Open/closed system dissolution of calcite and isotopes

[Joonas Pärn]

Dear all,

I am trying to model a range of recharge waters for different climatic conditions in order to be used in inverse modelling exercise of groundwater evolution. For this I am considering a scenario where soil CO2 with a presupposed partial pressure and pre-calculated delta-13C values, 14C activities equilibrates with pure water at presupposed temperature together with dissolution of calcite. I am trying to use iso.dat database for that.

To test the correctness of my model I have used values and results published in "Blaser, P.C., Coetsiers, M., Aeschbach-Hertig, W., Kipfer, R., Van Camp, M., Loosli, H.H., Walraevens, K., 2010. A new groundwater radiocarbon correction approach accounting for palaeoclimate conditions during recharge and hydrochemical evolution: The Ledo-Paniselian Aquifer, Belgium. Appl. Geochem. 25, 437–455." for comparison. In the article the PHREEQC iso.dat was not used, so I cannot implement any pre-existing input files for my exercise.

For OPEN system conditions I seem to be able to reproduce the published results quite closely, but the delta-13C value of calcite is not stable but decreases. In an open system case this is not a problem, because the delta13C of the solution is determined by soil CO2, but it becomes a problem when similar calculation is done in a closed system case. Could you please tell me how this change in calcite isotope composition can be avoided? An example of the input file I used for the calculations is the following:

DATABASE c:\phreeqc\database\iso.dat
SOLUTION 1 # soil water with predefined CO2 pressure and temperature together with calculated values for soil CO2 d13C and a14C
Temp 2
pH 7 charge
C 0.001 CO2(g) -3.6
Ca 0.001
[13C] -10.23
[14C] 99.81
GAS_PHASE 1 #equilibration of the CO2 phase containing isotopes with SOLUTION 1
-fixed_volume
-VOLUME 1e9
-equilibrate 1
-temp 2
CO2(g)
[13C]O2(g)
[14C]O2(g)
EQUILIBRIUM_PHASES 1 # Simultaneous equilibration of calcite solid solution defined for delta13C=+1.67 (Blaser et al., 2010) similar to Example 20A in PHREEQC 3 manual with solution 1
SOLID_SOLUTIONS 1
Calcite
-Comp Calcite 9.8283e-01
-Comp Ca[13C]O3(s) 1.1007e-02
-Comp CaCO2[18O](s) 6.0825e-03
-Comp Ca[13C]O2[18O](s) 6.8117e-05
-Comp CaCO[18O]2(s) 1.2548e-05
-Comp Ca[13C]O[18O]2(s) 1.4052e-07
-Comp CaC[18O]3(s) 8.6284e-09
-Comp Ca[13C][18O]3(s) 9.6629e-11
END

For CLOSED system calculations I was only able to arrive at similar results to the article when I used REACTION keyword (similar to example 20B in PHREEQC 3 manual) by modifiying the number of moles of calcite solid solution added by trial and error for every different CO2 pressure to reach equilibrium. Can these same results be achieved more efficiently by using EQUILIBRIUM_PHASES keyword which seems to be more correct one to use in this case? An example of the input file I used for this calculation:

DATABASE c:\phreeqc\database\iso.dat
SOLUTION 1 # soil water with predefined CO2 pressure and temperature together with calculated initial values for soil CO2 d13C and a14C
Temp 2
pH 7 charge
Ca 0.001
C 0.001 CO2(g) -3.6
[13C] -10.23
[14C] 99.81
INCREMENTAL_REACTIONS true
REACTION 1
Calcite 9.8283e-01
Ca[13C]O3(s) 1.1007e-02
CaCO2[18O](s) 6.0825e-03
Ca[13C]O2[18O](s) 6.8117e-05
CaCO[18O]2(s) 1.2548e-05
Ca[13C]O[18O]2(s) 1.4052e-07
CaC[18O]3(s) 8.6284e-09
Ca[13C][18O]3(s) 9.6629e-11
0.000111 moles 1 steps
RUN_CELLS
-cells 1
END

Because I am just getting started with the isotope modelling in PHREEQC it would also be helpful to know whether these input files make sense overall and not produce plausible results just by accident.

Thank you very much in advance!

[dlparkhurst]

I need a little clarification on the simulation. If you use SOLID_SOLUTION with a finite amount of solid and a very large GAS_PHASE, then the entire solid will take on the isotopic composition in equilibrium with the gas phase. Perhaps that is what you intended.

In many situations where there is pre-existing calcite, another end member is when the solid isotopic composition controls the gas phase. In that case, perhaps you want a smaller gas volume and a large solid solution mass.

Reality is somewhere between the two end members and depends on rates of reactions and rates of gas diffusion. PHREEQC does not have a gas diffusion capability, so you may need a different program to investigate these effects.

I assume you have seen the classic work by Cerling on unsaturated zone isotopes. I have a paper that was published in a Chinese proceedings volume (and probably hard to find) that considers the seasonal effects on gas isotopic profiles in the unsaturated zone. I can send it to you if you contact me.

[Joonas Pärn]

Dear Mr. Parkhurst,

Thank you very much for your answer. It is already very helpful. Actually it was not my intention initially to use a finite amount of SOLID_SOLUTION and a very large GAS_PHASE for the open system simulation and that is exactly the point where I get the problems. In my simulation I want pure water (infiltrating recharge water) to equilibrate simultaneously with soil CO2 and calcite (i.e. open system conditions). For that I want the SOLID_SOLUTION to also be very large similar to the GAS_PHASE but I have not been able to find a correct code sequence in PHREEQC to model that. Actually the components in the SOLID_SOLUTION used in my simulation right now represent the mole fraction of different isotopic components in calcite (calculated similar to Example 20A in PHREEQC 3 manual). My question would be, how can I increase the amount of the total SOLID_SOLUTION with a specified mole fraction of the isotopic components to be used in the simulation? If I would know that, then I could vary the relative amounts of SOLID_SOLUTION and GAS_PHASE to investigate the effects.

The reason why I am trying to establish a recharge water composition in this way is that I am investigating the evolution of palaeogroundwater in Estonia recharged from glacial meltwaters of the Scandinavian ice sheet in Pleistocene (by using inverse modelling). In this particular case the composition of recharge water cannot be estimated from modern recharge in the study area and I have to start from constructing hypothetical cases.

I hope my intentions are clearer now. If you could send me your paper published in a Chinese proceedings volume, it would be most helpful.

Thank you very much,

Joonas

[dlparkhurst]

You specify the moles of each component in the SOLID_SOLUTION data block. Just increase each by the same factor.

But,  in general, you can not simultaneously be in isotopic equilibrium with a fixed atmospheric isotopic composition and a fixed calcite isotopic composition. So, yes, the relative amounts of GAS_PHASE and SOLID_SOLUTION will determine the ultimate isotopic distribution.

Note that the solid solution will completely adjust to isotopic equilibrium, as if the solid completely dissolved and reprecipitated. It may be more reasonable that a small amount of solid solution at the surface of the calcite adjusts to isotopic equilibrium rather than the entire solid.

[Joonas Pärn]

Dear Mr. Parkhurst,

Thank you very much for your help and suggestions. I will try them out and they have been most helpful. Hopefully I am able to arrive at some realistic model solutions now.

Best regards,

Joonas Pärn