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Setting the CO2 quantity

Zsófia Busa

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I would like to model an interaction between a thermal water and a sandstone in the React menu. I set the minerals of the sandstone as the Reactants, and I would like to set the elements of the thermal water in the Basis, but I have a problem. The thermal water constists a significant amount of CO2. I could not find where I can set the quantity of CO2. Could you help me, where I can set it? Thank you in advance!

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Hi Zsófia,

The default carbon-bearing basis entry in your thermo dataset is likely HCO3- or CO3--. You would start by adding that entry to the basis. If your analysis is for dissolved carbon expressed as CO2, you would choose the "as CO2" option in the HCO3- units pulldown. For example, mg/kg HCO3- as CO2. If your analysis is for CO2 partial pressure or fugacity, you would swap CO2(g) into the basis in place of the HCO3- and set that value directly. For more information, please see 7.1 Example calculation in the GWB Essentials Guide and 6.1 <unit> in the GWB Command Reference.

Hop this helps,

Brian Farrell
Aqueous Solutions LLC

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Hi Brian,

Thank you for the helping, I have checked the GWB Essentials Guide and the GWB Command Reference but I have trouble about that. I think that my analysis is for CO2 partial pressure or fugacity, because the sample consists free CO2. But my sample constists also HCO3, so I would like to set the HCO3 and also the free CO2.

This is the composition of my sample:
     Mg++         = 50 mg/l
     Na+          = 470 mg/l
     Ca++         = 174 mg/l
     K+           = 12 mg/l
     Li+          = 0.25 mg/l
     HCO3-        = 1780 mg/l
     SO4--        = 144 mg/l
     Cl-          = 47 mg/l
     free CO2 gas: 1740 mg/l

In this case, how I should set it?

Zsófia Busa

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Hi Zsófia,

You can certainly set a constraint for the HCO3- component (which includes dissolved species such as CO2(aq), HCO3-, CO3--, NaHCO3, CaHCO3+, etc.) along with the CO2(g), as long as you swap the basis properly. It’s common to swap CO2(g) for H+, for example, so that the CO2 fugacity or partial pressure controls pH. As an example, please see section 7.2 Equilibrium models in the GWB Essentials Guide. As for your specific sample, I encourage you to double-check the units of the free CO2(g), as well as what exact analysis was done. Partial pressure would have units like bars or atms, not mg/l, so I'd be cautious. 


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Dear Brian,

Thank you for the answer, you helped me a lot. And yes, you are right, it can't be free CO2 if the unit is mg/l.

I have a last question, but it's not connected for my previous question. I've read that the program can calculate at one atm pressure below 100 °C. Is there a method that I can use, if I want to modeling under higher pressures, for example between 50 and 100 bar?


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Guest Melika Sharifi

Dear Zsofia,

The GWB's default thermo dataset, thermo.tdat, is compiled along the steam saturation curve from 0 -300 C. we don't currently account for the effects of pressure on the stability of species and you cannot control the pressure independently from the temperature, but you can certainly set the partial pressure or fugacity of gases in the initial system , but there are no "sliding partial pressure" paths. 

Hope this helps.



Melika Sharifi,

Aqueous Solutions, LLC

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