Calculating water chemisties in equilibrium with evaporite minerals

[Sara]

I am attempting to calculate water chemistries in equilibrium with several common evaporite assemblages specific to my area of study and then run further evaporation calculations on these waters. It seems as if the best way to do this would be to determine the water chemistries in SpecE8 and then run evaporation calculations in React.

For example, I am swapping in calcite for Ca++, gypsum for SO4--, glauberite for Na+, and halite for Cl-. Then I am including PCO2 calculated for 400ppm CO2 and a pH of 8. I continue to get an error stating "Ionic strength out of range".

I have also attempted inputting an initial water chemistry and then setting equilibrium with these specific minerals but it doesn't seem to be plausible when swapping in several minerals for ions that might be in solution.

I am still new to working with Geochemist's Workbench and unsure if this is the correct way to determine a water chemistry in equilibrium with several minerals. Much appreciated if you can shed some light on the subject.

[Jia Wang]

Hello Sara,

Swapping minerals into the basis pane sets the fluid in equilibrium with those minerals. However, you would need to be careful of which minerals you are setting your fluid in equilibrium to and whether or not they are all at equilibrium at the same time. If you have the initial water chemistry, you can run a SpecE8 run which will help you determine the mineral saturation indices of all possible minerals. You might find that some of the minerals that you were originally swapping out is far from equilibrium given the fluid chemical analysis and thus leading to the issue in the software that you see. Perhaps calcite, gypsum, glauberite, and halite are observed but may have precipitated at different times at different points of the evaporation process. For an example of evaporation with React, please see section 3.2 Flow-through model in the GWB Reaction Modeling User Guide. The React file corresponding to example can be found in Script folder where you installed GWB.

You might also be interested in the pickup fluid option in React as well. If you plan to use the equilibrated fluid with your minerals for an evaporation experiment, you can set up the basis pane in React as you would in SpecE8 and run the simulation without the evaporation. Then pickup the result as your new basis constraint as the starting point of your experiment. For more information on picking up results, please refer to section 3.10 Picking up the results of a run in the GWB Reaction Modeling User Guide.

Hope this helps,

Jia Wang

 

[Sara]

Hi Jia,

Apologies for my late response. I have already determined some of this on my own but I appreciate your solid explanation on the topic. Thank you for replying to my question and including relevant help sections.

Sara