Evaluating the Risks of Fukushima-Derived Radionuclide Concentrations in Pacific Ocean Seawater and Biota

Here’s a paper I wrote this quarter for an Ocean Environment class that I’m taking. I thought it might be cool to share! It was a fun research project. References are at the end (sorry, I didn’t link them in the body of the paper/post).

Evaluating the Risks of Fukushima-Derived Radionuclide Concentrations in Pacific Ocean Seawater and Biota

Ryan Gratzer UCLA Extension: Z3892: Ocean Environment Fall 2013

On March 11, 2011, a 9.0 earthquake off the coast of Japan triggered a tsunami that flooded the Fukushima Dai-ichi Nuclear Power Plant, resulting in a meltdown. During the meltdown, large amounts of freshwater and seawater were pumped through to cool the overheating reactors, and in the process 3 million gallons of contaminated water from the damaged reactor buildings were released into the ocean. There has been much concern over whether these radionuclides in the ocean will adversely affect both ocean and terrestrial life. Will they bio-accumulate up trophic levels in marine biota, and how may that affect humans who consume fish from the Pacific Ocean? The data so far seems to show that while radiocesium and other radioactive elements are certainly making their way around the Pacific Ocean, they are not concentrated at levels that present health concerns, and are, in most places, continually diffusing rather than accumulating.

Some context: Earth currently contains naturally-occurring stable isotopes (ex. carbon-14), naturally-occurring unstable (radioactive) isotopes (ex. potassium-40, uranium), and anthropogenic unstable isotopes (ex. cesium-137). We constantly expose ourselves to naturally-occurring radioactive materials (NORM) through mining, drilling, burning coal, building materials, flying on airplanes, food, sleeping next to people, the sun, and much more (many of these exposures are referred to as “background radiation”).[1] Radiation can damage DNA, and damaged DNA can lead to mutations (such as cancer, or aging). The isotopes we are exposed to are constantly decaying into other forms and thus becoming more stable, and so, barring an event that introduces a relatively large amount of radioactive materials into the environment, the radioactive level of our bodies stays pretty stable and our bodies are effectively handling damaged DNA.


The public has been very skeptical of the scientific and government reports about Fukushima. TEPCO and the Japanese government took two months to communicate to the public that a meltdown had occurred; and TEPCO did not admit until recently that 300 tons of contaminated water was leeching out into the ocean every day (though scientists had suspected it for a while, due to the concentrations of fast-decaying cesium-134 that was still present); and it is still unclear exactly how much radiation has entered the atmosphere, ocean, and land. There are also a lot of questions about “safe levels of radiation,” and the safety of nuclear power in general.

The Navies, scientists, and government bodies are doing their best to analyze and respond to the event given the uncertain data coming from TEPCO. But they’ve been slow to release their reports, and speculation abounds about “what is being hidden from the public,” and what the possible global ramifications could be. The bulletpointed conspiracy-laden articles are vastly more popular with the general public than, say, the scientific reports that focus on just one facet of the effects of the meltdown. The sensational “28 Signs That The West Coast Is Being Absolutely Fried With Nuclear Radiation From Fukushima” has gone “viral,” while the responses – “28 fallacies about the Fukushima nuclear disaster’s effect on the US West Coast“ and “More Fukushima Scaremongering Debunked“ – receive far less attention.[2, 3, 4]

Measuring the radionuclides

There are multiple variables to consider when assessing the concentrations of radionuclides in the ocean: the concentration levels of radionuclides in different areas and depths; the concentrations of radionuclides in primary providers and other marine biota; the rates of decay of the radioactive isotopes; ocean currents spreading and diffusing radionuclides; and migratory patterns of marine organisms.

The initial release of water during the meltdown contained an estimated 5,000-15,000 terabecquerels of radioactive isotopes cesium-134 and cesium-137.[5] And since then (2.5 years), it is estimated that 0.3 terabecquerels have leaked each month.[6] Becquerels measure the activity of radioactive material, in decays per second in the nucleus of an atom. It does not measure dosage. But most studies of the concentrations of radiocesium in marine biota use this measurement. In organisms, it is measured by concentration per kilogram of weight. In the open ocean, it is measured in concentration per cubic meter of water.

It is difficult to hear “tera” (trillion) and not think “that’s a lot.” And the proper context is often elusive. Concentrations in the immediately vicinity were found to be 4,000 to 10,000 Bq m3, which can be compared to permissible drinking water limits for cesium-137: in the US, it’s 7,400 Bq m3. Still, according to Ken Buesseler at the Woods Hole Oceanographic Institution, Fukushima “has become the largest accidental source of radionuclides to the ocean in terms of measured radionuclide concentrations.”[7] It exceeds Chernobyl in this regard because of its proximity to the ocean; not because of overall quantity of radionuclides released. But the Pacific Ocean, containing 187 quintillion gallons of seawater, has a tendency to dilute water soluble substances. Even though 300 metric tons of radioactive water are still being discharged into the ocean every day, the same study reported that “even at the discharge point, there were >1000 times lower radionuclide activities as quickly as one month after peak releases and even lower activities off shore.”

