Alkalinity input and then output interpretation

[Toph]

Hello,

I followed a few suggestions on a previous post here about converting alkalinity into meq/L but after conversion and looking over the output file, I am unable to connect it back to the original concentration in the initial input. The data I have provides concentrations of HCO3- ( and CO32- in mg/L and I used the following equation to put it into meq/L as CaCO3 and confirmed this by using the conversion shortcuts provided by USGS (https://pubs.usgs.gov/twri/twri9a6/twri9a66/twri9a_6.6.pdf page 43; multiply Alkalinity mg/L as CaCO3 by 0.01998 to get Alkalinity in meq/L).

I have an example:
    temp      5.1
    pH        9.6
    units     mol/L
    density   1
    Alkalinity 89.9 meq/L
    Ca        0.00027
    Cl        0.155
    F         0.00076
    K         0.010103
    Mg        0.00667
    Na        0.3341
    S(6)     0.0575
    Si         0.000113
END

And the output looks like this:
   Elements           Molality       Moles

   Alkalinity        1.832e-03   1.832e-03
   Ca                2.723e-04   2.723e-04
   Cl                1.585e-01   1.585e-01
   F                 7.730e-04   7.730e-04
   K                 1.030e-02   1.030e-02
   Mg                6.797e-03   6.797e-03
   Na                3.407e-01   3.407e-01
   S(6)              5.860e-02   5.860e-02
   Si                1.152e-04   1.152e-04

With this output, I convert the data back into mg/L to compare with field data. The other ions are converted back to mg/L but alkalinity becomes a bit more confusing as the conversion is not as simple as I had initially thought as compared with the other ions (e.g. for Si 1.152e-04*1000*28.085=3.2 mg/L - this matches up with field concentrations). What is the solution species for alkalinity that PhreeqC bases the final output on?

Thank you ahead of time for your assistance!
-Sandy

[dlparkhurst]

First, you can enter your data directly as mg/L as CaCO3. By default phreeqc.dat uses the equivalent weight of CaCO3 to convert mass to moles.

SOLUTION
Alkalinity 50 # mg/L as CaCO3

You will get the same result with phreeqc.dat if you remove the "#".

Second, your input and output do not match. I believe you entered 89.9 mg/L for alkalinity, which would correspond to ~1.8 meq/L alkalinity (89.9/50).

As for converting output back to input, you should take the value of alkalinity (1.832e-3) and multiply it times the equivalent weight of your desired unit--50 g/eq for CaCO3 or 61 g/eq for HCO3-, and then multiply by 1000 mg/g to obtain mg/L.

If you want to see the species that contribute to alkalinity, you can add

PRINT
-alkalinity

which will produce a table of aqueous species that contribute to the total alkalinity.