17. Radionuclides ¹³⁷Cs and ⁹⁰Sr in Components of the Black Sea Ecosystems: Contemporary Status and Prognosis

Over the period from 1986 to 2016 the data for the ⁹⁰Sr and ¹³⁷Cs horizontal and vertical distribution in the surface water and in the water column of the Black Sea, accordingly, were analyzed. The results of full-profile measurements of ¹³⁷Cs in the deep water column were obtained in 2015 for the first time for the whole post-Chernobyl period. The time constants of exponential functions were determined and the forecast of ⁹⁰Sr and ¹³⁷Cs concentrations reduction to the pre-accident levels in the components of the ecosystem of the Sevastopol bay was made. The estimations of the balance components of these radionuclides in the Black Sea were obtained. It is shown that to the present time the steady-state equilibrium between entry and elimination of ⁹⁰Sr and ¹³⁷Cs in the oxygen zone of the Black Sea was formed, at the same time their accumulation in the hydrogen-sulphidous zone has been increasing. By 2013 the ⁹⁰Sr and ¹³⁷Cs content in the Black Sea was evaluated as 1670 and 2843 TBq, respectively, that was in 3.6–6.4 and in 1.2–1.6 times higher, than the content of these radionuclides at the sea in 2000. At the same time 66% of ¹³⁷Cs and 67% of ⁹⁰Sr of the total content of radionuclides in the whole water column was in the hydrogen-sulphidous zone of the Black Sea. The period of circulation of ¹³⁷Cs and ⁹⁰Sr in the oxygen and hydrogen-sulphidous zones of the Black Sea is about 14 and 94 years, respectively.