Alkalinity Confusion

[swhitman]

I'm confused with how GWB is handling alkalinity, both in GSS and in calculations with SPECE8.

The data from my lab in mg/L CaCO3 eq. They report separately Carbonate, Bicarbonate, Hydroxide, and Total alkalinity. (results are <2, 100, <2, 100 respectively).

In their reporting system, it looks like Carbonate alkalinity refers only to alkalinity from CO32--, whereas it seems as though in GWB carbonate alkalinty refers to alkalinity from HCO3- AND CO3--.  Is that correct.

Next, I am unsure if when I input data into GSS, for Bicarbonate Alkalinity, Carbonate Alkalinity, or Hydroxide Alkalinity if the implied units are as mg/L CaCO3.

Finally, when I use the GSS data to run a SPECE8 analysis, the basis field for HCO3- is already filled with a value not equal (<) to bicarbonate alkalinity. As a test, I set all alkalinity values equal to zero in the GSS file, and then re-ran SPECE8. The prepopulated HCO3- value was virtually unchaged!

Can you please explain 1) if GSS expects values as CaCO3 for alkalinity, 2) If carbonate alkalinity  = bicarbonate alkalinity + carbonate alkalinity (as reported by lab), 3) what calculations SPECE8 is doing to arrive at a alkalinity numbers different from input values. ?

 

Thank You,

ColumnSamples_thermotdat.gss

[Brian Farrell]

Yes, the GWB’s “Carbonate alkalinity” refers to the alkalinity from all carbonates, so it includes what your lab reports separately as “CO3-- alkalinity” and “HCO3- alkalinity”.

Looking at your spreadsheet, keep in mind that the user analytes and user equations are just that – analytes for user reference that are disconnected from the program’s speciation calculations. Also, instead of the “concentration” dimension, you might consider using the “alkalinity” dimension, which can be found under the “Chemical parameters” variable type. You can find mg/l_as_CaCO3 among the options. A good use of the feature would be to store your various reported alkalinities as user analytes, then create a user equation to combine your HCO3- and CO3-- alkalinities, then copy the resulting values into GSS’s predefined “Carbonate alkalinity” variable (click +analyte > Chemical parameters > Carbonate alkalinity) that gets used for speciation calculations. 

As for constraining a speciation calculation in GSS, you can enter the total concentration of the HCO3- component (i.e. the sum of concentrations of CO2, HCO3-, CO3--, and complexes), the free concentration of the HCO3- species (just the concentration of the HCO3- species; do so by choosing the “free” option), or GSS’s Carbonate alkalinity, described above. If both are present, GSS will use whichever of the HCO3- or Carbonate alkalinity is higher up in the order of the spreadsheet. In SpecE8, by contrast, you don’t choose from separate HCO3- or Carbonate alkalinity variables. You only add HCO3- to the basis, then choose appropriate units. The concentration units (e.g. mmol/kg, mg/l, eq/kg, and so on) imply a concentration constraint, whereas the alkalinity units (mmol/kg_as_CaCO3, mg/l_as_CaCO3, eq_acid/kg) imply an alkalinity constraint. For more information, please see the <unit> and alkalinity commands in the SpecE8 chapter of the GWB Command Reference.

Hope this helps,

Brian Farrell
Aqueous Solutions
 

[swhitman]

Hi Brian,

Thank you, this was very helpful!

A couple notes/questions:

Is there any way to change the visualization so that it is apparent which parameters are user defined in GSS? This would probably help me from making that mistake again if so.

When I set up the user defined eq. for carbonate alkalinity, it seems that it is adding the values to the total, even if they are preceded by a "<" character. Is there any way to change this behavior?

 

Thanks Again,

[Brian Farrell]

Glad to hear this helped. Currently there is no way to change the appearance of analytes. You can right-click on an analyte and see different options, though (e.g. the user analytes and equations have an “edit” option). You might try naming the user analytes and equations to make their type more apparent.  

As for ignoring “less than” values in user equations, unfortunately GSS is not currently set up to do this.

Regards,
Brian