5 Results and discussion 
In this section, primary attention has been given 
to Cs-137 because it was the main long-lived 
radionuclide in the Chernobyl fallout. The role of 
Cs-137 is particularly important in sedimentologi- 
cal studies because the affinity of caesium to 
clay particles is well known. Certain amounts 
of Cs-137 already occurred in the sediments of 
the Baltic Sea before the Chernobyl accident, 
resulting from the nuclear weapons tests in the 
1950s and 1960s. Nevertheless, the proportion of 
the “old” caesium is beginning to be insignificant, 
particularly since the caesium peak of the global 
fallout is already buried into deeper sediment 
layers. 
The sampling and analysis activities were coor 
dinated by STUK (Finland) so that the Baltic Sea 
area was covered as fully as possible. The Ris0 
National Laboratory (Denmark) contributed to the 
project by analysing samples taken by the other 
participants forSr-90, Tc-99, Np-237 and Pu-239, 
Pu-240 and Pu-239 + 240. 
5.1 Cs-137 
A large number of additional caesium-137 results 
were reported in the Sediment Baseline Study 
and used in the present update of inventory calcu 
lations. New data were supplied especially for the 
Swedish side of the Bothnian Sea and Bothnian 
Bay, and for the southeastern part of the Baltic 
Proper. Unfortunately, however, the western Baltic 
Proper (Gotland west and south) still remained 
relatively poorly investigated. The new data also 
provided additional information from coastal areas 
and from hard bottoms. 
The present evaluation of the Cs-137 inventory is 
based on the data reported by all the Contract 
ing Parties to the HELCOM/MORS database, 
enhanced with additional data from STUK for 
1995-2005 and with Swedish data from the Gulf 
of Bothnia. Data were reported from 190 stations 
in the Baltic Sea, excluding 20 stations in the 
Belt Sea and the Danish Straits, which were not 
included in the inventory calculations due to the 
lack of data on the distribution of soft and hard 
bottoms in these sub-regions. Since many of the 
stations were sampled annually in 2000-2005, 
and the total amounts per square metre were 
based on numerous superimposed sediment 
slides, the total number of results was consider 
able. In total, the inventory was based on the 
results of 309 sediment cores taken from different 
sub-regions of the Baltic Sea. 
Prior to starting the calculations, the quality of the 
data was checked and the obviously questionable 
values were eliminated. Questionable values were 
identified, for example, by comparing the results 
reported by different laboratories for the same 
sampling station. The most recent observations 
reported by the laboratories for each station were 
selected for manual checking of the results. After 
checking, the accepted values were used in cal 
culating averages for each station. The sampling 
stations were grouped according to the respective 
sub-regions of the Baltic Sea, and the median 
value for each sub-region was chosen to repre 
sent the area in question. The median was used 
because the averages were dominated by a few, 
very high “hot spot” values, which were shown to 
misrepresent the results. 
Sediment samples are usually taken from soft 
bottoms and, thus, a majority of the results in the 
database represent concentrations of radionu 
clides in soft sediments. However, hard bottoms 
contain much less particle-bound radionuclides 
than soft bottoms. Therefore, two alternative 
ratios (1:5 or 1:20) were used to calculate Cs-137 
values for hard bottoms analogously with the 
earlier calculations. The values for hard bottoms 
were calculated from the above-mentioned 
median values for each sub-region. The content 
of Cs-137 (Bq nr 2 ) on soft and hard bottoms in 
the different sub-basins was multiplied by the area 
of soft and hard bottoms in each, according to the 
values given by Salo et al. (1986) (see Table 1). 
These values were measured planimetrically from 
maps of Quaternary deposits in the Baltic Sea 
(Winterhalter et al., 1981). 
According to recent Finnish results from the 
southeastern Baltic Proper, the ratio 1:5 (20%) 
seems to overestimate the amounts of Cs-137 on 
hard bottoms compared to those on soft bottoms. 
At the eight stations taken in 2004 by the STUK 
from the southeastern Baltic Proper, the total 
amounts of Cs-137 on hard bottoms were 1-14%