HCO3, CO3, and alkalinity inputs for simple Piper, Stiff, and Durov plots

[ahanson]

I am trying to plot simple Piper, Stiff, and Durov plots of groundwater samples collected over a field season. Laboratory results report HCO₃^- (mg/L), CO₃^2- (mg/L), and alkalinity as CaCO₃ (mg/L). For samples with pH > 8.1, CO₃^2- is non-zero, and I would like to represent it accurately in the HCO₃^- + CO₃^2- axes of the plots. However, on the geochemist spreadsheet, it doesn’t appear that HCO₃^- and CO₃^2- can be input separately as free variables.

What is the best way to input the necessary data for the dissolved inorganic carbon system in order for the plots to be most accurate? Is it best to put HCO₃^- as a free variable and include alkalinity as CaCO₃ as another analyte, and then GWB will calculate the HCO₃^- + CO₃^2- in the plot? Or should I convert both CO₃^2- (mg/L) and HCO₃^-(mg/L) to meq/L as HCO₃­^- and add them together, and use that as my HCO₃^- as a bulk variable?

Thanks!

[Jia Wang]

Hello,

The GSS spreadsheet allows user to add bulk component concentration as basis species. In this case, I think you are very close with the idea of adding the bulk amount of HCO3- and CO3-- as your carbon component represented by the HCO3- entry. If you want, you can select the unit for HCO3- as mg/l of carbon. That way, you can calculate the bulk elemental equivalents of carbon in your system (from HCO3- and CO3--) and enter that value directly into the spreadsheet. You can see an examples of concentrations entered as elemental equivalents in Example2.gss in the Scripts folder that comes with the GWB installation. 

Hope this helps,
Jia Wang
Aqueous Solutions LLC

[ahanson]

Great! Thank you. That helps!

[Jia Wang]

You're welcome. I am glad that was helpful.