Cesium is water soluble, and mimics potassium. It basically competes with the potassium in the water for a place in the cell walls of plants and primary providers.[8] It is expected that less than 1% of the cesium in the ocean will adhere to plankton. A few research studies shortly following the meltdown measured the quantities of radiocesium in the ocean. Kitamura et al (2013) detected radiocesium in water and zooplankton at 10 stations located 500-2100km from the Dai-ichi nuclear power plant.[9, 10] And Cs and AG110m isotopes were found in phytoplankton and zooplankton sampled 30–600 km off Japan in June 2011.[11] Radiocesium concentrations were highest around the transition area between the Kuroshio and the Oyashio currents. Studies at Stanford detected cesium-137 in Pacific Bluefin Tuna (PBFT) caught off of San Diego four months after the meltdown [12]. Bluefin is a migratory fish, beginning its life off of Japan’s coasts, and then migrating east toward the US west coast as it matures.

PBFT captured off of Japan in late 2011 were shown to have 15 times the radiocesium levels of those caught off of California. Other fish had much higher concentrations. As a result, many inshore fisheries off of Japan are closed to fishing right now.

Since radiocesium adheres to vertical migrators and sinking substances, benthic marine biota around the disaster area have shown elevated concentrations of radionuclides (though there is a lack of thorough monitoring on this stratum of the ocean).

So far only migratory piscivores, like PBFT (and they are exceptionally migratory), have proven to transport the isotopes across the Pacific. The rest will sink or disperse on ocean currents. The uncollected 137-C is dispersing across the Pacific Ocean at a slow rate. An MIT model (Finite Volume Community Ocean Model, or FVCOM) estimates that the plume will reach the west coast of the US around 5 years after the meltdown, and at that time will be diluted by four orders of magnitude.[13] Its presence is estimated to double the amount of anthropogenic radioactive substances off the west coast (cesium-137, uranium, and other radioisotopes were already present in the ocean, due to fallout from hydrogen bomb testings in the 1960s). That is to say, there are already radioactive substances in the water – both naturally-occurring and anthropogenic. Doubling the level of anthropogenic radioactive substances will raise the overall radioactive levels on the west coast less than 1%. Buesseler expects that “Fukushima contaminants in the ocean will be many thousands of times lower after they mix across the Pacific and arrive on the West Coast of North America some time in late 2013 or 2014.”[14]

Possible effects on marine biota and humans

The FDA establishes acceptable levels for radioactive substances found in water and food. The amount found in the bluefin caught off of San Diego were, according to the FDA, “roughly 300 times lower than levels that would prompt FDA to investigate further to determine if there were a health concern.”[15] It’s been reported by Fisher et al. that at these levels, a person who eats it multiple times per day (roughly five times more than the average person) would be exposed to the same level of radiation in a year’s time as that of one dental x-ray.[16] And at this rate, this would result in 2 out of 10,000,000 similar eaters of bluefin getting cancer as a result of the exposure. The Stanford team noted, anecdotally, that the amount of radioactive substances found in one serving of bluefin tuna (7.8 nanosieverts) is 5% of what is naturally occurring in a single banana.

Radiocesium concentrations around Fukushima are decreasing as time passes, and as the ocean disperses it, and as it decays, but it is also bio-accumulating in higher trophic biota. But even for those biota, both time and distance dilutes the concentrations. For example, as PBFT migrate, they grow in size and their concentrations of Cs diminish; and they also excrete some of the assimilated isotopes; and the isotopes decay (cesium-137 has a half-life of 30 years, but a biological half-life of 70 days).

PBFT generally migrate east across the Pacific Ocean in their first year or early in their second. It was estimated that the ones caught off San Diego took 4 months to cross the Pacific, and had been exposed to radiation for less than a month. They also tested bluefin a year later, in August 2012, and found that they contained less than half the levels of Cs isotopes that were tested in August 2011.[12] The researchers consider the presence of cesium-137 to be a great way to track migratory patterns of fish.

