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
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