Okay, any number of natural (or man-made) disasters could stop the water from flowing safely from the taps and contaminate the local groundwater and streams.
But what about the rainwater?
It seems to me that many (but definitely not all) of these scenarios would leave the rainwater safer than anything already on the ground. I'm considering harvesting my roof's rainwater using a collection barrel. In case of emergency, it should require less filtering than the local creek.
In non-emergency times, I could use the barrel to water the garden, defraying the cost of the city water.

Living partway between Portland (OR) and Mt. St. Helens, I have plenty of rainwater, but if we have a repeat of the last Mt.St.Helens eruption, the ash plume would cover the roof and render the rainwater an ashy mess.

Any thoughts?