Gas Partial Pressures

[agion18]

I was wondering how gas partial pressures are calculated in GWB?  I've read elsewhere that they are determined using Henry's Law and if so is there a database of Henry's Law constants that is used?

 

 

[Brian Farrell]

The thermodynamic datasets used by the software contain reactions for aqueous species, minerals, and gases. Each reaction has an equilibrium constant, expressed as a log K at different temperatures (or polynomial coefficients in a temperature expansion). The fugacity of a gas is calculated using the activity of each species in the reaction and the equilibrium constant. The fugacity coefficient is by default calculated according to the model specified on the fourth line of the thermodynamic dataset (or in the TEdit program’s header pane). The fugacity model can be overridden in the modeling programs.

For more information on the theory, please see sections 3.3.7 Gas fugacities and 3.3.8 Partial Pressure in the Geochemical and Biogeochemical Reaction Modeling textbook. For a description of the thermodynamic database format, see the Thermo Datasets chapter in the GWB Reference Manual. And for controlling the fugacity coefficient model, please see section 7.5 Gas partial pressure in the GWB Essentials Guide.

Hope this helps,

Brian Farrell
Aqueous Solutions LLC