Re: Question about PHAST

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

I see a couple inconsistencies in the input files, but they both look
wrong. So I'm not exactly sure what the problem is.

(1) the units are slightly different between the PHAST chem file and the
PHREEQC file, mol/kgw and mol/L, which should not make a significant
difference, and the concentrations are slightly different. Still, the
results should be pretty similar.

(2) I think the problem is that you have defined 0 moles of calcite in
equilibrium_phases 2 in the PHAST run. The moles of a mineral in
EQUILIBRIUM_PHASES defaults to 10 moles. Thus

Calcite 0 indicates 10 moles of calcite (PHREEQC file), whereas
Calcite 0 0 indicates 0 moles of calcite are present (PHAST chem file).

Water 2 in the PHAST file should dissolve both calcite and dolomite, but
there is no calcite to dissolve in the PHAST run. The PHREEQC run has both
calcite and dolomite present to react with initial solutions 1-100. This is
the fundamental reason for the discrepancies between the two runs.

(3) EQUILIBRIUM_PHASES 0 has no function in the PHREEQC calculation because
the result of equilibrating solution 1 with calcite (10 moles) is not
saved. The following couple of additional statements may be appropriate for
the PHREEQC file, I'm not sure. You explicitly equilibrate with
equilibrium_phases 1 in the PHAST file, but there is no calcite to
dissolve. It looks to me like you wanted the infilling solution to be in
equilibrium with calcite, but this is not the case in the PHAST run
(because there is no calcite) nor the PHREEQC run (because the result is
not saved).

SOLUTION 0
    temp       25
    pe         4
    redox      pe
    units      mol/kgw
    density    1
    H(0)       1.1e-7
    Ca         0.0042
    Cl         0.00024
    Na         9.9e-005
    Mg         7.12e-005
    C(4)       5.5e-005
    S(6)       0.00415
    water      100

EQUILIBRIUM_PHASES 0
    Calcite   0

# You should add the following two lines if you want to save SOLUTION 0
after it has equilibrated with calcite

SAVE solution 0
END

(3) The phreeqc file specifies 100 kg water, whereas phast uses 1 kg water.
This makes a difference in the ratio of dolomite and calcite that are
present. In one case it is 0 calcite and 13.9 dolomite mole per kilogram of
water and in the other it is 10 calcite and 13.9 mole per 100 kilogram of
water. But if calcite doesn't dissolve and dolomite is in excess, this may
not make a difference in the results.

I think you need to put 10 moles of calcite in the phast run. You need to
fix both files if you want the infilling solution to be in equilibrium with
calcite. And you probably should fix the ratios of solution to mineral to
be consistent (phast requires 1 kilogram of water, so you need to change
phreeqc). These changes would make phast and phreeqc consistent. You need
then to be sure that the selected values are consistent with the MIN3P
definitions, but you should be able to get phast to match MIN3P as well.

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