Contents lists available at ScienceDirect Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad Transport and fate of 137 Cs in the Mediterranean and Black Seas system during 1945–2020 period: A modelling study R. Bezhenar a , V. Maderich a,* , A. Schirone b , F. Conte b , V. Martazinova c a Institute of Mathematical Machine and System Problems, Kyiv, Ukraine b ENEA Marine Research Centre “S. Teresa”, La Spezia, Italy c Ukrainian Hydrometeorological Institute, Kyiv, Ukraine ARTICLE INFO Keywords: Mediterranean sea Black sea Marine environment 137 Cs Chernobyl accident Food chains Compartment modelling ABSTRACT The compartment model POSEIDON-R with an embedded dynamic food web model was used to assess 137 Cs distributions in the Mediterranean and Black Seas during 1945–2020 due to the weapon testing and accident at the Chernobyl nuclear power plant. Three maximums of contamination of surface waters can be identified from 1950 in the Mediterranean Sea system. Two of them (in 1959 and 1963) were caused by atmospheric deposition due to the nuclear weapon testing. Third maximum in 1986 was related with the Chernobyl accident. Maximum of inventory of 137 Cs in the Mediterranean Sea (11461 TBq) was achieved in 1968, whereas secondary maximum caused by Chernobyl accident in 1986 was almost the same (11460 TBq). The corresponding maximum in the Black Sea (3703 TBq) was reached in 1986. It is approximately two times larger than nuclear weapon tests maximum. The results of simulations conducted with generic parameters agreed well with measurements of 137 Cs concentrations in the water, bottom sediments, and in marine organisms. The inventory in the Mediterranean Sea is most sensitive to the global deposition, whereas water exchange with Atlantic Ocean and the Black Sea plays minor role. The cumulative individual dose for the period 1945–2020 from consumption of marine products contaminated by 137 Cs was in the range 41–130 μSv in the Mediterranean Sea and 213–274 μSv in the Black Sea. The dose increased up to 40% due to Chernobyl accident in the Mediterranean countries and 66–103% in the Black Sea countries comparatively with dose from the global deposition. A useful application of the modelling for monitoring purposes was selection of representative regions in the Mediterranean Sea (5 regions) and in the Black Sea (4 regions) using “etalon” method for classification. 1. Introduction The Southern European Seas form a unique chain of semi-enclosed sea basins connected by narrow and shallow straits with two-way ex- change. The Mediterranean Sea is connected with the Black Sea and the Azov Sea through the Turkish Strait system (Strait of Dardanelles, Sea of Marmara and Strait of Bosphorus). The Strait of Gibraltar connects the Mediterranean Sea with the Atlantic Ocean whereas two main ba- sins in the Mediterranean Sea (Western and Eastern basins) are in turn subdivided on several sub-basins also connected by narrows. With the exception of the very shallow Sea of Azov, the remaining marine basins are deep with relatively small continental shelf: 10% in the Mediterranean Sea and 20% in the Black Sea. The Mediterranean Sea is a concentration basin where evaporation exceeds precipitation and river runoff (main rivers are Ebro, Rhone, Po and Nile). Deficit of the water is covered by influx of Atlantic water (AW) through the Strait of Gibraltar. This water propagates from Western to the Eastern Mediterranean in surface layer gradually increasing salinity and density due to the evaporation and descends into water layer between 200 and 600 m where it returns to the Western Mediterranean as a Levantine Intermediate Water (LIW). Finally, intermediate water leaves Mediterranean Sea through the lower layer of the Strait of Gibraltar. The deepest parts of the Mediterranean Sea are filled by water formed due to the cooling episodes in the Gulf of Lions, the Adriatic Sea, and the Cretan Sea. A complexity of shoreline and bottom topography in Mediterranean basins results in complexity of circulation and pathways of transport of pollutants. In contrast, the Black Sea is a dilution basin where precipitation and river runoff from main rivers Danube, Dnieper, Dniester and Don exceeds evaporation that results in stable stratifica- tion and very weak mixing in deep sea. The excess of water is trans- ported through the upper layer of the Turkish Straits to the Mediterranean Sea, whereas dense water from eastern Mediterranean https://doi.org/10.1016/j.jenvrad.2019.106023 Received 26 April 2019; Received in revised form 25 June 2019; Accepted 22 July 2019 * Corresponding author. E-mail address: vladmad@gmail.com (V. Maderich). Journal of Environmental Radioactivity 208-209 (2019) 106023 0265-931X/ © 2019 Elsevier Ltd. All rights reserved. T