Processes > Inverse modelling

How to add Fertilizer formular in the Phreeqc.dat Database for Inverse modelling

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evansmanu:
Thank you for your thoughtful comments.
 
Yes, phosphorus concentration in groundwater was below the detection limit. This means that its concentration in the groundwater system is relatively low to be detected, and assuming that fertilizer is a source of phosphorus may not be a step in the right direction.

In general, sulphate is higher in the source solution (rainwater) affected by marine aerosols. The concentration in groundwater is far lower than that measured in rainwater. It may be that the groundwater system is a reduced environment causing sulphate to be reduced by other processes. A preliminary estimate of the pe was found to be -5, which is favourable for sulphate reduction to occur.

From here it is only fair to suppress the influence of fertilizers in my evolutionary analysis as field data may not support such a conceptual model.

evansmanu:
Dear Phreeqc users

I've been using Phreeqc for a while now and just realized I still have more to learn. I am currently running a forward reaction pathway model, and the final step is to include a cation exchange reaction to assess the effect of cation exchange on the Na, K, Mg and Ca concentrations. The modeling process consists of the following.

1. The inverse modeling was performed using a RedModRphree package, which leverages the r programming language to perform combinatorial inverse modeling. It uses the PHREEQC as a simulator to perform combinatorial inverse modelling. The combinatorial inverse model assumes an equilibration of primary minerals and precipitation of secondary minerals. Based on the rainwater chemistry, I was able to determine plausible mineral compositions that are responsible for the chemical development of the groundwater.

2. I then insert the identified mineral compositions into a forward model to reproduce the end member solution. After going through a series of reaction steps, I got very close to the observed concentrations for most of the major ions. However, there is still a significant discrepancy between the observed and modeled concentrations for Ca, Mg, Na and K, which I think could be due to cation exchange.

Incorporating the cation exchange reaction has become a problem, and for now, I've used a few examples that I don't have much confidence in their implementation. After performing the cation exchange, I found that all other concentrations, including Al, Fe, HCO3 and others, were significantly reduced and further deviated from the observed concentrations.

My original model shows that Na and K are underestimated relative to the observed concentrations, while Ca and Mg are overestimated.

How can I best implement cation exchange in my forward model to ensure that Na and K increase while Ca or Mg decrease the cation deviations?

Please find the input script below.

TITLE Northern evolution with new conceptual model
SOLUTION   0
    temp      25
    pH        3.749
  #  pe        14 #-4.944438
    redox     O(0)/O(-2)
    units     mol/kgw
    density   1
    O(0)      1.0     O2(g) -0.7
    C(4)      1.147e-03
    Ca        1.504e-03 charge
    Cl        1.321e-03 #charge
    K         1.559e-04
    Mg        1.081e-04
    Na        1.692e-04
    S(6)      1.216e-03
EQUILIBRIUM_PHASES 1
#   O2(g)      -0.7
   CO2(g)   -3.5
   Kaolinite   0.0
#   Hematite   0.0

SAVE Solution 1
END
USE Solution 1
REACTION 1
   Formalin 1
   0.001 moles
SAVE Solution 2
END   
USE Solution 2
EQUILIBRIUM PHASES 2
   Albite 0 8.017e-04
   Phlogopite 0 1.669e-04
   Pyrite 0 2.166e-09
   K-feldspar 0 0
   Kaolinite 0 0
   Fe(OH)3(a) 0
   pe_Fix -4.944438 O2(g) 0.5
SAVE Solution 3
END
USE Solution 3
EXCHANGE_SPECIES
 X- + K+ = KX; log_k 0.7
 2X- + Ca+2 = CaX2; log_k 0.8
 Na+ + X- = NaX; log_k 0
EXCHANGE 1
 NaX 0.417    # exchangeable Na and K in mol
 KX 0.1871   
MIX 3; 1 1e-9 #
SAVE Solution 4
END

dlparkhurst:
In your final calculation, you have USE solution 3 and MIX 3; 1 1e-9. These are conflicting definitions, and PHREEQC will use MIX 3 in preference to USE solution 3. The solution volume will be 1e-9 liters and the system will have only a tiny amount of Ca and Mg. I don't think that is what you intended.

I think you should reconsider the initial condition of your exchanger. If you start with fairly large amounts of just KX and NaX you are trying too hard to fix the system. In most cases, you would expect the exchanger to be in equilibrium with the pre-existing groundwater composition (say, EXCHANGE 1;  -eq 1 ...). Then if the infilling water composition changes or there are reactions, the exchange composition will respond.

