Processes > Inverse modelling

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

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Dear Phreeqc Users,

I come your way again with this task. I am running an inverse modelling and would want to account for the influence of fertilizer on the chemistry of the water under investigation. Here I provide you with the composition of the type of the fertilizer.

The formula to produce the ASAASE WURA Fertilizer is given as bellow

P = 22% (0.22), K = 18% (0.18), CaO = 9% (0.09), MgO = 6% (0.06), S = 7% (0.07)

All the above percentages sum up to 62% (0.62). The other nutrients which I have been informed so far is chlorine which comes in the form of KCl. There are also other micro nutrients which am not preview to. In the below equation, I attempted to find a balance for the main constituents of the fertilizer plus dissolving with H2O.

##   K0.18O0.8P0.2Ca0.09Mg0.06(SO4)0.07 + 0.26 e - = 0.18 K+ + 0.2 PO4-3 + 0.09 Ca+2 + 0.06 Mg+2 + 0.07 SO4-2
##   -log_k 0.0 for inverse modelling only

I included this equation in the data base and used fertilizer as part of the minerals by constraining it to dissolve only. I had only one model solution at the end of the inverse modelling which did not load Fertilizer as part of the modeled solution. I am not sure if the approach I used to form the equation above is right or wrong.

Thanks and hope to get input from you all


It is difficult to know how to formulate the reaction for your fertilizer. I would probably try to use some well-defined formulas for compounds. I'm not sure what would be right, but formulas like Ca3(PO4)2, KCl, CaCO3, MgCO3, Na2SO4 would make it easy to write a balanced reaction. I expect the percentages are weight percentages, so the stoichiometric coefficients of the compounds need to be adjusted so that you get the correct mass percentages of the elements.

I don't know why your fertilizer did not appear in a model. You must have had other sources for all of the elements so that fertilizer was not needed. The e- in the formula may be part of the problem because it would need an electron donor to dissolve the fertilizer. If you use the balanced compounds as suggested, you would not need e- and it may make a difference.

Thanks very much. I will work from that angle and update the forum

Dear Phreeqc users,

In my previous post, I tried to formulate a balanced stoichiometric equation for fertilizer which I used in my inverse model. I have tried to update my equation as shown below:

   K0.18O0.72P0.22Ca0.09Mg0.06Na0.18(SO4)0.07Cl0.18 + 0.16H2O = 0.18K+ + 0.09Ca+2 + 0.06Mg+2 +
        0.18Na+ + 0.32H+ + 0.22PO4-3 + 0.18Cl- + 0.07SO4-2
   -log_k 0.0

The inverse model produced only one model solution without fertilizer. First of all, I would like a comment on my fertilizer formula if it is correct according to the information in my earlier post. I have tried to come out with this balanced equation for the fertilizer. Can I say that in the given model solution fertilizer is not an important component in the development of groundwater? or that my stoichiometric equation is wrong? This is an area where fertilizers are widely used.

Any comments from you will be appreciated

Find below Phreeqc script

SOLUTION 1 Rain_Water Chemistry - here we evaporated the rainwater until the Cl concentration of the Northern zone is matched.
    temp      25
    pH        3.749
    pe        3.017
    redox     pe
    units     mol/kgw
    density   1
    C(4)      1.147e-03 as HCO3
    Ca        1.504e-03 charge
    Cl        1.321e-03
    K         1.559e-04
    Mg        1.081e-04
    Na        1.692e-04
    S(6)      1.216e-03 as SO4
    -water    1 # kg
SOLUTION 2 Northern Zone CaHCO3 - Water type
    temp      25
    pH        6.4635
    pe        4
    redox     pe
    units     mg/l
    density   1
    Al        4.419314e-03
    C(4)      1.085457e+02 as HCO3
    Na     1.190286e+01
    Ca     2.635481e+01
    Cl        8.449450e+00 charge
    K         7.085174e-01
    Mg        6.433950e+00
    S(6)      1.133550e+00 as SO4
    Si        2.394551e+01 as SiO2
    Fe     1.918470e-01
    -water    1 # kg
    KMg3(AlSi3)O10(OH)2 + 10 H+ = 1 Al+3 + 1 K+ + 3 Mg+2 + 3 H4SiO4
    log_k        41.082
    -delta_H        -360.123    kJ/mol    # References :92cir/nav
    -analytic    -1.7201279e+03    -2.6579576e-01    1.0718208e+05    6.1999929e+02    -4.7275095e+06
          Na0.62Ca0.38Al1.38Si2.62O8 + 8H2O = 0.62Na+ + 0.38Ca+2 + 1.38Al(OH)4- + 2.62H4SiO4
          -log_k  -18.65256
          -delta_h 20.45592 kcal
          -Vm 103.6288
   CH2O + H2O = 4 e- + 4 H + + CO2
   -log_K = 4.8
   K0.18O0.72P0.22Ca0.09Mg0.06Na0.18(SO4)0.07Cl0.18 + 0.16H2O = 0.18K+ + 0.09Ca+2 + 0.06Mg+2 + 0.18Na+ + 0.32H+ + 0.22PO4-3 + 0.18Cl- + 0.07SO4-2
   -log_k 0.0
   CH2O + H2O = 4 e - + 4 H + + CO2
   -log_K = 4.8

    -solutions      1        2
    -uncertainty    0.05     0.05
        Albite            dis
        Phlogopite        dis
        Fertilizer        dis
        K-feldspar        #pre
        Al          0.1      0.1
        Na          0.1      0.1
    -range             1000
    -tolerance         1e-10
    -mineral_water     true

Best regards

The fertilizer formula has both sulfur and phosphorus. However, sulfur actually decreases from initial to final solution, and phosphorus is zero in both solutions. Therefore, there is not really a need for fertilizer in an inverse model to account for the change in water chemistry. Sulfur is removed in pyrite, and you cannot add fertilizer (without an additional P sink) and end up with zero phosphorus.

So, consider whether your analyses are representative of the overall evolution of the water chemistry, and whether you should see a fertilizer signal.


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