Paper: P.P. Povinec, L. Liong Wee Kwong, J. Kaizer, M. Molnár, H. Nies, L. Palcsu, L. Papp, M.K. Pham, P. Jean-Baptiste, Impact of the Fukushima accident on tritium, radiocarbon and radiocesium levels in seawater of the western North Pacific Ocean: A comparison with pre-Fukushima situation, Journal of Environmental Radioactivity, Volume 166, Part 1, January 2017, Pages 56-66, http://doi.org/10.1016/j.jenvrad.2016.02.027
Snapshots in time
What do poodle skirts, shoulder pads, and butterfly clips have in common? They are all iconic snapshots of fashion for a particular decade. Anthropogenic chemicals serve the same purpose in the ocean. Scientists date water masses based on radionuclides (isotopes that undergo radioactive decay) measured. Nuclear weapon testing released massive amounts of tritium (3H) and radiocarbon (14-C) into the ocean (Figure 1, peak ocean concentration in 1965). Using this and other information scientists can estimate where a water mass came from, as well as how long it had been moving through the ocean. After any major radionuclide additions to the atmosphere and ocean, the radioactivity snapshot parameters have to be reevaluated to not misinterpret water masses histories. For example, if you see teen girls in choker necklaces and crop tops today, you know you probably didn’t travel back in time but the “fashion clock” has reset in 2017 with similar fashion elements to the 90s.
Global Weapons Testing
Scientists refer to “global fallout background” as the period of time from the 1940s to the early 1960s where the United States and other countries conducted more than 500 nuclear weapon tests in the atmosphere (cdc.gov). Overall, there have been over 2000 detonations since 1945. The radioactive particles and gases released spread over the globe, depositing in the ocean and on land (i.e. the fallout) before decaying and becoming more dilute. Estimates of the radionuclide totals along with the rates of decay help mark the age of deep-water masses in the ocean since the calculations pinpoint when masses were last at the surface.
On a Friday afternoon in 2011, a 9.0 magnitude earthquake off the coast of Japan resulted in a 15-meter tsunami that killed about 19,000 people (world-nuclear.org). As if that wasn’t bad enough, the tsunami damaged a Japanese nuclear power plant (NPP), Fukushima Daiichi, preventing multiple units from cooling properly. The reactor was flooded with seawater, which subsequently leached radioactive contaminants back into the ocean as the water receded. Radionuclides were also released into the atmosphere. Atmospheric radionuclides have the ability to travel long distances quickly via air currents and deposit both in the ocean and on land. After the Fukushima accident, scientists all over the globe measured local coastal waters and the atmosphere as part of an effort to understand the fate of radionuclides from the Fukushima disaster. If you want to learn more about radioactivity in the ocean after Fukushima visit www.ourradioactiveocean.org. You can learn more about radiation and look at data collected from 2011 – 2017 in the ocean and on land.
Focus of this study
There are numerous studies measuring radionuclides in the water, soil, atmosphere, and biota as a result of the Fukushima NPP release. In this study, scientists onboard a US expedition (June 2011) collected seawater samples east of Japan from the surface down to 500 m. They hoped to better understand the distribution of important radionuclides in the ocean. Another goal of this work was to use these new measurements to update tracer databases used by scientists to help age water masses.
The radioactive players
Scientists measured seawater samples for 137-cesium, tritium, and radiocarbon. The most important radionuclide released during the Fukushima disaster was 137-cesium (137Cs). Released in large quantities, this radionuclide has a relatively long half-life (~30 years) and high bioavailability (the ability to be broken down and used by organisms). There was a significant amount of 134-Cesium released as well, but its much shorter half-life (2 years) lessens its long-term impact and scientists did not include it in the analysis. Tritium (3-hydrogen) and radiocarbon (14-carbon) were released as well, although in much smaller quantities than during the weapon testing of the ‘50s and ‘60s (the global fallout background).
Measured 137-cesium was at least 200 times higher than previous background measurements, with levels remaining elevated even at 600 km distance from Fukushima. Figure 2 compares this group’s measurements to established background 137-cesium (red line). There is a decrease (6 times) in 137-cesium from measurements taken soon after the Fukushima disaster highlighting the ocean’s role in mixing and diluting the water mass from the Fukushima accident—this is supported by reports from stations showing a maximum 137-cesium level below the surface, indicating waters are mixing away from the source.
The maximum tritium found was at 10 m water depth at a station 30 km offshore. The tritium seawater profiles were similar to 137-cesium with high values 100 m deep showing mixing into the deeper layers of the ocean. Overall, there was less tritium released relative to 137-cesium and there is only a factor of 6 increase compared to the global fallout background.
The radiocarbon depth profiles were similar to both 137-cesium and tritium with the highest concentrations about 100 m down. Unfortunately, there is no data available about the concentration of radiocarbon released after the Fukushima accident. The radiocarbon produced due to Fukushima should be tiny when compared to both the naturally- and nuclear-produced levels. Scientists used radiocarbon measurements from 400 – 500 m as the background concentration because there was insufficient time for Fukushima altered water masses to have reached that depth. They saw only a 9% increase in radiocarbon that could be attributed to Fukushima.
Time to update the history books
137-Cesium was the largest radionuclide addition to the ocean after the Fukushima accident but oceanic concentrations of tritium and radiocarbon changed as well. As more time passes, their concentrations will decrease through dilution and decay but the Fukushima accident has altered the established snapshot of the ocean. Figure 3 shows the known oceanic 137-cesium as of 2008 and needs to be updated post-Fukushima. Moving forward, scientists will need to consider Fukushima-influenced water masses when using radionuclides as water mass tracers. If this is done effectively, these radionuclides will be effective tools to study changes in deep water formation and biogeochemical processes in a warming ocean.