By Jay T. Cullen

@JayTCullen and @FukushimaInFORM

This short blog summarizes an open access paper published today reporting results from a Canadian monitoring program tasked with documenting the arrival of ocean borne Fukushima contamination along the North American Pacific coast. This diary is part of an ongoing effort to communicate the best science available on the impacts of the Fukushima Dai-ichi meltdowns on the environment. High quality measurements to look for Fukushima derived radiocesium were made in seawater in the North Pacific and Arctic Oceans from 2011 to early 2014. The authors concluded that:

1. Fukushima derived radiocesium was first detected 1500 km west of British Columbia Canada in June 2012
2. Contamination was detected on the continental shelf (near coastal waters) in June 2013
3. By February 2014 Fukushima radiocesium was present at levels similar to preexisting weapons testing derived 137-Cs
4. The timing of the arrival and levels of radiocesium in the contaminated plume are in reasonable agreement with existing ocean circulation model predictions
5. These same models predict that total radiocesium levels from weapons testing fallout and Fukushima will likely reach maximum values of ~3-5 Becquerel per cubic meter (Bq m^-3 of seawater in 2015-2016 and then decline to fallout background level of ~1 Bq m^-3 by 2021
6. Fukushima will increase northeastern Pacific water to levels last seen in the 1980’s but does not represent a threat to environmental or human health

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.

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.

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 20^th 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.

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 a 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.