I recall assumptions made by pro-nuc apologists that it's a big ocean out there, and that once in ocean current, dilution and dispersion would take care of any high concentrations of reactor contaminants coming from Fukushima. Nevertheless, the ever vigilant (and pesky) Greenpeace continues to gather its own independent data, and recently discovered some pretty high levels of radioactivity in certain varieties of sea life further than 20km from the plant. AFP:
"Greenpeace said it detected radiation levels in seaweed 50 times higher than official limits, which it charged raised "serious concerns about continued long-term risks to people and the environment from contaminated seawater".
It also said that tests, which it said were independently verified by French and Belgian laboratories, showed above-legal levels of radioactive iodine-131 and caesium-137 in several species of fish and shellfish.
"Our data show that significant amounts of contamination continue to spread over great distances from the Fukushima nuclear plant," said Jan Vande Putte, a Greenpeace radiation expert, at a Tokyo news conference.
Japan's seafood safety limit for caesium-137 is 500 Becquerels per kilogram (227 per pound).
Greenpeace said it found levels of 740 Becquerels per kilogram in oysters, 857 in a fish species, 1,285 in sea cucumber and 1,640 in seaweed.
The maximum iodine-131 limit is 2,000 Becquerels per kilogram for seaweed, but Greenpeace said it found a level of 127,000 Becquerels per kilogram in the seaweed species Sargassum Horneri (please see below).
The group said that "eating one kilo of highly contaminated seaweed sampled by Greenpeace could increase the radiation dose by 2.8 millisievert -- almost three times the internationally recommended annual maximum".
The amount of radioactivity released into the ocean from Dai-ichi is not trivial. In early May alone, 250 tons containing about 20 terabecquerel leaked from a pit near reactor 3. From April 1-6, 500 tons of contaminated water containing 4,700 terabecquerel leaked from reactor 2.
I think it's foolish to assume that once the isotope-laden water mixes with ocean current, it gets evenly distributed in nice, conveniently negligible quantities. High concentrations of isotope are certain to drift around without appreciably dispersing, and will end up depositing in random areas. Sea life, possessing typical biology that depend on different elements for survival, take in cesium and iodine and continue to do so regularly. This problem will continue until Tepco figures out a way to decontaminate and re-circulate reactor water, and that's where Areva comes in.
Here's some comic relief - I had trouble finding out if the species of seaweed Sargassum Horneri was actually edible. Resorting to YouTube, i typed in "Sargassum Horneri Japan". The results of that query: