Radioactive cesium in Tokyo Bay

In the summer of 2021, Japan will most likely host the delayed Summer Olympics. In addition to the many sporting disciplines on land, rowing and canoeing championships will also be held in Tokyo Bay. With the ongoing Fukushima nuclear disaster and radioactive contamination of large parts of the country with radiating particles such as cesium-137, many people are asking about the radiation exposure of Tokyo Bay. A 2018 study examined this very question and came up with some disturbing results.

Tokyo Bay covers an area of approximately 1,380 square kilometers (70 km from north to south and 20 km from east to west), is on average 15 meters deep and is connected to the Pacific Ocean by a 7 km wide strait. The bay is surrounded by major cities such as Chiba, Tokyo and Yokohama, which are home to a total of 38 million people and are the economic and political center of Japan.

Seven rivers feed the bay: Edogawa, Old Edogawa, Sakagawa, Arakawa, Tamagawa, Sumidagawa and Tsurumigawa. The "Sea Forest Waterway" regatta course, which will host the rowing and canoeing championships of the Summer Olympics, is located on a small artificial island offshore from the city of Tokyo (point '21' on Fig. 1), originally made of heaped up waste and designed as a breakwater for the inner city. It is located exactly between the mouths of the Arakawa River, which flows through the eastern suburbs of Tokyo, and the Sumidagawa River, which flows through downtown Tokyo.

Fig. 1: Olympic sites around Tokyo Bay

Between 2011 and 2016, a group of scientists from Osaka, Kyoto and Shiga Universities studied water and sediment samples in and around Tokyo Bay. In addition to the concentrations of cesium 134 and cesium 137 in surface and deep water layers, they also examined the radiation levels of sediment samples down to a depth of one meter and compared the levels over the course of five years.

They also based their research on measurements from Japan's Ministry of Science, MEXT, which published numerous studies of surface contamination following the Fukushima nuclear disaster that showed, among other things, a high rate of radioactive precipitation over the northeastern suburbs of Tokyo and the watershed of the Edogawa River system. This region received about the same amount of radioactive precipitation as Fukushima City (30-100 kBq/m2) in the days between March 16-22, 2011, due to northerly winds blowing radioactive clouds toward Tokyo. Of the cesium particles dispersed by radioactive clouds over this area, a substantial portion (about 10-22%) reached Tokyo Bay via the Edogawa, Old Edogawa, and Sakagawa rivers, and to a lesser extent via the Arakawa, Tamagawa, Sumidagawa, and Tsurumigawa rivers. The radioactive substances are deposited mainly in the estuaries of rivers, since the flow velocity of the water decreases here and the particles sink from the water surface into the river bed.

Fig. 2: Concentration of radioactive cesium as a result of the multiple nuclear meltdowns of Fukushima (Nov 1, 2011)

In total, the scientists repeatedly sampled 77 sites in Tokyo Bay, 10 sites in the Edogawa River, and 6 sites in the Sakagawa River over a 5-year period. These samples were analyzed for radioactive cesium (Cs-134 and Cs-137) using gamma spectrometry. In the water samples, the scientists found concentrations in surface water to be twice as high as in lower water layers. The measurements in the course of the rivers increased steadily up to the estuary, then decreased continuously the further one got into the bay. Activity of cesium-134 and cesium-137 was highest in the Old Edogawa River estuary (Zone X, see Fig. 3), where about 70% of the total inventory of radioactive cesium in the bay is located, and decreased steadily through Zones Y, W, and V to Zone Z. The highest activity of cesium-134 and cesium-137 was measured in the Old Edogawa River estuary (Zone X, see Fig. 3), where about 70% of the total inventory of radioactive cesium in the bay is located. Zone W contains the regatta course "Sea Forest Waterway" (near measuring point 57).


Activity Cs134/Cs137


X (Estuary of Old-Edogawa)

78-1340 Bq/kg

424 Bq/kg

Y (Confluence of Old-Edogawa/Sumidagawa)

40-371 Bq/kg

131 Bq/kg

W (Estuary of Sumidagawa)

32-374 Bq/kg

103 Bq/kg

V (Estuary of Tamagawa)

5-234 Bq/kg

57 Bq/kg

Z (Central Tokyo Bay)

1-162 Bq/kg

17 Bq/kg

Tab. 1: Cesium concentrations Konzentration in Tokyo Bay


Fig. 3: Examination zones in Tokyo Bay

It must be kept in mind that some amount of radioactive cesium was already present in Tokyo Bay before the Fukushima nuclear disaster: due to the bombing of Hiroshima and Nagasaki in 1945, hundreds of above-ground nuclear weapons tests in the Northern Hemisphere from 1946-1980 and nuclear disasters such as the Mayak explosion in 1957 or the Chernobyl meltdown in 1986. However, due to the different decay rates and the different ratios of cesium-134 and cesium-137, this "old" contamination can be distinguished from "newer" contamination. For comparison: in the German North Sea, the radioactive caesium contamination is 1-15 Bq/kg, while in the German Baltic Sea, which was significantly more contaminated by Chernobyl, it is 1-140 Bq/kg ("Environmental Radioactivity and Radiation Exposure Annual Report 2005", Federal Ministry for the Environment, Nature Conservation and Nuclear Safety, p.46), i.e. roughly comparable to the contamination in the center of Tokyo Bay. The following figure shows the values for radioactive cesium concentrations on March 16, 2011:

Fig. 4: Activity of radioaktive cesium at different measurement points

The scientists also examined sediment samples with the help of boreholes. Of particular interest in these investigations is the temporal component. The following figure illustrates the chanegs of the deposits of radioactive cesium over time: In August 2011, the highest radiation concentration was found in the bottom sediment at a depth of 5 cm. In October 2013, the highest radiation levels were already at a depth of 13 cm, and in 2014 between 10 and 30 cm. Heavy rainfall in September 2015 pushed the radioactive cesium from Fukushima into even deeper layers: to a depth of 70 cm in July 2016.

Fig. 5: Concentration of radioactive cesium at different sediment depths 2011-2016

Based on the concentration ratios of Cs-134 to Cs-137, the scientists were also able to determine that radioactive substances continuously flowed into Tokyo Bay via the Old Edogawa River over the entire five years of the study period, albeit to a lesser extent than at the beginning: while in August 2011, an additional 0.13 kBq/m2 of radiation flowed into the bay every day, in 2016 this figure was only 0.053 kBq/m2 per day. Accordingly, the radioactive inventory in the estuary increased continuously during this period: from an initial 20 kBq/m2 in 2011 to 104 kBq/m2 in July 2016. According to the authors of the study, it can be assumed that new radioactive radiation will continue to accumulate in the future, because the reservoirs of soil contamination in the watersheds of the tributary rivers are far from exhausted.

A recent study by scientists from Tokyo and Shizuoka Universities re-examined the radioactivity of Tokyo Bay in 2019 and this time found elevated radiation levels (71-163 Bq/kg, mean value of 117 Bq/kg) especially in the Arakawa River estuary, while only 4-36 Bq/kg (mean value of 20 Bq/kg) were measured at other monitoring points in the bay. The mouth of the Arakawa River is in close proximity to the Olympic Games regatta course.

These two studies illustrate the complexity of radioactive particle migration after a nuclear catastrophe and help us better understand the health and environmental consequences of the nuclear disaster. Thanks to the scientists' work, we now know that over many years, radioactive particles from contaminated sites entered Tokyo Bay via groundwater and major rivers, and that substantial amounts of radiation accumulate, especially at the mouths of rivers. How long the radioactive inventory of Tokyo Bay will continue to increase, how the flow and sedimentation rates of radioactive cesium will develop in the coming years, and what consequences these developments could have for the flora and fauna as well as the residents of the bay, for fishermen, water sports enthusiasts, and ultimately also the consumers of marine products, are questions which scientists do not yet have all the answers to.

What is clear, however, is that Tokyo Bay has become a depository for radioactive fallout originating in the region northeast of Tokyo after the multiple Fukushima meltdowns in 2011. It is also clear that the Olympic rowing and canoeing championships will be held at a regatta course located not far from the river mouth where the highest radiation levels in the entire bay were measured. Although the majority of radioactive particles should now be in a layer of sediment about 1 m deep, it cannot be ruled out that seismic activity or severe storms could resuspend radioactive materials and cause them to rise to the surface. Careful monitoring of radiation levels in and around the bay is therefore still necessary, not least before the first canoeists enter the water in July.



Dr. med. Alex Rosen
Pediatrician and Co-Chair of the German Section of IPPNW



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