With recent reports that there have been further radioactive leaks from the Fukushima nuclear power plants, a new study has assessed the level of radioactivity in the ocean in the first months after the disaster.
Woods Hole Oceanographic Institution chemist Ken Buesseler and two Japanese colleagues report that discharges from the power plants peaked a month after the March 11 earthquake and tsunami, and continued through at least July.
The disaster was the largest accidental release of radiation to the ocean in history. Concentrations of cesium-137 at the plants' discharge points to the ocean peaked at more than 50 million times normal levels.
The team found that releases to the ocean peaked in April, decreasing in May by a factor of 1,000 thanks to ocean mixing and a dramatic fall in emissions.
But while concentrations of some radionuclides continued to decrease, by July they were still 10,000 times higher than levels measured in 2010 off the coast of Japan.
This indicates that the plants remain a significant source of contamination to the coastal waters off Japan, the researchers say.
"There is currently no data that allow us to distinguish between several possible sources of continued releases," says Buesseler.
"These most likely include some combination of direct releases from the reactors, or storage tanks or indirect releases from groundwater beneath the reactors or coastal sediments, both of which are likely contaminated from the period of maximum releases."
Buesseler says that at levels indicated by these data, the releases are not likely to be a direct threat to humans or marine life in the surrounding ocean waters.
There could be an issue, however, if the source remains high and radiation accumulates in marine sediments.
"We don't know how this might affect benthic marine life, and with a half-life of 30 years, any cesium-137 accumulating in sediments or groundwater could be a concern for decades to come," he says.