Chemical model of Amazon river water

MOOSE input file: no minerals

The MOOSE input file contains the usual GeochemicalModelDefinition that specifies the database file to use, and in this case just the basis species and equilibrium minerals (that are prevented from precipitating, but for which the saturation indices are of interest). The flag piecewise_linear_interpolation = true in order to compare with the Geochemists Workbench result.

[UserObjects]
  [definition]
    type = GeochemicalModelDefinition
    database_file = "../../../database/moose_geochemdb.json"
    basis_species = "H2O H+ SiO2(aq) Al+++ Fe++ Ca++ Mg++ Na+ HCO3- SO4-- Cl- O2(aq)"
    equilibrium_minerals = "Nontronit-Ca Nontronit-Mg Nontronit-Na Hematite Kaolinite Beidellit-Ca Beidellit-H Beidellit-Mg Beidellit-Na Pyrophyllite Gibbsite Paragonite Quartz"
    piecewise_linear_interpolation = true # for comparison with GWB
  []
[]

To instruct MOOSE to find the equilibrium configuration, a TimeIndependentReactionSolver is used:

• The pH is fixed by setting the activity of H.

• The bulk mole number of the aqueous species is also fixed appropriately. The numbers are different than the concentration in mg.kg given in the above table, and may be worked out using the TDS.

• The prevent_precipitation input prevents any minerals from precipitating when finding the equilibrium configuration, even if their saturation indices are positive.

• The other flags enable an accurate comparison with the Geochemists Workbench software.

[TimeIndependentReactionSolver]
  model_definition = definition
  charge_balance_species = "Cl-"
  constraint_species = "H2O              H+            O2(aq)             SiO2(aq)         Al+++            Fe++             Ca++             Mg++             Na+              HCO3-            SO4--            Cl-      "
  constraint_value = "  1.0              -6.5          1.813E-4           0.0001165        2.5945E-6        1.0744E-6        0.0001073        4.526E-5         7.83E-5          0.0003114        3.1233E-5        1.383E-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"
  constraint_unit = "   kg               dimensionless molal              moles            moles            moles            moles            moles            moles            moles            moles            moles"
  max_initial_residual = 0.1 # this small value is needed so that the charge-balance species is not switched to Ca++ (which, by the way, does not make a huge difference to the result)
  prevent_precipitation = "Nontronit-Ca Nontronit-Mg Nontronit-Na Hematite Kaolinite Beidellit-Ca Beidellit-H Beidellit-Mg Beidellit-Na Pyrophyllite Gibbsite Paragonite Quartz"
  stoichiometric_ionic_str_using_Cl_only = true # for comparison with GWB
  mol_cutoff = 1E-7
  abs_tol = 1E-15
[]

GWB input file: no minerals

The analogous GWB input file is

# React script that is the GWB equivalent to amazon.i
data = thermo.tdat verify
conductivity = conductivity-USGS.dat
temperature = 25 C
H2O          = 1 free kg
HCO3-        = .0003114 mol
SiO2(aq)     = .0001165 mol
O2(aq)       = .0001813 free molal
Ca++         = .0001073 mol
SO4--        = 3.1233e-5 mol
Cl-          = .0001383 mol
balance on Cl-
Na+          = 7.83e-5 mol
Mg++         = 4.526e-5 mol
Al+++        = 2.5945e-6 mol
Fe++         = 1.0744e-6 mol
pH           = 6.5
suppress all
printout  species = long
epsilon = 1e-12

Results: no minerals

Fixing mineral volumes

An alternate model assumes the water is in equilibrium with kaolinite and hematite, and that these control the aluminum and iron concentrations. That is, instead of Al, kaolinite is used in the basis, and instead of Fe, hematite is used in the basis. The free volumes of each of these is rather unimportant, but Bethke (2007) suggests using the values in Table 3.

MOOSE input file: with minerals

[TimeIndependentReactionSolver]
  model_definition = definition
  swap_out_of_basis = "Al+++ Fe++"
  swap_into_basis = "Kaolinite Hematite"
  charge_balance_species = "Cl-"
  constraint_species = "H2O              H+            O2(aq)             SiO2(aq)         Kaolinite    Hematite     Ca++             Mg++             Na+              HCO3-            SO4--            Cl-      "
  constraint_value = "  1.0              -6.5          1.813E-4            0.0001165       1E-2         0.033        0.0001073        4.526E-5         7.83E-5          0.0003114        3.1233E-5        1.383E-4"
  constraint_meaning = "kg_solvent_water log10activity free_concentration bulk_composition free_mineral free_mineral 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"
  prevent_precipitation = "Nontronit-Ca Nontronit-Mg Nontronit-Na Hematite Kaolinite Beidellit-Ca Beidellit-H Beidellit-Mg Beidellit-Na Pyrophyllite Gibbsite Paragonite Quartz"
  ramp_max_ionic_strength_initial = 0 # not needed in this simple problem
  stoichiometric_ionic_str_using_Cl_only = true # for comparison with GWB
  mol_cutoff = 1E-7
  abs_tol = 1E-15
[]

[UserObjects]
  [definition]
    type = GeochemicalModelDefinition
    database_file = "../../../database/moose_geochemdb.json"
    basis_species = "H2O H+ SiO2(aq) Al+++ Fe++ Ca++ Mg++ Na+ HCO3- SO4-- Cl- O2(aq)"
    equilibrium_minerals = "Nontronit-Ca Nontronit-Mg Nontronit-Na Hematite Kaolinite Beidellit-Ca Beidellit-H Beidellit-Mg Beidellit-Na Pyrophyllite Gibbsite Paragonite Quartz"
    piecewise_linear_interpolation = true # for comparison with GWB
  []
[]

GWB input file: with minerals

# React script that is the GWB equivalent to amazon_with_minerals.rea
data = thermo.tdat verify
conductivity = conductivity-USGS.dat
temperature = 25 C
H2O          = 1 free kg
HCO3-        = .0003114 mol
SiO2(aq)     = .0001165 mol
O2(aq)       = .0001813 free molal
Ca++         = .0001073 mol
SO4--        = 3.1233e-5 mol
Cl-          = .0001383 mol
balance on Cl-
Na+          = 7.83e-5 mol
Mg++         = 4.526e-5 mol
swap Kaolinite for Al+++
Kaolinite    = .01 free mol
swap Hematite for Fe++
Hematite     = .033 free mol
pH           = 6.5
suppress ALL
unsuppress  Hematite Kaolinite
printout  species = long
epsilon = 1e-15

Results: with minerals

According to this model, only nontronite clay is supersaturated by any significant amount. The geochemistry module produces identical results to the GWB software.

References