The current evaluation of whether good status is achieved, using concentrations of 137Cs in biota and seawater, is based on data from the period 2011-2016.
The results indicate that the activity concentrations of 137Cs in biota and seawater still reflect not-good status in many HELCOM assessment units. Currently the activity concentrations of 137Cs are approaching the pre-Chernobyl levels, representing good status boundary in all matrices. 137Cs in fish and seawater reached their maximum values in the late 1980s and early 1990s (Results figures 1-3). Since then concentrations of 137Cs in fish and seawater have continued to decrease in all regions of the Baltic Sea, it is expected that good status may be reached in all the HELCOM assessment units of the Baltic Sea by about 2020-2025.
Based on data from 2011 to 2016, good status for 137Cs in herring, is not achieved in most of the HELCOM sub-basins, except for the Arkona Basin, where the average concentration of 137Cs (calculated for whole assessed period) is equal to 1.4 Bq kg-1 wet weight and sub-basins west of the Arkona Sea (Kiel Bay - 0.8 Bq kg-1 wet weight, Bay of Mecklenburg – 0.7 Bq kg-1 wet weight and Kattegat – 0.7 Bq kg-1 wet weight). In the Gulf of Finland, the Gdańsk Basin, the Eastern and Western Gotland Basins, the Bornholm Basin and the Åland Sea the average concentrations calculated for the period 2011-2016 were in the range from 3.5 to 4.4 Bq kg-1 wet weight, while the highest mean value was found in the Bothnian Sea – 5.8 Bq kg-1 wet weight.
The annual mean values of concentrations of 137Cs in herring muscle in each sub-basin since 1984 are compared to the threshold value, showing a steady downward trend after the Chernobyl accident in 1986 (Results figure 1). In 2016, in the western parts of the Baltic Sea (i.e. the Kattegat and the Arkona Basin), the mean values were well below the target value of 2.5 Bq kg-1 wet weight, which was calculated as the average of pre-Chernobyl (1984-1985) concentrations. In the remaining Baltic Sea sub-basins, the target value is still not reached as the concentrations are higher than the pre-Chernobyl levels. In 2016, the highest mean concentrations of 137Cs in herring (whole fish without head and entrails or flesh without bones) were in the range of 4.0 to 4.6 Bq kg-1 wet weight in the Bothnian Bay, the Bothnian Sea and the Quark. In the Åland Sea the concentration remained at the level of 3.6 Bq kg-1 wet weight, in the Gulf of Finland mean concentration was equal to 2.9 Bq kg-1 wet weight, while in the Bornholm Basin concentration decreased in 2016 (as compared to 2015) to the 2.5 Bq kg-1 wet weight reaching the threshold level.
Results figure 1. 137Cs mean concentrations (in Bq kg-1 wet weight) in herring (whole fish without head and entrails or flesh without bones) in 1984–2016, as annual mean by sub-basin. Red line indicates the good status boundary (threshold value 2.5 Bq kg-1) calculated as average of pre-Chernobyl (1984–1985) concentrations. Notice variable scale in graphs.
Based on data from the period 2011-2016 for flatfish (flounder), good status is achieved in the Arkona Basin, where the average concentrations is equal to 2.6 Bq kg-1 wet weight, in the Kiel Bay (0.8 Bq kg-1 wet weight) and in the Kattegat (0.3 Bq kg-1 wet weight), while 137Cs concentrations above the threshold value were found in the Eastern Gotland Basin (with average value equal to 3.7 Bq kg-1 wet weight), in the Gdańsk Basin (3.5 Bq kg-1 wet weight) and in the Bornholm Basin (3.7 Bq kg-1 wet weight).
Results figure 2 shows time series of 137Cs concentrations in flatfish samples (whole fish without head and entrails, or flesh without bones), which include flounder (Platichthys flesus) and plaice (Pleuronectes platessa). In 2016, the mean values of 137Cs concentrations were below target value 2.9 Bq kg-1 wet weight in two basins. They were at the levels of 0.1 Bq kg-1 wet weight in the Kattegat and 0.7 Bq kg-1 wet weight in the Kiel Bay. In the Bornhom Basin (4.1 Bq kg-1 wet weight) and in the Eastern Gotland Basin (3.4 Bq kg-1 wet weight) the concentration exceeded the threshold value, while in the Arkona Basinin it was equal to 2.9 Bq kg-1 wet weight.
