True, but you seem to be overlooking the fact that the energy from the blast itself, especially in a groundburst, converts a *huge* amount of surrounding material (dirt, stone, steel, etc.) into radiactive isotopes. The great majority of fallout comes not from the original core of the bomb itself, but from the secondary conversions. This is why groundbursts are much "dirtier", fallout-wise, than airbursts. They also stir up more of the irradiated material and send it up into the atmosphere, to fall... somewhere.
Don't forget your decay ratios (seven-ten rule). After about two weeks, most of the "hot" stuff would be at an acceptable range.