Updated summary of knowledge concerning the impact on
the marine environment of radioactive release from thedamaged nuclear site of Fukushima Dai-ichi
13 July 2012

This memo presents and comments on the latest information gathered by IRSN since the previous
summary published on 26 October 2011 on the same subject and IRSN report entitled “Fukushima,
one year later – Initial analyses of the accident and its consequences” published on 12 March 2012.
As a reminder, the Fukushima accident led to a pollution of the marine environment caused by two
main sources:-

Radioactive liquids discharged directly into the ocean initially composed of various
radionuclides. These were mainly iodine-131, iodine-132, tellurium-132 and caesium-134,
caesium-137 and caesium-136. IRSN estimates that more than 80% of these discharges were
produced between 21 March and 8 April 2011, and 27.1015 Bq of caesium-137 weredischarged up to July 2011;-

Polluting atmospheric deposition on the surface of the ocean represents approximately 10%of the total radioactive discharges into the ocean, according to the latest IRSN estimates
described in this memo,

From June 2011 onwards, caesium-134 and caesium-137 were the only remaining detectableradionuclides in seawater, and their concentrations off the coast beside the Fukushima nuclear
power plant have been steadily decreasing over the course of 2011 as they disperse in the ocean.
These concentrations declined more slowly in the second half of 2011 and the first half 2012,
however, indicating that radioactive caesium is still regularly entering the ocean along the coast,
particularly in the immediate vicinity of the damaged nuclear plant. This is the result of discharges
from the Fukushima plant, which are significantly lower than those in March 2011, and intake fromrun-off due to leaching of the contaminated land.
In the coastal area, the sediment was initially contaminated through direct contact between
surface and polluted seawater; the contamination levels now change as a result of the intake of
sediment particles by the rivers, particularly after rainy periods.
Significant concentrations of radioactive caesium are still measured in various marine species (fish,
molluscs and sea urchins) caught in the coastal waters of the Fukushima, Miyagi and Ibaraki
prefectures, sometimes exceeding the food standards which were tightened on 1 April 2012. These
concentrations are gradually decreasing in filter-feeder species such as mussels, oysters and clams
or grazing organisms such as sea urchins and abalones, but they are virtually unchanged in
sedimentary species (ainames, halibuts, gurnards, rays, rockfish, etc.). As a result, the monitoringof marine species caught in the north-eastern coastal waters of Japan should be continued.
The scientific publications of 2011 and 2012, as well as the analyses conducted by IRSN on a wahoosample caught in October 2011 to the north of one of the Hawaiian islands (Oahu), show that tracesof caesium-134, which can be attributed to the Fukushima accident, can be found in pelagic species(such as some species of tuna) that were in the waters off the Japanese coast at the time of theaccident and then migrated over large distances (to the Californian coast, for example). Although
the levels found in these migratory species are very low and not dangerous to human, thecontinuation of such studies would provide a better understanding of the importance of thistransport on the oceanic scale.

1/13

1. CHANGES IN THE RADIOACTIVE POLLUTION IN SEAWATER
1.1. Main radionuclides observed in seawater since 21 March 2011
On 21 March 2011 and the following days, considerable radioactive pollution was detected in themarine environment near to the Fukushima Dai-ichi nuclear power plant. This pollution was mainly
characterised by means of measurements conducted on samples of seawater, sediment and species
living in the marine environment. The results have been published by MEXT1 and TEPCO. IRSN, which
could not conduct measurements in Japan, has regularly collected and analysed these results inorder to monitor changes in the radioactive pollution in the marine environment. The latest
measurements date from June 2012.
The measurement results published in Japan mainly concern the gamma-emitter radionuclides listed
in Table 1.

Table 1 – Main radionuclides measured in seawater
Main radionuclides detected
Radionuclide

Half-life

Iodine-131 (131I)
Caesium-137 (137Cs)
Caesium -134 (134Cs)
Caesium-136 (136Cs)
Tellurium-132 and Iodine-132 (132Te and 132I)

8 days30.15 years
2.1 years
13.1 days
78 hours

Other artificial radionuclides, most of which have a short half-life, have also been detectedoccasionally, but at lower concentrations. There are fewer measurements concerning pure betaemitter radionuclides: Nine results for strontium-90 in seawater, at concentrations ranging from
1 to 10 Bq/L, representing between 1 and 20% of the radioactivity of caesium-137 measured in thesame samples, i.e. a higher proportion than that observed in the atmospheric fallout from theaccident onto land areas of Japan, which is in the region of 0.1%.
Iodine-131 (131I) and caesium-137 (137Cs) are the main radionuclides monitored in 2011 in the marine
environment. Although it was predominant at the time of the accident, the amount of iodine-131
declined sharply in the following weeks as a result of its rapid radioactive decay. As a result, it has
no longer been detectable since the end of May.
This radioactive pollution has had two main sources:1-

Liquid radioactive discharges from the damaged site: in interpreting the seawaterradioactivity measurement results published in Japan, IRSN has estimated an overall
discharge of 27.1015 Bq of caesium-137 into the sea between March and July 2011, themajority of which (82%) occurred before 8 April 2011, the date on which a rapid and regularreduction (50% every 6.9 days) was observed in the caesium-137 levels detected in the
coastal waters near to the damaged plant up to summer 2011;-

Atmospheric fallout on the surface of the sea when the radionuclides are discharged
into the air: the resulting diffuse pollution has been difficult to single out in the sea
because the surface water receiving the atmospheric fallout rapidly mixes with the rest of

Ministry of Education, Culture, Sports, Science and Technology in Japan.

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the seawater as the result of advection and dispersion. Only the concentrations measured inseawater more than 10 km away from the plant before 24 March 2011 (or a little later,
further away), when the direct liquid discharges were still relatively low, can be attributed
to these atmospheric discharges. IRSN’s work in 2011 to estimate the atmosphericdischarges caused by the accident and model their atmospheric dispersion has led it torevise its estimate of the cumulative deposition of caesium-137 on the sea within an 80-km
radius to 2.6x1015 Bq, representing approximately 10% of the overall activity of caesium-137
directly discharged into the sea by the Fukushima Dai-ichi plant.
With regard to the atmospheric fallout onto the sea, it should be emphasised that the air
contamination or the depositions onto the marine environment are not measured directly, making
any precise estimate of this intake impossible, unlike the Japanese terrestrial environment, inwhich many environmental measurements are performed. IRSN’s study of caesium-137
concentrations in seawater and its modelling of the radionuclide’s dispersion in the Japanese
coastal waters seem to support its latest estimates of atmospheric fallout, however (see paragraph
1.3, below). In any event, the existing uncertainties regarding the estimated fallout should be
relativised, as this fallout forms only a secondary part of the radioactive pollutants entering themarine environment during the course of the accident, which mainly consisted of liquids directly
discharged from the Fukushima Dai-ichi plant.

1.2. Changes in seawater contamination on the coast near to the Fukushima
Dai-ichi plant
The measurements performed near the power plant revealed considerable contamination of themarine environment from 21 March 2011 onwards as a result of highly-contaminated water flowing
from the damaged reactors into the sea.
From June 2011 onwards, caesium-134 and caesium-137 remained the only detectable radionuclides
in the seawater, with an activity ratio of approximately 1 relative to the date of the accident. The
temporal and spatial changes in the caesium-137 concentrations are representative of those of all of
the radionuclides measured in the sea.
Figure 1 shows the changes in the concentrations measured near to the damaged plant between theend of March 2011 and the beginning of May 2012, where the detection limit was revised from
10 Bq/L to 1 Bq/L in October 2011; these concentrations are representative of the flow ofradionuclides discharged by the plant. Although there are still some significant values, increasing
numbers of the concentrations are below the detection limit. The levels decrease regularly over thecourse of time, with an apparent decay period of approximately two months starting from June
2011. The same is true of the measurements performed along the coast at a distance from thedamaged plant, albeit with a slightly longer apparent decay period of approximately three months.
These kinetics are much slower than those identified immediately after the direct discharges. They
reflect a continuous yet declining intake of caesium-137 in solution into the seawater. At a distancefrom the coast, the concentrations are approximately 0.01 Bq/L, five times higher than thosemeasured before the accident.