Results figure 2. 137Cs concentrations (in Bq kg-1) in flatfish - plaice and flounder (whole fish without head and entrails, or flesh without bones) in 1984–2016, as annual means by sub-basin. Red line indicates the threshold value-2.9 Bq kg-1 calculated as average of pre-Chernobyl (1984–1985) concentrations.
Based on the averages concentrations of 137Cs calculated for period 2011-2016 for surface seawaters, none of the HELCOM sub-basins achieve good status. The highest average concentration of 137Cs equal to 29.3 Bq m-3 was found in the Bothnian Sea. Slightly lower values were calculated for the Eastern Gotland Basin (28.2 Bq m-3), Western Gotland Basin (27.2 Bq Bq m-3), Northern Baltic Proper (26.4 Bq m-3) and Arkona Basin (26.5 Bq m-3). The lowest average concentration of 137Cs, equal to 17.4 Bq m-3, was specific for the Kattegat.
In 2016, the concentrations of 137Cs in seawater were significantly more uniform in their range (Results figure 3) as compared to the distribution of 137Cs in surface waters after the Chernobyl accident, when in the most contaminated areas (the Bothnian Sea and the Gulf of Finland) activities exceeding 500 Bq m-3 were observed while in the western parts of the Baltic Sea they were close to 100 Bq m-3. In 2016, the highest concentrations were found in the Eastern Gotland Basin (26 Bq m-3) and in the Bothnian Sea (24 Bq m-3). Slightly lower values were found in the Northern Baltic Proper and Arkona Basin (23 Bq m-3) and in the Western Gotland Basin and Bornholm Basin (22 Bq m-3). Concentrations at the level of 20 Bq m-3 were specific to four sub-basins: The Sound, Kiel Bay, Gdańsk Basin and Bay of Mecklenburg. Lower mean concentrations of 137Cs, but still above the target level, were specific for the Gulf of Finland (16 Bq m-3) and Great Belt (18 Bq m-3). In 2016, the mean concentrations of 137Cs reached the pre-Chernobyl level in two basins: the Kattegat (14 Bq m-3) and The Bothnian Bay (14 Bq m-3).
Results figure 3. 137Cs concentrations (in Bq m-3) in seawater (sampling depth less than 10 m) in 1984‑2016, as annual mean values by sub-basin. Red line indicates the threshold value (15 Bq m-3) calculated as average of pre-Chernobyl (1984‑1985) concentrations.
Based on the inventory estimates, the effective half-life of 137Cs in Baltic seawater has been 10.2 years during the period 1986 - 2016. The effective half-life of a radioactive contaminant is the time required for its concentration to decrease by 50% as a result of physical, chemical and biological processes which are specific of each radionuclide and each environment where they may occur. With this decay rate, the 137Cs inventory in the Baltic Sea would reach the pre-Chernobyl levels (250 TBq) by the year 2020, presuming that the effective half-life would remain constant, and no substantial remobilization of 137Cs from sediments should occur.
It should be pointed out that the 137Cs input from Fukushima fallout (the Fukushima Dai-chii disaster took place in 2011) did not result in an increase in 137Cs concentration in seawater and biota of the Baltic Sea (Kanisch & Aust 2013).
The confidence of the indicator results is high.
Quality assurance is a fundamental part of radioanalytical procedures, needed to confirm the precision and the long-term repeatability of analyses. The radiochemical procedures and counting techniques used by laboratories are well tested, up-to-date, and similar to those used by laboratories worldwide. The laboratories from eight countries bordering the Baltic Sea and IAEA MEL have contributed to the intercomparison programme during the years 2011 – 2014 (results have been presented in the thematic assessment of radioactivity. The intercomparison exercises confirmed that the data provided by the MORS group are of very good quality and can be considered comparable. The inter-comparison exercise of analyses of Baltic Sea water has continued in 2015 and 2016 as organized by STUK, Finland (sample were sampled and delivered by Germany on 23rd June 2015). Several laboratories have also participated in the other inter-comparison exercises organized by the IAEA or other bodies.