fixed-volume gas phase

[bgilbert]

I am modeling a couple of biogeochemical systems in which solution species (CO2, H2, CH4) can exchange with a fixed volume of gas phase (e.g., headspace in a vial). After developing a reasonable geochemical and microbial reaction network, I came to the conclusion that GWB cannot model this specific system.

1. Can you please confirm this is indeed correct? I can introduce a kinetic gas-fluid exchange and monitor fluxes at a fluid-gas interface, but that gas phase must be at a fixed "reservoir" composition.  I have the Professional edition and hoped that I could define a simulation cell as 'headspace' and other cells as fluid. But I don't think I can change the gas-phase saturation (indeed, I think that all GWB cells must be 100% fluid saturated, right?) 

2. Is there any work-around? For example, adding a script to update the gas phase composition. Again, I don't think so as so few GWB variables are allowed to be modified by a script. 

3. If the above is impossible, what is the best way to export a functioning GWB input file to PHREEQC that seems to have this capability? 

Many thanks, 

Ben 

[Jia Wang]

Hello Ben,

The software currently accounts for gases as partial pressure or fugacity in equilibrium with your fluid. It does not allow for you to set a fixed volume gas phase and specify the composition of the gas to exchange with your fluid.

With regards to your second question, I think you're referring to the fugacity or partial pressure of the external reservoir and whether or not that can be altered during the simulation. There is an option to alter this through compiling your own rate law using a C++ script. Within React, there is a reactant status class for each reactant. The value for the fugacity of partial pressure of the gas reservoir is stored as the variable named "amount". You can view these variables in the header file "gwb_context.h" that comes installed with the GWB. For details on custom and compiled rate laws, please see chapter 5 of the GWB Reaction Modeling User Guide.

There is not a converter available to convert input files from the GWB to PhreeqC. However, you can right-click and open any saved GWB input file in a text editor, such as note pad, to see the commands stored to help you configure your own PhreeqC run.

Hope this helps,
Jia Wang
Aqueous Solutions LLC