Among the water bodies, exposed to the Chornobyl Nuclear Power Plant accident, the cooling pond (CP) is one of the most contaminated with radionuclides. Currently, over 320 TBq of biologically hazardous, long-lived radionuclides are concentrated in the CP ecosystem. Since the end of 2014, the drawdown of water level in the CP continues, which leads to the drainage of radioactively contaminated bottom sediments. CP is considered as a potential source of additional radiation exposure to the environment. The decrease of the water level in the CP at the present stage has led to an essential change in the hydrological, hydrobiological and hydrochemical regimes, which in the future will inevitably change the physicochemical forms of radionuclides, their mobilization and redistribution between the components of the aquatic ecosystem. The purpose of the research was to establish the features of migration and distribution of physicochemical forms of ⁹⁰Sr and ¹³⁷Cs in the components of the CP ecosystem. During 2017-2018 samples of coastal soils, bottom sediments, aquatic plants and bivalve mollusks were taken. The physicochemical forms of ⁹⁰Sr and ¹³⁷Cs were determined by sequential extraction. Measurement of the activity of radionuclides in fractions was performed using β-radiometric and γ-spectrometric methods.

As a result of the studies performed, it was found that in silty and sandy-silt bottom sediments of the CP deposits at depths of 3–7 m, up to 55% of ⁹⁰Sr cations were in potentially exchangeable forms capable of passing into solution and absorbed by hydrobionts. The exchange activity of ⁹⁰Sr in the soils of the coastal strip did not exceed 10% of the total amount. The carbonate form of ⁹⁰Sr, not involved in active metabolic processes, was 2–8% and 6–32% for silty and sandy-silty bottom sediments, respectively, and up to 46% for soils. The content of ⁹⁰Sr in organic form in soils and bottom sediments was 34-50%, which indicates the presence of a certain reserve of the radionuclide, which can be absorbed by plants during the root feeding process. The distribution of potentially exchangeable forms of ¹³⁷Cs in bottom sediments is characterized by insignificant radionuclide content in all studied forms and does not exceed 16%, with the prevalence of the exchange form at depths up to (14%). Thus, about 80% of the radionuclide is inaccessible for metabolic processes in the ecosystem. ⁹⁰Sr in aquatic plants was predominantly found in potentially exchange forms, namely, 25–30% in the form of adsorbed extracellular cations. ¹³⁷Cs prevails in fixed forms (up to 50%), which indicates the ability of plants to deposit the radionuclide. In the shells of bivalve mollusks, radionuclides are mainly found in fixed forms (50-80%). In the soft tissues of mollusks, the main share of ⁹⁰Sr is in the form associated with organic matter (about 50%) and ¹³⁷Cs is mainly concentrated in fixed forms (more than 60%). The еimplementation of such studies will allow us to evaluate the processes and parameters of physicochemical transformations of radionuclides in the system of "bottom sediments - water - hydrobionts", as well as to establish indicators for the remobilization of radionuclides and their subsequent redistribution between the biotic components of the aquatic ecosystem. This, in turn, will allow developing migration models and improving existing methods for forecasting the behavior of radionuclides in water ecosystems in a context of decrease of the water level.