Re: an interesting water

["David L Parkhurst" <dlpark@xxxxxxxx>]

>I ran this file through PHREEQC to get a CO2 number. I wanted to plot the
water on a Phase diagram which uses pCO2 , to find out if We are in the
calcite or the dolomite field. at 25 deg the file was not converging, then
I realized that the field temperature was 15 degs C. It runs at 15 and at
every temperature up to and including 20. At 21 degrees it no longer
converges. I thought this was curious. I am wondering if increasing the
allowable iterations will allow convergence. Or do I recalculate the phase
diagram for 15 deg C?

Hi Jorie,

The problem you are running into is related to alkalinity. At pH 12.3,
there is a lot of OH- alkalinity, nearly equal to the alkalinity that you
input (903 mg/L). As you raise the temperature, the OH- plus smaller
contributions from Al, Fe, NH3, and OH- complexes is greater than the input
alkalinity. The program is trying to find the right amount of carbon for
the given alkalinity and by the time you get to 21 degrees, there can be no
carbon contribution to the alkalinity with the given pH and total
alkalinity. The program just fails.

I'm not sure what the best way to go about this is. If you enter TIC [C(4)]
instead of alkalinity, you will not have a problem, but you may not know
TIC. You could set it by calcite equilibrium or fix the pCO2, but that may
not be appropriate either. Anyway, the problem is that the alkalinity and
the pH become incompatible at higher T. You can write or give me a call if
you want to talk about it.

Finally, you can look directly at saturation indices for calcite and
dolomite to see whether they should form, rather than going through some of
the assumptions needed for the activity-activity diagram.

David

David Parkhurst (dlpark@xxxxxxxx)
U.S. Geological Survey
Box 25046, MS 413
Denver Federal Center
Denver, CO 80225