The purpose of this post is to summarize a recently published, peer-reviewed study that documents levels of Fukushima derived contamination in surface waters of the Pacific Ocean. This post is part of an ongoing series aimed at communicating scientifically derived information about the impact of the disaster on marine environmental and public health. Michio Aoyama and colleagues measured the activity of Cesium-137 (137Cs, half life ~30 years) and Cesium-134 (134Cs, half life ~ 2 years) in seawater collected from the western Pacific Ocean including waters off the coast of Fukushima Prefecture from 2011-2017. They found the following:
- Contamination decreased dramatically and rapidly in waters offshore of the Fukushima Daiichi Nuclear Power Plant (FDNPP) from maximum values of ~3000 Becquerel per cubic meter (Bq m-3) of seawater in 2011 to values in 2015-16 of ~2-3 Bq m-3. This precipitous decline is consistent with the ongoing but relatively low rates of release of radionuclides from the site compared to the bulk of contamination that was released in March-April 2011.
- Levels of 137Cs close to FDNPP now are similar to levels of contamination present there before the disaster occurred (1.5-2 Bq m-3) owing to atmospheric nuclear weapons testing in the middle of the 20th century.
- Levels in the western Pacific were around 1-7 Bq m-3 in 2011-2012 but stabilized at lower values in 2017.
Levels being measured in nearshore and offshore waters in the western Pacific near to Japan do not approach levels known to represent a credible risk for ocean or public health. These results in the western Pacific are consistent with what the Integrated Fukushima Ocean Radionuclide Monitoring (InFORM) project is finding in the eastern Pacific off of North America.
Aoyama and others recently published their study in the Journal of Environmental Radioactivity. The collected and analyzed surface seawater for the presence of radiocesium isotopes between 2011 and 2017 in waters of the western Pacific in the following locations:
Boundaries of areas (boxes) sampled by Aoyama et al. (2018) in the western and central Pacific Ocean.
The activity of 137Cs and 134Cs in Bq m-3 with time that they found are summarized in the following figure:
Long term trends (2011-2017) in radiocesium activity in boxes defined in the first figure. Solid blue squares are 137Cs activity concentration and open red circles represent 134Cs.
The researchers found that in Box 2 (closest to the FDNPP) contamination in surface waters offshore were highest in early 2011 coincident with the largest releases from the site in March-April of that year when the vast majority of radionuclides were released to the atmosphere and directly to the ocean. Values dropped dramatically so that by 2014-2016 levels were ~3 Bq m-3 and similar to levels of contamination measured before the disaster occurred owing to nuclear weapons testing that occurred in the 1950s-60s. Note that the concentrations of 134Cs diminish relative to 137Cs, and the red symbols on the figure diverge from the blue symbols, because 134Cs has an ~2 year half life and is decaying away from the environment much more rapidly. Indeed, it is becoming increasingly challenging analytically to detect Fukushima 134Cs in environmental samples. Contamination farther offshore in Boxes 4-6 indicate that maximum levels of contamination from Fukushima approached by did not exceed 200 Bq m-3 in 2011 and are now ~2-3 Bq m-3.
Based on best estimates of how much radiocesium was released from FDNPP in March-April 2011 the authors used a model of the water circulation and mixing in the Pacific to predict the levels and movement of Fukushima 134Cs in the Pacific from April 2012 until October 2016. The results of the modeling study are summarized in the following figure:
Horizontal distribution of 134Cs from Fukushima for the period April 2012 to October 2016. Open circles represent observations/measurements of 134Cs while shading reflects model results.
What the model and observations indicate is that the bulk of contamination from the site went into the Pacific Ocean in 2011 and that rates of release from the site after that time are very small in comparison. Most of the Fukushima contamination is now in the eastern Pacific near to North America and that levels in behind the main body of contamination are difficult to detect. Similarly, the lack of appreciable 134Cs and 134Cs/137Cs activity ratios close to FDNPP indicate that there is little evidence for ongoing fission in the reactors at the site as is commonly speculated by those with little scientific training. The levels the scientific community is measuring close to FDNPP and those expected and measured in waters close to North America do not represent a significant risk to the marine ecosystem or public health.
The Fukushima InFORM project will continue its monitoring activities in the eastern Pacific until Spring 2019.
