> Problem: Product water is injected into an aquifer and begins to mix, simulated by progressive amounts of product water added to 1 liter of groundwater. What are the effects on manganese concentrations assuming birnessite solubility and redox are the controlling factors? Questions: > 1. If I assume N2 and NO2 are stable, as in this file, manganese concentrations are held to 1 nM or less. As far as I can tell, in this file, N2, NO3, and NH3 are all allowed to react to equilibrium. This means that most of the nitrogen probably ends up as N2. Using llnl.dat is a little overkill, I would stick with wateq4f.dat or phreeqc.dat (augmented with birnessite), with fewer species and minerals to deal with. I think I explained before that you may want to redefine N2(aq) to have a much smaller K to effectively remove it from the calculation. I've included the K's to do this in the following input file. > However, assuming only nitrate and ammonia are stable results in >5 orders of magnitude increase in manganese concentration. Why does this occur and which state is more realistic? I don't know exactly, you need to look at the results of the simulation. Many of the redox reactions are caused simply my mixing the two waters, so look at the difference between the mixture and the mixture plus birnessite. Then you need to consider MnO2 + 2e- + 2H2O -> Mn+2 + 4OH- At high pe, very little MnO2 dissolves, at lower pe more MnO2 dissolves and raises the pH. Note that MnO2 is reduced, so something must be oxidized in the process. That something is probably N(-3), so enough MnO2 reacts to oxidize the amount of N(-3) available. It makes some difference whether it is oxidized to N2 or NO2 or NO3, but not that much. > 2. Is the problem posed correctly in this file? No. Your mixing fractions make 2 liters * 2 *3*4*5, etc, by which you are making several thousand liters of water, which is why the program did not converge. The following file simply mixes the waters in different fractions. David solution_species 2NO3- + 12H+ + 10 e- = N2 + 6H2O # original log K log_k 207.08 # This log K makes N2 negligible # log_k 0 NO3- + 2H+ + 2e- = NO2- + H2O # original log K log_k 28.57 # This log K makes NO2 negligible # log_k 0 TITLE West Basin Simulation 1 SOLUTION 2 Barrier Product Water, December 2001 (Mg: 2001 avg) units mg/L pH 7.7 pe 7 temp 25 redox N(5)/N(-3) Na 15 K 1.3 Ca 19 Mg 0.07 Alkalinity 69 Cl 22 S(6) 3.3 N(5) 0.5 N(-3) 3.2 END SOLUTION 3 Groundwater 17B, 2/26/02 units mg/L pH 8.25 pe 11 temp 22.6 redox O(0)/O(-2) O(0) 2.64 Fe 0.028 Mn 0.08 Alkalinity 81 Ca 28 Mg 11 Na 49 K 3.3 Cl 53.3 S(6) 69.5 N(5) 0.31 END EQUILIBRIUM_PHASES 2 Birnessite 0.0 1.0 END MIX 2 0 3 1 END MIX 2 0 3 1 USE equilibrium_phases 2 END MIX 2 .25 3 .75 END MIX 2 .25 3 .75 USE equilibrium_phases 2 END MIX 2 .5 3 .5 END MIX 2 .5 3 .5 USE equilibrium_phases 2 END MIX 2 .75 3 .25 END MIX 2 .75 3 .25 USE equilibrium_phases 2 END MIX 2 0 3 1 END MIX 2 0 3 1 USE equilibrium_phases 2 END 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
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