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

 

	Author: Erkki Ilus, STUK

 

 

Key message

 The most significant source of artificial radioactivity in the Baltic Sea is the fallout from the Chernobyl accident. The distribution pattern of Chernobyl derived Cs-137 in the Baltic Sea sediments is very scattered, with the highest values occurring in the Bothnian Sea and the eastern Gulf of Finland. The total inventory of Cs-137 in the Baltic Sea sediments was estimated at 1900-2200 TBq in 1998.

 

Results and assessment

Relevance of the indicator for describing developments in the environment

The most significant source of artificial radioactivity in the Baltic Sea is the fallout from the Chernobyl accident (April 1986), and the most significant long-lived radionuclide in the fallout was Cs-137. In the course of time a significant share of that caesium has sunk to the bottom and accumulated into the sediments.

Figure 1 illustrates the total amounts of Cs-137 in bottom sediments at nine sampling stations of the Baltic Sea since 1984. The nine stations were chosen to describe the differences between the sub-regions of the Baltic Sea. Another criterion of choosing was, that these stations belong to the permanent monitoring network being representative for the sub-regions, and there are more or less complete time-series of Cs-137 results available from these stations for the whole period of 1984-2004.

Click on the image to enlarge!

 

Figure 1. Cs-137 in bottom sediments at 9 sampling stations in 1984-2004, Bq/m².

 

Assessment

The distribution pattern of Chernobyl-derived Cs-137 in the drainage area of the Baltic Sea was very scattered, with the highest deposition values occurring in the areas surrounding the Gulf of Bothnia and the Gulf of Finland. The highest total amounts of Cs-137 (Bq per square metre) in bottom sediments also occurred in these gulfs, and the scattered nature was further emphasized as a consequence of river discharges, sea currents and varying sedimentation rates on hard (erosion) and soft (sedimentation) bottoms.

Figure 1 shows that the strongest sedimentation of Chernobyl-derived Cs-137 occurred in the Bothnian Sea and eastern Gulf of Finland. This is in agreement 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 focusing 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 continued through the 1990s, reaching its peak in 1999, and started to decrease in 2000. 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.

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].

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

 

 

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. Baltic Sea Environment Proceedings No.85, pp. 55-75. Helsinki Commission, Helsinki.

[3] Ilus E, Ilus T, Ikäheimonen TK, Niemistö L, Herrmann J, Suplinska M, Panteleev Y, Ivanova L, Gritchenko ZG, Neumann G, 2000. Intercomparison of sediment sampling devices using artificial radionuclides in Baltic Sea sediments - The MOSSIE Report. Baltic Sea Environment Proceedings No.80, 1-69. Helsinki Commission, Helsinki.

[4]  Ikäheimonen TK, Mulsow S, 2003. Data quality. In: Radioactivity in the Baltic Sea 1992-1998. Baltic Sea Environment Proceedings No.85, pp. 23-48. Helsinki Commission, Helsinki.

 

Data

The report is based on data reported by the Contracting Parties to the HELCOM/MORS database. The data are given as total amounts of Cs-137 in Bq/m².

 

Meta data

The monitoring programme of HELCOM/MORS-PRO consists of 49 permanent sampling stations for bottom sediments. The samples are taken annually. The nine stations used in this report were chosen to represent the different sub-regions of the Baltic Sea, and secondly because there were Cs-137 results available for a long period from these stations. In recent years, some countries have carried out extensive sediment studies in different parts of the Baltic Sea (e.g. Sweden, Finland, Russia, Germany), but since the results are not based on long time-series, they will be considered separately in later indicator reports.

 

Technical information

The samples were taken with different types of sediment corers tested and compared in an intercomparison exercise “MOSSIE” arranged by the MORS Group in 1992. The details of the corers are given in the report of the exercise [3]. The diameters of the coring tubes in the corers used today are 80-90 cm (sampled areas 60-100 cm²).  The sediment cores were sectioned into slices of 1-5 cm, the slices were freeze-dried and homogenised.

The dried samples were analysed by gamma-ray spectrometry and the total amounts per square metre were calculated from the dry weight values.

 

Quality information

In addition to the “MOSSIE” exercise, concerning the sediment sampling methods and devices [3],  the Contracting Parties of the MORS-PRO Group have made significant efforts to verify the analytical quality of the environmental data submitted  to the database. During the period 1992-1998 the IAEA organised 7 intercomparison exercises covering radionuclides in seawater, sediments and fish. The results have demonstrated that the data from the MORS-PRO Group have been of good quality [4].

 

 

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.