Current contamination changes of 137Cs and 90Sr in black alder forests elements in the Chernobyl nuclear power plant exclusion zone

Abstract

In the Chernobyl nuclear power plant exclusion zone for the period 2019–2025, current changes in the accumulation of ¹³⁷Cs and ⁹⁰Sr by the organs and tissues of plants of the main elements (stand, undergrowth and understory, background species of living ground cover) of high-age phytocenoses of black alder, goutweed, fern and sedge forest types were studied. Over the past 6 years, the current radiation situation in black alder plantations has improved significantly. The density of ¹³⁷Cs soil contamination in 2019 was 364.3–655.8 kBq m⁻², in 2025 – 323.9–496.8 kBq m⁻², ⁹⁰Sr – 88.9–219.9 and 76.4–130.1 kBq m⁻², respectively. During this period, the γ-radiation dose rate decreased annually by 3.4–6.3 %, the specific activity of ¹³⁷Cs in 20 cm soil layers – by 3.1–5.5 %, ⁹⁰Sr – by 0.6–9.9 %, the density of soil contamination in most plantations of ¹³⁷Cs – by 1.6–4.0 %, ⁹⁰Sr – 2.3–6.8 %. In some plantations, the density of soil contamination in most plantations of ¹³⁷Cs and ⁹⁰Sr soils increased. Changes in the indicators of current soil contamination are due to the peculiarities of the hydrological regimes of soils in different types of forests under the influence of weather and climatic conditions and the behavior of ¹³⁷Cs and ⁹⁰Sr in them. In 2019–2025, the specific activity of ¹³⁷Cs in the structural components of black alder trees was at the level of 0.66–9.67 kBq kg⁻¹, ⁹⁰Sr – 0.07–2.57 kBq kg⁻¹, the transfer coefficients of ¹³⁷Cs to its organs and tissues were (2.0–27.2)×10⁻³ m² kg⁻¹, ⁹⁰Sr – (0.7–13.2)×10⁻³ m² kg⁻¹, ¹³⁷Cs in the organs of plants of undergrowth and understory – (3.1–37.5)×10⁻³ m² kg⁻¹, ⁹⁰Sr – (2.0–27.9)×10⁻³ m² kg⁻¹. During the analyzed time, the indicators of contamination of plant parts with these radionuclides in the elements of black alder phytocenoses changed at different rates and in different directions, varying by year, type of forest and among themselves within the observation stages. In total, in all plants of all types of forest, the ¹³⁷Cs transition coefficients decreased in 50.6 % of the organs and tissues, ⁹⁰Sr – in 30.3 %. The average ranked rows of parts of black alder trees by their contamination are ¹³⁷Cs (wood < bark < branches < leaves < roots) and ⁹⁰Sr (wood < branches < roots < leaves < bark), the most frequent distributions of organs of undergrowth and understory (stems < roots < leaves); ranked rows of species of undergrowth and understory and rows background species of living ground cover by the transition coefficients of ¹³⁷Cs and ⁹⁰Sr to their organs or parts of the phytomass. The different levels and vectors of radionuclide transfer to plant organs and tissues are determined by edaphic and weather-climatic conditions, the specifics of their deposition by different plant species.