Artificial radionuclides in Black Sea sediments

The present work is a summary of the Moscow State University's activities under the IAEA's RER 2/003 Project 'Marine Environment assessment of the Black Sea Region'. The Black sea is a highly-contaminated water body. The atmospheric nuclear tests in the 50-60s and the accident at the Chernobyl NPP (1986) are the main sources of man-made radionuclide contamination of the Black Sea. Some impact of the contemporary nuclear activities of European countries is possible as well. The aim of this work was to estimate the level and to reconstruct chronology of anthropogenic radioactive pollution of the Black Sea ecosystem. Sediment samples (11 sediment cores) were collected by multicorer and gravity corer during the 55th Cruise of the RV 'Professor Vodyanitsky' (2000) and during cruises of the RV 'Akvanavt' (1997- 2000) in the eastern part of the Black Sea and near Turkish, Russian and Ukrainian (Crimea) shores. Sediment samples were cut into 1 cm layers and analyzed for 137Cs specific activity. The vertical distribution of 137Cs in the Black Sea sediments in most cases has a sharp maximum, corresponding to the Chernobyl accident. Sometimes a second maximum caused by the atmospheric nuclear tests also occurs (corresponds to early 60s). The specific activity of 137Cs in the bottom sediments of the Black Sea reaches the level of 200 Bq/kg of dry weight. Recently 137Cs fluxes to marine sediments dramatically decreased. It means, that self-cleaning of water column takes part during the last years. In some cases the chronology of contamination by 137Cs was proved by independent tracer - excess 210Pb. In order to study the origin of the radioactive contamination the plutonium isotope ratios were used. The 241Pu with a half-life period of 14.5 years served as sedimentation monitor together with 238Pu/239,240Pu and 241Pu/239,240Pu ratios. The obtained data for Danube avandelta indicate that sediments are mixed and the radionuclides have both Chernobyl and fallout origin. The upper layers were mostly contaminated with Chernobyl plutonium whereas the relative content of fallout radionuclides increased in deeper layers. For the first time 237Np specific activity data for the Black Sea sediments are presented