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

	Author:  Erkki Ilus,  STUK - Radiation and Nuclear Safety Authority, Finland

The indicator shows the total amount of the man-made radionuclide Cs-137 accumulated in the seabed of the Baltic Sea.

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 2 100 - 2 400 TBq in the beginning of the 2000s. However, in 2000-2005 the concentrations of man-made radionuclides in the sediments were generally at or below the concentrations of naturally occurring radionuclides, and were not expected to cause harmful effects to the Baltic Sea wildlife.

Introduction

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. The total input of Cs-137 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. The role of Cs-137 in sedimentological studies is important, because the affinity of caesium to clay particles is well known. Certain amounts of Cs-137 occurred in the sedi­ments of the Baltic Sea already before the Chernobyl accident as a consequence of the nuclear weapons tests performed in the 1950s and 1960s. Nevertheless, the proportion of the ‘old’ caesium begins to be insignificant, particularly since the caesium peak of the global fallout is already buried into deeper sediment layers. 

Results and 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 eastern Gulf of Finland. The highest total amounts of Cs-137 in bottom sediments (Bq per square metre) 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.

The total amounts of Cs-137 at different sampling stations in the Baltic Sea are shown in Figure 1. The highest amounts in sediments were probably not caused by highest site-specific deposition values, but are due to particle transport and the focusing of particle-bound caesium in the deepest parts of the accumulation basins [1].

Figure 1. Total amounts of Cs-137 (Bq m^-2) at different sampling stations of the Baltic Sea in the beginning of the 2000s [2]. Click image to enlarge!

The highest total amounts of Cs-137 per m² were detected in the Bothnian Sea. The maximum value, 125 000 Bq m^-2, was recorded in 1998 from the northernmost part of the Bothnian Sea [1], and values exceeding 110 000 were also recorded in the sea area off Gävle. The median total amount of Cs-137 was 36 400 Bq m^-2 on the soft bottoms of the Bothnian Sea. The corresponding value for the soft sediments in the Bothnian Bay was 9 700 Bq m^-2.

In the Gulf of Finland, the maximum amount of Cs-137 was 42 700 Bq m^-2 in the middle of the eastern part of it in 2003, and values exceeding 38 000 were also recorded in the most eastern parts of the gulf. The median total amount in the soft sediments of the Gulf of Finland was 10 400 Bq m^-2. In the Gulf of Riga, the maximum total amount of Cs-137 was 21 800 Bq m^-2 at a soft bottom station at the middle of the gulf. In the Baltic Proper, the amounts of Cs-137 in sediments varied considerably; from 40 to 14 700 Bq m^-2 and the median value was 1 970 Bq m^-2. In the Belt Sea, Kattegat and The Sound the median value was 1 370 Bq m^-2.

Figure 2 shows vertical distribution of Cs-137 in sediments at some sampling stations of the Baltic Sea. The caesium profiles reflect large differences in sedimentation processes at different stations, which depend on the topography bottom, sedimentation rate, bioturbation (mixing of sediment layers by bottom animals), etc.

The total inventory of Cs-137 in the seabed of the Baltic Sea was estimated at 2 100 - 2 400 TBq in the beginning of the 2000s. This is about 8-9% more than in the previous evaluation in 1998 [1]. The difference is explained with the additional data collected in the Sediment Baseline Study of HECOM MORS-PRO, which have particularised the calculations, and with the fact that Chernobyl-derived caesium has continued to deposit into the seabed. Besides the sedimentation, considerable amounts of caesium have been transported from the Baltic Sea to the North Sea via Kattegat in out-flowing water masses.

The Sediment Baseline Study showed that the concentrations of man-made radionuclides are still higher than the target of the HELCOM ecological objective “radioactivity at pre-Chernobyl level”. This is particularly true for the Bothnian Sea and the eastern Gulf of Finland, which received the largest amounts of fallout in the Baltic Sea from Chernobyl accident in 1986. However, in 2000-2005 the concentrations of man-made radionuclides in sediments were generally at or below the concentrations of the naturally occurring radionuclides, and were not expected to cause harmful effects to the Baltic Sea wildlife.

References

[1] 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.

[2] Ilus E, Mattila J, Nielsen SP, Jakobson E, Herrmann J, Graveris V, Vilimaite-Silobritiene B, Sup-linska M, Stepanov A, Lüning M, 2006. Final report of the Sediment Baseline Study of HELCOM MORS-PRO in 2000-2005 (in preparation).

[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.

Figure 2. Vertical distribution of Cs-137 (Bq kg^-1 d.w.) at some sampling stations of the Baltic Sea in 2003: Bothnian Bay (CVI), Bothnian Sea (EB1), Gulf of Finland (LL3a) and Baltic Proper (BY15) [2].

Meta data

Description of data

The monitoring programme of HELCOM/MORS-PRO consists of 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 annexed with additional data collected for the Sediment Baseline Study of the HELCOM MORS-PRO in 2000-2005. Data were reported from 210 stations consisting of more than 300 sediment cores taken from all sub-regions of the Baltic Sea. 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 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 diameter of the coring tubes in the corers varied between 50 and 90 mm.  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 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 [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 2006. Online. [Date Viewed], http://www.helcom.fi/environment2/ifs/en_GB/cover/.
 

Last updated 6.9.2006