3 Previous inventories of long-lived 
radionuclides in the Baltic Sea sediments 
In recent years, several investigations have 
been carried out to evaluate total inventories of 
certain long-lived radionuclides in the Baltic Sea 
sediments. Sale et al. (1986) estimated the total 
amounts of Sr-90, Cs-137, and Pu-239,240 bound 
to bottom sediments in the Baltic Sea at the begin 
ning of the 1980s. They used two different methods 
in their calculations. In the first method, they 
estimated the average contents of radionuclides 
(Bq nr 2 ) in soft and hard sediments in various sea 
areas and multiplied them by the areas (Table 1) of 
soft and hard bottoms in each. The second method 
was based on an estimation of approximate mean 
concentrations of radionuclides in sinking matter 
and assumed sedimentation rates in different areas 
of the Baltic Sea. They concluded that at the begin 
ning of the 1980s the total amounts of Cs-137, Sr- 
90, and Pu-239,240 bound in Baltic Sea sediments 
were 277 TBq, 12 TBq, and 15 TBq, respectively. 
Inventory 
An inventory is the estimate of the total amount 
of a radionuclide in a certain compartment, for 
example, sediment or water. 
The uneven distribution of Chernobyl fallout in 
the drainage area of the Baltic Sea has made the 
calculations more difficult. On the other hand, 
although the global fallout of the nuclear weapons 
tests was more or less evenly deposited in the 
northern hemisphere, the deposition of radionu 
clides into sediments was not evenly distributed 
at that time either owing to differences in the 
sedimentation rate in different areas. 
The first preliminary calculations of the Cs-137 
inventory in the Baltic Sea sediments, made by the 
MORS Group soon after the Chernobyl accident, 
resulted in values of 520 TBq in 1986 and 856 TBq 
in August 1987 (HELCOM, 1989). These figures 
were based on measurements of Cs-134 and an 
initial activity ratio of 0.52 between Cs-134 and 
Cs-137. Direct calculations from measured Cs-137 
concentrations in sediments gave an inventory of 
1466 TBq in August 1987, as the sum of Cs-137 
from all sources. However, these calculations 
were based only on soft-bottom values and the 
fact that lower values are associated with hard 
bottoms was not taken into account. 
Sediment 
Sediment covers the seafloor. It can consist of 
sand, gravel or mud, depending on the differ 
ent conditions in the environment. In general, 
hard bottoms consist of sand and gravel, and 
the soft bottoms are muddy. 
Sedimentation 
As a result of a combination of biological 
and physical processes, solid material is 
transported from the water column towards the 
seafloor. This material accumulates in layers 
which can provide information about the his 
tory of the water body. 
Using the same method as Salo et al. (1986) 
based on the average contents of radionuclides 
(Bq nr 2 ) in soft and hard sediments in various sea 
areas, llus et al. (1995) estimated that the total 
amount of Cs-137 in the Baltic Sea sediments was 
1 400 TBq in 1990-1991 and that of Pu-239,240 
was 18 TBq in 1987-1988. In comparison with 
the values given by Salo et al. (1986), this meant 
that the Chernobyl fallout had increased the total 
amount of Cs-137 by a factor of 5, whereas the 
increase of Pu-239,240 was negligible. Salo et 
al. (1986) assumed that Sr-90 and Cs-137 were 
distributed between hard and soft bottoms in 
Sub-region 
Total 
(km 2 ) 
Soft bottom 
(km 2 ) 
Hard bottom 
(km 2 ) 
Bothnian Bay 
37 000 
16 000 
21 000 
Bothnian Sea 
79 000 
40 000 
39 000 
Gulf of Finland 
30 000 
16 000 
14 000 
Baltic Proper 
209 000 
99 000 
110 000 
Gulf of Riga 
19 000 
7 000 
12 000 
Total 
374 0 00 
178 000 
196 000 
Table 1. 
Areas of soft and 
hard bottoms (km 2 ) 
in different regions of 
the Baltic Sea (Salo et 
al., 1986). The values 
were measured plani- 
metrically from maps 
of Quaternary deposits 
in the Baltic Sea (Win 
terhalter et al., 1981). - 
9 
Long-lived radionuclides in the seabed of the Baltic Sea