Change in plutonium sedimentation fluxes into the bottom sediments of the Sevastopol Bay before and after the Chernobyl NPP accident

The Sevastopol Bay located in the northern Black Sea was exposed to radioactive contamination by anthropogenic radionuclides, inter alia 238,239+240Pu, and to other types of anthropogenic load. One of them was the construction of breakwaters at the bay mouth in 1975–1986, which resulted in a change in the hydrological regime. The aim of this work was to assess the change in 238Pu and 239+240Pu sedimentation fluxes into the bottom sediments of the Sevastopol Bay mouth in the period before and after the Chernobyl NPP accident (1962–1986 and 1986–2013, respectively). Plutonium in sediments was determined by the radiochemical method, followed by measurement of the activity of radioisotopes with an alpha spectrometer. The sedimentation rate was defined by geochronological dating of sediment layers, accessing the change in 238Pu/239+240Pu activity ratio in the bottom sediment core. Then, the mass accumulation rate in the bottom sediments was calculated. As established, in the period after 1986, the mean annual sedimentation rate and mass accumulation rate in the bay mouth increased by 63 and 70 %, respectively. Assessment of 238,239+240Pu sedimentation fluxes during two research periods showed as follows: after 1986, 238Pu and 239+240Pu fluxes into the bottom sediments increased by 150 and 49 %, respectively. The increased 238Pu percentage in plutonium sedimentation flux after 1986 indicates Chernobyl origin of plutonium in the bottom sediment layers above 11 cm. In the post-Chernobyl period, the cumulative effect of an increase in 238,239+240Pu sedimentation flux into the bay mouth results both from an increase in the radioactive fallout intensity after the Chernobyl NPP accident and an effect of breakwaters on the regime of sedimentation processes in the water area (an increase in the mass accumulation rate).