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

	Author: HELCOM MORS-PRO Corresponding author: Jukka Mattila, STUK - Radiation and Nuclear Safety Authority, Finland

Key message

The most significant source of Cs-137 in the Baltic Sea sediments is the fallout from the Chernobyl accident. The distribution pattern of Chernobyl derived Cs-137 in the Baltic Sea sediments is scattered due the uneven deposition and the sediment accumulation to the bottoms. The highest amounts occur in the sediments of the Bothnian Sea and the eastern Gulf of Finland. Recently the total inventory of Cs-137 in the Baltic Sea sediments was estimated to be about 2 100 - 2 400 TBq in the beginning of the 2000s. Nonetheless, most of the radioactivity in the sediments of the Baltic Sea originate from naturally occurring radionuclides. At present the radioactivity in the sediments is not expected to cause harmful effects to the Baltic Sea wildlife. The continuous monitoring work and observations of time trends of the radioactive substances are the bases for knowing and understanding the state of the radioactivity in the Baltic Sea.

Results and assessment

The main sources of Cs-137 have been the fallout from the Chernobyl accident in 1986 and the global fallout in the 1950s and 1960s [1]. The total input of Cs-137 activity from Chernobyl to the Baltic Sea was estimated at 4 700 TBq. In the course of time a significant share of this caesium has sunk to the bottom and accumulated into the sediments [2,3]. Certain amounts of Cs-137 occurred in the sediments of the Baltic Sea already before the Chernobyl accident as a consequence of the atmospheric nuclear weapons tests performed in the 1950s and 1960s. Nevertheless, the proportion of the ‘old’ caesium begins to be insignificant, particularly since the caesium of the global fallout is buried into deeper sediment layers. 

The distribution pattern of Chernobyl-derived Cs-137 in the drainage area and in the sediments of the Baltic Sea was very scattered [1,2,3]. The highest deposition values occured in the areas surrounding the Gulf of Bothnia and the eastern Gulf of Finland and also the highest total amounts of Cs-137 activities (Bq per square metre) were observed in the bottom sediments of these areas (figure 1, table 1). In general, the total amounts of Cs-137 in the bottom sediments are clearly highest at the stations of the Bothnian Sea and at the stations of the eastern Gulf of Finland [1,2]. The highest registered amount in the Bothnian Sea has been 125 000 Bq m^-2 in year 1998. In the Baltic Proper, Belt Sea, Kattegat and Sound the amounts are considerably lower than in the northern parts of the Baltic Sea (Table 1). Also, the variability inside the sea areas is large. In addition to uneven deposition, the total amounts have also been affected by the type of the bottom sediments, bottom topographies, sediment accumulation rates in different sea areas, unstable sedimentation processes and river discharges [2,3].

Table 1. Median values of total amounts of Cs-137 at locations describing the regional distribution of Chernobyl derived Cs-137 activity in the bottom sediments of the Baltic Sea [2,3]. Variabilities within the sea areas are large [3].

The amounts of the artificial radioactivity have not increased notably in the bottom sediments during present years. The total inventory of artificial Cs-137 in the seabed of the Baltic Sea was estimated to be about 2 100 - 2 400 TBq in the beginning of the 2000s [2]. This is 8 - 9 times higher compared to the pre-Chernobyl level. Nowadays Chernobyl-derived caesium has continued to deposit into the seabed, but also considerable amounts of caesium have been transported from the Baltic Sea to the North Sea via Kattegat in out-flowing water masses.

The concentrations of artificial radionuclides are still higher than the target of the HELCOM ecological objective “radioactivity at pre-Chernobyl level”. In the Baltic Sea sediments, there are still considerable amounts of artificial radioactivity due to radionuclides with longer half-lives. Although, the artificial radioactivity in the sediments of the Baltic Sea is not expected to cause harmful effects to the Baltic Sea wildlife.

Figure 1. Total amounts of Cs-137 (Bq m^-2) at different sampling stations of the Baltic Sea in 1984 - 2006. The present level is generally about 8 times higher compared to the pre-Chernobyl levels. Click image to enlarge!

References

[1] HELCOM, 2003. Radioactivity in the Baltic Sea 1992-1998. Baltic Sea Environment Proceedings No.85, Helsinki Commission, Helsinki.

[2] Ilus E, Mattila J, Nielsen SP, Jakobson E, Herrmann J, Graveris V, Vilimaite-Silobritiene B, Suplinska M, Stepanov A and Lüning M, 2007. Long-lived radionuclides in the seabed of the Baltic Sea. Baltic Sea Environment Proceedings No.110, pp. 1-41. Helsinki Commission, Helsinki.

[3] Mattila J, Kankaanpää H and Ilus E, 2006. Estimation of recent sediment accumulation rates in the Baltic Sea using artificial radionuclides ¹³⁷Cs and ^239,240Pu as time markers. Boreal Env. Res. 11: 95–107.

[4] HELCOM, 2000. Intercomparison of sediment sampling devices using artificial radionuclides in Baltic Sea sediments – The MOSSIE Report. Baltic Sea Environment Proceedings No.80, Helsinki Commission, Helsinki.

 [5] Ikäheimonen TK and Vartti VP, 2007. An Intercomparison of Radionuclide Analyses in a Baltic Sea Sediment Sample, in Ilus E (ed). Long-lived radionuclides in the seabed of the Baltic Sea. Baltic Sea Environment Proceedings No.110, pp. 1-41. Helsinki Commission, Helsinki.

Meta data

Description of data

The monitoring programme of HELCOM MORS-PRO includes 49 permanent sampling stations for bottom sediments. This report is based on the regular data reported by all the Contracting Parties to the HELCOM MORS database. The data are given as total amounts of Cs-137 in Bq/m².

Technical information

The samples were taken with different types of sediment corers tested and compared [4]. 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.

Quality information

The quality of the analytical data submitted to the MORS database is tested through on-going intercomparison exercises, which show that the quality of data is very good [1, 5].

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

[Author’s name(s)], [Year]. [Indicator Fact Sheet title]. HELCOM Indicator Fact Sheets 2007. Online. [Date Viewed], http://www.helcom.fi/environment2/ifs/en_GB/cover/.
 

Last updated 14.6.2007