American and Japanese scientists say they have found elevated levels of radioactive cesium throughout a 150,000 square kilometer area of the Pacific Ocean off Japan.
Scientists say some radioactive cesium levels in seawater are higher farther away than adjacent to Japan's crippled Fukushima nuclear power plant.
Marine chemist Ken Buesseler, a senior scientist at the Woods Hole Oceanographic Institution in the U.S. state of Massachusetts, was among those carrying out the research in Pacific waters last June.
"Some of those concentrations were up to 100-1,000 times higher than what had existed off Japan before the accident," said Buesseler. "Those levels are still low relative to direct impacts on humans, in terms of exposure, and organisms living in them. And, even if you eat the seafood from these waters off shore, that would be the primary pathway by which you might be affected at this point in time."
The findings appear in this week's edition of the Proceedings of the U.S. National Academy of Sciences.
Elevated concentrations of Cesium-134 at levels up to 325 becquerels per square meter were found more than 600 kilometers from the Fukushima nuclear plant.
A circular pattern of ocean current, known as an eddy, is responsible for the highest ocean radioactive hot spot of 3,900 becquerels per square meter, located 130 kilometers off the Fukushima coast.
Buesseler says the marine radiation levels are comparable to those seen after past accidents, such as Chernobyl.
"People still eat the fish in the Baltic and the Black seas," said Buesseler. "So I am not too concerned about the offshore fisheries in Japan. I still have some concerns about the near shore fish or the bottom dwelling or shellfish that live on the bottom."
That is because of accumulation of radiation in sediment.
There is also concern among marine biologists about the continuing runoff of contaminated water from the Fukushima plant, as well as ongoing radiation emissions from its crippled reactors drifting into northern Pacific waters.
Three reactors at the Fukushima-1 nuclear power plant suffered core meltdowns last March after the area was hit by a magnitude 9.0 earthquake that triggered a massive tsunami.
Scientists from Woods Hole, as well as Stony Brook University in New York state and Tokyo University, measured a dozen different isotopes on and below the ocean surface, and collected plankton and fish for analysis.
Buesseler says one particular radionuclide of concern is strontium-90, which has a half-life of nearly 30 years.
"It accumulates more in bones," continued Buesseler. "It replaces calcium in your bones or in fish bone. And in fish, in particular, there is a concern of this accumulation. And if you were to eat small fish, as is done commonly more in Japan, then you would accumulate strontium-90 that would then affect your dose through the accumulation in your bone."
Research on the strontium-90 levels off the Japanese coast is due for publication in a few months.
Such reports are considered significant because there has been little information about the distribution and movement of radiation in the north Pacific and independent confirmation of whether the levels raise significant health concerns.
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