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Water activity

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I'm wondering if you could provide some information about the water activity calculation in Debye-Huckel/Bdot thermodynamic datasets.  How exactly are the coefficients in the header of the file derived and utilized?  I have scanned the reference manual and the Bethke (2010) book, but cannot find this information in either. Any relevant reference would be greatly appreciated.



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Hello Ben,

The water activity in the Bdot method is calculated using the mole number of water, stoichiometric or actual ionic strength, and osmotic coefficient. The equation to calculating the osmotic coefficient is given as power series in equation 8.8 in section 8.1.2 'B-dot model' of the Geochemical and Biogeochemical Reaction Modeling text. The water activity coefficients given in the thermodynamic dataset is used in this equation to calculate water activity. The text does not discuss the derivation of the values given but a good place to start might the papers and other text books referenced in this section.

Denbigh, K., 1971, The Principles of Chemical Equilibrium, 3rd ed. Cambridge University Press.

Pitzer, K. S. and L. Brewer, 1961, Revised edition of Thermodynamics, by G. N. Lewis and M. Randall, 2nd ed. MacGraw-Hill, New York.

Helgeson, H.C., 1969, Thermodynamics of hydrothermal systems at elevated temperatures and pressures. American Journal of Science 267, 729–804.

Hope this helps,
Jia Wang

Edited by Jia Wang
corrected reference paper citation
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  • 1 year later...

Hi Ben,

GWB 2022 includes more flexibility in how water activity is calculated within the various Debye-Huckel methods (b-dot, phreeqc/wateq4f/minteq, davies). With the B-dot model, specifically, you can now use the method of Wolery from newer versions of EQ3/6, the method of Garrels and Christ used in PhreeqC, or simply set water activity to 1. The methods are notable in that they do not require parameterization of the "H2O" tables as a function of temperature (and pressure) as in the standard method of Helgeson (1969). Note, the tables still need to be included, although the values won't matter if an alternative method is chosen. The methods are set in the thermodynamic dataset following the value for H2O's "ion size" parameter. More information is available in section 3.4.2, Basis species, in the Thermo datasets chapter of the GWB Reference Manual.

These options are only available in GWB 2022 and in datasets with the "mar21" format. As a final note, the "debye-huckel" label can be replaced with "b-dot" in GWB 2022 beginning with the "mar21" format. The old label is from a time when the B-dot Debye-Huckel was the only non-virial model included in the GWB.


Brian Farrell
Aqueous Solutions

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