New data on the character of 137Cs lateral and vertical migration in soil-litter-moss cover within undisturbed elementary landscape geochemical systems on the test site in the Chernobyl abandoned zone, Russia

<p>A study of <sup>137</sup>Cs distribution in two new 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, has been performed in 2020. The test site (70×100 m) is located on the Iput’ river terrace in a pine forest characterized by undisturbed soil-plant cover. The soil cover is presented by sod-podzolic sandy illuvial-ferruginous soils. The initial level of <sup>137</sup>Cs contamination of the area varied from 1480 kBq/m<sup>2</sup> to 1850 kBq/m<sup>2</sup>. Cs-137 activity was measured in the soil, moss and litter cover along two parallel (the distance was 5 m) cross-sections with 1 m step. Moisture content was also determined in the studied objects to roughly assess the influence of water regime on radiocaesium migration. Surface <sup>137</sup>Cs activity was measured in field conditions by adapted gamma-spectrometer Violinist-III. Cs-137 content in the soil and plant samples was determined in laboratory conditions by Canberra gamma-spectrometer with HPGe detector.</p><p>Analysis of the obtained data showed that a major part of <sup>137</sup>Cs is now fixed in the soil layer 2-10 cm deep while the highest specific 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.</p><p>Along the first cross-section we observed positive correlation between <sup>137</sup>Cs surface activity and the content radiocaesium in the top soil layer 0-2 cm (r<sub>0.05</sub>=0.643, n=15). Cs-137 activity in the moss samples correlated with the radionuclide activity in soil samples 4-6 cm deep (r<sub>0.05</sub>=0.627; n=15). In the moss samples the highest correlation was observed between the green and rhizoid moss parts (r<sub>0.01</sub>=0.704, n=60). Correlation between radiocaesium activity of the green part of mosses and the underlain litter samples was lower, but also significant (r<sub>0.01</sub>=0.612, n=60). Values of <sup>137</sup>Cs activity in the rhizoid part of moss and in litter were also positively linked, but to a lower degree (r<sub>0.01</sub>=0.402, n=60).</p><p>Along the second cross-section correlation between <sup>137</sup>Cs surface activity and the content radiocaesium in the top soil layer 0-2 cm equaled to r<sub>0.05</sub>=0.507 (n=7). Radiocaesium content in moss samples (green, rhizoid parts) and litter was higher correlated with <sup>137</sup>Cs content in soil layer 2-4 cm (r<sub>0.05</sub>=0.640; 0.410; 0.460, n=7). Similar to the first cross-section the highest correlation was observed between the green and rhizoid parts of moss (r<sub>0.01</sub>=0.780, n=39). Relation between <sup>137</sup>Cs activity in green part of moss and litter samples appeared smaller than along the first cross-section but still significant (r<sub>0.01</sub>=0.419, n=39) while that between the rhizoid part of moss and litter was higher (r<sub>0.01</sub>=0.509, n=39).</p><p>Performed study showed that<strong> </strong>in the studied objects <sup>137</sup>Cs secondary migration has a specifically stable character which may be related to spatial peculiarities of radionuclide migration in soil-plant cover controlled by water regime in the ELGS system. In our opinion this may form a characteristic cyclic variation of <sup>137</sup>Cs activity observed along cross-sections of ELGS.</p><p> </p><p>The reported study was funded by RFBR according to the research project № 19-05-00816.</p>