Spatial pattern of 137Cs in soils in a mixed deciduous forest in Fukushima, Japan

Abstract:

Spatial heterogeneity of ¹³⁷Cs contamination was studied in a forest floor of Fukushima region, c.a. 40 km NW of Fukushima Daiichi Nuclear Power Plant (FDNPP) focusing on downwards flow from forest canopy via stemflow and throughfall which play major role in determining spatial contamination of ¹³⁷Cs after the FNDP accident. Setting a study plot (400 m²) in a secondary mixed deciduous forest, dominated by Quercus crispula and Abies firma in canopy layer in August and November 2014, we sampled the souk from surface to 5 cm in depth of soils and measured ¹³⁷Cs in every 2 m grids and at tree stem bases. The total estimated activity of ¹³⁷Cs in soil within the study plot was approximately 210 kBq/m², but showed large spatial heterogeneity showing 30 times of difference between the lowest and highest activities. The activity decreased with increasing distances from tree stem bases. High activity around tree stem bases was presumably due to the stemflow containing ¹³⁷Cs seeped into soil only around tree stem bases that raised radioactivity in soil locally in the areas. Relatively low activity away from trees (outside canopies) may be due to small effects of stemflow and throughfall. Activity of ¹³⁷Cs around bases of deciduous broadleaf trees increased with increasing the tree size. Because larger trees have higher potentials to capture larger amount of ¹³⁷Cs on the tree surface, cumulative activity of ¹³⁷Cs included in stemflow may increase with increasing the tree size. However evergreen coniferous tree species (Abies firma) did not show such a pattern relating to the tree size. The difference is assumed to be affected by phenological characteristics as the accident happened in winter and deciduous broadleaf trees did not have leaves and ¹³⁷Cs deposited on tree bodies, while evergreen coniferous tree had leaves and ¹³⁷Cs was intercepted by the canopies.