a ratio of 1:5, while the ratio was about 1:10 for 
Pu-239,240. Later results from the Finnish coastal 
areas have shown that the ratio for Cs-137 may be 
much smaller (1:20), which would lower the total 
inventory. By using this value, the estimate for 
Cs-137 in 1990-1991 was 1 200 TBq. 
Because very few data are available on radionu 
clide concentrations in sinking matter and the use 
of concentrations observed in the surface sedi 
ment layer is doubtful for this purpose, the method 
based on concentrations in sinking matter and the 
sedimentation rate was not used. Furthermore, 
it was not possible to estimate the total amount 
of Sr-90 in the seabed because very few data on 
strontium in sediments have been reported since 
1986. However, it is well known that the proportion 
of Sr-90 was quite small in the Chernobyl fallout. 
The number of observations used in the above 
evaluation was relatively small. In the second 
evaluation of llus et al. (1999), the study material 
was more comprehensive, consisting of 129 sam 
pling stations and 180 sediment cores. These were 
taken by STUK and the Finnish Institute of Marine 
Research in 1993-1997 from different sub-regions 
of the Baltic Sea. In this evaluation, the activity 
concentrations were time-corrected to 26 April 
1996 (tenth anniversary of the Chernobyl accident) 
and the ratio of 1:20 was used to calculate Cs-137 
values for hard bottoms. According to this investi 
gation, the total inventory of Cs-137 in Baltic Sea 
sediments was 2 140 TBq in 1996. The significant 
difference between this value compared with that 
given before was supposed to result from the addi 
tional data on Cs-137 in sediments and the fact 
that Chernobyl-derived caesium had continued to 
be deposited onto the seabed. 
In the third evaluation of llus et al. (2003), the total 
inventory of Cs-137 in the seabed of the Baltic Sea 
was estimated at 1 940-2 210 TBq in 1998. This 
was about eight times higher than the inventory 
made at the beginning of the 1980s (277 TBq) 
and about one and one halftimes higherthan 
our estimate made in 1990-1991. The study was 
based on the Cs-137 data reported by all the Con 
tracting Parties to the HELCOM/MORS database, 
enhanced with additional data from STUK and the 
Finnish Institute of Marine Research. Before the 
calculations were made, the quality of data was 
checked and the obviously questionable values 
were eliminated. The questionable values were 
identified, e.g., by comparing the results given 
by different laboratories for the same sampling 
stations. Then the latest observations reported by 
the laboratories for each station were chosen for 
manual checking of the results. After checking, the 
accepted values were used for calculating aver 
ages for each station. The sampling stations were 
grouped according to the sub-regions of the Baltic 
Sea and the median value of each sub-region 
was chosen to represent the area in question. The 
median was used because the averages were 
dominated by a few, very high “hot spot” values. 
In this study (llus et al., 2003), two alternative 
ratios (1:5 and 1:20) were used to calculate Cs-137 
values for hard bottoms. The values for hard bot 
toms were calculated from the above-mentioned 
median values of each sub-region. The content 
of Cs-137 (Bq nr 2 ) on soft and hard bottoms in 
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). The Belt Sea, 
the Kattegat, and the Sound were not included in 
the inventory owing to a lack of quantitative data 
on the area of soft and hard bottoms. Bojanowski 
etal. (1995a, 1995b) estimated that the total 
inventory of Cs-137 in the sediments of the Polish 
Economic Zone increased from 10 TBq to 45 TBq 
as a consequence of the Chernobyl accident. This 
area forms about 8% of the total area of the Baltic 
Sea. This estimation was in good agreement with 
the total inventory, taking into account that the 
Chernobyl fallout was clearly lower in the area 
surrounding the southern Baltic Properthan, e.g., 
in the areas surrounding the Bothnian Sea and the 
Gulf of Finland. 
Sediment samples are usually taken from soft 
bottoms, i.e., from real sedimentation bottoms of 
sedimentation basins. Soft bottoms very often act 
as “sinks” for radionuclides, whereas hard bottoms 
are regarded as transport bottoms with very little 
accumulation of sinking matter. However, erosion 
bottoms are very seldom truly uncontaminated 
because bioturbation caused by benthic fauna may 
transfer contaminants and organic material into 
deeper sediment layers. Studies carried out on the 
Polish coast have shown that Cs-137 penetrates 
effectively into nearshore sandy sediments, and 
that rapidly accumulating sediments affected by 
river discharges have much higher contents of 
exchangeable radio-caesium than slowly accumu 
lating marine sediments (Knapinska-Skiba et al., 
1994, 1995, 1997). 
It should be kept in mind that the calculations pre 
sented above are very rough because the uneven 
distribution of the Chernobyl fallout has created an 
additional difficulty in the calculations.