Regularities of the 137Cs secondary distribution in the soil-moss cover of elementary landscape-geochemical systems and its dynamics within 6 years on the test site in the Chernobyl abandoned zone, Russia

A study of ¹³⁷Cs distribution in a landscape cross-section characterizing the ELGS system (top-slope-closing depression) in the “Vyshkov-2” test site located in the Chernobyl abandoned zone, the Bryansk region, Russia, has been performed in 2015 and 2021. The test site (70×100 m) is located on the Iput’ river terrace in a pine forest characterized by the undisturbed soil-plant cover. Sod-podzolic sandy illuvial-ferruginous soils present the soil cover. The initial level of ¹³⁷Cs contamination of the area varied from 1480 kBq/m² to 1850 kBq/m². Up to now, 89-99 % of the total ¹³⁷Cs is fixed in the upper 20 cm soil layer with 70-96 % in the upper 8 cm. It allows field spectrometry data to study the structure of the ¹³⁷Cs contamination field. The ¹³⁷Cs activity was measured in the soil and moss cover along cross-sections with 1 m step by adapted gamma-spectrometer Violinist-III (USA). Cs-137 content in the soil cores’ and plant samples was determined in the laboratory by Canberra gamma-spectrometer with HPGe detector. It was shown that there is no unidirectional movement of ¹³⁷Cs both in the soil and in the vegetation cover of the ELGS from the top to the closing depression. On the contrary, the data obtained allow us to state a pronounced cyclical variation of the ¹³⁷Cs activity in ELGS, which can be traced in the soil and the vegetation. The variation appeared to be rather stable in space 29 and 35 years after the primary pollution. Cyclic fluctuation (variation) of ¹³⁷Cs activity was described mathematically using Fourier-analysis, which was used to model the observed changes by the revealed three main harmonics. High and significant correlation coefficients obtained between the variation of ¹³⁷Cs activity and the model for the soil-vegetation cover (r_0,01= 0,868; n=17 - 2015; r_0,01= 0,675; n=17 - 2021), soils (r_0,01= 0,503-0,859; n=17) and moss samples (r_0,01= 0,883; n=17 - 2015; r_0,01= 0,678; n=17 - 2021) proved satisfactory fitting of models. The character of ¹³⁷Cs variability in moss cover was generally similar to surface soil contamination, but the level of contamination and amplitude was specific.

The performed study confirmed specific features of ¹³⁷Cs secondary migration in ELGS, which periodic functions describe. We infer that the observed cyclicity reflects elements’ migration in the ELGS system with water.

The reported study was funded by the Vernadsky Institute federal budget (research task #0137-2019-0006). The field works were supported partly by RFBR No 19-05-00816.