By Geoff Brumfiel | Nature

Radioactivity is persisting in the ocean waters close to Japan’s ruined nuclear power plant at Fukushima Daiichi.

New data presented at a conference held on 12–13 November at the University of Tokyo show that levels of radioactivity in the sea around the plant remain stable, rather than falling as expected.

Researchers believe that run-off from rivers, as well as continued leaks from the plant, may be partially to blame. But contaminated sediment and marine organisms also seem to be involved.

The level of contamination is not likely to pose a significant health risk to humans. But it could have long-term economic consequences for fishermen along Japan’s east coast.

On 11 March 2011, a magnitude-9 earthquake struck off the coast of Japan. The quake sparked a massive tsunami that hit the Fukushima Daiichi nuclear power plant. Three of the plant’s six units suffered meltdown, releasing large quantities of radioactivity into the atmosphere. In the days after the accident, emergency cooling water leaked into the sea, adding to ocean contamination.

The Fukushima disaster caused by far the largest discharge of radioactivity into the ocean ever seen. A new model presented by scientists from Woods Hole Oceanographic Institution in Massachusetts estimates that 16.2 petabecquerels (10¹⁵becquerels) of radioactive caesium leaked from the plant — roughly the same amount that went into the atmosphere.

Most of that radioactivity dispersed across the Pacific Ocean, where it became diluted to extremely low levels. But in the region of the ocean near the plant, levels of caesium-137 have remained fixed at around 1,000 becquerels, a relatively high level compared to the natural background. Similarly, levels of radioactive caesium in bottom-dwelling fish remain pretty much unchanged more than 18 months after the accident.

Triple blow

Researchers at the conference are convinced that something is preventing the radiation levels from dropping. “There must be a source,” says Scott Fowler, an oceanographer at Stony Brook University in New York.

In fact, a fresh analysis by oceanographer Jota Kanda at the Tokyo University of Marine Science and Technology suggests that not one source, but three, are responsible. First, radioactivity from the land is being washed by rainfall into rivers, which carry it to the sea. Second, the plant itself is leaking around 0.3 terabecquerels (10¹² becquerels) per month, he estimates.

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