Radiation from the Fukushima-Daiichi nuclear plant in Japan has reached the North American coast as scientists have found low levels of radioactive isotopes off the coast of Canada.

Scientists monitoring the waters off British Columbia found some traces of cesium-134 and cesium-137 isotopes in the waters off Vancouver Island near the town of Ucluelet, according to a statement by the Woods Hole Oceanographic Institution (WHOI). The organization, based in Massachusetts, has been monitoring the radiation residue coming from Japan since the disaster happened with the help of citizen scientists.

In 2011, following a massive earthquake and tsunami, the Fukushima-Daiichi nuclear power plant in northern Japan experienced a meltdown and for days radiated water emptied into the Pacific Ocean. Four years later the radiation has finally began reaching North American shores.

With the help of the Ucluelet Aquarium, scientists were able to test water off the Canadian coast in late February and found the isotopes. However, the amounts are not dangerous.

"Radioactivity can be dangerous, and we should be carefully monitoring the oceans after what is certainly the largest accidental release of radioactive contaminants to the oceans in history," said Ken Buesseler, a marine chemist at WHOI. "However, the levels we detected in Ucluelet are extremely low."

The Canadian sample only contained 1.4 Becquerels per cubic meter (Bq/m3) (the number of decay events per second per 260 gallons of water) of cesium-134 and 5.8 Bq/m3 of cesium-137. Neither of these quantities is considered harmful.

According to CBC, the Canadian Department of Fisheries and Oceans says "normal levels of cesium-137 in the Pacific Ocean are about one Becquerels per cubic meter." However, the quantity was expected to increase due to the disaster and will continue to increase until returning to normal levels by 2021.

"Today's report is not alarming at all. It's kind of to be expected," Buesseler told CBC. "We knew four years later it would be reaching our shoreline, and we had seen it offshore, and these numbers are quite small."

The WHOI statement explained that the organization is using the cesium-134 isotopes to test the fallout from the nuclear plant as this isotope has a half-life of two years, making it easier to measure. Cesium isotopes do not form naturally.

"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," said Buesseler, "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."

Buesseler is working through a crowd-funded effort but has gained help from the Scripps Institution of Oceanography in La Jolla, California, which will establish 10 more collection stations off California. Another initiative will gather 250 samples from Hawaii to Alaska.