The equilibrium constants for CaX2, NaX, and KX will determine the relative affinity for the ions on the exchanger, and in turn the concentrations remaining in solution.

Finally, I do not like fixing the pe. You can force the system out of the stability field of water if you are not careful. Better to fix the O2(g) partial pressure.

evansmanu:
I made some changes to the exchange equation and this time put it in equilibrium as shown in the script below. My intention is to equilibrate the final solution 3 with the exchanger and observes the changes in the cation concentrations (Ca2+, Na+, Mg2+ and K+. I do not see many changes in the ion concentrations after bringing the final solution 3 in equilibrium with the exchanger.

I may be wrong, however, if am right then the model may suggest that cation exchange may not have much influence on the evolution of groundwater. I also do not understand the interpretation of the exchange composition below. Other exchangers like FeX2 and AlOHX2 are also included and would like to understand their role.

The final pH was also reduced and this suggests the influence of H+ ion, how can this be controlled

-----------------------------Exchange composition------------------------------

X                1.000e-03 mol

                                  Equiv-    Equivalent      Log
   Species             Moles      alents      Fraction     Gamma

   CaX2              4.647e-04   9.293e-04   9.293e-01    -0.000
   FeX2              2.026e-05   4.053e-05   4.053e-02    -0.138
   NaX               1.095e-05   1.095e-05   1.095e-02    -0.000
   MgX2              8.856e-06   1.771e-05   1.771e-02    -0.139
   KX                1.503e-06   1.503e-06   1.503e-03    -0.000
   AlOHX2            2.118e-14   4.235e-14   4.235e-11     0.036

The objective is to use ascertain the influence of cation exchange in the system.


TITLE Northern evolution with new conceptual model
SOLUTION   0
    temp      25
    pH        3.749
  #  pe        14 #-4.944438
    redox     O(0)/O(-2)
    units     mol/kgw
    density   1
    O(0)      1.0     O2(g) -0.7
    C(4)      1.147e-03
    Ca        1.504e-03 charge
    Cl        1.321e-03 #charge
    K         1.559e-04
    Mg        1.081e-04
    Na        1.692e-04
    S(6)      1.216e-03
EQUILIBRIUM_PHASES 1
   O2(g)      -0.7
   CO2(g)   -3.5
   Kaolinite   0.0
#   Hematite   0.0

SAVE Solution 1
END
USE Solution 1
REACTION 1
   Formalin 1
   0.001 moles
SAVE Solution 2
END   
USE Solution 2
EQUILIBRIUM PHASES 2
   Albite 0 8.017e-04
   Phlogopite 0 1.669e-04
   Pyrite 0 2.166e-09
   K-feldspar 0 0
   Kaolinite 0 0
   Fe(OH)3(a) 0
#   pe_Fix -4.944438 O2(g) 0.5
SAVE Solution 3
END
USE Solution 3
EXCHANGE_SPECIES
 X- + K+ = KX; log_k 0.7
 2X- + Ca+2 = CaX2; log_k 0.8
 Na+ + X- = NaX; log_k 0
EXCHANGE 1
   -equilibrate 1
   X   1.e-3
SAVE Solution 4
END

dlparkhurst:
A few comments.

You have no Ca source or sink in your set of minerals other than calcite. At one point you had a definition of plagioclase, which seems a likely reactant if you have K and Na feldspar. The reacted water of your last post is supersaturated with calcite, which would mean calcite is likely to precipitate.

The "Formalin" definition is not wrong but seems misleading. I prefer CH2O or Organic_matter, where CH2O is a simplification of something like cellulose.

If this is a soil-zone or saturated zone weathering, soil-zone pCO2 is usually greater than atmospheric--in the range 10-3 to 10-1 depending on productivity. If you are modeling soil zone, you should probably include CO2(g) in the equilibrium phases and perhaps O2(g).

I think it is difficult to assess the effect of exchange. I think a TRANSPORT calculation may be more appropriate, where the exchange composition evolves as water percolates through the system. Still it is hard to know how to define the system. If you start with rain and a pure primary mineral assemblage and weather to secondary clays and minerals, then I think you might have a well-defined  system that would exchange ions as they were created by weathering. But if you start at some intermediate weathering point, I am not sure what the correct exchange composition would be.

I might go back to the inverse modeling approach to try to get a net reaction. Before, we got hung up on the fertilizer composition. Now, you might reconsider the minerals that are weathered and the secondary minerals that form. You could start with pure water (rainwater) and run to one of your water compositions. It still may be difficult to arrive at anything definitive, but I think it would help to formalize what you think the important reactions are.



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