The purpose of this diary is to report on a recently published (Jan 2015) open-access, peer reviewed study which examined the activities of ¹³⁷Cs (half life 30.2 yr), ¹³⁴Cs (half life ~2.1 yr) and ⁹⁰Sr (half life ~28.8 yr) in the northwest Pacific off the coasts of Japan and China. The diary is part of a ongoing effort to communicate the results of scientific research into the impact of the Fukushima Dai-ichi nuclear disaster on environmental and public health. Men and colleagues report on how activities of these fission produced isotopes changed between three research expeditions in June 2011, December 2011 and June 2012. Activities in seawater decreased dramatically through time for all three isotopes consistent with very high release rates measured from the Fukushima site in March-April 2011 followed by ongoing but many orders of magnitude (10,000 – 100,000 fold) lower releases from the site thereafter. By 2012 the impact of the Fukushima releases could be still be detected in most samples for Cs isotopes however ⁹⁰Sr distributions were much more uniform with the highest measured activity only slightly above the pre-Fukushima background. These results are consistent with:

1. the relatively small source term for ⁹⁰Sr from compared with the Cs isotopes from Fukushima as determined by measurements of air, soil and water after the disaster
2. the much lower Fukushima derived activities for these isotopes in the eastern Pacific off of North America being measured given decay and mixing of the contamination as it is transported by ocean currents

Seawater samples have been collected on eight research expeditions from 2011-2014 by scientists working at the State Oceanic Administration of China to understand the impact of the Fukushima Dai-ichi triple meltdowns on the Pacific. The paper my Men and colleagues reports results from the June 2012 work and compares these results with two previous occupations of the same stations in June and December 2011. Station locations where surface and subsurface seawater samples were collected are shown in the figure below:

In Case You Are Interested (ICYI): Methods
(You are free to skip ahead to the results if this is too dry)

Detecting these low levels of artificial radionuclides in environmental samples is a significant analytical challenge. Details of the methodology used in this study can be found here and, but are summarized briefly below.

¹³⁷Cs and ¹³⁴Cs Measurements

60 liters of seawater are collected and the Cs isotopes are concentrated from the sample by co-precipitation with ammonium phosphomolybdate (AMP) followed by gamma spectrometry. Along with a stable CsCl tracer to help determine how effective the preconcentration step is the AMP, which is sticky for Cs⁺, is added to the 60 L, the pH is adjusted down with acid to improve recovery and the sample is mixed for 30 min and then left to sit for at least 24h for all the Cs to bind with the AMP. The Cs-AMP residue is then filtered from the seawater, ashed at 450 degrees C for 2h and the residue place in the high purity germanium detector for many hours to determine Cs isotope activities.

⁹⁰Sr Measurements

40 liters of seawater are collected for each data point and ⁹⁰Sr in the sample is determined by di(2-ethylhexyl) phosphoric acid (HDEHP) extraction followed by beta counting of its daughter isotope 90-Yttrium (⁹⁰Y). Both stable Sr and Yttrium (Y) are added to the seawater to determine recovery efficiency. The samples pH is adjusted up to cause the cations to precipitate as hydroxides and oxyhydroxides. The precipitate is recovered by filtration redissolved in acid and this solution is then extracted twice with HDEHP. This extraction is then treated to precipitate the Y from solution, isolating it from other beta emitters, which is filtered out and placed in a beta counter to determine the activity of ⁹⁰Y (a daughter of ⁹⁰Sr) which is used to calculate the amount of ⁹⁰Sr originally in the sample. Insufficient water was available to allow for ⁹⁰Sr to be detected in the 200 m seawater samples.

Results

The activities of the radionuclides are summarized in the following table.

