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Comparing SI values from Flash Diagram to GSS


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Hi GWB Forum,

I am trying to mix two seawater samples with the same chemistry, but different temperatures and calculate a flash diagram for aragonite saturation state using React following the "Fluid Mixing and Scaling" instructions.  I have also calculated the endmember fluid Log(Q/K) values for aragonite using GSS.

I am getting a very different answer for the end member compositions depending on which approach I take. I checked that I am using the same thermo database (phreeqpitz) for both, and am having trouble understanding why they are so different.

I've attached my React file, and my gss file for your reference.

Thanks for any insights,

Amanda

JFEndmembers.gss

JanFeb-Mixing-AssumeSWinboth.rea

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Hi Amanda,

I took a look at your GSS file and your React script and noticed a few things. The React script was using the default thermo.tdat dataset, not the Phrqpitz dataset. You can check the dataset loaded in any app by going to File -> File Properties -> Thermo Data. It's possible you changed the thermo dataset in the Preferences dialog, but this only applies a preferred setting for every subsequent time you open a blank instance of an app. You need to use File -> Open to actually load a different dataset in an already open app.

GSS does not allow supersaturated minerals to precipitate, so you should set up React to be consistent by disabling precipitation. You should similarly disable charge balancing in React to be consistent with GSS (or set GSS to balance on Cl-, to be consistent with React). For more information, please see 2.3 Initial system in the GWB Reaction Modeling Guide.

The "flash" configuration is not enabled by default, so you need to turn that on from the Config -> Stepping dialog. Please see 3.7 "Flash diagrams" in the GWB Reaction Modeling Guide.

I think the unit for Na+ in GSS is supposed to be mol/kg, not mmol/kg. You also have F- and B(OH)3 components in React, but not in your GSS spreadsheet. Just a tip, you can drag and drop a sample from GSS into React's Basis pane, or from React's basis pane into GSS. That makes it easy to quickly copy chemical data from one app to another.

By the way, you can implement polythermal mixing in React by clicking the pulldown next to the temperature unit and changing it to "reactant mixing". Then, set the reactant temperature to the temperature of your reactant fluid. For more information, please see 3.4 Polythermal reaction paths in the GWB Reaction Modeling Guide.

Hope this helps,

Brian Farrell
Aqueous Solutions
 

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Hi Brian,

Thanks for your suggestions. When I followed them, and converted the LogQ/K values to Q/K values in GSS, I was able to get the React results to match the GSS results. This will save a lot of time! Thanks also for the reminder that I can copy the graph in GTplot and paste results into excel. Really helpful!

Thanks,

Amanda

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