> I'm trying to execute a transport modelling of 1D-flowtube, containing an ammonium contamination, that is flushed by pristine background water (contains oxygen, but no ammonium). Ammonium is retarded by cation exchange, which I can succesfully simulate. But now I wanted to incorporate kinetic ammonium oxidation (inflowing solution contains oxygen). I defined rates and kinetics in Phreeqc, but the simulation allways ends after 7 transport steps. > Is it possible that this is due to the fact that the time to travel a 1m cell (=1 shift) is high (8.3333... days= 720 000s, because the effective groundwater velocity = 0.11875m/d)? I tried step_divide > 1 (till 1000), but this didn't seem to help. > Are these problems typically occurring when dealing with kinetics? have you any suggestions to resolve this problem or did I make some error in my kinetic formulation? Your formulation looks fine. The problem is in the numerical method and redox. My guess is that the problem is at the edge of the front where N concentrations are small. There is no dissolved O2 or H2. The redox is defined by NO3/NO2/N2, which have concentrations 1e-10 or less. The solver has a hard time with this, but usually has some strategies to keep going. There are a few options you can try: (1) Adding SOLUTION_SPECIES H2O + .01e- = H2O-.01 log_k -9 This gives a little redox buffer under most conditions and makes the solver happier. It should affect concentrations very little and not more than 1e-9. I think this is the best solution. (2) Your problem runs on my machine if you use KNOBS; -tol 1e-16. I'm a little worried that this could cause other numerical problems. (3) You can delete N2 and NO2- (and N2(g)) from the data base. I think this will alleviate the redox problem, but you lose some of the nitrogen redox states. You may consider whether you want to form N2 or not anyway. Finally, I would prefer the -formula for the KINETICS reation to be -formula Amm -1 NH3 +1 The -formula only considers elements added or removed from solution, the charge on species is completely ignored. There is a tendancy to want to write some of the aqueous equilibria or electron acceptor into the formula (including the oxygen in the reaction) but it is not a good idea to force oxygen to be the electron acceptor. It is better to simply put in the "NH3" and let thermodynamics decide where it should end up and what should accept the electrons. The N must end up as N2, NO2, or NO3 because these are the only N species, and the program will reduce whatever is thermodynamically favored (O2 in this case). 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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