Detailed study of 137Cs distribution in soil-litter-moss cover within the undisturbed woodland test site in the Chernobyl abandoned zone, Russia

Cs-137 distribution was studied in the regular grid and in landscape cross-sections characterizing the ELGS system (top-slope-closing depression) in the “Vyshkov-2” test site located in the Chernobyl abandoned zone, the Bryansk region, Russia. The test site (70×100 m) located on the Iput’ river terrace in a pine forest formed on sod-podzolic soils is characterized by a pronounced elevation gradient (ca 4.5 m). In 1986 the level of ¹³⁷Cs contamination of the area varied from 1480 kBq/m² to 1850 kBq/m². The distribution of ¹³⁷Cs was studied in the topsoil, soil, moss and litter cover in a grid manner with a step of 10 m and along cross-sections with 1 m step. To assess the influence of water regime on radiocaesium migration moisture content was also determined in the studied objects. The data showed that the major part of ¹³⁷Cs is now fixed in the soil layer 2-10 cm thick while the highest activity of radiocaesium is found at a depth of 2-8 cm that can be explained mainly by the burial of the initially contaminated layer under the annual leaf fall. In the ELGS system the content of ¹³⁷Cs in the top soil layer (0-2 cm) determined in laboratory conditions by Canberra gamma-spectrometer with HPGe detector correlated with the surface Cs-137 activity measured by field gamma-spectrometer Violinist III (r=0.643, n=15). Cs-137 activity in the green part of moss samples correlated with the radionuclide activity in soil layer samples 4-6 cm deep (r=0.627, n=15). In the moss samples the highest correlation was observed between the green and rhizoid moss parts (r_0.01=0.718, n=60, along cross-sections), (r_0.05=0.858, n=20, in regular grid). Correlation between radiocesium activity of the green part of mosses and the underlain litter samples was lower, but also significant (r_0.01=0.617, n=60, along cross-sections), (r_0.05=0.481, n=20, in regular grid). Cs-137 activity in the rhizoid part of moss and in litter were also positively linked, but to a lower degree (r_0.01=0.402, n=60). Studies performed in 2015-2019 confirmed our earlier finding that radiocaesium had not moved in unidirectional manner down the slope to the closing depression, but formed a more complex polycentric structure in ELGS.

The study was financially supported by the Russian Foundation for Basic Research (grant # 19-05-00816).