3/13

1E5

Source: IRSN, based on TEPCO and MEXT
measurements

Caesium-137 (in Bq/l)

1E4

1E3

1E2

1E1

1E0

Mar – 20110

May – 2011

Jul – 2011

Sep – 2011 Nov – 2011

Jan – 2012 Mar – 2012

May - 2012

Caesium-137

Figure 1 – Changes in 137Cs concentrations in seawater within 500 m of the Fukushima Dai-ichi
nuclear power plant
Figure 2 shows the spatial distribution of average caesium-137 concentrations between January and
May 2012. This map shows that concentrations have decreased by a factor of approximately 1,000
since the accident. The source of this residual contamination seems to be spread along the coast
50 km to the north and south of the damaged plant. The distribution’s similarity with that of thesediment contamination (see map in Figure 5 in Chapter 2) suggests that the caesium comes from asimilar source. It may be due to the leaching of soils contaminated by meteoric water, bringing a
steady flow of water loaded with caesium to the sea, or to the release of the caesium deposited on
the sediment in 2011.
The changes in the radionuclide concentrations in the seawater indicate that radioactive
caesium is still regularly entering the sea along the coast, particularly in the immediate vicinityof the plant. This is the result of discharges from the nuclear power plant and run-off due toleaching of the contaminated land.

4/13

137

Cs in seawater, January -> May 2012

Figure 2 – Spatial distribution of average concentrations in 137Cs in seawater,deduced from all measurements available between January and May 2012

1.3. Simulation of caesium-137 dispersion in seawater off the coast of Japan
IFREMER was approached by IRSN to run simulations of discharge dispersion from Fukushima Daiichi. It used the Mars-3D model, Mercator-Ocean hydrodynamic limit conditions and NCEP
meteorological forcing.
The caesium-137 direct discharge measurement used in this simulation is taken from thecalculations shown in the previous summaries and in IRSN’s report “Fukushima, one year later”
published on 12 March 2012: 27 PBq.
As stated in paragraph 1.1 above, the caesium atmospheric fallout onto the surface of the sea
within 80 km of the Fukushima Dai-ichi nuclear power plant has been revised to 2.6 PBq as a resultof IRSN’s work in 2011 to estimate the atmospheric discharges caused by the accident and model
their dispersion (see IRSN’s report “Fukushima, one year later” published on 12 March 2012). These
discharges are mainly in the form of wet deposits (during rain), and their spatial distribution
estimated by IRSN by means of modelling is shown on the map in Figure 3.
When the dispersion of the caesium 137 discharged into the sea (direct liquid discharges andatmospheric fallout) was modelled, IRSN adopted the hypothesis that the radionuclide was solely insoluble form.

5/13

38.2

Total wetdepositions of 137Cs

38.1
38

4.0E+006
3.8E+006

37.9

3.5E+006

37.8

3.3E+006

37.7

3.0E+006

37.6

2.8E+006

37.5

2.5E+006

Fukushima
Dai-ichi

37.4

2.3E+006

37.3
2.0E+006

37.2

1.8E+006

37.1
371.5E+006

36.9

1.3E+006

36.8

1.0E+006

36.7

7.5E+005

36.6

5.0E+005

36.5

N

36.4
36.3

0

20

2.5E+005

Source: IRSN
0.0E+000

40

60

80

100 kmBq.m-236.2
140.1 140.35 140.6 140.85 141.1 141.35 141.6 141.85 142.1 142.35

Figure 3 – Distribution of the total atmospheric fallout of 137Cs onto the sea as at 23 March
2011, estimated by IRSN by modelling the atmospheric dispersion of discharges from the
Fukushima Dai-ichi accident
The caesium-137 marine dispersion model has been modified in order to reproduce the renewal
kinetics of the water masses deduced from the caesium-137 measurements (Figure 4). The model
does not tend to underestimate or overestimate the concentrations (see Figure 5); the average
difference between the simulated and measured values is 55%. The obtained results, whencompared with the measured caesium-137 concentration values in seawater at a distance from thedamaged nuclear power plant or even further from the coast, also confirm the estimatedcaesium-137 quantities resulting from atmospheric fallout.

