> I tried to use the Ngas to simulate the air component disolution in water. the result is very close (almost the same) to the simulation with only CO2(g) in pH 2-9.5. I think this may be because the N2 (maybe O2 too) is just not disoluted in the water during the blowing process. However there is no further analysis of the outlet component to verify. But it makes me feel better that the simulation is somehow very close to the real world. The O2 and N2 are not reactive in this simulation, so there is only a small effect through the activity of water. > However after pH increases over 9.5, the total CO2 (as well as CO3-2 and HCO3-) seems to deviate (decrease sharply) from the "right track". Even REACTION data block seems ineffective. Attached is the CO2 and carbonate species distribution in solution. There is an activity coefficient for uncharged species that is dependent on the ionic strength. As the ionic strength gets larger, the activity coefficient gets larger. The activity of dissolved CO2 is fixed, so the molality of CO2 must decrease. If you plot the activity of CO2(aq) it should be constant over the entire pH range. Above pH 10, NaCO3- is calculated to be increasingly important. I'm not sure if this is reality or we have passed the limits of the aqueous model. In summary, the effects you see are the result of activity corrections and aqueous speciation. The calculations are correct given the aqueous model, but the ion-association model may not be adequate. David 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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