My plan is to let it rain for at least 15 min (hard rain,,longer in a light rain), and then start collecting the runoff. I'd still run all of it through my charcoal filter and then boil it before using.

Drinking the stuff (sump water), even with chemical treatment, seems to me a "last-ditch desperate measure."

But it occurs to me that a raw water source may have other uses in a difficult situation.

If people are confined to a shelter, for example, this water could (if not seriously contaminated by sewage bacteria or chemicals) be used to wash out socks and shirts and hair, soap out the latrine bucket, swab the shelter floor, provide evaporative cooling (wet towel over the neck), and generally make people's lives a little less miserable.

As long as they don't ingest it, and it doesn't raise a rash on contact ...