According to the available data, so far it has been appropriate to close fisheries in some areas around Japan and to regularly monitor radiocesium concentrations in the marine biota. Some organisms seem to escape ingestion, while others ingest it at high concentrations. Due to the short half-life of cesium-134 (2 years), it’s easy to tell if the radionuclides being detected are from continued run-off, or from the initial surge. Additionally, it’s important to gather better data on exactly what substances have been discharged from Fukushima, and where (atmosphere, land, ocean). The Woods Hole Oceanographic Institution estimates that 80% of discharged radiation from Fukushima has ended up in the ocean.[17] But due to the large size of the Pacific Ocean coupled with its tendency to diffuse materials, the concentrations of radionuclides released from the Fukushima Dai-ichi meltdown do not pose a threat to human health outside of the immediate disaster area. And the concentrations in high trophic biota are not bio-accumulating at a rate that will pose a significant increased health risk if consumed.


  1. World Nuclear Association. Naturally-occurring radioactive materials (NORM). Retrieved 11/11/13.

  2. Snyder M. 28 Signs That The West Coast Is Being Absolutely Fried With Nuclear Radiation From Fukushima. Retrieved 11/11/13.

  3. Thaler AD. 28 fallacies about the Fukushima nuclear disaster’s effect on the US West Coast. Retrieved 11/8/13.

  4. Rothschild M. More Fukushima Scaremongering Debunked. Retrieved 11/8/13.

  5. National Geographic. Fukushima’s radioactive water leak: what you should know. Retrieved 11/12/13

  6. Kanda J. Continuing 137Cs release to the sea from the Fukushima Dai-ichi Nuclear Power Plant through 2012. Biogeosciences Discuss., 10, 3577-3595, 2013.

  7. Buesseler K, Aoyama M, Fukasawa M. Impacts of the Fukushima nuclear power plants on marine radioactivity. Environmental Science & Technology, 2011, 45.

  8. Rowan DR, Rasmussen JB. Bioaccumulation of radiocesium by fish: the influence of physiochemical factors and trophic structure. Can. J. Fish. Aquat. Sci. 1994, 51.

  9. Kitamura M, Kumamoto Y, Kawakami H, Cruz EC, Fujikura K. Horizontal distribution of Fukushima-derived radiocesium in zooplankton in the northwestern Pacific Ocean. Biogeosciences Discuss., 10, 5729-5738, 2013.

  10. Kaeriyama H, Ambe D, Shimizu Y, et al. Direct observation of 134Cs and 137Cs in surface seawater in the western and central North Pacific after the Fukushima Dai-ichi nuclear power plant accident. Biogeosciences Discuss., 10, 1993-2012, 2013.

  11. Honda MC, Kawakami H, Watanabe S, Saino T. Fukushima-derived radiocesium in western North Pacific sediment traps. Biogeosciences Discuss., 10, 2455-2477, 2013.

  12. Madigan DJ, Baumann Z, Fisher NS. Pacific bluefin tuna transport Fukushima-derived radionuclides from Japan to California. Proceedings of the National Academy of Sciences of the United States of America, 2012, 109:24.

  13. MIT. Modeling the spread of radioactivity in seawater. Retrieved 11/11/13.

  14. Buesseler K. FAQ: radiation from Fukushima. Woods Hole Oceanographic Institution. Retrieved 11/12/13.

  15. U.S. Food and Drug Administration. FDA Response to the Fukushima Dai-ichi Nuclear Power Facility Incident. Retrieved Nov. 11, 2013.

  16. Fisher NS, Beaugelin-Seiller K, Hinton TG, et al. Evaluation of radiation doses and associated risk from the Fukushima nuclear accident to marine biota and human consumers of seafood. Proceedings of the National Academy of Sciences of the United States of America, 2013, 110:26.

  17. Pacchioli D. Radiation health risks: how can we assess impacts of exposures? Oceanus Magazine. Retrieved 11/12/13.

Feel free to ask me any questions!

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Have you found any on going monitoring? Growing up in Utah, milk was monitored for radioactive iodine - with about a 9day half-life x10 = 90 days to dissipate. Why isn’t USA and Canada monitoring milk?

Have you read in the last few months (11-2013 to 2-2014) of TEPCO under reporting current radiation contamination levels? Cesium is at least twice as high and Strontium 90 five times higher. See:

Sadly, we are not being told what is happening and more importantly we are not monitoring, so we have no idea what the risks are. To learn more see:

Above is a really great source of basic information I encourage everyone to watch/listen to this two day seminar.

Thanks. Yes, I know there were three. My paper wasn’t focusing on the meltdowns, so I tried to simplify that part. Maybe I simplified it too much.

And I hadn’t found evidence that the cores melted through the containment vessels. Last I saw, the tops were blown off two, and the cores were melting into the containment vessels, but it wasn’t clear that they had gone all the way through. I think the data I looked at was one year old.

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Founded 2005. Over the years I've posted writing, comics, ringtones, and stuff about maps, bikes, programming, pinball. And I had a robust music blog mostly about '90s hardcore punk (category = music).