Downward migration of 137Cs within the humus layer under temperate coniferous stands in the Czech Republic

Abstract

This study examined the current distribution and downward migration of 137Cs in the humus horizon under temperate coniferous stands in the Czech Republic. Depth distribution profiles of lithogenic alkali metals (K, Rb, and Cs) were also traced to find any indication regarding the 137Cs dynamics within the humus horizon. Soil (bulk soil) samples were collected manually from the uppermost down to a depth of 20 cm at three points situated 10–20 m apart in three locations diversely affected by the Chernobyl accident in 1986. Humus samples (about 6 cm in thickness) were separately collected at three points adjacent to the bulk soil sampling. The humus samples were divided into three fractions (upper, middle, and lower) denoted to Hu, Hm, and Hl, respectively, depending on their depths. Activity concentration of 137Cs and the amount of alkali metals (K, Rb, and Cs) in each humus fraction were determined with gamma spectrometry and ICP spectrometry, respectively. Some properties (pH, density, SOM, and identification of clay minerals), of both bulk soils and humus horizons were also investigated. The highest activity concentration of 137Cs (Bq kg−1) appeared in the upper portion of the soil (mainly humus horizon) under three coniferous stands about 30 years after the Chernobyl accident. Increasing activity of 137Cs (Bq) was found in the lower humus fraction (Hl, 4~6-cm depth range) with no appreciable amounts of clay minerals like illite and smectite in the investigated sites. The findings suggest that the fallout 137Cs moves downward at a speed of 0.13~0.19 cm year−1 with degrading organic matter within the humus horizon. Possible association of 137Cs with alkali metals (K, Rb, and Cs) was suggested by depth distribution profiles of lithogenic alkali metals (K, Rb, and Cs) in humus. The humus horizon under temperate coniferous stands plays an important role in retaining fallout 137Cs for a long time. Comparing the depth distributions of the fallout radionuclide 137Cs with the depth distributions of lithogenic alkali metals (K, Rb, and Cs) gives valuable information for clarifying mechanism of 137Cs movement in humus. Further investigation is needed to elucidate mechanism of 137Cs migration within humus horizon by tracing 137Cs speciation and decomposing soil organic matter simultaneously.