Saturation index with ppm or molarity

[HayatNottsUK]

Dear All

thank you for this helpful forum, I'm using PHREEQC for my groundwater research, and I came upon this issue:

I'm trying to get the saturation index for gypsum for 60 wells, I uploaded my Data into the solution spreadsheet, once in ppm (mg/L) and again in Molarity (mol/L) and run both programs, theoretically the saturation index should be the same for both runs, but its not, mainly the activity for SO4 is different among other things, for example, the "solution description" below shows how they give different results:

ppm run:


                                       pH  =   6.800   
                                       pe  =   4.000   
      Specific Conductance (µS/cm,  25°C)  = 4255
                          Density (g/cm³)  =   0.99923
                               Volume (L)  =   1.00333
                        Activity of water  =   0.999
                 Ionic strength (mol/kgw)  =   5.571e-02
                       Mass of water (kg)  =   1.000e+00
                 Total alkalinity (eq/kg)  =   3.098e-04
                       Total CO2 (mol/kg)  =   3.922e-04
                         Temperature (°C)  =  25.00
                  Electrical balance (eq)  =   7.179e-03
 Percent error, 100*(Cat-|An|)/(Cat+|An|)  =   9.05
                               Iterations  =  11
                                  Total H  = 1.110127e+02
                                  Total O  = 5.553204e+01

Molarity run:
                   pH  =   6.800   
                                       pe  =   4.000   
      Specific Conductance (µS/cm,  25°C)  = 5183
                          Density (g/cm³)  =   1.00024
                               Volume (L)  =   1.00360
                        Activity of water  =   0.999
                 Ionic strength (mol/kgw)  =   7.015e-02
                       Mass of water (kg)  =   1.000e+00
                 Total alkalinity (eq/kg)  =   1.577e-03
                       Total CO2 (mol/kg)  =   1.997e-03
                         Temperature (°C)  =  25.00
                  Electrical balance (eq)  =  -1.879e-02
 Percent error, 100*(Cat-|An|)/(Cat+|An|)  = -19.91
                               Iterations  =   9
                                  Total H  = 1.110140e+02
                                  Total O  = 5.558597e+01

For gypsum activity for Ca+2 and SO4 are important to me, Ca activity is not that different, but SO4 really is as can you see from the table below.

             (ca2+) Ion activity   (SO+4) Ion activity      (ca2+) Ion activity   (SO+4) Ion activity
                      ppm                                ppm                               molarity                    molarity
well 1      3.64E-03                          1.81E-03                            2.74E-03                 5.64E-03
well 2      3.02E-03                           1.22E-03                      2.34E-03                 3.77E-03



I recond it has something to do with how PHREEQC speciate sulfare, but not sure which was to run ppm or Molarity?


Thanks for reading and hoping someone can help


Regards
Hayat

[dlparkhurst]

Somehow, you have not done equivalent calculations. I would need to see the two SOLUTION definitions, but when you enter mg/L SO4, it is converted to moles using an gram formula weight of SO4 (~96).

Note that ppm is mg/kg solution. For mg/L, density is used to calculate mg/kgs, and then the sum of the solute masses is used to convert to mg/kg water, which is the unit for all PHREEQC solution calculations.

[HayatNottsUK]

Hi

Thanks for the reply, yeah I agree it seems that the calculations don't match, I have sulfur value, so I converted using the formula weight of (32.06) and when choosing the element in the spreadsheet I choose (S)

Regarding PHREEQC converting to mg/Kg, I just thought since my solution is basically water then kg equal L??

For example well 1 uploaded data for mg/L:
SOLUTION_SPREAD
    -units    mg/l
  C(4)             Ca         Cl              K             Mg        N             Na      pH       S(6)
 mg/l           mg/l       mg/l           mg/l           mg/l     mg/l           mg/l                  
 23.87    363.3397781     973.17    78.44593641    119.2042429     5.24    397.9054757     6.8     592.31


for mmol/L

SOLUTION_SPREAD
    -units    mmol/l
          Cl              N           C(4)             Na             Mg              K             Ca      pH              S
     mMol/l         mMol/l         mMol/l         mMol/l         mMol/l         mMol/l         mMol/l                      
 27.45190409    0.373975732    1.989166667    17.30776319    4.903506495    2.006289934    9.065363725     6.8    18.47502569

what do you think?

Thanks

[dlparkhurst]

You need to know the gram formula weights that PHREEQC uses. They are defined in the SOLUTION_MASTER_SPECIES block of the database that you use. Values are defined for each element or redox state, either the gram formula weight (g/mol) or a formula (like SO4) from which a gram formula weight is calculated. Check out SOLUTION_MASTER_SPECIES in the manual for details.

For phreeqc.dat, these are the definitions for sulfur and carbon are given below. C(4) is converted from grams to moles with gfw 60, and S and S(6) are converted with gfw 96. You used 12 and 32, which accounts for the difference in output. You can add a SOLUTION_MASTER_SPECIES block in your input file, or you can define the conversion factor to use with gfw or "as", for example:

SOLUTION_SPREAD
      C(4)           S(6)
mg/l as C    mg/l gfw 32
        12             32

Code: [Select]

C CO3-2 2.0 HCO3 12.0111
C(+4) CO3-2 2.0 HCO3
C(-4) CH4 0 CH4
Alkalinity CO3-2 1.0 Ca0.5(CO3)0.5 50.05
S SO4-2 0 SO4 32.064
S(6) SO4-2 0 SO4
S(-2) HS- 1.0 S


[HayatNottsUK]

Yes, based on your advice I did change the gfw and it is fine now, so thank you so much for your advice and for this helpful forum


Hayat