Even though you may have already computed the union of your analytes, you cannot use this to compute an overburden. Overburden computations are based upon the concentrations of an individual analyte. When you create the union of multiple analytes, your are creating a volumetric region (subset), and you retain the data of each of your analytes, but it does NOT create any special data that is related to the union.

In order to perform overburden on the union of all analytes, you need to compute the overburden of each individual analyte and then do the union of each of the individual overburdens using their respective thresholds. 

The union of the overburden data components will be the overburden of the union of multiple analytes.