As far as drinkingwater plant treatment goes. They remove sediment and kill organisms. Usually with chlorine.
Chemicals, heavy metals, hormones -are mostly untouched.
I know in some provinces ( I think Alberta) municipal sewage waste has to be dumped into the river upstream from the drinking water intake. Makes you do a good job treating the waste.
Still, lots of things get thru.
Increased waterflow spring/fall dilutes it all more. But midsummer/winter.......