I can well believe it--I attended a minor scientific conference after Chernobyl (a few months after) and talked to one researcher who specialized in water plants. He used e-mail and phone to recruit colleagues around the world to survey the amount of radiation that showed up in one common --kelp I think; something with a very wide distribution. When he got of the phone he picked up his own kelp sample he'd gathered that morning and, just for grins ran a geiger counter over it. When he heard the hiss, he quit grinning.

The actual counting was done on carefully dried, ashed samples so that different water weights wouldn't throw things off--but he said he threw the rest of the kelp in the radioactive disposal.