For the first time, scientists have detected small amounts of radioactivity in seawater along the shores of British Columbia that can be traced back to the 2011 Fukushima Nuclear Power Plant disaster. But the levels are so low they are likely of little concern.

Scientists with the Woods Hole Oceanographic Institution (WHOI) in Massachusetts reported Monday that a water sample collected in mid-February from a dock in Ucluelet, on British Columbia's Vancouver Island, contained trace amounts of cesium-134 and cesium-137 – isotopes that only come from human sources.

However, the levels detected in the sample were so low that even if someone were to swim for six hours a day, every day of the year in water containing twice as much cesium, the radiation received would still be 1,000 times less than what they would receive from a single dental X-ray.

Still, researchers say this is the first detectable of radioactivity from Fukushima found in a water sample taken from the U.S. and Canadian West Coast.

They say they are certain the radioactivity came from Fukushima because cesium-134 has half-life of only two years, meaning the cesium decays relatively rapidly, and the only recent source of cesium-134 has been Fukushima.

The discovery comes a full four years after the nuclear disaster, when three nuclear power reactors damaged by a tsunami began to melt down, spilling contaminated water into the sea.

WHOI scientists have been collecting water sample from more than 60 sites along the U.S. and Canadian West Coast and Hawaii over the past 15 months, with the help of volunteer citizens, looking for traces of the radiation on this side of the ocean.

Last November, the team found Fukushima radioactivity in a sample taken about 150 kilometres offshore of Northern California. However, no radiation had been found, until now, in any samples taken near Canadian or American shorelines.

Ken Buesseler, a marine chemist at WHOI who has been measuring radioactivity in Pacific seawater since 2011, says it's been important to carefully monitor the oceans, given that the Fukushima disaster saw the largest accidental release of radioactive contaminants to the oceans in history.

“We expect more of the sites will show detectable levels of cesium-134 in coming months, but ocean currents and exchange between offshore and coastal waters is quite complex,” he said in a statement.

“Predicting the spread of radiation becomes more complex the closer it gets to the coast and we need the public’s help to continue this sampling network."

More information on the water sampling work is available at