As radiation levels in the seas surrounding the Fukushima Daiichi power plant in Japan remain at toxic levels, scientists look to the sea floor for growing sources of radiation.

The data, which was presented at the Fukashima Ocean Impacts Symposium at the University of Tokyo (November 12-13), addresses the recent reports that—despite the natural dissolution of the ocean in the twenty months since the Fukushima nuclear disaster—levels of caesium-137 have remained at around 1,000 becquerels in the waters surrounding the plant. 

According to Nature Magazine, Tokyo University oceanographer Jota Kanda, suggested three potential sources:

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. But Kanda thinks that the third source, marine sediment, is the main cause of the contamination. Around 95 terabecquerels of radioactive caesium has found its way to the sandy ocean floor near the plant.

Though the source is unknown, Kanda speculates that the radioactive caesium was either absorbed by the sediment directly or that it was deposited via excretions of tiny marine organisms such as plankton. Regardless of how it got there, he adds, "there must be some loaded organic material somewhere in the sediment.”

Though the seas' radiation levels are not likely to pose a risk to human health, the contamination is finding its way into the food chain. An earlier analysis by Ken Buesseler, senior scientist at Woods Hole Oceanographic Institution of the United States, found that 40% of bottom feeding fish near the site still show elevated levels of radiation.

Nature Magazine writes, "the implications are serious for the fishing industry, which lost an estimated ¥100 billion to ¥200 billion (US$1.3 billion to $2.6 billion) in 2011 as a result of the accident." They add:

Many fisheries remain closed, and because of the persistent contamination "we can't answer the basic question of when these fisheries will be able to open", says Woods Hole oceanographer Ken Buesseler. "Much more must be done to understand the accident."

Scientists estimate that the March 2011 disaster caused record level discharge of radioactivity into the ocean; the Woods Hole Oceanographic Institution in Massachusetts now estimates that 16.2 petabecquerels (1015 becquerels) of radioactive caesium leaked from the plant, roughly the same amount that went into the atmosphere.