October 25, 2012
http://www.scientificamerican.com/article.cfm?id=fukushima-fish-remain-radioactive-suggesting-ongoing-radionuclide-release
Bottom-dwelling fish continue to be found with high levels of radioactive elements, potentially coming from leaking radioactive water or contaminated sediments
ByDavid Biello
The fish off Fukushima remain radioactive more than a year after the earthquake and subsequent tsunami triggered three meltdowns at the Daiichi nuclear power plant. In fact, bottom-dwelling greenling fish caught in August 2012 bore the highest levels of radioactive particles seen to date—25,000 Becquerels per kilogram. (A becquerel is a unit of the rate of radioactive decay—or radiation emitted by a substance.) That is 250 times higher than current Japanese safety standards, a key reason fishing off Fukushima remains prohibited.
RADIOACTIVE FISH: Some source of radioactive contamination is causing bottom-dwelling fish, like the greenling pictured here, to absorb high levels of radionuclides.Image: Flickr.com / Brian Gratwicke
The findings suggest that contaminated water is still leaking from the stricken power plant, the sea bottom itself is now laced with radionuclides, or both. Concentrations in the ocean water itself remain below any human health concern but they do pass into fish that swim through those waters.
"When fish 'drink' they take [cesium] and other salts up from the water they are swimming in, that accumulates in the muscle tissue," explains marine chemist Ken Buessler of the Woods Hole Oceanographic Institution, who compiled the analysis of publicly released Japanese fisheries data and published it in Science on October 26. But the fish also shed that cesium if they swim in uncontaminated waters, as has been seen in tuna that migrated from near Japan to near San Diego, suggesting that levels in fish should decrease over time. For this reason, most of the fish caught off Japan's northeastern coast are not radioactive. But roughly 40 percent of bottom-dwelling fish, such as flatfish or halibut, caught off the coast adjacent to Fukushima bear radionuclides above the Japanese food safety standard of 100 becquerels per kilogram.*
According to a response to questions from Scientific American that was prepared by staff at the U.S. Nuclear Regulatory Commission, ingesting fish at that level "would only produce a dose that is a small fraction of the dose that people receive from natural levels." For example, as Buessler notes, fish caught off Japan in June 2011 boasted levels of potassium-40—a naturally occurring radionuclide—10 times higher than those of radioactive cesium from Fukushima.
Radioactive cesium decays by emitting what's known as a beta radiation, a negatively charged particle that is easily blocked by metal, plastic or wood—but not skin. In particular, ingesting beta-emitting radioactive elements is "a concern," according to the NRC. "Beta particles released directly to living tissue can cause damage at the molecular level, which can disrupt cell function." Plus, beta particles are small enough to travel far in the body, causing damage far and wide.
U.S. safety guidelines from the Food and Drug Administration permit foods to bear 1,200 becquerels per kilogram of radioactive cesium, but the FDA declined to comment for this article. "The more restrictive action taken by the Japanese seems reasonable for the population living close to Fukushima because they receive radiation doses from other sources, including non-fish food, drinking water and land surface contamination," the NRC staff writes. "Based on the FDA and [World Health Organization] recommendations, eating fish contaminated at 100 Bq/kg would result in a small and acceptable exposure to radioactive cesium."
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http://www.scientificamerican.com/article.cfm?id=fukushima-fish-remain-radioactive-suggesting-ongoing-radionuclide-release&page=2
Because it takes decades for radionuclides to decay, fishing off Fukushima is likely to remain prohibited for many years. Exactly how long is uncertain, especially as the Tokyo Electric Power Company continues to struggle to contain and clean up millions of liters of contaminated waters at the Fukushima Daiichi nuclear power plant. And local waterways continue to wash radionuclides out to sea. "For the record, I was in Japan this past July and ate all types of seafood," says Buessler, who will hold a public colloquium on the findings November 14 at the University of Tokyo with colleagues. "The fisheries data like [those] shown here are used to keep certain areas and types of fish out of the markets. The question we can't answer is when will this no longer be of concern."
*Correction (10/26/12): It is 40 percent of bottom-dwelling fish, not all fish, caught near Fukushima that bear concentrations above Japanese safety standards.
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http://www.bbc.co.uk/news/science-environment-19980614
25 October 2012 Last updated at 18:36 GMT
Fukushima fish still contaminated from nuclear accident
Levels of radioactive contamination
in fish caught off the east coast of Japan remain raised, official data
shows.
"It all points to this issue being long-term and one
that will need monitoring for decades into the future."
Prof Buesseler is affiliated to the US Woods Hole Oceanographic Institution (WHOI).
His evaluation covers a year's worth of data gathered by the Japanese Ministry of Agriculture, Forestry and Fisheries (MAFF).
Its monthly records detail the levels of radioactive caesium found in fish and other seafood products from shortly after the March 2011 Tohoku earthquake and tsunami - the double disaster that triggered the Fukushima crisis.
The caesium-134 and 137 isotopes can be traced directly to releases from the crippled power station.
MAFF uses the information to decide whether certain fisheries along five east-coast prefectures, including Fukushima, should be opened or closed (it is not a measure of contamination in actual market fish).
The caesium does not normally stay in the tissues of saltwater fish for very long; a few percent per day on average should flow back into the ocean water. So, the fact that these animals continue to display elevated contamination strongly suggests the pollution source has not yet been completely shut off.
He notes that although caesium levels in any fish type and on any day can be highly variable, it is the bottom-dwelling species off Fukushima that consistently show the highest caesium counts.
For the WHOI researcher, this points to the seafloor being a major reservoir for the caesium pollution.
"It looks to me like the bottom fish, the fish that are eating, you know, crabs and shellfish, the kinds of things that are particle feeders - they seem to be increasing their accumulation of the caesium isotopes because of their habitat on the seafloor," he explained.
Prof Buesseler stresses however that the vast majority of fish caught off the northeast coast of Japan are fit for human consumption.
And while the 40% figure for unsafe catch in the Fukushima prefecture may sound alarming, the bald number is slightly misleading.
Last April, the Japanese authorities tried to instil greater market confidence by lowering the maximum permitted concentration of radioactivity in fish and fish products from 500 becquerels per kilogram of wet weight to 100 Bq/kg wet.
This tightening of the threshold immediately re-classified fish previously deemed fit as unfit, even though their actual contamination count had not changed.
It is also worth comparing the Japanese limit with international standards. In the US, for example, the threshold is set at 1,200 Bq/kg wet - significantly more lenient than even the pre-April Japanese requirement.
And Prof Buesseler makes the point that some naturally occurring radionuclides, such as potassium-40, appear in fish at similar or even higher levels than the radioactive caesium.
Nonetheless, the contamination question is a pertinent one in the Asian nation simply because its people consume far more fish per head than in most other countries.
"At one level, there shouldn't be any surprises here but on another, people need to come to grips with the fact that for some species and for some areas this is going to be a long-term issue; and with these results it's hard to predict for how long some fisheries might have to be closed," said the WHOI scientist.
Prof Buesseler, with Japanese colleagues, is organising a scientific symposium in Tokyo on 12/13 November to present the latest thinking on Fukushima and its impacts on the ocean. The information will then be shared with the public in a free colloquium on 14 November.
Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos
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