22 
6 Conclusions 
The goal of this study was to complement the 
knowledge and inventories of long-lived radionu 
clides in the seabed of the Baltic Sea by providing 
additional data from so-called “white areas”, and 
from nuclides which were not recorded in the 
earlier surveys. According to the project plan, the 
sampling and analysis activities should mainly 
be based on the permanent HELCOM/MORS 
Monitoring Programme, but samples should 
also be taken from coastal areas and areas with 
hard bottoms, as well as from anoxic sediments. 
Especially in the case of Cs-137, the targets 
were fulfilled very well, although some parts of 
the western Baltic Proper still remained relatively 
poorly investigated. A large amount of data on 
certain other nuclides were compiled, but many 
gaps still remained concerning our knowledge on 
long-lived radionuclides in the seabed of the Baltic 
Sea as a whole. 
The results show that about one half of the total 
input of Cs-137 from Chernobyl into the Baltic 
Sea area has accumulated in the seabed. The 
accumulation was strongest in the first 5-6 years 
after the fallout, but it is still in progress. Cs-137 
is still transported from the drainage area to the 
sea, and in the sea from the water column to the 
bottom. In recent years, the accumulation of 
Cs-137 has become slower and the total inven 
tories of this radionuclide in the seabed have 
stopped increasing, which means that the accu 
mulation rate of Cs-137 and its radioecological 
half-life in the sediments are essentially attaining 
balance at present. 
Chernobyl-derived Cs-137 was very unevenly 
distributed in the seabed of the Baltic Sea. The 
largest amounts were measured in the bottom 
sediments of the northern parts of the Bothnian 
Sea, the southern parts of the Bothnian Bay, 
and the eastern parts of the Gulf of Finland. In 
addition to the patchy nature of the atmospheric 
deposition, the distribution and accumulation in 
the sediments was affected by the character of 
the bottom, the sedimentation conditions, and 
the sedimentation rate in different areas. The 
maximum value in Baltic Sea sediments (125 000 
Bq nr 2 ) was recorded in the northernmost part 
of the Bothnian Sea and was clearly higher than 
the highest deposition values recorded, e.g., from 
the Finnish territory in 1987. The dominant role 
of the Bothnian Sea as an accumulation basin of 
Chernobyl caesium is clear; its portion was 73% 
of the total inventory of the entire Baltic Sea. 
The total inventories of certain long-lived radionu 
clides in the Baltic Sea sediments were estimated 
as follows: 
Man-made radionuclides 
TBq 
Natural radionuclides 
TBq 
Cs-137 
2 100-2 400 
K-40 
8 500 a ' b 
Sr-90 
26 c 
Ra-226 
420 a ' b 
Pu- 
239,240 
15.3 c 
Np-237 
0.02 c 
a in the surface (0-10 cm) sediment layer. 
b supposing that the amounts are equal on hard/ 
soft bottoms. 
c rough estimate based on relatively few data. 
The Sediment Baseline Study results show 
that the concentrations of naturally occurring 
radionuclides in Baltic Sea sediments remain at 
background levels. However, the concentrations of 
man-made radionuclides in 2000-2005 remained