On the other hand, and bear in mind I am not a medical professional:

These compounds are present in effluvia. The wastewater treatment system (I’m generalizing here) tries to follow the most basic downstream and upstream procedures but can still end up with significant amounts of stuff from effluent in its clean source. If we know that stuff that ought to go through the out pipe makes its way to the in pipe already, and we know of a specific chemical present in the out pipe, it seems reasonable to test for that chemical in the in pipe.

Alternately: I don’t think finally testing something people have been saying for twenty years is a bad precedent.