26 
Table 1 : 
Details of some naturally 
occurring and artificial 
radionuclides observed in the 
sediments of the Baltic Sea and 
discussed in the text. The half- 
lives (T„_) of these radionuclides 
generally vary from 370 hours 
to 14,000,000,000 years. 
The main sources of artificial 
radionuclides have been the 
fallout from the Chernobyl 
accident in 1986 and the 
global fallout from atmospheric 
nuclear weapons testing in 
the 1950s and 1960s. Rivers 
are still transporting fallout 
nuclides from their drainage 
areas to the sea. Other sources 
have only had minor impacts 
on radioactivity levels, either 
in the southern Baltic Sea 
(West European Nuclear 
Reprocessing Plants) or only 
very locally (nearby NPPs). 
Nuclide 
Origin 
T, 
40 K 
natural 
300,000,000 yrs 
210 Pb 
natural 
22.2 yrs 
226 Ra 
natural 
1,600 yrs 
232 Th 
natural 
14,000,000,000 yrs 
60 Co 
artificial 
5.3 yrs 
103 Ru 
artificial 
39.3 days 
106 Ru 
artificial 
372.6 hrs 
110m Ag 
artificial 
249.8 days 
,25 Sb 
artificial 
2.8 yrs 
90 Sr 
artificial 
28.5 yrs 
90 Tc 
artificial 
210,000 yrs 
,37 Cs 
artificial 
30.2 yrs 
237 Np 
artificial 
2,100,000 yrs 
238 Pu 
artificial 
87.7 yrs 
239 Pu 
artificial 
24,000 yrs 
240 Pu 
artificial 
6,563 yrs 
241 Pu 
artificial 
14.4 yrs 
241 Am 
artificial 
432.7 yrs 
was mostly found in the bottom sediments 
of the Gulf of Bothnia (especially in the 
Bothnian Sea) and in the eastern part of the 
Gulf of Finland (Figure 1). The amounts of 
137 Cs have remained relatively unchanged 
during the reporting period (Figure 2). 
Artificial radionuclides from the Chernobyl 
and global fallouts are partly going to be 
buried deeper into the sediments on the 
accumulation bottoms. Nonetheless, part of 
the activity bound up in the sediments will still 
be transported from the erosion/transportation 
bottoms towards the accumulation bottoms. 
In spite of careful planning of the monitoring 
programmes, there are still a lot of factors 
Table 2: 
Total inventories (TBq) of 
90 Sr, ,37 Cs and 239240 Pu in the 
sediments of the Baltic Sea 
have been estimated several 
times. The total amount of 
,37 Cs in the bottom sediments 
increased considerably after 
the Chernobyl fallout, and today 
is about 8-9 times higher than 
pre-Chernobyl levels in the 
beginning of the 1980s. The 
90 Sr and 239249 Pu inventories are 
only rough estimates because 
of the limited amount of data. 
Year 
“Sr 
137 Cs 
239,240p u 
Reference 
Early 1980s 
12 
277 
15 
Salo et al. 1986 
1990-1991 
- 
1200-1400 
18** 
llus et al. 1995 
1998 
- 
1940-2210 
- 
llus et al. 2003 
2000-2005 
26* 
2100-2400 
15 
llus et al. 2007 
* = rough estimate 
** = 1987-1988 
affecting the results of the sediment studies 
and causing variation in the results and time 
trends (llus et al. 2000 and 2003, Mattila 
et al. 2006). For example, variations inside 
the sedimentation basins and around the 
monitoring stations can be large, due to the 
heterogeneity of soft sediment deposits. The 
differences in sampling techniques, as well as 
in analysis methods, may also increase the 
variability in the results. The bottom dynamics 
have an influence on the accumulation, 
transportation/erosion and hydraulic sorting 
of the sediments. As yet we evidently know 
relatively little about the variability in the 
sediments around our monitoring stations. 
Most of the radioactivity in the sediments 
of the Baltic Sea originates from naturally 
occurring radionuclides. In recent years, the 
activity concentrations of naturally occurring 
radionuclides with long half-lives, such as 40 K, 
226 Ra and 232 Th (Table 1), have been reported 
into the database. In the surface sediments, 
most of the 40 K concentrations (in the 0-10 
cm layer) varied between 200 and 1,400 Bq 
kg -1 d.w.; concentrations of 226 Ra (in the 0-10 
cm layer) amounted to 10-100 Bq kg 1 d.w.; 
and 232 Th concentrations (in the 0-30 cm 
layer) varied between 10 and 50 Bq kg 1 d.w.. 
The activity levels of these nuclides depend 
on the type of the sediment. In the sediment 
baseline study, we tried to estimate the total 
amounts of 40 K and 226 Ra in the seabed of the 
Baltic Sea, but at this time the estimations 
had to be limited only to the uppermost layers 
of recently accumulated sediments. The total 
amounts in the 0-10 cm layer were estimated 
to be roughly 8,500 TBq for 40 K, and 420 TBq 
for 226 Ra. In the 0-20 cm layer, these amounts 
were over twice as high as in the uppermost 
10 cm. 
Although there are considerable amounts 
of artificial radioactivity due to the presence 
of long-lived radionuclides in the Baltic Sea 
sediments, they are not expected to cause 
harmful effects to marine life in the Baltic 
Sea. After the Chernobyl fallout, elevated 
concentrations were also detected of many 
other radionuclides, such as 60 Co, 103 Ru, 106 Ru, 
110m Ag and 125 Sb (Table 1), but because of 
their short half-lives, the activities of these 
radionuclides have decreased considerably, or 
they have vanished (llus et al. 2003). 
In the sediment baseline study (llus et 
al. 2007), it was also possible to get new 
knowledge of "Tc and 237 Np (Table 1) in Baltic 
Sea sediments by using mass spectrometric 
analysis methods. In surface sediments, the 
concentrations of "Tc (in layer 0-10 cm) varied 
between 0.04 and 1.30 Bq kg 1 d.w., whereas 
the activities of 237 Np (in layer 0-20 cm) varied 
between 0.2 and 6.5 mBq kg 1 d.w.. The total 
amount of 237 Np in the sediments of the Baltic 
Sea was estimated to be about 0.02 TBq (llus 
et al. 2007). 
Over the period 1999-2006, the activities 
of 137 Cs remained relatively unchanged at 
different monitoring stations in the Baltic Sea, 
although there were considerable differences 
between the stations (Figure 2). After the 
Chernobyl accident, the activity of 137 Cs has 
been intensively studied, since there was a 
lot of caesium activity in the fallout, it has a 
long half-life and high K d values (Bq kg 1 in