By Jay T. Cullen

The purpose of this diary is to share the results of a recently published, peer reviewed, study that examined the radiological impact of the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) meltdowns on invertebrates living in coastal waters close to the disaster site in Japan.  The paper by Sohtome and colleagues measured the activity of artificial radionuclides 134-Cesium (¹³⁴Cs half life ~ 2 years) and 137-Cesium (¹³⁷Cs half life ~ 30 years) in 592 specimens representing 46 different taxonomic families collected in coastal waters in Fukushima Prefecture between July 2011 and August 2013.  The authors found ¹³⁷Cs in 77% of the samples with highest activities in the organisms near to or south of the FDNPP where highest levels in seawater and marine sediments were found after the large initial releases in 2011.  Levels of ¹³⁷Cs decreased exponentially with time with differences between the various taxonomic groups. 

This is consistent with lower seawater activities with time after the large initial releases of radionuclides from the plant diminished exponentially in Spring 2011.  Observations of the decreasing ¹³⁷Cs in the organisms allowed the biological half life (time it takes for the half of the ¹³⁷Cs activity in the organism to go away) to be determined which was found in the range of 102 to 487 days.  Longer biological half life and higher, more persistent ¹³⁷Cs levels in organisms that tend to feed on organic matter in the sediments suggest that contaminated sediments in the area are the most likely source of continued contamination of organisms living on or in the sediments. Even the maximum activities measured within 4 months of the of the disaster and within 20 km of the power plant did not reach levels known to cause negative health impacts for invertebrates. The authors note that benthic marine communities in these coastal waters were not changed greatly by the earthquake and tsunami or by the FDNPP accident.

The study by Sohtome and colleagues collected animals living on or in the seafloor off the coast of Japan close to the FDNPP site starting in July 2011 about 4 months after the disaster.  The collection locations are indicated on the following map:

For each sample the activity of radiocesium in the whole organisms was determined using high sensitivity, high purity germanium detectors.  Of the 592 organisms examined 77% had detectable radiocesium levels (lowest detectable activities were 0.33 — 28 Bq kg^-1 wet and 0.50 — 24 Bq kg^-1 wet for ¹³⁴Cs and ¹³⁷Cs respectively and depended on counting time and sample quantity and density).  Only four samples of urchin exceeded 100 Bq kg^-1 wet with the maximum activity being 290 Bq kg^-1 wet weight.  Trends in ¹³⁷Cs in individual taxa are shown in the graph below and are typically decreasing exponentially with time.

 

This decrease is owing to the short (<1 year) biological half life of radiocesium in the organisms, the rapid dissipation, dilution and mixing of the contaminated seawater and dramatic decrease of radionuclide release from the FDNPP site following high rates of release in March and April 2011.  The following graph shows how radiocesium activities in seawater, sediments and various benthic invertebrates have changed in area D to the south of the FDNPP with time since the meltdowns:

The lack of decreasing trend in radiocesium with time in deposit feeding species suggest that contaminated sediments are the major source of contamination to bottom dwelling species as of 2013.  It is likely that local anomalies in ¹³⁷Cs levels in sediments will continue to cause variability in these species in the future.

Take home messages from the study are:

1. Few organisms sampled in Japanese coastal waters near to the FDNPP site exceeded 100 Bq kg^-1 wet weight ¹³⁷Cs as soon as 4 months after the peak of releases from the site
2. The most contaminated organisms were found in areas close to or south of the plant site where maximum seawater and sediment contamination have been found
3. Levels in almost all species decreased exponentially with time consistent with releases of radionuclide contamination from the site
4. Levels in deposit feeding organisms have not decreased as rapidly and indicate that contaminated sediments are the most important source of radiocesium to benthic animals by August 2013
5. The benthic community is largely unchanged when compared to pre-Fukushima ecosystem structure