Radioactive Strontium and Cesium in Fish From the Harbor at the Fukushima Dai-ichi Nuclear Plant

By Jay T. Cullen

The purpose of this post is to report on a recent peer-reviewed study that investigated the radionuclide content of fish caught in the harbor of the Fukushima Dai-ichi Nuclear Powerplant (FDNPP) in 2012 and 2013. The post is also written in part to address questions like:

Why don’t you measure 90Sr in fish you catch off of North America?

This post is part of an ongoing series dedicated to summarizing results from scientific research into the impact of the FDNPP disaster on the environment. Fujimoto and colleagues measured the activity of Cesium-134 (134Cs half life ~2 years), Cesium-137 (137Cs half life ~30 years) and Strontium-90 (90Sr half life ~29 years) in fish collected from the FDNPP harbor and just outside the port in 2012 and 2013. Fish were most contaminated in the harbor and had radiocesium activity concentrations (in whole body without internal organs, Bq kg-1 – wet weight) that were ~200-330 times higher than measured 90Sr levels. The much lower 90Sr levels compared to radiocesium in the fish is consistent with much lower releases of 90Sr to the Pacific Ocean compared to radiocesium in the aftermath of the meltdowns at FDNPP (see here, here and here for example). The activity of radiocesium in fish diminishes dramatically with distance from the harbor and as of April-June 2015 none of the fish caught in Fukushima prefecture waters exceeded the stringent 100 Bq kg-1 Japanese safety standard. Across the Pacific, we have yet to detect Fukushima derived radiocesium in salmon and steelhead trout caught in British Columbian waters as part of the Fukushima InFORM monitoring effort. 90Sr is much more difficult and costly to analyze in environmental samples than are the cesium isotopes. The results of the Fujimoto study suggest that 90Sr from Fukushima is unlikely to be found at detectable levels in marine organisms in the northeast Pacific and that resources to monitor the impact of the disaster on our marine environment should focus on the detection of the cesium isotopes.

Fujimoto and colleagues collected Japanese rockfish, brown hakeling and fat greenling using small cage traps and gill nets inside the harbor at the FDNPP and at locations in the coastal ocean nearby. Sampling locations are shown on the map below.

Map of monitoring area around the FDNPP: 1−4 indicate the locations of reactor units 1−4; IC, intake canal of units 1−4; a, sampling point with cage at unloading deck; b, sampling point with cage at north jetty; c, sampling point with cage at south jetty; and d, sampling point with gill net at entrance of harbor. Pods indicate sampling conducted with a research vessel.

Full results of the study can be accessed in pdf here.

Radiocesium in Fish Near to FDNPP

The highest concentration of radiocesium was found in fat greenling caught at the harbor entrance on February 12, 2013 which was measured at 202,000 Bq kg-1 wet weight. The lowest was a brown hakeling caught near the south jetty on January 30, 2013 at 104 Bq kg-1 wet weight. The results comparing values for fish caught at the FDNPP compared with fish caught in other parts of Fukushima prefecture are shown in the figure below.

Box plots of the concentration of radiocesium in muscles of fish caught in the main harbor of FDNPP and around Fukushima Prefecture. Each box indicates the range between the first and third quartiles. A band inside the box indicates the median. Whiskers show the minimum and maximum concentrations observed. Open circles indicate outliers. The data set of Fukushima Prefecture was obtained from website

Rockfish consistently had the highest radiocesium activities which may reflect diet choice differences among fish species. Activities of 134Cs and 137Cs in the fish dropped dramatically outside the harbor as the results for “Pod” stations below indicate.

Box plots of the concentration of radiocesium in muscles of brown hakeling caught around FDNPP. Each box indicates the range between the first and third quartile. A band inside the box indicates the median. Whiskers show the minimum and maximum concentrations observed. Open circles indicate outliers.

This is consistent with radionuclide activities that drop dramatically in seawater and sediments with distance from the FDNPP site owing to ocean transport and mixing processes.

Comparing Radiocesium and 90Sr in Fish Near FDNPP

The figure below shows the relationship between radiocesium and 90Sr in the fish.

Relationship between the concentration of radiocesium and 90Sr in the whole body without internal organs of Japanese rockfish.

The 90Sr activity in the fish in the fish tended to be about 300 fold less intense than the radiocesium activity. This likely reflects the lower activity of 90Sr in seawater and sediments in close proximity to the FDNPP. The maximum activity of 90Sr in the fish from the harbor was 170 Bq kg-1 which is approximately 4 orders of magnitude higher than values measured before the FDNPP accident. All fish caught outside the harbor and measured had 90Sr levels -1. Given that 90Sr was released in much lower total activity when compared to radiocesium these relative values in fish with distance from the FDNPP site make sense.

The lack of significant concentrations of 90Sr compared to Cs in fish close to Japan highlight the fact that deploying significant resources to test samples near to the coast of North America would not likely provide useful information about the impact of the disaster on our marine ecosystem. Indeed, given the amounts of 90Sr released to this point by the disaster there is likely to little in the way of measurable impact on levels in seawater or fish in the eastern Pacific. This is why monitoring efforts like the Fukushima InFORM project tend to focus on the cesium isotopes as together they were released in significant quantities, serve as an unequivocal tracer of Fukushima impact and tend to bioconcentrate in marine organisms. They provide the most information relative to the cost of sampling and analysis.

In Case You’re Interested (ICYI): Methods to detect radiocesium and 90Sr in fish

Radiocesium – Fish samples (~2-300 grams) are frozen and then minced with a knife before placing in gamma counting vials. Samples are kept frozen until counting. Gamma emissions are counted using a high purity germanium detector coupled to a multichannel spectroscope. Samples are counted for 7200 – 16000 seconds depending on radiocesium activity to achieve desired detection limits.

90Sr – Fish, minus internal organs, are dried at 105 C for 48 hours and then ashed for a further 48 hours at 500 C. 20 grams of ash are dissolved in 6 milliliters of nitric acid and then digested at high temperature and pressure in a laboratory microwave. To isolate Sr from this digest solution divalent cations are concentrated in a precipitate using carbohydrate and oxalic acid precipitation steps. The precipitate is redissolved in 200 milliliters of 0.5 M hydrochloric acid and then loaded onto a cation ion exchange resin column (Dowex). Strontium is purified by subsequently washing the column with ammonium acetate/ethanol and ammonium acetate solutions. 90Sr is detected by measuring the beta decay of its progeny isotope yttrium-90 (90Y half life ~64 hours) two weeks after the first preconcentration steps are taken. 90Sr activity is decay corrected to the time of sampling.

This highlights the fundamentally different effort/resources that must be deployed to monitor 90Sr in biological samples compared to radiocesium and why 90Sr measurements are more rare in the literature.

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