Concentrations of the artificial radionuclide caesium-137 in Baltic Sea fish and surface waters

	Authors: HELCOM MORS Project Group

The indicator shows the concentrations of the man-made radionuclide Cs-137 in herring, flounder and plaice from the Baltic Sea since 1984.

Key message

Overall the levels of radioactivity in the Baltic Sea water, sediments and biota have shown declining trends since the Chernobyl accident in 1986, which caused significant fallout over the area. Radioactivity is now slowly transported from the Baltic Sea to the North Sea via Kattegat. The concentrations of Cs-137 in water as well as in herring and flounder muscle are still above the pre-Chernobyl levels in major part of the Baltic Sea.

Minor amounts of radioactivity from Sellafield are transported in the opposite direction. Routine discharges of radioactivity from nuclear power plants into the Baltic Sea area are small and only detectable locally.

Results and assessment

The most significant source of artificial radioactivity in the Baltic Sea is the fallout from the Chernobyl accident. The direct total input of Cs-137 from Chernobyl to the Baltic Sea was estimated at 4700 TBq.   Secondary riverine input from Chernobyl fallout added further 300 TBq of Cs-137.

Other important sources are global fallout from atmospheric nuclear weapons tests performed during the late 1950s and early 1960s and discharges from nuclear reprocessing plants in Western Europe, Sellafield and La Hague. The latter sources have become of minor radiological importance, due to significant reduction of discharges from Sellafield in the past two decades.

The predominant radionuclide in terms of activity in the discharges from the nuclear power plants and research reactors in the Baltic Sea region is H-3. The total discharges of H-3 from local sources have amounted to 2400 TBq and those of other beta-gamma emitting radionuclides to about 20 TBq. The total discharges of alpha-emitting radionuclides have been 0.0001 TBq.

The dose rates to individuals from the regions of the Bothnian Sea and Gulf of Finland are predicted to be larger than from any other area in the Baltic Sea due to the pattern of fallout from the Chernobyl accident. The dose rates are predicted to have peaked in 1986 at a value of 0.2 mSv y^-1.

The total collective dose from man-made radioactivity in the Baltic Sea is estimated at 2600 manSv of which about two thirds (1700 manSv) originate from Chernobyl fallout, about one quarter (650 manSv) from fallout from nuclear weapons testing, about 8% (200 manSv) from European reprocessing facilities, and about 0.04% (1 manSv) from nuclear installations bordering the Baltic Sea area.

Dose rates and doses from natural radioactivity dominate except for the year 1986 where the individual dose rates from Chernobyl fallout in some regions of the Baltic Sea approached that from natural radioactivity.

The maximum annual dose since 1950 to individuals from any critical group in the Baltic Sea area due to artifical radioactivity is estimated at 0.2 mSv y^-1, which is below the dose limit of 1 mSv y^-1 for the exposure of the general public set out in the EU Basic Safety Standards, 1996. It is unlikely that any individual has been exposed from marine pathways at a level above this dose limit considering the uncertainties involved in the assessment. Doses to man due to liquid discharges from nuclear power plants in the Baltic Sea area are estimated at or below the levels mentioned in the Basic Safety Standards to be of no regulatory concern (individual dose rate of 10 µSv y^-1 and collective dose of 1 manSv). It should be noted that the assumptions made throughout the assessment were chosen to be realistic and not conservative. Consequently, this also applies to the estimated radiation doses to man.

Figure1. ¹³⁷Cs concentrations (in Bq/kg) in herring muscle in 1984-2004, as annual mean values by basin. Target values have been calculated as averages of pre-Chernobyl (1984-1985) concentrations. (Note: variable scales in the graphs. Click image to enlarge. Click on the following sub-basins to view relevant plot in more detail: Bothnian Bay, Bothnian Sea, Archipelago Sea, Gulf of Finland, Northern Baltic Proper, Bornholm Sea and Southern Baltic, Arkona, Sound and Belt Sea and Kattegat.)

Figure 2. ¹³⁷Cs concentrations (in Bq/kg) in plaice and flounder muscle in 1984-2004, as annual mean values by basin. Target values have been calculated as average of pre-Chernobyl (1984-1985) concentrations. (Click on the following sub-basins to view relevant plot in more detail: Kattegat, Gotland East and West, Arkona, Sound and Belt Sea, and Bornholm Sea and Southern Baltic.)

Figure 3. ¹³⁷Cs concentrations (in Bq/kg) in surface water (sampling depth <=10m) in 1984-2004, as annual mean values by basin. Target values have been calculated as average of pre-Chernobyl (1984-1985) concentrations. (Note: variable scales in the graphs. Click image to enlarge. Click on the following sub-basins to view relevant plot in more detail: Bothnian Bay, Bothnian Sea, Archipelago Sea, Gulf of Finland, Gulf of Riga, Northern Baltic Proper, Gotland East and West, Bornholm Sea and Southern Baltic, Arkona, Sound and Belt Sea and Kattegat)

Meta data

Technical Information

1) Source

Data have been collected by the Contracting Parties of HELCOM and submitted to the MORS (Monitoring of Radioactive Substances) database. The database is located in Finnish Environment Institute (SYKE).

2) Description of data

The data are based on ¹³⁷Cs concentrations of a) herring (Clupea harengus L.), b) flounder (Platichthys flesus L.) and plaice (Pleuronectes platessa L.) and c) surface seawater (samples 0-10 m). Analyses have been made either from round fish (without head and entrails) or filets (herring), and for plaice and flounder from filets, only. Concentrations have been calculated from wet weight of the samples.

Sea water concentrations have been analyzed from water sampled between 0-10 meters.

Data of each media (herring, plaice and flounder and sea water) have been averaged by basin and by year.

Average concentrations before the Chernobyl accident have been used as target values. These are:

• herring: 2.5 Bq/kg

• flounder and plaice: 2.9 Bq/kg

• seawater: 14.6 Bq/m³    

3) Spatial and temporal coverage

Herring data covers all the areas except Gulf of Riga and the area of Gotland East and West as there are data only of 2003-2004. Only a few years are missing.

Plaice and flounder data are very scarce both temporarily and spatially covering only four sea areas and several years missing. Sampling on plaice and flounder takes place only in some of the countries.

Sea water data coverage is almost complete, except the missing years in the Gulf of Riga and in the Archipelago Sea.

4) Missing data

Biota data

Russian biological data are all missing and Latvian and Lithuanian data are missing in 2004.

Seawater data

Swedish seawater data are missing throughout the years (1984-2004) as no sampling has been carried out.

For reference purposes, please cite this indicator fact sheet as follows:

[Author’s name(s)], [Year]. [Indicator Fact Sheet title]. HELCOM Indicator Fact Sheets 2006. Online. [Date Viewed], http://www.helcom.fi/environment2/ifs/en_GB/cover/.

Last updated:  20.9.2006