re: Enquiry on water pH

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

> I would be most appreciated if you could provide information related to
USGS's expertise in modelling the dependence of water pH over temperature,
pressure and salinity.

We have a number of models designed to investigate the chemistry of natural
waters: phreeqc, wateq4f, phrqpitz, and solmineq. PHREEQC is the most
versatile of the three, but solmineq has more capabilities related to high
pressure and high salinities, and phrqpitz is also a better model for high
salinities. PHREEQC is limited to relatively low ionic strengths (< 1
molal), but allows for a wider range of geochemical reactions (silicates
and redox).  Pressure effects are relatively small unless you are at
several km depth.

The pH of natural waters depends predominantly on CO2 and carbonate
equilibria, but may be controlled by a wide variety of reactions. PHREEQC
has capabilities to calculate speciation given a pH and water composition.
PHREEQC also has capabilities to predict pH given assumptions about initial
water composition, mineral equilibria, mass-transfers of reactants, and, if
necessary, kinetic reactions.

PHRQPITZ is an older FORTRAN program, which has the Pitzer formulation for
activities of ions. Solmineq, EQ3/6, Geochemists Workbench, and TEQUIL also
include this thermodynamic model and may be simpler to use if they are
available.

PHREEQC and phrqpitz can be found at the web site given below. Wateq4f is
available from https://wwwbrr.cr.usgs.gov/projects/GWC_chemtherm. You will
have to contact Yousif Kharaka (ykharaka@xxxxxxxx) for information about
solmineq.

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