Total amounts of the artificial radionuclide caesium -137 in Baltic Sea sediments

	Author: Erkki Ilus, STUK

Key message

The amounts of Cs-137 in sediments are so low that they do not cause any hazard to man or aquatic organisms.

Results and assessment

Relevance of the indicator for describing developments in the environment

The indicator shows the total amounts of Cs-137 in bottom sediments of the Baltic Sea since 1984. The comparison of the Cs-137 amounts at different sampling stations shows that the strongest sedimentation of Cs-137 occurred in the Bothnian Sea and eastern Gulf of Finland. This is in aggreement with the distribution pattern of Chernobyl fallout in the drainage area of the Baltic Sea. However, the highest amounts in sediments are probably not caused by highest site-specific deposition values, but are due to particle transport and the concentration of particle-bound caesium in the deep areas of the accumulation basins [2]. In the central Bothnian Sea the maximum was reached in 1992 and the values have decreased after that, but in the eastern Gulf of Finland the accumulation of Cs-137 continued through the 1990s. In the southern parts of the Baltic Sea the amounts of Cs-137 were about one tenth of those in the Bothnian Sea and the Gulf of Finland.

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Figure 1. Cs-137 in bottom sediments at 9 sampling stations in 1984-2000, Bq/m2.

Assessment

The most significant source of artificial radioactivity in the Baltic Sea is the fallout from the Chernobyl accident. The total input of Cs-137 from Chernobyl to the Baltic Sea was estimated at 4700 TBq. The total inventory of Cs-137 in the Baltic Sea sediments was estimated at 1900-2200 TBq in 1998 [1,2].

Description of data

The data are obtained from the monitoring programmes carried out by the countries around the Baltic Sea and give the total amounts of Cs-137 in sediments in Bq m-2.

Methodology and frequency of data collection
Dried sediment samples are analysed by gamma-ray spectrometry and the total amounts per square metre are calculated from the dry weight values. Samples are collected annually.

References

[1] Ilus E, Mattila J, Kankaanpää H and Laine A, 1999. Caesium-137 in Baltic Sea sediments since the Chernobyl accident. Marine pollution, Proceedings of a symposium held in Monaco, 5-9 October 1998, IAEA-TECDOC-1094, pp. 379-380. IAEA, Vienna.
[2] Ilus E, Suplinska M and Mattila J, 2003. Radionuclides in sediments. In: Radioactivity in the Baltic Sea 1992-1998. To be published in Baltic Sea Environment Proceedings.

Summary

The most significant source of artificial radioactivity in the Baltic Sea is the fallout from the Chernobyl accident. The total inventory of Cs-137 in the Baltic Sea sediments was estimated at 1900-2200 TBq in 1998.