[bpodgursky]

That's technically true I guess?

But that does not happen unless the municipal water source changes day-to-day, or is catastrophically badly run in a really inconsistent manner... which is almost never the case in the US. Even in Flint, it's not like the water changed day-to-day; it just became consistently acidic, which pulled lead from pipes.

Aquifer-pumped water is consistent, reservoir-fed water is consistent, groundwater is consistent.

[hanniabu]

Anecdotal but in my area (NJ) we're notified numerous times a year not to drink the water. The kicker is that many times we recieve the mail after the time where we supposedly shouldn't be drinking it. Also messed up that we're still charged for that water, but that's another topic.

[bpodgursky]

Yeah E. coli is the one thing that can change seasonally, that's fair. Still testable or filterable if you want to get a cheap purifier.

(also personally the contaminant I'm least fussed about)

[hanniabu]

Usually the notices are for lead. Other times they don't specify and just say don't use it so I can only assume it's something worse if they don't want to mention why.