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 Baltic Sea sediments was 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
Click to enlarge the image.
Figure 1. Cs-137 in bottom sediments at 9
sampling stations in 1984-2002, Bq/m2. Click the image to enlarge
.
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 distribution
pattern of Cs-137 at different sampling stations 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 deposi-tion 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 continued through the 1990s, reaching
its peak in 1999, and started to decrease in 2000. In the south-ern
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.
Assessment
The distribution pattern of Chernobyl-derived
Cs-137 in the catchment 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 bot-tom
sediments also occurred in these gulfs, but the scattered nature
has been further emphasized as a consequence of river discharges,
sea currents and varying sedimentation rates on hard (erosion) and
soft (sedimentation) bottoms.
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/m2.
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. The 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 Bal-tic Sea
1992-1998. Baltic Sea Environment Proceedings No.85, pp. 55-75.
Helsinki Commission, Helsinki.
Last update 30 September 2003
Helsinki Commission