Scientists have released the results of a study on just how long it took the radioactive elements released from Japan's Fukushima nuclear plant into the ocean by the 2011 earthquake and tsunami to reach the west coast of North America.
Cesium 137 and cesium 134 that spilled into the waters of the Pacific Ocean from the damaged Fukushima Daiichi plant took about 2.1 years to show up in measurable amounts on the shores of North American, Canadian researchers report in the Proceeding of the National Academy of Sciences.
Computer models of ocean currents could have been used for an approximate prediction that, but the disaster provided a way to perfectly track those currents, the researchers say.
"We had a situation where the radioactive tracer was deposited at a very specific location off the coast of Japan at a very specific time," says John Smith if the Bedford Institute of Oceanography in Nova Scotia, and the lead author of the paper.
"It was kind of like a dye experiment," he added. "And it is unambiguous -- you either see the signal or you don't, and when you see it you know exactly what you are measuring."
Smith and fellow researchers started gathering samples of ocean water from as far as 930 miles off the coastline of British Columbia in June 2011, just 3 months after the disaster, then collected samples from the same sites every June through 2013.
The 2011 sampling found no signature of the Fukushima disaster, but by 2012 small amounts of radiation were detected in samples from the westernmost sampling site.
By June 2013 the radiation had reached Canada's continental shelf, the researchers say.
However, the amount of radiation detected was small, they say -- below 1 Becquerels per cubic meter. (A Becquerel is the number of radioactive decades event per second for each 260 gallons of water.)
That level is at least 1,000 lower than what the U.S. Environmental Protection Agency considers acceptable in drinking water.
It is also lower than the levels present in the Pacific Ocean in the 1980s due to fallout from testing of nuclear weapons, the researchers point out.
"[Cesium] levels in the eastern North Pacific from Fukushima inputs will probably return eastern North Pacific concentrations to the fallout levels that prevailed during the 1980s but does not represent a threat to human health or the environment," they wrote in their published paper.
Computer models and the measurements gathered so far suggest the levels of radiation off British Columbia will peak between this year and 2016, but will remain below 5 Becquerels per cubic meter.
"Those levels of cesium 137 are still well below natural levels of radioactivity in the ocean," Smith says.