Gennady Matishov1,2, Irina Usyagina1, Nadezhda Kasatkina1
1Murmansk Marine Biological Institute, Murmansk, Russia
2Southern Scientific Centre of the Russian Academy of Sciences, Rostov-on-Don, Russia
Abstract
This publication summarizes long-term data on the radioactive contamination in the waters of the Barents and Kara Seas. It also describes current patterns in the distribution of radionuclides in the study area.
Keywords: Barents Sea, Kara Sea, radionuclides, 137Cs, 90Sr, long-term trend
The research interest in marine radioactivity decreased significantly in the first decade of the 21st century due to the general decrease of radionuclide levels after the cease of large-scale nuclear tests.
Discovery of traces of Fukushima NPP discharges in high-latitude Arctic areas indicated the importance of regular monitoring of artificial radionuclide background in the environment.
Long-term trends in the radioactive contamination of the Barents-Kara region were reconstructed using a radioecological data base of the Murmansk Marine Biological Institute (MMBI), published information (AMAP 1997; Matishov, Matishov, 2004; Sivintsev et al., 2005; Matishov et al., 2009) and results of the latest marine studies of MMBI.
In 2015–2017 surface water samples were collected during cruises to the Barents and Kara seas using the research vessel Dalnie Zelentsy and other ships of opportunity Talnakh, Nadezhda, and Norilskiy Nickel. Radiological measurements were performed at MMBI. The activity of 137Cs and
90Sr were measured in the samples. The cellulose inorganic ANFEZH sorbent was used to concentrate 137Cs from the sea water (Remez et al., 1998); the volume of each sample was not less than 100 L. The activity of 137Cs was measured using the “InSpector-2000” γ-spectrometer and the multichannel “b13237” γ-spectrometer for measuring X-ray and gamma radiation with pure germanium detectors (“Canberra”, USA). The spectra were analyzed using the “Genie-2000”
software. Each sample was measured for 24 hours and more. To determine the activity of 90Sr we used the oxalate-radiochemical preparation method (Outola et al., 2009) followed by measuring the activity of equilibrium 90Y in the multi-purpose scintillation counter “LS-6500” (“Beckman Instruments Inc.”, USA).
Barents Sea
In 2015−2017, no short-lived anthropogenic γ-nuclides associated with recent radioactive releases were registered. The activity of 137Cs varied from 0.2 Bq⋅m–3 to 4.6 Bq⋅m–3. The range of measured activities of 90Sr was 0.05−13.5 Bq⋅m–3.The waters of Atlantic origin have the greatest influence on the hydrological regime of the Barents Sea in comparison with other arctic seas. The Atlantic waters contribute the most part of the artificial radionuclides to the ecosystem. In 2017, the studies were carried out at two transects along the western boundary of the Barents Sea and at the Kola Meridian Transect. The comparison of average concentrations of 137Cs in different parts of the Barents Sea makes it obvious that the Atlantic waters are still characterized by the highest concentration of radionuclides. The main flux of cesium enters the Barents Sea with the Atlantic waters through the North Cape−Bear Island border. The maximum concentration of this isotope was registered in the upper layers of the Atlantic water flow in all branches of the North Cape Current. At the Kola
Meridian Transect, in the central part of the Barents Sea the concentration of cesium is markedly reduced (1−1.5 Bq⋅m–3).
Long-term dynamics of the activity of 137Cs and 90Sr in the waters of the Barents Sea are presented in Figure 1. By the period of 1979−1980, the increase in 137Cs activity up to 45 Bq⋅m–3 (Kershaw, Baxter, 1995) was registered in the waters of the Barents Sea. Since that time, the long-term dynamics of the volumetric activity of 137Cs has been showing the trend to decline exponentially: y
= 36.906e–0.099x, where x is the number of years elapsed after the maximum of contamination (R² = 0.91). The corresponding average environmental half-life (T1/2) is about 7.0 years. The decrease in the volumetric activity of 90Sr after the maximum of contamination is less pronounced: y = 9.9354e–0.04x (R² = 0.58). The environmental half-life (T1/2) is about 17 years. The estimated environmental half-lives for 137Cs and 90Sr are much less than the physical half-lives of these radionuclides. This fact can be well-explained for a dynamic marine environment where radionuclide redistribution occurs under the influence of hydrological conditions, water exchange with adjacent seas, geochemical processes in the water column, and at the ‘water-bottom sediment’ boundary.
Figure 1. Long-term dynamics of 137Cs and 90Sr activities in water of the Barents Sea in 1979−2017.
Kara Sea
In 2015−2017, the activity of 137Cs varied from 0.4 Bq⋅m–3 to 4.3 Bq⋅m–3. Spatial differences in
137Cs concentrations in the Kara Sea waters are hard to be determined because of the unavailability of data. Reduction of the 137Cs content in seawater was registered at low depths. The area of the lowest 137Cs concentrations is allocated in the southern part of the sea, near the estuary zones of the Yenisei and Ob rivers. The range of 90Sr activity varied from 0.5 to 11.6 Bq⋅m–3. The distribution of
90Sr activity in the Kara Sea is significantly affected by the river runoff. The highest concentrations of the radionuclide were observed in the shallow parts of the sea most affected by the runoffs of the Ob and Yenisei rivers.
A comparative analysis of the long-term dynamics of 137Cs and 90Sr activity in the waters of the Kara Sea showed a sharp decrease in the 137Cs activity compared to the 1990s. The concentration of
90Sr in the water varied in a relatively wide range and there were no obvious decrease compared to
137Cs. Long-term tendencies for decreasing of 137Cs and 90Sr concentrations in the Kara Sea water can also be described by the exponential curves but the degree of reliability is lower than in the Barents Sea.
Comparative analysis of water contamination on the continental shelf and in the coastal zone of the Barents and Kara seas suggests that due to the natural oceanological processes and isotope decay the concentration of artificial radionuclides has multiply decreased over 60 years. At present, the influence of regional and local contamination sources (such as discharges from West-European radiochemical plants, Russian nuclear industrial enterprises Mayak and Tomsk-7, nuclear fleet bases, burials at the Novaya Zemlya), which were active in the past, has become almost indistinguishable from the background.
The reported study was funded by RFBR according to the research project № 18-05-60249
“Radioactive contamination and secondary sources of man-made isotopes in seas of the Arctic Ocean at the turn of the XX–XXI centuries”.
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