Estimating soil water solution

[FlouretAlex]

Hello everyone.

I'm trying to estimate soil water solution on basis of clay content and the exchangeable cation measured for this soil. Solution concentration where estimating given the estimated porosity and soil apparent density. Those concentrations are defined in SOLUTION 1. The pCO2 is estimated at 10^-2 atm based on (Calvet, 2003) and thus equilibrated with the solution depending if the soil is basic or acidic calcite or gibbsite are equilibrated with the same solution. With basic soil, this approach is working really fine as my result are close to what he is measured for both ions in solution and pH. But with acidic soil convergence failure occur. I think it's a problem of exchange mass balance with Ca but I don't know how to deal with this problem.

code input with basic soil

Code: [Select]

DATABASE C:\Users\International\Documents\PhD\AlexF\Modelo alex\Fitted\000_Database_PHREEQC\PKDLM_BRGM_database_phreeqc_ThermoddemV1.10_06Jun2017.dat

#Soil-E (basic pH 7.2) 60% porosity

SURFACE 10
-sites_units density
Illitex 1.80E-04 97 1666.67
Montx 1.81E-06 800 1666.67
Kaolinitex 3.13E-06 10 1666.67
# ^site/nm^2 ^SSA (m^2/g) ^M/V (g/L)
EXCHANGE 10

Illi 2.50E-02
Mont 7.29E-02
Kao 6.96E-04

SOLUTION 1
units mol/L
pH 7  charge
Na 8.33E-04
K 9.50E-04
Ca 1.14E-02
Mg 1.55E-04
N(5) 3.20E-03
Cl 4.86E-04
S(6) 2.56E-04
USE SURFACE 10
USE EXCHANGE 10
EQUILIBRIUM_PHASES 1
Calcite 0.00
CO2(g) -2

SELECTED_OUTPUT
-reset false
-file C:\Users\International\Desktop\VF_model\RHIZO_moy\Eau Porale au champ\SOL-E_EAU-P_PORO60%.sel
-high_precision false
USER_PUNCH
-headings pH Na K Ca Mg NO3 Cl SO4 HCO3
-start
10 Caaq = tot("Ca") #(mol/L)
20 CO3aq = tot("C(4)") #(mol/L)
30 Kaq = tot("K") #(mol/L)
40 Naaq = tot("Na") #(mol/L)
50 Mgaq = tot("Mg") #(mol/L)
60 Claq = tot("Cl") #(mol/L)
70 NO3aq = tot("N(5)") #(mol/L)
80 SO4aq = tot("S(6)") #(mol/L)

200 punch     -LA("H+"), Naaq, Kaq, Caaq, Mgaq, NO3aq, Claq, SO4aq, CO3aq,
-end

END

Code input for the acidic soil

Code: [Select]

DATABASE C:\Users\International\Documents\PhD\AlexF\Modelo alex\Fitted\000_Database_PHREEQC\PKDLM_BRGM_database_phreeqc_ThermoddemV1.10_06Jun2017.dat

#Soil-H (acidic pH 5) 60% porosity

SURFACE 10
-sites_units density
Illitex 4.48E-05 97 1666.67
Montx 2.64E-06 800 1666.67
Kaolinitex 6.63E-05 10 1666.67
# ^site/nm^2 ^SSA (m^2/g) ^M/V (g/L)
EXCHANGE 10
Illi 6.22E-03
Mont 1.06E-01
Kao 1.47E-02

SOLUTION 1
units mol/L
pH 7 charge
Na 3.81E-03
K 1.57E-03
Ca 1.08E-03
Mg 1.99E-04
N(5) 5.75E-04
Cl 7.20E-03
S(6) 1.87E-03
Al 1.59E-04
USE SURFACE 10
USE EXCHANGE 10
EQUILIBRIUM_PHASES 1
Gibbsite 0.00
CO2(g) -2.00

SELECTED_OUTPUT
-reset false
-file C:\Users\International\Desktop\VF_model\RHIZO_moy\Eau Porale au champ\SOL-H_EAU-P_PORO60%.sel
-high_precision false
USER_PUNCH
-headings pH Al Na K Ca Mg NO3 Cl SO4 HCO3
-start
10 Caaq = tot("Ca") #(mol/L)
20 CO3aq = tot("C(4)") #(mol/L)
30 Kaq = tot("K") #(mol/L)
40 Naaq = tot("Na") #(mol/L)
50 Mgaq = tot("Mg") #(mol/L)
60 Claq = tot("Cl") #(mol/L)
70 NO3aq = tot("N(5)") #(mol/L)
80 SO4aq = tot("S(6)") #(mol/L)
90 Alaq = tot("Al") #(mol/L)

