Re: Modelling saine (as opposed to insaine) environments

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

> In my reading it seems the software is not very good at modelling saline
environments. I am in a rather interesting predicament, where one side of
my
the sudy area is fresh and the other is briny (up to 4x seawater).

The Pitzer specific interaction approach for activity corrections is better
at higher ionic strengths. PHREEQC uses an ion-association model, which is
less reliable as the ionic strength increases.

> The waters are Na and Cl dominated, and I am hoping phreecqc will
encompass
such a wide range in salinity, with the purpose of modelling solution
pathways, aragonite-strontianite solid solution exchanges and some basic
speciation calculations.

The activity coefficients of PHREEQC are in part based on chloride salts of
cations. Thus, it should work better at high ionic strengths of NaCl
solutions than in other electrolytes. However, you probably should make
some comparison calculations using the Pitzer approach. There are a number
of programs that use this approach, EQ3/6, Geochemist's Workbench,
SOLMINEQ, TEQUIL. PHRQPITZ is an old code of ours that is pretty difficult
to use. If none of the other programs are options, then you could use it to
see how much difference there is in the results of the two approaches.

> Also, I have no direct pe data and have not assessed
ammonium or any iron speciation data for my samples. Will phreecqc be
limited by my lack of redox data ? Any tips on how to get around such a
dilema ??

It does not sound as if you are doing much with redox, so the lack of a pe
may not make much difference. For speciation, the pe (or a redox couple) is
necessary if you have total concentration of a redox element and you need
to distribute it among its redox states, for example total iron to be
distributed between ferrous and ferric. The pe will then affect the
saturation indexes of minerals that have Fe(II) or Fe(III) in their
formulae. Even with measured Fe(II) and Fe(III) the pe will be used in the
calculation of SI for some minerals that have unusual redox states, pyrite
for instance. The pe only affects the initial speciation calculation. If
you put redox elements in their proper state, for example, you can assume
all of the dissolved iron is ferrous at pH 7, then again, you do not need a
pe. Pe is probably overrated as an input parameter.

Note that the Pitzer approach does not include redox reactions in any
event.

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