Cs-137 distribution in forest floor and surface soil layers from two mountainous regions in Bulgaria

Abstract

The functional characteristics of forest soils in the mountainous regions in Bulgaria were assessed in order to obtain a better understanding of cesium distribution in soil. The aim of this paper is to describe the small-scale spatial variability of Cs-137 contamination in forest floor and surface soil layers in relation to regional and local characteristics. The mountainous regions of Bulgaria were strongly and inhomogeneously contaminated with cesium-137 due to the Chernobyl accident in 1986. The study confirmed that the cesium in forest soils from two mountainous regions of Bulgaria, SW Rila Mountain and Central Balkan, is located in the forest floor and upper 0–5 cm of soil. In a few cases from Central Balkan the maximum activities are found at a soil depth 5–10 cm. The measured values of cesium activity concentration in Aoh in Rila Mountain region are between 287.9 and 827.1 Bq kg − 1 . Greater variations in cesium activity concentration in forest floor are determined for the plots from Central Balkan, where the measured values in Aof are between 85.7 and 2543 Bq kg − 1 . The contamination density of Cs-137 in fine soil (< 1 mm) from Rila varies from 0.6 to 6.0 kBq m − 2 while in the Central Balkan plots it is between 1.2 and 19.6 kBq m − 2 . The forest floor remains the main reservoir for cesium even years after pollution. The profile distribution of Cs-137 in soil systems shows a tendency of decrease toward deeper layers. The mountainous ecosystems with different types of vegetation were compared on the basis of estimated factor of accumulation (FA). The FA for conifers in Rila Mountain is from 0.28 to 0.69 and in the Central Balkans the FA is from 0.20 to 0.61. The data showed that the spatial distribution of Cs-137 in soil system depends on type of vegetation, humus type and altitude. Due to their low clay content and the high content of organic matter, the mountain forest soils can be considered an excellent ecosystem in which to study the mobility and behaviour of Cs-137 and its transfer into the soil-plant system. Due to the high levels of radiocesium contamination within the mountainous regions of the Central Balkan and the characteristics of soils, the highland mountain herbaceous ecosystems located in this region, in the authors’ opinion, could be considered as risk zones to easier soil-to-plant transfer of cesium.