Distribution and migration of radiocesium in sloping landscapes three years after the Fukushima-1 nuclear accident

Abstract

The results of the study are presented on the distribution and migration of radiocesium in mountainous (580–620 m a.s.l.) landscapes in the northeast of Honshu Island (Tohoku Region, Miyagi Prefecture) subjected to radioactive contamination after the nuclear accident at Fukushima-1 NPP. In July 2014, the average contamination density with radiocesium (¹³⁴Сs and ¹³⁷Сs) over the territory (150 km to the northwest from NPP) was equal to 16 kBq/m². This contamination is estimated at the acceptable level according to both Japanese and Russian standards and legislation. Three years after the accident, radiocesium is found to be unevenly distributed by the biogeocenosis components, i.e. 45% in litter, 40% in plants, 10% in soil, and 5% in roots. As for the distribution of total radiocesium (Cs tot = ¹³⁴Сs + ¹³⁷Сs) by the profile of volcanic podzolic-ocherous soil (Dystric Aluandic Andosols), its maximal content (about 80%) was found in the surface layer (0–2.5 cm), with the specific activity ranging from 250 to 10070 Bq/kg and sharply decreasing with the depth. Radiocesium amount in the soils of forest ecosystems was on average by 20% higher than in meadow ecosystems. Accumulation of radionuclides in soils of lower and middle parts of a slope with an insignificant vertical migration was found to be the most general regularity. The air dose rate did not exceed the maximal permissible level, and the snow cover acted as an absorbing and scattering screen.