200 punch     -LA("H+"), Alaq, Naaq, Kaq, Caaq, Mgaq, NO3aq, Claq, SO4aq, CO3aq,
-end

END

Error message

Code: [Select]

WARNING: Trying reduced tolerance 1e-16 ...

WARNING: Maximum iterations exceeded, 200

WARNING: Numerical method failed with this set of convergence parameters.

WARNING: The program has failed to converge to a numerical solution.

The following equations were not satisfied:
ERROR:                   Ca has not converged. Total: 1.080567e-03 Calculated: 2.432248e-02 Residual: -2.324192e-02

ERROR:                   Mg has not converged. Total: 1.991045e-04 Calculated: 4.408719e-03 Residual: -4.209615e-03

ERROR:                 Illi Exchanger mass balance has not converged. Residual: 2.593439e-03

ERROR:                 Mont Exchanger mass balance has not converged. Residual: 5.452718e-02

ERROR:                  Kao Exchanger mass balance has not converged. Residual: 6.954154e-03

Using solution 1.
Using exchange 10.
Using surface 10.
Using pure phase assemblage 1.
Thank you for your help

[dlparkhurst]

If you attach your input and database file to a post, I will take a look.

[FlouretAlex]

Here the database.
I was thinking about simplifying the database by removing all the species that I don't take into consideration but I do not know if this is useful.

Thank you for your help!

[dlparkhurst]

Your initial conditions for EXCHANGE and SURFACE are not well defined.

For EXCHANGE, you should define the cations that are initially present. You can either define something like

Code: [Select]

EXCHANGE 10
NaIlli 6.22E-03
Ca0.5Mont 1.06E-01
Na0.5Ca0.25Kao 1.47E-02

to give the initial exchange composition, or you can define the exchanger to be in equilibrium with a solution with "-equil n", where n is a SOLUTION user number.

The same goes for SURFACE. It would be best to define the surface either as uncharged with no sorbed ions

Code: [Select]

SURFACE 10
-sites_units density
IllitexOH0.5 4.48E-05 97 1666.67
MontxOH0.5 2.64E-06 800 1666.67
KaolinitexOH0.5 6.63E-05 10 1666.67

or with -equil to set the initial surface composition in equilibrium with a known solution composition.

[FlouretAlex]

Hello,

Thank you for your help. Ho, I did not know that it was possible to subdivide the charge on my exchanger like that, this is really interesting.
In your example.

Code: [Select]

EXCHANGE 10
NaIlli 6.22E-03
Ca0.5Mont 1.06E-01
Na0.5Ca0.25Kao 1.47E-02
Mont is charged at 50% with Ca is that correct?

The script on the previous message had two mains objective:
 1) approximate ions concentration into the soil solution at the sampling site.
 2) find how my surface and exchanger are charged with this solution.
The idea was to create an initial solution that represents total exchangeable cation (calculated from cobalt hexamine trichloride soil desorption) and some labile ions (calculated with pure water desorption at 4 different m/V).
And then initiate batch reaction exchange between this solution and the uncharged surface and exchanger with the right m/V ratio. The hypothesis was that the non sorbed ions would represent ions in soil solution and the ion adsorbed the charged exchanger and surface found into the soil.
Therefore, I did not define the exchanger to be in equilibrium with the solution. I will try with the uncharged definition and see what happens.
Thank you for your help and Merry Christmas.
Alex

[dlparkhurst]

All of these EXCHANGE definitions are equivalent:

Code: [Select]

Na0.8Ca0.05Mg0.05X  1


Code: [Select]

Na0.8X0.8    1
Ca0.05X0.1   1
Mg0.05X0.1   1


Code: [Select]

NaX  0.8
CaX2 0.05
MgX2 0.05


[FlouretAlex]

Thank you for the tips!
And for my problem, it's all good. The problem was some miss calculations for the definition of my initial solution.
Thank you!