In 2012 ¹³⁴Cs activities were undetectable is some seawater samples and as high as 9.7 Bq m^-3 with average activity of 1.2 Bq m^-3. The longer lived ¹³⁷Cs was in the range of 0.41 to 18 Bq m^-3 and an average of 3.4 Bq m^-3. Both Cs isotopes were present at average activities about the pre-Fukushima background of 1.7 Bq m^-3 of ¹³⁷Cs and undetectable levels of ¹³⁴Cs before the March 2011 disaster. ⁹⁰Sr activities were less spatially variable than Cs isotopes with a range of 0.6 to 3.6 Bq m^-3. The average ⁹⁰Sr for this expedition in 2012 was 1.5 Bq m^-3. This average was not different than the pre-Fukushima background ⁹⁰Sr activity of 1.2 +/- 0.4 Bq m^-3 in this part of the Pacific owing to the presence of weapons testing fallout from the 20^th century. Only one sample exceeded the pre-Fukushima ⁹⁰Sr background in the northwest Pacific by the 2012 sampling.

The horizontal and vertical variability of ¹³⁴Cs (a marker for Fukushima contamination) and ⁹⁰Sr are shown in subsequent figures:

 

 

The activity of Fukushima derived ¹³⁴Cs is more variable than the ⁹⁰Sr with maximum Cs activities found at 200 m depth both north and south of 30N latitude. ¹³⁴Cs was virtually absent from samples collected near to Taiwan and mainland China while ⁹⁰Sr activities there were similar to values measured in samples collected closer to the Fukushima Dai-ichi nuclear power plant site in Japan.

Compared to samples collected in June and December 2011 activities for all the isotopes were dramatically lower in the 2012 samples. These results are summarized in the following figure:

The highest and average activities of the isotopes decreased precipitously from June 2011 to June 2012. Maximum ¹³⁷Cs activities of ~800 Bq m^-3 compared to maximum ⁹⁰Sr of ~30 Bq m^-3 in 2011 fell 44-fold for ¹³⁷Cs and ~8-fold for ⁹⁰Sr by 2012.

Conclusions

1. the presence of Fukushima derived ¹³⁷Cs and ¹³⁴Cs in 2012 in waters of the northwest Pacific were above pre-Fukushima background indicative of Fukushima impact in the area
2. maximum activities for Cs isotopes were found at 200 m depth in the area consistent with previous measurements showing that sinking of surface waters during winter cooling (also known as mode water formation) were transporting Fukushima isotopes into the ocean interior
3. by 2012 the vertical and horizontal variability and activity of ⁹⁰Sr was virtually indistinguishable from pre-Fukushima background in waters off the coast of Japan consistent with the smaller source term of ⁹⁰Sr compared to Cs isotopes
4. the dramatic decrease of Fukushima derived radionuclides in the northwest Pacific and close to the Japanese coast between 2011 and 2012 is consistent with known release rates from the Fukushima Dai-ichi site which were very high in March-April 2011 but dropped many orders of magnitude thereafter continuing at rates roughly 10,000 – 100,000 fold lower to this day

Editorial comment

The activities of Fukushima derived isotopes in the northwest Pacific help to illustrate why investigators in the northwest Pacific off of North America tend to focus on the Cs isotopes in seawater and the biota in order to monitor the impact of the disaster there. Given a roughly 30-fold higher activity of the initial source term of the Cs isotopes relative to ⁹⁰Sr to waters of the Pacific and given the mixing and dilution of the signal as waters are transported west by currents it would be very difficult to detect the ⁹⁰Sr signal from Fukushima relative to weapons testing background. We are currently measuring about 3-4 Bq m^{-3 137}Cs from Fukushima offshore of British Columbia and the Pacific northwest. There would be approximately 30-fold less ⁹⁰Sr in the contaminated plume or about 0.1 – 0.13 Bq m^-3 on top of the ~1.2 Bq m^-3 weapons testing background. This increase in lost in the precision of the measurement and variability in background resulting in an immeasurable increase from Fukushima in the northeast Pacific. Given that ⁹⁰Sr tends to concentrate less in marine organisms than do the Cs isotopes it is less likely that analytical techniques could detect the presence of any Fukushima ⁹⁰Sr in fish or algae or other marine organisms.

For the information gained and the effort and resources required to make measurements the Cs isotopes offer the best tracer we have of Fukushima impact and given their propensity to concentrate in organisms and the total activities released represent the most probable radiological health risk to the environment and the public.

More results will be posted here as they become available.