Momo Takada

Temporal changes in vertical distribution of 137Cs in litter and soils in mixed deciduous forests in Fukushima, Japan

ABSTRACT Downward migration of 137Cs in soils was studied in three mixed deciduous forests c.a. 40 km northwest of the Fukushima Daiichi Nuclear power plant (FDNPP), Japan. We selected three different types of forest regarding to environmental condition such as slope inclinations and snow coverage conditions at the time of deposition. We examined temporal changes in the vertical distribution of 137Cs from litter layers to 10 cm soil depth for two years (2.3 to 4.3 years after the FDNPP accident in 2011). At all three study sites, the 137Cs in the litter layer had largely migrated to surface soil by 2013. After 2014, about 80% of the 137Cs in forest soils (litter layer to 10 cm soil depth) remained within 0–5 cm soil layer. The vertical distribution had not changed substantially since 2014, suggesting that changes to the downward migration rates of 137Cs in soils drastically decreased with time. In addition, small amounts of migrating 137Cs could not be detected by the present method because there was a large spatial variation in the distribution of soil 137Cs. The results showed similar patterns of soil 137Cs distribution among the three study sites although there were differences in the environmental conditions.

Spatial heterogeneity of radiation emission on a secondary mixed forest floor in northeastern Japan after the Fukushima Daiichi Nuclear Power Plant explosions

ABSTRACT The spatial distribution of radiation air dose was assessed on the floor of a secondary mixed forest located 35 km northwest of the Fukushima Daiichi Nuclear Power Plant. All trees with diameter at breast height ≥5 cm were measured and mapped within two plots. Radiation air doses were measured on the forest floor at each 1-m grid point and at stem bases and were analyzed in relation to bark texture, tree strata position, and leaf phenology. Radiocesium concentration measured from soils sampled in a similar manner (2-m grid point and at stem bases) in one of the two plots showed significant correlation with the air dose rate. Air dose rate showed a highly heterogeneous spatial distribution and tended to increase toward the stem bases. This trend was distinctive in some tree functional groups, with the rate being higher in some smaller deciduous trees with smooth bark (e.g. Fraxinus lanuginosa, Acer rufinerve). The trends were indistinctive in larger canopy trees with thick, coarse bark (Quercus mongolica subsp. crispula): in such trees, air dose rate become lower at stem bases than those in the outer range of the concentric zone in some large evergreen conifer trees (Abies firma). However, radiocesium concentration measured near tree stem bases (1 m away from the base) did not vary in relation to tree functional groups. Bark smoothness, which facilitated immediate vertical runoff of contaminated water to lower positions, may have increased air dose rates at the tree bases but would not have enhanced the heterogeneity of radiocesium concentration in the forest floor soils.