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React: unphysical behavior redux

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Hello again,

I am investigating carbon speciation in water-rock reactions with changes in temperature. I use React 8.0.10 with thermo.com.v8.r6+.dat as my database. I suppress 32 aqueous species because they don't have data past 25C, so that GWB loads the same amount of species throughout my temperature runs (10C-70C). Yet, CO2 concentrations (mmol/kg) drop by 20 orders of magnitude between 37C and 38C, and I can't figure out what is changing between the runs. All i'm changing is temperature. I'd value any insights!



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It looks like your initial step size (set to the default value) is the problem. Oftentimes the initial stages of a reaction process see the most change. If the steps taken by the program are too big then very small differences in chemistry between 37 C and 38 C end up having a large effect on the final solution.


If you follow output in the Results pane (Run pane in older versions), you'll see that the solution at 38 C is much "easier" (fewer iterations per step, fewer swaps) than at 37 C. If you lower the step size delxi, however, you'll see that the results at 37 C and 38 C begin to look more alike. Try plotting the minerals formed (and their saturation state) vs. Reaction progress (or mass reacted) to see how step size influences the solution. Since you're interested in CO2(aq) concentration, try plotting that too. What is initially a blocky solution at 37 C begins to look more like the smooth curve at 38 C when you use smaller step sizes.


I would recommend you look into the delxi and dx_init variables in the GWB User's Guides. You might try uniform small steps, or small initial steps that get bigger as the reaction proceeds (you can change the step size linearly or logarithmically).


Hope this helps,


Brian Farrell

Aqueous Solutions LLC

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