-----Original Message-----
From: David L Parkhurst [mailto:dlpark@xxxxxxxx]
Sent: Monday, October 01, 2001 9:58 AM
To: LaRiviere, Daniel
Subject: Re: PHREEQC questions



> 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