How to correct degasing CO2 from Calcite saturation Index

[genIm]

Dear all,
Please I have a question about saturation Index and PhreeqC, I really hope that you may help me.
When I calculated the Saturation Index of my water samples analyses, it seems that some IS are oversaturated to Calcite SI>0, but in my study area this case is not plausible and it is may be due to degassing of CO2 during sampling. So I have to correct the composition of these analyses before any interpretation.
To do so, some friends told me to use PhreeqC IN THE Equilibrium Phase: Calcite 0.0  CO2(g), in this case I can add the lost CO2 back to solution and the pH and the Ca are then corrected.
I tried this solution in my samples, and I found two results: The first SI where still the SI Calcite >0 and an assempblage phase where I found SI Calcite=0 but the other SI results (Dolomite, Aragonite, ...) are modified. In this case which results should I take into consideration plz??? Which one are correct??? I really feel lost and hope that you can help me solving this problem.

[dlparkhurst]

There can be two numerical solutions where SI(calcite)=0. In one, the one you expect, CO2 is added to the solution to lower the pH and a(CO3-2), resulting in calcite equilibrium. In the other, CO2 is removed to the point where TDIC is sufficiently small to result in a(CO3-2) that produces calcite equilibrium. Which result is found can be random.

Not sure why you got a result with SI(calcite) > 0. Perhaps you ran out of CO2.

In all cases, by adding (or removing) CO2 you are adjusting the pH and carbonate equilibria, which will necessarily change the saturation indices for all carbonate minerals.

[genIm]

Thank you very much
So in the case where I have SI>0, and I added the CO2 to the solution like what I explained, I have to use the second SI results with SI Calcite=0?

[dlparkhurst]

I cannot tell from the information you give, but normally, the pH should end up between 7 and 9 for natural waters in equilibrium with calcite. I think an incorrect result, where CO2 is calculated to be removed from solution to produce calcite equilibrium, would have a much higher pH.

[genIm]

The pH did not change so much and is still betwwen 7 and 9 when SI Cacite=0. But the SI Aragonite is the same in all the water samples is equal to -0.15, it seems stange!!!
Also please, if I have SI Calcite <0 how can I correct this?
I REALLY Can't undrestand why the SI Calcite  MUST be equal to 0 (equilibrium); I am working on a sandystone aquifer.

[dlparkhurst]

Aragonite has the same IAP (ion activity product) as calcite [a(Ca+2)*a(CO3-2)], so the difference in SI [log(IAP/K)] between calcite and aragonite is due to the difference in log K. This difference is fixed at a given temperature.

The SI of Calcite is your choice. I thought your intent was to use equilibrium with calcite to correct your data. If you want equilibrium with calcite, then you choose SI=0. I am not sure how you would justify any other choice, but it is up to you.

[genIm]

Thank you very much for your answer, especially for the aragonite.
As for the SI calcite<<<0, I tried to increase pH and it worked I get a correct value of SI Calcite, but are there any way to correct the pH with phreeqC, because what I did is just a try. I wanna get right value of pH. Are there any way to correct pH?? Thank youuuu

[dlparkhurst]

Here is a script that that generates solution 2 in equilibrium with calcite at log(PCO2)=-1.

Solution 3 is the solution after degassing to atmospheric PCO2. EQUILIBRIUM_PHASES is used to ingas enough CO2 to reach equilibrium. The result is the equal to solution 2.

SOLUTION 4 demonstrates a way to adjust solution composition in the SOLUTION data block. The results of solution 3 are redefined in the SOLUTION 4 definition, and the pH is adjusted to reach calcite equilibrium. The result is again equal to solution 2.



SOLUTION 1
    temp      25
    pH        7
    units     mmol/kgw
END
USE solution 1
EQUILIBRIUM_PHASES 1
    CO2(g)    -1 10
    calcite   0 10
SAVE solution 2
END
USE solution 2
EQUILIBRIUM_PHASES 2
CO2(g) -3.4
SAVE solution 3
END
USE solution 3
EQUILIBRIUM_PHASES 3
Calcite 0 CO2(g) 10
END
SOLUTION 3
    pH        8.977     Calcite   0.0
    units     mol/kgw
    Alkalinity 0.007288
    Ca        0.003644
END

[genIm]

Thank you, hope that I can do it because I am still a bigginer with PhreeqC and I don't know how to add scripts but I will search in the help.
I will try it and I will let you know thank youu

[tOehler]

Dear all,

I have a following question regarding the CO2 correction. I tried to correct my data for CO2 degassing by setting the SI of calcite to 0 using equilibrium_phases. It seems like in my case PhreeqC removes TDIC from the solution in order to reach an SI of 0.

Is there any way to "force" PhreeqC to add CO2 to the solution in order to reach an SI = 0 for calcite instead of removing TDIC?

[dlparkhurst]

If you attach an input file, I will look at it.

[tOehler]

Thank you very much! I attached the input file.

[dlparkhurst]

The mole transfer of CO2(g) is negative for all of your solutions. That means the amount of CO2(g) decreases (it dissolves), and the amount of dissolved CO2 increases. Similarly calcite dissolves for all four solutions. So, TDIC does in fact increase, which is what you said you wanted to happen.

It is possible to specify "dissolve_only" or "precipitate_only" in the definition of a phase in EQUILIBRIUM_PHASES. Also, if you give zero moles of a phase, then it can only precipitate.

[tOehler]

Ah OK I see. Then I just misinterpreted these results. Thanks a lot for the clarification!

What I actually want to know is how much CO2 needs to be dissolved in my water until the SI of Calcite is 0 (without dissolving Calcite). Is there a convenient way to do this in PHREEQC?

[dlparkhurst]

You can use the following. It works for the first three solutions. The fourth solution fails because it is undersaturated with calcite, so adding CO2 will not produce equilibrium.

Code: [Select]

EQUILIBRIUM_PHASES 1-4
     Calcite   0 CO2(g) 10 dissolve


[tOehler]

Thanks a lot! That's exactly what I wanted.