Calculate a carbonate system with Spec8

[Tobias]

Dear GWB developers and users,

I am unsure how to create the correct system in Spec8 to calculate my solution.
I am creating a supersaturated solution by mixing two initial solutions.

Initial solution A: Aqueous solution with 3 mmol/L CaCl2
Initial solution B: Aqueous solution with 3 mmol/L NaHCO3
Mixed solution: Ratio 1:1 of the initial solutions

The solution is supersaturated for some minerals which start to nucleate after an induction time.
I want to calculate the Supersaturation with GWB Spec8.

To add 1.5 mmol/L Ca++ and 3 mmol/L Cl- is quite trivial. Also the H+ <-> CO2(g) change to state the environment is clear.
Unclear is the NaHCO3 in my solution. The Na+ should be 1.5 mmol/L (no problem) but what about the HCO3-? Due to the carbonate system of CO2(aq), H2CO3, HCO3- and CO3-- I have clearly less than 3 mmol/L. Do I have to change HCO3- to NaHCO3? Is that change in conflict with the Na+ entry?

Summary question:
How do I determine the Na+ and HCO3- content accurately in a carbonate system if I add NaHCO3?

Best regards,
Tobias

[Jia Wang]

Hello Tobias, 

The program SpecE8 requires the user to provide a set of initial thermodynamic component concentration and chemical constraints to calculate the distribution of dissolved species in the system. Unless specified in the unit, the concentration set in the basis pane is the bulk concentration for the component. For example, when you set the concentration for HCO3-, the component includes other carbonate species present in the system such as H2CO3, CO3--, CO2(aq), NaHCO3. The program will use the component concentration to calculate the each species concentration at equilibrium. For your calculation, I think you can enter the HCO3- and Na+ as bulk concentrations separately. If you would like more information on this, please refer to section 7.2 Equilibrium Models in the GWB Essentials User Guide. 

You might be interested in the Mixing Samples feature in the GSS spreadsheet. You can mix two or more different fluids in GSS using this feature and launch SpecE8 directly for the results. This might be particularly useful if you are trying to mix more complex fluids. For more informatio, please refer to section 3.4.4 and 3.5 of the GWB Essentials User Guide. 

A couple of other things to note.. SpecE8 is simply calculating the distribution of aqueous species given the system constraints.  The program will calculate the fluid's saturation with respect to minerals but it will not allow precipitation. Also, SpecE8 does not have any concept of time, which means you can't account for kinetic reactions that might be responsible for mineral precipitation over time. You would need React to allow for precipitation and rate relevant calculations.

Hope this helps,
Jia Wang