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How to correct degasing CO2 from Calcite saturation Index
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Topic: How to correct degasing CO2 from Calcite saturation Index (Read 1591 times)
genIm
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Posts: 6
How to correct degasing CO2 from Calcite saturation Index
«
on:
October 06, 2016, 03:17:40 PM »
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.
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dlparkhurst
Top Contributor
Posts: 1129
Re: How to correct degasing CO2 from Calcite saturation Index
«
Reply #1 on:
October 06, 2016, 03:47:54 PM »
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.
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genIm
Contributor
Posts: 6
Re: How to correct degasing CO2 from Calcite saturation Index
«
Reply #2 on:
October 06, 2016, 03:58:51 PM »
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?
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dlparkhurst
Top Contributor
Posts: 1129
Re: How to correct degasing CO2 from Calcite saturation Index
«
Reply #3 on:
October 06, 2016, 04:39:00 PM »
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.
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genIm
Contributor
Posts: 6
Re: How to correct degasing CO2 from Calcite saturation Index
«
Reply #4 on:
October 10, 2016, 09:03:17 AM »
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.
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dlparkhurst
Top Contributor
Posts: 1129
Re: How to correct degasing CO2 from Calcite saturation Index
«
Reply #5 on:
October 10, 2016, 04:42:59 PM »
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.
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genIm
Contributor
Posts: 6
Re: How to correct degasing CO2 from Calcite saturation Index
«
Reply #6 on:
October 10, 2016, 05:47:12 PM »
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
«
Last Edit: October 10, 2016, 06:17:21 PM by genIm
»
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dlparkhurst
Top Contributor
Posts: 1129
Re: How to correct degasing CO2 from Calcite saturation Index
«
Reply #7 on:
October 10, 2016, 06:58:42 PM »
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
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genIm
Contributor
Posts: 6
Re: How to correct degasing CO2 from Calcite saturation Index
«
Reply #8 on:
October 10, 2016, 07:27:33 PM »
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
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How to correct degasing CO2 from Calcite saturation Index