Bill Heller's book A Good Day Has No Rain points to a cloud of radioactive fallout raining onto the Capital District region of New York.

http://www.amazon.com/Good-Day-Has-No-Rain/dp/0878755462/ref...

Wood ash from burning firewood can concentrate radioaction levels found in some parts of the Eastern US. Enough above background levels to trip various radiation sensors. Here's what US EPA has to say about this:

http://www.epa.gov/rpdweb00/marssim/docs/marsame/appendixB.p...