6/13

Quantities of dissolved caesium-137 (Bq)

1E17

1E16

1E15

1E14

1E13

1E12
20/03

30/03

09/04

19/04

29/04

09/05

19/05

29/05

08/06

18/06

28/06

Date
Environmental half-life of 6.9 days

Simulated quantities
Measured quantities

Figure 4 – Quantities of 137Cs in the measured and Mars-3D-simulated seawater between
15 April and 26 July 2011 within a 50-km radius of the plant

Cs137

1E5

1E4

Mars simulation

1E3

1E2

1E1

1E0

1E-1

1E-2
1E-2

1E-1

1E0

1E1

1E2

1E3

1E4

1E5

Measurem

Figure 5 – Comparison of measured and simulated 137Cs concentrations
in seawater between March and June 2011

7/13

2. RADIONUCLIDES IN SEDIMENT
The materials in suspension in the seawater tend to fix some of the radionuclides dissolved in thewater column according to the levels of radioactivity in the surrounding water. These particles end
up as sediment on the sea floor, creating a contaminated superficial deposit.
Figure 6 shows a map of caesium-137 concentration distribution in sediment. The actual
concentrations are generally less than 1000 Bq/kg. In the coastal area, the sediment was initiallycontaminated through direct contact between its surface and the polluted seawater; its
contamination level now changes as a result of sediment particles carried by the rivers, particularly
after rainy periods.
The finer caesium-bearing particles found in the sedimentary mix are likely to carry the adsorbed
radionuclides for long distances.
Benthic organisms living in direct contact with sediment, together with filter feeders, are mostseriously affected by the residual pollution in sediment.

38.5

8.7

5.1

3000
2100
1500

15

19
14

1100
830

390

600

38

430
310

8480
12
20
21
25
12
15
32 20
250
44
17
18

Fukushima
37.5 Dai-ichi

54
27
70
81
500
32
63
1600
760
930
910
1800
1100
830
810

85
90

100

220
160

34

38
17

2.7

28
21
34
25

140

32

220
150
56
550
1400
240
330
200
170
160
2300 110
200
330
450
270
230
120
200
130 96
84

81

64

86

220
280
210
160

62

190 7492110
95
91 13013099660
150 52
250
160

37

230
140
210190200
290

87

2 0280

23

110

45
32

44
43
56

23
17
12
9
6
3

36.5
60

48

33

0

Bq.Kg-1
9

36

18: Sampling location

140.5
141
141.5
142
142.5
Figure 6 – Map of average 137Cs concentrations in sediment,
measured between January and May 2012

8/13

3. RADIONUCLIDES IN MARINE SPECIES
3.1. Marine species exceeding the maximum permitted consumption levels for
caesium
In the last few months and before 1 April 2012, the date on which the new, more stringent foodstandards were introduced by the Japanese authorities, the only marine organisms whose caesium
concentration exceeded the standard in force at the time (500 Bq/kg wet) were fish, all of whichwere from the Fukushima prefecture. No organisms caught in the sea outside the area close to thepower plant or off the coast exceed this standard.
After this standard was tightened to 100 Bq/kg wet, it was sometimes exceeded not only by various
species of fish, but occasionally also by clams and sea urchins. This concerns samples caught along
the coast of the Fukushima, Ibaraki and Miyagi prefectures.

3.2. Concentrations observed in marine animals
The graph in Figure 7 shows the measured caesium concentration levels in various marine species
between March 2011 and March 2012, which are published on a regular basis.
Caesium (Bq/kg wet)

AinameCongerHalibut
Gurnard

100000Japanese sand lance

10000

Poacher
Ray
RedfishHorse mackerel
MackerelSardineAbalones
Sea urchins
Clams

1000

100

10

1

0,1
18/0

1
3/20

1
07/0

1
5/20

1
26/0

1
6/20

1
15/0

1
8/20

1
04/1

1
0/20

1
23/1

1
1/20

1
12/0

1
1/20

2
02/0

1
3/20

2
21/0

1
4/20

2

Figure 7 – Changes over time of 137Cs+134Cs (Bq/kg wet) concentrations in some marine
products, between March 2011 and March 2012
Of all of the sampled marine products, the Japanese sand lances were found to have the highest
contamination levels, which were detected early in the fishery product monitoring programme.
Caesium-137 and caesium-134 were detected in all Japanese sand lance samples caught in the
Fukushima and Ibaraki prefectures; the maximum concentrations were 12,500 Bq/kg wet(134Cs+137Cs) off the Iwaki coast on 13 April.
The Japanese catch and eat Japanese sand lances (Ammodytes personatus) in their larval andjuvenile stages, both of which are pelagic (i.e. live in the water column) in the period from January

