Highly Diluted Fukushima Radioactivity Becomes an Ocean Tracer, Providing New Insights into Ocean Circulation
There are a variety of ways to study ocean circulation—most are planned though some are occasionally unplanned, as in the case of a tracer that accidentally ended up in the Pacific Ocean on March 11, 2011. That was the day a severe, undersea earthquake off the northeast coast of Japan triggered a tsunami, causing extensive damage to a nuclear power plant in Fukushima, Japan. The natural disaster resulted in the release of radioactive cesium-137 (137Cs) and cesium-134 (134Cs)—common products of nuclear fission—directly into the western North Pacific Ocean.
The plume of radioactive cesium eventually flowed northeast, swept along in the offshore Kuroshio Current. Computer modelling of ocean transport predicted that the radioactivity would take several years to approach the Canadian coastline.
Following the March 2011 tsunami that caused extensive damage to a nuclear power plant in Fukushima, Japan, and the release of large amounts of radioactive cesium-137 (137Cs) and cesium-134 (134Cs) into the western North Pacific Ocean, Fisheries and Oceans Canada established a monitoring program to track the movement of radioactivity across the Pacific to the eastern North Pacific and Arctic Oceans.
Canadian Ocean Monitoring Program
As part of the overall Canadian response, Fisheries and Oceans Canada embarked on a monitoring program to track the movement of Fukushima radioactivity across the Pacific to the eastern North Pacific and Arctic Oceans
Established by researchers Dr. John N. Smith of the Bedford Institute of Oceanography and Robin Brown of the Institute of Ocean Sciences, the monitoring revealed some interesting findings, including the observation that the radioactivity moved more quickly than projected.
"Despite the Fukushima incident, measurements of radioactive cesium in ocean water are well below levels that could harmfully affect the environment,=" says Smith. "Our interest is in using the cesium as a tracer to learn more about ocean circulation and to test the ocean transport models that were used to predict where the radioactivity will go.="
Sampling ocean water on Line P
During regular missions of the CCGS John P. Tully in June of 2011, 2012 and 2013, the research team collected water samples along a series of oceanographic monitoring stations known as Line P, which extend approximately 1,500 kilometres westward into the Pacific Ocean from Victoria, British Columbia. Water samples of 30 to 50 litres were analyzed for the presence of radioactivity from Fukushima. In 2012, samples were also gathered at several Arctic stations to evaluate the inflow of radioactivity from the Pacific Ocean to the Beaufort Sea.
Water samples gathered by the research team along the Line P series of oceanographic monitoring stations, in the Pacific Ocean west of Victoria, B.C., were analyzed for the presence of artificial radioactivity from Fukushima. The goal of monitoring ocean transport of the radionuclides 137Cs and 134Cs is to learn more about ocean circulation and to test the ocean transport models that were used to predict where the radioactivity will go.
"It is important to note that background radioactivity is present throughout the environment=" says Smith. There are two types of background radioactivity: the kind that occurs naturally in the environment, which comes from radioactive elements of uranium and thorium that are found in rocks, soils, water and air; and artificial radioactivity from the fallout of atmospheric nuclear weapons testing, conducted by several countries beginning in the early 1950s.
"Whenever we measure radioactivity in the ocean, we also have to consider that there is already background radioactivity as well,=" says Smith. "Our monitoring program needs to determine what proportion is from the Fukushima incident rather than natural sources or from past nuclear weapons testing.=" Enter 134Cs.
Background levels of 137Cs have been measured in the ocean since the 1950s. However, Smith says Fukushima also produced as much 134Cs as 137Cs. Given that 134Cs has a half-life of two years, meaning half of it decays every two years, scientists know there isn’t any left in the environment from past nuclear weapons testing. By contrast, 137Cs, with a half-life of 30 years, remains in the environment much longer and there is a substantial residual fallout amount of this isotope from past nuclear weapons tests.
The distribution of 137Cs from Fukushima along Line P in June, 2013 (above). Levels of 137Cs were highest at Station P26—in the Pacific Ocean, 1,500 kilometres west Victoria, B.C. —and gradually decreased eastward to Station P1 on the continental shelf. Seawater sampling along Line P so far shows that the measured levels of Fukushima 137Cs are highly diluted and well below levels that could harmfully affect the environment; specifically, they are more than 1,000 times below the Canadian maximum acceptable concentration (MAC) in sea water.
"We know unequivocally that if we detect 134Cs in the water samples we gather in the eastern Pacific, it is coming from Fukushima,=" says Smith.
Among the findings of the monitoring program to date:
- Analysis of seawater sampled off the coast of British Columbia along Line P so far shows that the concentration of Fukushima 137Cs in sea water is highly diluted. The measured levels of 137Cs from Fukushima are more than 1,000 times below the Canadian maximum acceptable concentration (MAC)Footnote 1 in sea water.
- In June 2011, 137Cs levels measured to depths of 1,000 metres along Line P were consistent with the amount of background radioactivity from fallout sources that is known to exist in the North Pacific. No 134Cs was detected in the 2011 water samples. This means that cesium from Fukushima had not yet arrived in the eastern Pacific by June, 2011. In June 2012, very low levels of 134Cs were detected in water samples from the upper 50 metres at Station P26, located 1,500 km west of Victoria, B.C. This indicated that radioactivity from Fukushima was just beginning to arrive. As the level was not yet detected at Station P4, just south of Vancouver Island, it had not yet reached that area.
- The time for this radioactivity to first arrive in the eastern Pacific was faster than predicted by computer modelling of ocean transport. For example, one computer modelling scenario anticipated that water-borne cesium from Fukushima would not arrive in the eastern North Pacific until 2015.
- In September 2012, water samples from the Chukchi Sea and southern Canada Basin, gathered during a regular mission of the CCGS Louis S. St. Laurent, did not have any detectable 134Cs. This indicates that there had not been significant transport of radioactivity from Fukushima through the Bering Strait by that time.
- June 2013, water samples revealed 134Cs in the upper 100 metres at all of the monitoring stations along Line P, indicating that the Fukushima radioactivity signal had fully arrived on the continental shelf of North America, although at very low levels. The levels of 134Cs decreased gradually from west to east (highest at Station P26 and lowest at Station P1 on the continental shelf), which is consistent with the transport of Fukushima cesium from the ocean interior onto the eastern Pacific continental shelf.
By subtracting the background levels of 137Cs from the levels measured following the accident, the team was also able to determine how much of the 137Cs in the eastern Pacific water sample was from Fukushima and how much from fallout.
No threat to the environment
Scientists predict that ocean-borne radioactive material from Fukushima will almost entirely shift from the western to the eastern North Pacific during the next five years. Smith says, however, it is important to note that the 137Cs in the ocean from Fukushima is now about equal to the background level of 137Cs from fallout that already existed in the ocean, which is extremely low.
"Even though future radioactive cesium levels in the eastern Pacific are expected to increase due to ocean transport, the levels will remain low and stay well below levels that are of concern to human health and the environment, including marine flora and fauna,=" says Smith.
Computer modelsFootnote 2 of ocean transport predict that cesium concentrations off the North American coastline will increase modestly in the future, says Smith, "likely peaking in the next two years before levelling off.="
Fisheries and Oceans Canada will continue sampling for radioactivity from Fukushima off the Pacific coast of Canada along Line P. The goals of the water sampling program are to identify the arrival of peak levels, verify what those peak levels are, and compare the subsequent decline in levels with that projected by various computer models.
"Data from continued sampling will also be used to assess and validate the transport models, which are used for a wide range of oceanographic research,=" says Smith.
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