Radioactivity in the Pacific Ocean from the nuclear accident at Fukushima-Daiichi will dilute rapidly from peak values to about 10 becquerel per cubic metre (Bq/m3) during the first two years, followed by a gradual decline to 1 to 2 Bq/m3 over the next four to seven years, a numerical model study has shown.
The study, by the German Geomar Helmholtz Centre for Ocean Research, concludes that because the Paciﬁc Ocean is so vast, and is stirred by “energetic, ﬂuctuating currents”, this results in “an effective dilution of the contaminated body of sea water arising from the short-term discharge from the stricken Fukushima-Daiichi reactors”.
The goal of the study was to provide “a clearer perspective” of the evolution of that dilution process over 10 years.
The study says the total peak radioactivity levels after four to seven years would still be about twice the pre-Fukushima values. It says this is “below the level of concern”.
For the study, a sequence of global ocean circulation models was used to estimate the long-term dispersion by ocean currents of a slowly decaying tracer with a half-life of 30 years, comparable to that of caesium-137, from local waters off the Fukushima-Daiichi nuclear plant.
The tracer was continuously injected into the coastal waters over some weeks and its subsequent spreading and dilution in the Pacific Ocean was simulated for 10 years.
An abstract of the study says after two to three years the tracer cloud has penetrated to depths of more than 400 metres, spanning the western and central North Pacific, leading to “a rapid dilution of concentrations”.
The rate of dilution declines in the following years, while the main tracer patch propagates eastward across the Pacific Ocean, reaching the coastal waters of North America after about five to six years.
The study says: “Tentatively assuming a value of 10 petabecquerel (PBq) for the net caesium-137 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 two years, followed by a gradual decline to 1 to 2 Bq/m3 over the next four to seven years.”
According to agencies including the US Environmental Protection Agency, contamination of drinking water with caesium radioisotopes would still be considered safe below 100 Bq/m3.
Radioisotopes discharged from the plant included both short-lived isotopes like iodine-131 (with a half-life of eight days) and long-lived isotopes such as caesium-137 (with a half-life of 30.2 years).
Citing earlier research, the study says some fraction of these long-lived isotopes is associated with particles and accumulates in the food chain, with as yet unknown consequences for marine organisms in the area.
A substantial part of the caesium-137 input is expected to be carried away by the swift near-coastal currents and dispersed over broader regions.
A study last year by the Woods Hole Oceanographic Institution in Massachusetts showed that radioactivity levels in marine life sampled around the Fukushima area were well below levels of concern for humans and marine organisms, although the question of whether radioactive materials are accumulating on the seafloor sediments was still open.
The study, published in the scientific journal ‘Environmental Research Letters’ is online: