> I use Phreeqc to simulate fish breathing to see what happen when I dissolve CO2 into seawater and freshwater, with and without NH3. (NH3 is a gas from nitrogen break down in the metabolism). If I, in seawater only use CO2 (for instance 0.3 mmol/l) the reaction is normal with expected fall in pH. But if I also adding NH3 (0.03 mmol/l) the pH falls further? If I simulate the same procedure in fresh water the opposite happen, which I think is normal. I suspect you are getting oxidation of ammonium to N2 or nitrate. This will happen thermodynamically if there is any oxidant in the system, probably O2 or nitrate in your case. To avoid ammonium oxidation, you need to separate ammonia species from the N2 and NO3- species in the database. Effectively, ammonia species need their own element through definitions in SOLUTION_SPECIES and SOLUTION_MASTER_SPECIES data blocks. Tony Appelo has written a database that contains "Amm" and has N(-3) removed. With this database, ammonia species can not be oxidized (unless you do it manually by REACTION or KINETICS). All you need to do is enter the ammonia as "Amm" in the SOLUTION, REACTION, and other data blocks when using this database. If this is not the problem send me your input file and I'll take a look. David (See attached file: tony.dat) 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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