This US aircraft should be permanently located near Japan because what the Japanese are regularly flushing into the Pacific (and that finally reaches the US West Coast) is million times higher that those nano levels detected in Europe.
In Europe it could come from lots of sources: even a very shallow search shows that a similar situation was in autumn 2011 when there was a small leak of some stuff in Hungary at the plant where medical sensors were manufactured using radioactive isotopes. This could happen everywhere. Oh, sorry, of course the Russians are to be blamed: they developed Katrina and sent it to New Orleans and also they are boiling El Nino from time to time :-)