How could it be the culprit? If nuclear material entered the groundwater supply it would be immediately noticeable and a major environmental catastrophe. It wouldn't just affect bees.
I understand, what I'm saying is that if this did happen it would have been obvious to everyone that it did. The fact that nobody's drinking water is irradiated right now proves that it couldn't have happened...
(Sidenote: is it really possible that after all these years they've just left a smashed nuclear bomb in a marsh? I would imagine they've gone back with better technology and recovered the pieces, at least to avoid the material falling into the hands of terrorists?)
If the US military couldn't recover it with the legal right to be there and no concern about having to be clandestine, I doubt a terrorist group will successfully recover it.
Probably the Air Force decided it would be cheaper to keep the site under constant surveillance for a few centuries than to go through the nightmare of trying to excavate a swamp well enough to extract the thermonuclear stage.
