REGULARITIES OF 137CS MIGRATION ON GEOCHEMICAL BARRIERS OF MARGINAL ZONE OF MEZOTROPHIC BOG IN UKRAINIAN POLISSYA

Abstract

It was shown that in the narrow marginal zone (about 6-8 m wide) of a mezotrophic bog, typical for the northern part of Ukrainian Polissya, a transsuperaquatic landscape is formed between transacumulative and superaquatic landscapes. In the transsuperaquatic landscape a complex biogeochemical barrier has been formed. Here, at a short distance, barriers of different types are spatially connected – physico-chemical (sorption, gley etc.) and phytocenotic (all vegetation layers), lateral and radial. It was found that due to lateral 137Cs redistribution among the geochemically connected landscapes of the catena, there was observed the concentration of the radionuclide on the investigated complex biogeochemical barrier. Three biogeochemical barriers were found in the soil – layers of semi-decomposed residuals of mosses, trees and grasses (H0dt), peat (Т1-Т5) and iluvial humic-ferruginous gley horizon (Ihgl), which slow down 137Cs migration both in lateral and radial directions. Generalization of datа on 137Cs radial distribution in the soil of the geochemical barrier shows that analogs of the forest litter (H0 non-decomposed and layer of H0dt – semi-decomposed plant residuals) contained 8.16% of the total 137Cs activity in the soil; peat (0-25 cm) – 17.14%, and the mineral horizons (Ihgl and Pigl) – 74.7%. This distribution testifies about high speed of radial 137Cs migration in the studied soil, insofar as the main part of the total activity of 137Cs has already migrated beyond the boundaries of the peat layers to the mineral horizons. The calculations showed that the main part of the total stock of 137Cs in biogeocenosis on the biogeochemical barrier in the transsuperaquatic landscape was retained in soil – 86.59%, accordingly phytocenosis contained 13.41% of the total stock of the radionuclide. The geochemical transfer coefficient was 0.16, which was close to the corresponding values for the superaquatic landscapes. Significant interspecific differences in 137Cs accumulation were observed in all layers of vegetation. The migration ability of 137Cs in the chains «soil – plant» and «soil – fungi» in the transsuperaquatic landscape was evaluated based on the mean 137Cs transfer factor (TF) values to be 2-20 times higher than that for the eluvial and transeluvial landscapes. This leads to significant excess of the permissible levels of the 137Cs specific activity in medicinal plants, wild berries and edible mushrooms at soil contamination density with 137Cs which is not officially regarded as radiationally hazardous (about two times less than 37 kBq·m-2).