water evaporation

[mauricio]

Hello everyone, I have chemical (major and some minor elements) and isotopic (deuterium, 18O) data from water samples. With this i want to know if is it possible to simulate evaporation, with both components at the same time, so that way i could know isotopic enrichment and final chemical concentration. Thanks for this forum 😄 

[Jia Wang]

Hello,

I think a good place for you to start is the section working with isotope fractionation in SpecE8. You would need to define the isotope system and then allow the program to calculate the composition of the solvent and dissolved species to honor the fluid's bulk composition assuming isotopic equilibrium. If you require evaporation in your simulation, then you would need to use the React application to simulate reaction path changes. For more information and examples, please take a look at section 7.7 Fractionation of stable isotopes in the GWB Essentials Guide and section 3.9 in the GWB Reaction Modeling Guide. 

 

Hope this helps,
Jia Wang

[mauricio]

Hello, thanks for the answer. Im modeling as you told me but when i see the results, only Oxygen 18 get enriched, not Deuterium (it ends with the same inicial composition at the end of simulation, with almost all water removed).

Any idea of what im missing?, i believe both isotopes must grow while water is still evaporating.

[Jia Wang]

Hello,

You're welcome. I am not sure the exact details of your file setup so it's difficult to know. One suggestion is to check the isotope data file for the species that are important in your simulation. The isotope.dat file provided with the installation is a good start but you may have to add to it fractionation factors for species that are not accounted for but expected in your simulation. If the species is missing in the database, the program assumes a fractionation factor of 0. If you are still encountering issues, please attach your input script so we can take a closer look. It would also be very helpful if you can provide more explanation of what you're trying to model and the trends you expect. 

Best regards,
Jia Wang

[mauricio]

 

Hi, I took a look at the database, but i found the species there). Below is the input script. It contains the chemical composition, deuterium and oxygen 18 of a water sample (superficial). What I hope is to be able to simulate the increase in oxygen 18 and deuterium as the water evaporates up to 100% (almost).

My result is that simulation gives me a very small isotopic enrichment of oxygen 18 (that is what I think of, based on more samples that I have from the sector); and also theres no hydrogen enrichment in almost the entire simulation.

Thanks for the help and best regards!

Mauricio

Test.rea

[mauricio]

Algo, as i have more water samples and the real isotopic composition of these samples while they loss water (by evaporation). I want to compare the simulation with the real samples, so i can take conclusions about samples that do not evaporate in the zone because they are underground (so with the simulation i can make and idea of the evaporation process for those samples).

Greetings 😛

Mauricio

[Jia Wang]

Hello Mauricio,

Thanks for attaching your file and explanation. Here are a couple of ideas/suggestions.

It's important to note that the software only accounts for equilibrium fractionation effects on isotopic speciation. If your isotopes are affected by kinetic processes, then the simulation won't account for that. If the kinetic effect is large, then this will certainly affect the results.

Also note that as fluid is removed from your system, the ionic strength increases, especially at the end of your simulation. The thermo.tdat dataset uses the the Bdot activity model to calculate species activity coefficients and is rather limited to systems' with low or moderate ionic strengths. The seawater evaporation example (chapter 3.2 Flow through model in the GWB Reaction Modeling User Guide) uses the HMW dataset, which utilizes the Pitzer activity model for calculating species activity coefficient and generally more accurate in a wider range of ionic strength than the Bdot. 

Another suggestion is to consider adding a gas buffer in your system. If you can assume your fluid is at equilibrium with the atmosphere, you may be able to use this to provide additional information that might affect the speciation calculation in your system. Often times, users may use a O2(g) fugacity or partial pressure measurement and set it in equilibrium to the fluid so that the software can use it as a constraint. You can see an example for this in section 7.2 Equilibrium models of the GWB Essentials User Guide. To maintain equilibrium over the course of a reaction path in React (or Phase2/X1t/X2t), you would need to fix the initial fugacity for the gas in the Reactants pane. 

Hope this helps,
Jia

[mauricio]

Hello, sorry for delay... 

Thanks for the advice ... i tried to fix it but 2H is still not getting enriched.. About kinetic processes, that means that evaporation wont account for modelling path by isotopic fractionation, right? ... If that`s true then it doesn`t make sense to keep trying the simulation i think 😧 

Best regards,

Mauricio

[Jia Wang]

Hello Mauricio,

Kinetic isotope fractionation is pretty common with evaporation problems. The software won't be able to model kinetic isotope fractionation. If this is the main driver responsible for the change you are expecting, it won't be account for in an equilibrium only model. 

Best,
Jia