⁹⁰Sr and ¹³⁷Cs distribution in Chornobyl forests: 30 years after the nuclear accident

The primary aim of this study was to quantify patterns in the distribution of ⁹⁰Sr and ¹³⁷Cs activity in pine (Pinus sylvestris L.: 18 sites) and birch (Betula pendula Roth.: 2 sites) forests within the Chornobyl exclusion zone, 30 years after the Chornobyl nuclear power plant (NPP) accident (1986). To achieve this, radionuclide activity concentrations were measured in the mineral and organic soil horizons, the green forest floor (grasses, mosses, and lichens), and trunk wood in forest plots dominated by either pine or birch trees.Our results showed that the geometric mean of the ⁹⁰Sr aggregated transfer factors from mineral soil to the trunk wood of pine trees (T_ag) for Scots pines was 24 x 10^-3 m² kg^-1, which is an order of magnitude higher than the IAEA Handbook (2010)reference value (1.7 x 10^-3 m² kg^-1), which were based on studies conducted after the Kyshtym accident (Ural region, Russia) and in the first years following the Chornobyl accident (Ukraine and Belarus). The observations suggest that the above-ground biomass (soil organic layers, green forest floor, and trees) may contribute more to the ⁹⁰Sr inventory than the mineral soil at the stand level. In contrast, the ¹³⁷Cs T_ag values for pine and birch stands were consistent with those reported in the literature (ranging from 0.1 to 10 and 0.5-1.1 x 10^-3 m² kg^-1, respectively). Both results align with the known bioavailability of radionuclides from previous studies: low for ¹³⁷Cs, leading to limited soil depth migration (less than 30 cm in the mineral horizon), and higher for ⁹⁰Sr, resulting in greater soil migration (up to 1 m in the mineral horizon). This study highlighted significant correlations between the radionuclides' activity concentrations in the litter layers and their content in the trunk wood of pine trees.