Wednesday, January 7, 2015, 8:09 PM – Radioactive isotopes originating from the Fukushima nuclear power plant in Japan have been slowly drifting across the Pacific Ocean since March 2011, and Canadian scientists have been using this to test some of their most basic ideas of how ocean currents work.
In March 2011, when the Fukushima Daiichi nuclear power plant was damaged by the magnitude 9.0 earthquake that struck off the east coast of Japan, it resulted in a ‘plume’ of radioactive cesium isotopes being released into the Pacific Ocean.
While regions closest to the plant came under intense scrutiny, due to the health risk posed by these isotopes – cesium-134 (134Cs) and cesium-137 (137Cs) to be exact – scientists from the Bedford Institute of Oceanography in Dartmouth, Nova Scotia began to closely monitor the plume as it was carried away by Pacific Ocean currents.
This wasn’t due to any risk to the ocean environment beyond Fukushima, though, or any radiation threat to the shorelines of North America (or any other part of the world). Even just months after the accident, ocean currents had so diffused the plume that radiation exposure levels had dropped to well below safety standards for drinking water, and by the time the plume had reached halfway across the ocean, the levels of radiation from the isotopes had dropped to such low levels that they would have been overwhelmed by the natural radioactivity of the ocean water itself.
DID YOU KNOW? Radioactive elements, including uranium-238, tritium, potassium-40, carbon-14 and rubidium-87, which are naturally found mixed into seawater, give the ocean an average natural radioactivity of 13,000 Becquerel (decays per second) per cubic metre. The safe radioactivity level for drinking water, in Canada, is 10,000 Bq per cubic metre. Even the human body gives off roughly 100 Bq/kilogram.
The reason these scientists, led by Bedford research scientist John Smith, tracked these cesium isotopes was that they represented a unique opportunity to see Pacific Ocean currents at work, and verify the computer models they are currently using.
“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,” Smith told Phys.org. “It was kind of like a dye experiment, 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.”
One of the models being tested by these findings is shown below, from the Woods Hole Oceanographic Institution, in Woods Hole, Massachusetts.
Note that the scale on this model, in Bq per cubic metre, only goes up to 10,000 (the safe level in Canadian drinking water), and levels by September 2011 – even closest to the power plant – are well below that, at around 1,000 Bq per cubic metre. As the plume spreads out, levels drop to below 100 Bq/m3, and the most concentrated areas represent only around 20-30 Bq/m3 (the scale is logarithmic, so the greens halfway between 10 and 100 are in the low 20s). This is well below any danger level to humans or wildlife.
Ken Buesseler, a marine chemist at Woods Hole who is currently working with Smith, as well as University of Victoria researcher Jay Cullen, on a monitoring program called InFORM, has been watching the waters along the northern coast of California for this same signal from Fukushima.
“We detected cesium-134, a contaminant from Fukushima, off the northern California coast. The levels are only detectable by sophisticated equipment able to discern minute quantities of radioactivity,” he said in a Woods Hole press release back in November. “Most people don’t realize that there was already cesium in Pacific waters prior to Fukushima, but only the cesium-137 isotope. Cesium-137 undergoes radioactive decay with a 30-year half-life and was introduced to the environment during atmospheric weapons testing in the 1950s and ’60s. Along with cesium-137, we detected cesium-134 – which also does not occur naturally in the environment and has a half-life of just two years. Therefore the only source of this cesium-134 in the Pacific today is from Fukushima.”
Smith and his team tested sites stretching out into the ocean up to 1,500 kilometres from the British Columbia coastline, gathering water samples and specifically looking for both cesium-134 and cesium-137.
According to the study, the first indications of the plume showed up in June 2012, at the stations furthest west of the coast. By the next year, in June 2013, it had spread further onto the Canadian continental shelf, and by February 2014, the radioactivity from the cesium isotopes had increased to around 2 Bq/m3 – extremely low, but still detectable, levels. These results showed that the radiation plume was arriving on the North American coastline around two years ahead of one computer model, which predicted the first signal to show up in 2015. However, the model presented in the video above, was much more accurate, giving ocean researchers a much better idea of how ocean currents behave in the Pacific.
As for what future levels will be like, the authors state the following in the study: “Ocean circulation model estimates that are in reasonable agreement with our measured values indicate that future total levels of 137Cs (Fukushima-derived plus fallout 137Cs) off the North American coast will likely attain maximum values in the 3–5 Bq/m3 range by 2015–2016 before declining to levels closer to the fallout background of about 1 Bq/m3 by 2021. The increase in 137Cs 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.”