47 
5 Radioactivity in the Baltic Sea 
compared to other sea regions 
lolanda Osvath 
International Atomic Energy Agency (IAEA) 
Marine Environment Laboratories 
A recently published study on worldwide 
marine radioactivity (IAEA, 2005) enables a 
comparison of levels of anthropogenic radionu 
clides in Baltic seawater against those in other 
areas of the world ocean. The Baltic Sea ranked 
highest for average Cs-137 levels in surface 
water in the year 2000 (Figure 1), closely fol 
lowed by the Irish Sea. In terms of Sr-90 levels, 
the Baltic Sea ranks third, after the Irish Sea 
and the Black Sea. Average Pu-239,240 levels 
in surface water in the Baltic Sea are similar to 
those measured in other areas of the northern 
ocean at large, but two orders of magnitude 
below those estimated for the Irish Sea. 
These levels clearly reflect the dominant 
regional sources for the respective radionu 
clides, as described in detail for the Baltic Sea 
in Chapter 2, as well as the processes affecting 
the fates of these radionuclides, quite different 
in terms of physico-chemical properties, after 
their release to the environment. A compari 
son with other semi-enclosed European Seas, 
such as the Black Sea and the Mediterranean 
Sea, helps set this in perspective. The follow 
ing comparison will focus on Cs-137, which 
has been considered as the most significant 
man-made radionuclide in the marine environ 
ment in terms of inventory, transfer and global- 
scale dosimetry (IAEA, 1995). Input from the 
Chernobyl accident is significantly higher for 
the Baltic Sea than the contribution of global 
fallout from nuclear weapons testing, while for 
the Black Sea these two sources contributed 
more equally to the inventory of Cs-137 (Table 
1). Although it drains a smaller land area than 
the Black Sea, the Baltic Sea also received 
larger long-term post-Chernobyl inputs of Cs- 
137 through rivers than the Black Sea. This 
reflects the pattern of contamination by fall 
out from Chernobyl of the respective drainage 
basins (OECD, 1996; EC, 1998) and the latitu 
dinal distribution of global fallout (UNSCEAR, 
2000). The contribution of global fallout is con 
siderably higher for the Mediterranean Sea, 
given the large surface area of this sea and of 
its drainage basin. Basin-specific freshwater 
inflow patterns, seawater circulation and mixing 
as well as geochemical processes further con 
trol the transport and transfer of radionuclides 
through the marine environment and explain 
the observed distributions and differences from 
region to region. 
During the period 2000-2005, Cs-137 levels 
of up to 125 kBq-nr 2 were found in Baltic Sea 
bottom sediments, with median levels of 36 
kBq nr 2 in the Bothnian Sea and 10 kBq nr 2 
in the Gulf of Finland (HELCOM, 2007). For 
comparison, Cs-137 inventories in Black Sea 
bottom sediments reported by Egorov et al. 
(2006) for the years 1990-1994 range between 
0.2 kBq m' 2 in the abyssal basin and continen 
tal slope areas, and 30 kBq m' 2 at the mouths 
of the River Danube. Sediment inventories up 
to 18 kBq m' 2 Cs-137 were reported for the lat 
ter area for 2003 (Laptev and Voitsekhovitch, 
2006). Western Mediterranean inventories 
of Cs-137 in deep-sea sediment of 0.2 - 0.3 
kBq m' 2 were reported for the early 1990s 
(Delfanti et al., 1995). 
A comprehensive study of radioactivity in the 
marine environment and doses from marine 
exposure pathways published in 1995 (IAEA, 
1995) shows that average levels of Cs-137 in 
fish in the Baltic Sea in 1990 were similar to 
Source 
Baltic Sea 
Black Sea 
Mediterranean Sea 
Chernobyl atmospheric deposition on the sea surface, PBq 
4.1-5.1 
1.7-2.4 
3-5 
Chernobyl river inflow (1), inflow from the Black Sea through the 
0.3 (1) 
0.03 (1) 
0.3 (2) 
Bosphorus Strait (2), PBq 
Global fallout/atmospheric nuclear weapons tests, PBq 
0.8 
1.4 
12 
Nuclear waste reprocessing plants, PBq 
0.25 
- 
0.03 
Sea surface area, 10 3 km 2 
415 
436.4 
2500 
(average depth, m) 
(55) 
(1253) 
(1500) 
Drainage basin surface area, 10 3 km 2 
1700 
2400 
3300 
Table 1: 
Contributions to 
cumulative Cs-137 
inventory: comparative 
table for semi-enclosed 
European seas (This 
report; Egorov et al, 
2007; IAEA 2005) 
Baltic Sea Environment Proceedings No. 117