Chemical models of Morro de Ferro groundwater

This example closely follows Section 7.3 of Bethke (2007).

A chemical analysis of the major element composition of Morro de Ferro groundwater is shown in Table 1. In addition:

  • the temperature is 22C;

  • the pH is 6.05;

  • EhmV.

Table 1: Major element composition of Morro de Ferro groundwater

SpeciesConcentration (mg.litre)
HCO1.8
Ca0.238
Mg0.352
Na0.043
K0.20
Fe (II)0.73
Fe (total)0.76
Mn0.277
Zn0.124
Cl
SO0.15
Dissolved O4.3

Assuming redox equilibrium

Assume redox equilibrium and that the oxidation state is set by the dissolved oxygen. Also:

  • the free concentration of O(aq) is set to 4.3mg.litre,

  • the bulk composition of Fe is 0.73mg.litre,

  • and charge balance is enforced on Cl

MOOSE input file

The MOOSE input file contains the GeochemicalModelDefinition and TimeIndependentReactionSolver. The bulk mole number of the aqueous species is also fixed appropriately in the latter. The numbers are different than the concentration in mg.l given in the above table, and may be worked out using the TDS. The other flags options ensure nice convergence and an accurate comparison with the Geochemists Workbench software.

[TimeIndependentReactionSolver]
  model_definition = definition
  temperature = 22
  charge_balance_species = "Cl-" # this means the bulk moles of Cl- will not be exactly as set below
  constraint_species = "H2O              H+            O2(aq)             Cl-               HCO3-             Ca++              Mg++              Na+               K+                Fe++              Mn++              Zn++              SO4--"
  constraint_value = "  1.0              -6.05         0.13438E-3         3.041E-5          0.0295E-3         0.005938E-3       0.01448E-3        0.0018704E-3      0.005115E-3       0.01307E-3        0.005042E-3       0.001897E-3       0.01562E-4"
  constraint_meaning = "kg_solvent_water log10activity free_concentration bulk_composition  bulk_composition  bulk_composition  bulk_composition  bulk_composition  bulk_composition  bulk_composition  bulk_composition  bulk_composition  bulk_composition"
  constraint_unit = "   kg               dimensionless molal              moles             moles             moles             moles             moles             moles             moles             moles             moles             moles"
  max_initial_residual = 1E-2
  ramp_max_ionic_strength_initial = 10
  stoichiometric_ionic_str_using_Cl_only = true # for comparison with GWB
  mol_cutoff = 1E-5
  abs_tol = 1E-15
[]

[UserObjects]
  [definition]
    type = GeochemicalModelDefinition
    database_file = "../../../database/moose_geochemdb.json"
    basis_species = "H2O H+ Cl- O2(aq) HCO3- Ca++ Mg++ Na+ K+ Fe++ Mn++ Zn++ SO4--"
  []
[]
(modules/geochemistry/test/tests/redox_disequilibrium/morro.i)

Geochemists Workbench input file

The equivalent GWB input file is

# React script that is equivalent to the morro.i MOOSE input file
data = thermo.tdat verify
conductivity = conductivity-USGS.dat
temperature = 22 C
H2O          = 1 free kg
Cl-          = 3.041E-5 mol
balance on Cl-
H+           = 6.05 pH
O2(aq)       = 0.13438E-3 free molal
HCO3-        = 0.0295E-3 mol
Ca++         = 0.005938E-3 mol
Mg++         = 0.01448E-3 mol
Na+          = 0.0018704E-3 mol
K+           = 0.005115E-3 mol
Fe++         = 0.01307E-3 mol
Mn++         = 0.005042E-3 mol
Zn++         = 0.001897E-3 mol
SO4--        = 0.01562E-4 mol
printout  species = long
suppress all
epsilon = 1e-13
go
(modules/geochemistry/test/tests/redox_disequilibrium/morro.rea)

Assuming redox disequilibrium

Assume redox disequilibrium for iron. Also:

  • the free concentration of O(aq) is set to 4.3mg.litre,

  • the bulk composition of Fe is 0.73mg.litre,

  • the bulk composition of Fe is 0.03mg.litre,

  • and charge balance is enforced on Cl

MOOSE input file

The MOOSE input file is very similar to the redox-equilibrium case. The differences are:

  • Fe is included in the basis;

