Claus Böning, Erik Behrens and colleagues from the Helmholtz Centre for Ocean Research in Kiel used global ocean circulation models to simulate the movement of a tracer which was continuously injected into Japanese coastal waters over several weeks. They then modelled its spreading and dilution in the Pacific Ocean for 10 years.

“We were of course not surprised that there is a mixing effect, but we were surprised at how quickly the tracer spread,” Claus Böning, co-author of the study, told environmentalresearchweb. “Within one year it will have spread over the entire western half of the North Pacific and in five years we predict it will reach the US West Coast.”

However, Böning points out that, due to considerable mixing and dilution, the level of radioactivity reaching the US will be much lower than that released by the Fukushima plant. “The levels of radiation that hit the US coast will be small relative to the levels released by Fukushima,” he said. “But we cannot estimate accurately what those levels will be because we do not know for certain what was released by Fukushima.”

Tentatively assuming a value of 10 petabecquerel (PBq) for the net 137Caesium (Cs) input during the first weeks after the Fukushima incident, the simulation suggests a rapid dilution of peak radioactivity values to about 10 Bq/m³ during the first 2 years, followed by a gradual decline to 1–2 Bq/m³ over the next 4–7 years. The total peak radioactivity levels would then be about twice the pre-Fukushima values. “While this may sound alarming, these levels are still lower than those permitted for drinking water,” said Böning.

The team found that using models with different resolutions to simulate eddy fields gave different results. “This shows that high-resolution models are needed,” said Böning. “We also did not model biological processes which might lead to accumulation of 137Cs in areas of high biological productivity, and potentially affect vertical tracer distributions due to adherence to sinking particles.”

The researchers published their work in Environmental Research Letters (ERL).