Equilibrium phases and isotopes

[Flopi]

Dear all,
I would like to model the intrusion of methane in an aquifer in terms of isotopes. I defined the solution and to model the intrusion of methane I use equilibrium_phases as follow:

USE SOLUTION 1
EQUILIBRIUM_PHASES 1
CH4(g) 1.35 0.15
calcite 1 10
END

However, I don't know how to include isotopes in this. The idea would be that the methane and calcite have different isotope signatures and I would like to follow it. Is it possible with equilibrium phases to define the isotopic value ? If no, is there another way ?

Thanks for you help


EDIT:
I read some info online and I obtained some answers but I'm facing another problem. When I try to equilibrate calcite with CO2(g) (delta=-23) with the following code:

SOLUTION 1
    temp      10
    pH        6 charge
    pe        4
    redox     pe
    units     mol/kgw
    density   1
    C(4)      1 CO2(g) -1.5
    [13C](4)      -23
    -water    1 # kg

SOLID_SOLUTIONS 1
    Calcite
       -comp Calcite 1
        -comp Ca[13C]O3(s) 0.0111802
END

I was expecting to obtain the classic value of delta_TIC = -15, but instead I obtain delta_TIC = -2.4. No matter how I rewrite the code, I never obtain the expected result. Could you also help me on that ?

[dlparkhurst]

To be honest, the isotope capabilities of PHREEQC are not used very often. The approach considers that 12C (referred to as C) and 13C (referred to as [13C]) react as separate elements. Thus, they are separate components in a GAS_PHASE, and separate components in a calcite solid solution. Although conceptually correct, it is probably too tedious for most applications.

Here is an example where 1 liter of methane gas at a partial pressure of 1 atm, delta(13C) = -10 equilibrates with 1 kilogram of water. Run it using the database iso.dat.

SOLUTION
END
USE solution 1
GAS_PHASE 1
CH4(g)         1
[13C]H4(g)     0.011068398 # -10 permil,  delta = (1 - Rsmp/Rstd)*1000, Rstd = 0.0111802
END

Attached is an example that considers 13C, D, T, and 18O  with both a gas phase and a calcite solid solution. You can simplify to remove the other isotopes.

[catafsi]

To be honest, the isotope capabilities of PHREEQC are not used very often. The approach considers that 12C (referred to as C) and 13C (referred to as [13C]) react as separate elements. Thus, they are separate components in a GAS_PHASE, and separate components in a calcite solid solution. Although conceptually correct, it is probably too tedious for most applications.

Here is an example where 1 liter of methane gas at a partial pressure of 1 atm, delta(13C) = -10 equilibrates with 1 kilogram of water. Run it using the database iso.dat.

SOLUTION
END
USE solution 1
GAS_PHASE 1
CH4(g)         1
[13C]H4(g)     0.011068398 # -10 permil,  delta = (1 - Rsmp/Rstd)*1000, Rstd = 0.0111802
END

Attached is an example that considers 13C, D, T, and 18O  with both a gas phase and a calcite solid solution. You can simplify to remove the other isotopes.

Hi, thank you very much for the answer, was very useful for me.
I tried to download the example attached but it appeared an error that said " 404 not found"
Would be excellent if the file can be available again

[dlparkhurst]

Yeah, it's a bummer that the attachments seem to be no longer available.

I expect this is the file that was attached:

Code: [Select]

#DATABASE ../database/iso.dat
SOLUTION 1
pH 8.2
Na 1 charge
Ca 1 Calcite .1
C 2
[13C] 0 # permil
[14C] 100 # pmc
T 10 # TU
D 0 # permil
[18O] 0 # permil
END
USE solution 1
GAS_PHASE
-fixed_volume 1
H2O(g) 0
HDO(g) 0
D2O(g) 0
H2[18O](g) 0
HD[18O](g) 0
D2[18O](g) 0
HTO(g) 0
HT[18O](g) 0
DTO(g) 0
CO2(g) 0
CO[18O](g) 0
C[18O]2(g) 0
[13C]O2(g) 0
[13C]O[18O](g) 0
[13C][18O]2(g) 0
[14C]O2(g) 0
[14C]O[18O](g) 0
[14C][18O]2(g) 0
SOLID_SOLUTION
Calcite
-comp Calcite          0
-comp CaCO2[18O](s)      0
-comp CaCO[18O]2(s)      0
-comp CaC[18O]3(s)        0
-comp Ca[13C]O3(s)        0
-comp Ca[13C]O2[18O](s)  0
-comp Ca[13C]O[18O]2(s)  0
-comp Ca[13C][18O]3(s)    0
-comp Ca[14C]O3(s)        0
-comp Ca[14C]O2[18O](s)  0
-comp Ca[14C]O[18O]2(s)  0
-comp Ca[14C][18O]3(s)    0
USER_PRINT
10 PRINT "13C units:            ", ISO_UNIT("[13C]")
20 PRINT "R(13C)_CO2(aq) units: ", ISO_UNIT("R(13C)_CO2(aq)")
END[code]

[catafsi]

Thank you very much!

[MichaelZ20]

Dear David,
When I change the solution composition in example 20a to real seawater composition, I get a warning "Element [13C] is contained in solid solution Ca[13C]O3(s) (which has 0.0 mass), but is not in solution or other phases". Isn't it a bug?

SOLUTION 1
units mol/kgw   
Temp   25
pH   8.2
pe   8.45
Ca   1.25E-02  Calcite  0
Mg   6.06E-02               
Na   5.29E-01 
K   1.15E-02
Cl   6.39E-01  charge
S(6)   2.90E-02
C(4)   2.54E-03
N(5)   1.37E-05
Br   9.14E-04
Density   1.02576
[13C]   -0.05
D   0
[18O]   0

[dlparkhurst]

Let's call it a "feature".

There are two options for defining the carbon isotopes isotopes: You can either define concentration of "C" and "[13C]" or "C(4)" and "[13C](4)"  (and "C(-4)" and "[13C](-4) if needed).

If you define "C(4)" and "[13C]", PHREEQC is not smart enough, and you get the result that you noted.

[MichaelZ20]

David, thank you!
In iso.dat in SOLUTION_SPECIES, alphas etc. only [13C] has been defined.
In your example, there was no this problem.
In any case, I got the solution with [13C], but is it correct?

[dlparkhurst]

I'm not sure what you are asking. With your seawater SOLUTION, I think you should define C(4) and [13C](4) .