Fukushima: Sea mud prolongs contamination

Radionuclides in the sediment are released decades later

Researchers recover one of the sediment traps they had exposed 115 miles off the coast of Japan. © Makio Honda / Japan Agency for Marine-Earth Science and Technology
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Contaminated Mud: Even more than a hundred kilometers off the coast of Fukushima, the ocean floor is contaminated with radioactive cesium. It did not get there by air or water directly after the nuclear disaster, but comes from the still heavily contaminated sediment near the coast. If this is stirred up by storms, it releases part of the radionuclides buried there - and this can continue for decades to come.

The Fukushima disaster has already been over for more than four years, but the consequences are far from over - even if the first evacuees are soon to return home. At the Fukushima Daiichi nuclear power plant, workers and engineers are still struggling to control the reactors and contaminated cooling water. Also the bird world around the nuclear power plant shows consequences.

Radioactive cesium also reached the sea via the contaminated cooling water and the air. By early 2014, these radionuclides had spread so far with the currents that they were detectable off the North American coast. The only good thing about it: Due to the distribution in the Pacific, the burden is thinned, so to speak.

Reservoir in the sediment

But as it turns out, there is a previously unrecognized reservoir that releases radioactive cesium again and again off the coast of Japan. "Almost 99 percent of cesium was carried out with the water into the open sea, " explains Ken Buesseler of the Woods Hole Oceanographic Institution (WHOI). "But a small part - about one percent - ended up as sediment on the seabed."

Marine chemist Ken Buesseler in front of the power plant vn Fukushima Ken Buesseler

In order to find out whether the cosium remains buried in the sediment or can be stirred up again, Buesseler and his colleagues took samples of the seabed at various points off the eastern coast of Japan. In addition, they set up sediment traps that trapped sand and mud over several months. Two of these traps were located fifteen miles southeast of Fukushima Daiichi - and thus off the shelf of Japan - at 500 and 1, 000 meters. display

Contamination even 100 kilometers before the coast

The evaluation of the samples revealed something unusual: Even at these locations, more than one hundred kilometers from the power plant, radioactive C sium was found in the sediment. "That was a small surprise, because normally sediment in the ocean is formed by what trickles down from above, " says Buesseler. And the open sea is no longer so heavily contaminated that C sium sinks from there to a greater extent.

But where did the cosium come from? The researchers went in search of clues wurden and came to the conclusion: "The only place where the material in our sediment traps can come from is the continental shelf and the sea exits in the coastal area", reports Buesseler. Obviously, there is still enough radioactive C sium in the coastal seabed to contaminate the sediment even years after the atomic leak.

Upset and distributed by storms

As the researchers report, the contaminated sediment on the coast is whirled up by storms and then carried southwards through the south. Above all, typhoons, which are not particularly rare in Japan, contribute to the contamination of these tombs.

According to the researchers, the sediment reservoir and this periodic transport could explain why marine animals are still measurably contaminated: "This contributes to the increased C sium values ​​of fish at especially at the bottom of the sea living fish off Japan, "said Buesseler. This can remain true for decades - because most of the C sium remains buried in the sediment and again and again only a small part of this radioactive reservoir is whirled up. (Environmental Science and Technology, 2015; doi: 10.1021 / acs.est.5b02635)

(Woods Hole Oceanographic Institution, 19.08.2015 - NPO)