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 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. Minor amounts of radioactivity from Sellafield are transported in the opposite direction. Routine discharges of radioactivity from nuclear power plants in 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 total input of 137Cs from Chernobyl to the Baltic Sea was estimated at 4700 TBq.

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. At present the latter sources have become of minor radiological importance, due to significant reduction of discharges from Sellafield during recent years.

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 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.

	Click the following images to enlarge!

Figure 1. Cesium-137 concentrations (in Bq/kg wet weight) in herring muscle in 1984-2003, as annual mean values by basin. Target value has been calculated as average of pre-Chernobyl (1984-1985) concentrations. (NOTE: variable scales in the graphs)

Figure 2. Cesium-137 concentrations (in Bq/kg) in plaice and flounder muscle in 1984-2003, as annual mean values by basin. Target value has been calculated as average of pre-Chernobyl (1984-1985) concentrations.  

Figure 3. Cesium-137 concentrations (in Bq/kg wet weight) in surface water (sampling depth = < 10 m)in 1984-2003, as annual mean values by basin. Target value has been calculated as average of pre-Chernobyl (1984-1985) concentrations. (NOTE: variable scales in the graphs)

Meta data

Technical Information:

The 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 the Finnish Environment Institute (SYKE).

Description of data: 

The data are based on ¹³⁷Cs concentrations of 1) herring (Clupea harengus L.), 2) flounder (Platichthys flesus L.) and plaice (Pleuronectes platessa L.) and 3) surface seawater (0-10m). Analyses have been made either as round fish (without head and entrails) or filets (herring) and for plaice and flounder from filets, only. Concentrations have been calculated from wet weight samples.

Sea water concentrations have been analyzed from surface water 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. The reference values are:

Herring: 2.5 Bq/kg

Flounder and plaice: 2.9 Bq/kg

Seawater: 14.6 Bq/kg     

Spatial and temporal coverage:

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

Plaice and flounder data are very scarce both in time and space. Only four sea areas are covered and data are missing for many years. Sampling of plaice and flounder takes place in some countries only.

Seawater data coverage is almost complete, except for the Gulf of Riga and the Archipelago Sea.

Data Quality:

The quality of the analytical data submitted to the MORS database is tested through on-going intercomparison exercises, which show that the quality is very good. 

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

[Author’s name(s)], [Year]. [Indicator Fact Sheet title]. HELCOM Indicator Fact Sheets 2005. Online. [Date Viewed], http://www.helcom.fi/environment2/ifs/en_GB/cover/.

Last updated 25 Nov 2005.