75% of American drinking water needs treatment to reduce particulate and parasites, and the treatment additive used to render the water safe is produced at a single chemical plant located in an area of severe flood risk -- which means that a flood could take it offline for a day or two, or damage it for weeks.
(Efforts to build a second site recently fell through due to ever-changing regulations. Of course they're stockpiling it in some mountain bunker, I'm sure)
The next Katrina could give us a brain-worms infestation via tap-water.