9/13

to April. The adults, however, live buried in the sediment from May to December and are no longer
caught, which explains why the data for this species practically vanish from late April 2011 onwards.
Monitoring was resumed at the beginning of 2012, when the pelagic Japanese sand lances (sandeels)
could be caught once more. Their levels range from 54 to 122 Bq/kg wet (134Cs+137Cs) in samples
from Iwaki city (Fukushima prefecture), while the samples caught off the coast of Soma city range
from 20 to 30 Bq/kg wet. These levels reflect the considerably lower concentrations in thesurrounding environment, as Japanese sand lances mainly feed on zooplankton, which is in strict
equilibrium with the environment in which it grows.
In addition to the concentrations found in Japanese sand lances (sandeels), Figure 7 shows theconcentration curves for both caesium isotopes in other regularly-sampled species. It is difficult tofind a trend in the contamination of fish over the course of time, given the wide range of results
obtained.
Overall, sedimentary fish (represented by orange symbols in Figure 7) such as ainames, halibuts,
gurnards, rays and rockfish, tend to have higher concentration levels than pelagic species(represented by bluish symbols in Figure 7) such as mackerel, sardines and horse mackerel;
In addition to fishes, it should be pointed out that the sea urchin, abalone, and clam samples caught
in the Fukushima prefecture sometimes also contained high levels.
Iodine-131 has not been detected in the organisms since mid-June 2011, reflecting its level in theseawater.

3.3. Expected changes for marine species
The concentrations found in filter feeders such as mussels, oysters and clams, or grazing organisms
such as sea urchins and abalones, are decreasing due to the absence of new discharges. Thechanging levels in these organisms over the course of time reflect the changes in the surrounding
environment quite well, and the trends clearly show the decreasing concentrations over time
(Figure 8).

10/13

Caesium in clams (Bk/kg-1 wet) – Iwaki city – Fukushima prefecture
134Cs

137Cs

Total Cs

1000
900
800
700
600
500
400
300
200
100
0

17

11
20
/
4
/0

06

11
20
/
6
/0

26

11
20
/
7
/0

14

11
20
/
9
/0

03

11
20
/
1
/1

23

11
20
/
2
/1

11

12
20
/
2
/0

01

12
20
/
4
/0

21

12
20
/
5
/0

Figure 8 – Changes in caesium concentrations in clams (Pseudocardium sachalinense) caught off
the Iwaki coast in the Fukushima prefecture over time, up to March 2012. The red lines
represent the maximum permitted levels for food consumption (solid line up to 1 April 2012:
500 Bq/kg wet; dotted line from 1 April 2012 onwards: 100 Bq/kg wet)

As expected, the change in fish contamination levels over time does not reflect the decrease
detected in the seawater. Figure 9 takes as an example the changes in concentrations in a sample
of a species of skate (Okamejei kenojei) caught in the coastal waters of the Fukushima prefecture.
A very slight trend towards a decline in activity seems to be emerging. The highest values are to befound on the Iwaki city coast. In fish, concentration variations could very well be partly due to their
movements in the case of species that are not strictly confined to the area in which they werecaught, but they also reflect their complex trophic regimes. In addition, the biological elimination
periods for caesium in fish are longer than in other marine organisms.

