A paper by Dr. John Smith (a member of the InFORM network monitoring program) and colleagues was published today in the Proceedings of the National Academy of Sciences of the USA. The paper is open access and can be found and read by anyone interested here. The paper reports some of the most recent results of a monitoring program initiated by Canada's Department of Fisheries and Oceans in 2011 to track contamination of Fukushima contamination in the Pacific and Arctic Oceans. To do this seawater samples were collected at specific stations between 2011 and 2014 indicated on the following map.
Map showing the location of the site of the Fukushima Dai-ichi Nuclear Power Plant accident in Japan. Stations are indicated at which seawatersamples were collected in 2011-2014 on Line P and in 2012 in the Beaufort Sea. Box B and point R represents the model domains for which Fukushima-derived 137-Cs time-series concentrations were estimated by Behrens and colleagues and Rossi and colleagues respectively. Inset shows sampling station locations along Line P. Dashed curves are time-averaged streamlines representing the mean dynamic height field for 2002–2012, indicating the northward geostrophic transport of the Alaska Current across Line P.
Smith and colleagues first detected Fukushima derived radiocesium offshore (~1500 km) in June 2012 and saw Fukushima near the continental shelf the following year. The results of the study are summarized in the figure below. Because 134-Cs (half life ~2 years) is so short lived all of the isotope from Chernobyl and weapons tests last century have decayed away making the presence of 134-Cs in a sample an unambiguous fingerprint of Fukushima impact.
In Panel A Water-depth profiles of 134- and 137-Cs measured at stations P4 and P26 in June 2011, June 2012, June 2013, and February 2014 (from left to right) tracking the arrival of 134-Cs and 137-Cs from the Fukushima accident on Line P. In 2011, 134-Cs was below the detection limit (dashed line) at both stations, but was measurable (concentrations and detection limit are decay-corrected to April 6, 2011) at station P26 in 2012 and at both stations P4 and P26 in 2013 and 2014. Panel B shows a water-depth section of Fukushima 137-Cs concentrations (calculated from decay-corrected 134-Cs concentrations) on Line P in June 2013 demonstrating that 137-Cs concentrations decrease toward the east from station P26 to station P1 in the surface mixed layer that reflects 137-Cs transport from Fukushima onto the continental shelf off Canada. Negligible Fukushima 137-Cs had been transported below 150 m by June 2013.
Over the time period of the study and up until February 2014 the levels of added Fukushima radiocesium ranged between 170% and 75% of the amount of 137-Cs remaining in the eastern North Pacific from weapons testing fallout delivered in the 20th century. You will likely find that news aggregator sites online that do a poor job reporting on Fukushima or newspapers looking for an eye catching headline will latch onto these numbers to suggest dire consequences for the Pacific owing to Fukushima. To put the numbers in the proper perspective Smith and colleagues put together a fine figure comparing their measurements to predictions made by computer models as to the timing of arrival and maximum activities of radiocesium in the contaminated plume.
Fukushima-derived 137-Cs concentrations in surface water at stations P4 and P26 are shown versus time. Fukushima 137-Cs was below the detection limit (illustrated by arrows) in 2011 but was measurable at station P26 in 2012 and measurable at both stations in 2013. Model results correspond to 137-Cs activities in surfacewater predicted by Behrens and colleagues (blue curve) for Box B in Fig. 1 and Rossi and colleagues (cyan curve) for cross shelf regime R in the papers first figure. The inset shows the ocean model simulations for 137-Cs (including an additional fallout background of 1.2 Bq/m3), which are compared with the historical measurements for 137-Cs fallout levels (brown symbols) in surface waters of the North Pacific Ocean.
The take home message of this paper is really given in the inset of the figure above where the historical record of 137-Cs in the North Pacific from weapons testing fallout (and to a lesser degree Chernobyl) is compared with model predictions that are broadly consistent with the Smith et al. (2014) data shown here. You can see that maximum activities in the eastern Pacific from weapons fallout was between 20 and 30 Bq m-3 before 1960. This signal diminished with time through decay and mixing of 137-Cs out of the surface ocean into the ocean interior. The impact of Fukushima on 137-Cs according to the models will likely bring maximum activities back up to about 5 Bq -3 or to levels similar to those last seen in the 1980's. By 2021 ocean mixing and transport will disperse the plume such that the impact of Fukushima on 137-Cs in our waters here will not be detectable. Given that other isotopes of concern with respect to environmental and public health (e.g. 90-Sr and Plutonium isotopes) were released in very low amounts relative to radiocesium the general conclusion reached for Cs can likely be extended to these isotopes as well.
Links to the Behrens et al. and Rossi et al. modeling studies are found below:
Behrens et al. (2012) and Rossi et al. (2013)
The levels of radiocesium expected and being measured offshore do not represent an environmental or public health threat. Ongoing monitoring will help to improve models of North Pacific circulation and keep the public up to date with respect to expected impacts on ocean and human health. Look for continued monitoring from the InFORM project and for new results to be written about here as they become available.
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