  • the sum of the bulk composition for Fe and Fe equals the bulk composition of Fe in the equilibrium case

[TimeIndependentReactionSolver]
  model_definition = definition
  temperature = 22
  charge_balance_species = "Cl-" # this means the bulk moles of Cl- will not be exactly as set below
  constraint_species = "H2O              H+            O2(aq)             Cl-              HCO3-            Ca++             Mg++             Na+              K+               Fe++            Fe+++             Mn++             Zn++             SO4--"
  constraint_value = "  1.0              -6.05         0.13438E-3         3.041E-5         0.0295E-3        0.005938E-3      0.01448E-3       0.0018704E-3     0.005115E-3      0.012534E-3     0.0005372E-3      0.005042E-3      0.001897E-3      0.01562E-4"
  constraint_meaning = "kg_solvent_water log10activity free_concentration bulk_composition bulk_composition bulk_composition bulk_composition bulk_composition bulk_composition bulk_composition bulk_composition bulk_composition bulk_composition bulk_composition"
  constraint_unit = "   kg               dimensionless molal              moles            moles            moles            moles            moles            moles            moles            moles            moles            moles            moles"
  max_initial_residual = 1E-2
  ramp_max_ionic_strength_initial = 10
  stoichiometric_ionic_str_using_Cl_only = true # for comparison with GWB
  mol_cutoff = 1E-5
  abs_tol = 1E-15
[]

[UserObjects]
  [definition]
    type = GeochemicalModelDefinition
    database_file = "../../../database/moose_geochemdb.json"
    basis_species = "H2O H+ Cl- O2(aq) HCO3- Ca++ Mg++ Na+ K+ Fe++ Fe+++ Mn++ Zn++ SO4--"
  []
[]
(modules/geochemistry/test/tests/redox_disequilibrium/morro_disequilibrium.i)

Geochemists Workbench input file

The equivalent GWB input file is

# React script that is equivalent to the morro_disequilibrium.i MOOSE input file
data = thermo.tdat verify
conductivity = conductivity-USGS.dat
temperature = 22 C
decouple Fe+++
H2O          = 1 free kg
Cl-          = 3.041E-5 mol
balance on Cl-
H+           = 6.05 pH
O2(aq)       = 0.13438E-3 free molal
HCO3-        = 0.0295E-3 mol
Ca++         = 0.005938E-3 mol
Mg++         = 0.01448E-3 mol
Na+          = 0.0018704E-3 mol
K+           = 0.005115E-3 mol
Fe++         = 0.012534E-3 mol
Fe+++        = 5.372E-7 mol
Mn++         = 0.005042E-3 mol
Zn++         = 0.001897E-3 mol
SO4--        = 0.01562E-4 mol
printout  species = long
suppress all
epsilon = 1e-13
go
(modules/geochemistry/test/tests/redox_disequilibrium/morro_disequilibrium.rea)

Results

The geochemistry results mirror those from Geochemists Workbench exactly.

Error and charge-neutrality error

The geochemistry simulations report an error of less than mol, and that the charge of the solution has magnitude less than mol.

Solution mass

The solution mass is 1.000kg.

Ionic strength and water activity

The ionic strength is 1.01E-4mol/kg(solvent water) for the equilibrium case and 1.29E-4mol/kg(solvent water) for the disequilibrium case. The water activity is 1.000 in both cases

pH and pe

The pH is 6.05 in both cases (as specified by the constraint), the pe is 14.7 in both cases.

Nernst Eh values

Bethke (2007) computes the Nernst Eh values for the redox half-reactions as: Both codes produce this result. The latter one is only relevant in the redox-disequilibrium case

Aqueous species distribution

The species distribution predicted by Bethke (2007) is shown in the right-hand column of Table 2. This result is obtained by ignoring any potential precipitation of minerals. Both codes predict the same result, up to precision.

Table 2: Calculated molalities (mol.kg) of iron species in Morro de Ferro groundwater, assuming redox equilibrium (central column) or disequilibrium (right-hand column)

SpeciesEquilibriumDisequilibrium
Fe
FeSO
FeHCO
FeCl
FeOH
Fe(OH)
Fe(OH)
FeOH
Fe(OH)

References

  1. Craig M. Bethke. Geochemical and Biogeochemical Reaction Modeling. Cambridge University Press, 2 edition, 2007. doi:10.1017/CBO9780511619670.[BibTeX]