11/13

Caesium in ocellate spot skates (Bk/kg-1 wet) – Fukushima prefecture
Cs134

Cs137

Cs total

10000

1000

100

10

1
0
/2
06
/
16

11

0
/2
08
/
05

11

0
/2
09
/
24

11

0
/2
11
/
13

11

0
/2
01
/
02

12

0
/2
02
/
21

12

0
/2
04
/
11

12

Figure 9 – Changes (and trend) in radioactive caesium concentrations over time, in a species ofspot skate (Okamejei kenojei) caught off the Iwaki coast in the Fukushima prefecture. The redlines represent the maximum permitted levels for food consumption (solid line up to 1 April
2012: 500 Bq/kg wet; dotted line from 1 April 2012 onwards: 100 Bq/kg wet)
Generally speaking, fish will be the best medium- and long-term indicators of caesium
contamination in the marine domain. Caesium has higher concentration factors in fish, and theytend to be greater in species that are higher in the trophic chain. As a result, although in the shortterm the highest concentrations tend to be found in species at the beginning of the food chain, thepredators at the top of the food chain should exhibit higher levels in the longer term, once thecaesium has been transferred through the various links in the trophic networks. These levels shouldbe even higher in the case of species whose way of life involves close contact with sediment andwhose habitat is close to the contaminated area.
As a result, the monitoring of marine species caught in the north-eastern coastal waters ofJapan should be continued.

3.4. Other data published in the standard literature
Between 4 and 18 June 2011, American researchers organised a sampling campaign in the North
Pacific (Buesseler et al., 20122). As well as measuring concentrations in the water, they also provide
various results regarding plankton and mesopelagic fish samples. The caesium-134 and caesium-137
concentrations range from the detection limit to 56.4 Bq/kg dry, depending on where the samples
were taken and their type. The 137Cs/134Cs activity ratios are usually approximately 1.
It should be noted that silver-110m was detected in the plankton in almost all cases, at levels
ranging from the detection limit to 23.6 Bq/kg dry. This element was not detected in the fish,

2

Buessler K.O., Jayne S.R., Fisher N.S., Rypina I.I., Baumann H., Baumann Z., Breier C.F.,
Douglass E.M., George J., Macdonald A.M., Miyamoto H., Nishikawa J., Pike S.M., Yoshida S.,
Fukushima-derived radionuclides in the ocean and biota off Japan. Proc Natl Acad Sci USA,109:5984-5988.

12/13

however. The presence of this radionuclide is not mentioned in the marine discharges, but it was
detected in the soil samples from around the plant.
Recently, another scientific article (Madigan et al., 20123) presented measurement results obtainedin tuna caught off the Californian coast. In this work, samples of two tuna species were measured:
firstly, the Pacific bluefin tuna (Thunnus orientalis), which is a migratory species and secondly, theyellowfin tuna (Thunnus albacares), which is also a migratory species but tends to remain off thecoast of California and Mexico. Whereas the bluefin tuna samples revealed the presence of both
caesiums (137Cs and134Cs), the yellowfin tuna samples contained only caesium-137. The levels are
low, in the order of a few Bq/kg dry. The caesium-134 found in the bluefin tuna is the result of
exposure while passing through the waters contaminated by discharges caused by the Fukushima
nuclear power plant accident, off the coast of Japan. The absence of this radionuclide in theyellowfin tuna leads us to conclude that this species of fish caught off the Californian coast was not
exposed to discharges caused by the Fukushima accident but contains only the residual caesium-137
resulting from old fallout due to old nuclear weapon tests in the atmosphere and from the
Chernobyl accident. Migratory species are therefore a global vector of radionuclide transport acrossthe oceans. Even though these migratory species exhibit low radionuclide levels, the continuation ofsuch studies would provide a better understanding of the importance of this transport and themigration of these species.
During the Tara’s campaign in the North Pacific, a sample wahoo was caught for IRSN in October
2011 to the north of Oahu, one of the Hawaiian islands. This sample was analysed by IRSN’s
Environmental Monitoring Laboratory (LESE) in Tahiti; it revealed traces of caesium-134(approximately 0.05 Bq/kg dry), confirming that it was contaminated by the discharges caused by
the Fukushima accident.

These various results emphasise the usefulness of additionally monitoring migratory species in
the South Pacific. To quote only the example of Pacific bluefin tuna, fish have been observed
migrating from the North-West Pacific toward the waters of New Zealand.

3

Madigan D.J., Baumann Z., Fisher N., Pacific bluefin tuna transport Fukushima-derived
radionuclides from Japan to California. Proc Natl Acad Sci USA, doi:10.1073/pnas.1204859109

13/13