Can I Plot Analytes Other than TDS with the "TDS Circles"?

Yes you can. This feature on Piper diagrams was originally designed around the convention of plotting TDS circles in the upper prismatic area. This is a standard convention, but there is no reason that you can't plot the scaled circles based on the value of other constituents.

Open the Tasks Pane on the right-hand side of the EnviroInsite interface, select Create Window and then select Piper Diagram. Piper diagrams can also be created by selecting Window> New> Piper Diagram from a plan view map. A new view window is generated and the Piper Diagram dialog box opens. On the TDS Circles tab, select the constituent that you want to plot from the dropdown box. You can also set the minimum and maximum value of the scaled values. In this case, the fraction and media values are ignored. Therefore, be careful if you have both dissolved and total values of some analyte.