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Radionuclides released and deposited because of the 2011 Fukushima Dai-ichi Nuclear Power Plant accident caused an increase in air dose rates in Fukushima Prefecture forests. Although an increase in air dose rates during rainfall was previously reported, the air dose rates in the Fukushima forests decreased during rainfall. This study aimed to develop a method to estimate rainfall-related changes in air dose rates, even in the absence of soil moisture data, in Namie-Town and Kawauchi-Village, Futaba-gun, Fukushima Prefecture. Moreover, we examined the relationship between preceding rainfall (Rw) and soil moisture content. The air dose rate was estimated by calculating the Rw in Namie-Town from May to July 2020. We found that the air dose rates decreased with increasing soil moisture content. The soil moisture content was estimated from Rw by combining short-term and long-term effective rainfall using half-live values of 2 h and 7 d and considering the hysteresis of water absorption and drainage processes. Furthermore, the soil moisture content and air dose rate estimations showed a good agreement with coefficient of determination (R 2 ) scores >0.70 and >0.65, respectively. The same method was tested to estimate the air dose rates in Kawauchi-Village from May to July 2019. At the Kawauchi site, variation in estimated value is relatively large due to the presence of water repellency in dry conditions, and the amount of 137Cs inventory was low, so estimating air dose from rainfall remained a challenge. In conclusion, rainfall data were successfully used to estimate soil moisture and air dose rates in areas with high 137Cs inventories. This leads to the possibility of removing the influence of rainfall on measured air dose rate data and could contribute to the improvement of methods currently used to estimate the external air dose rates for humans, animals, and terrestrial forest plants. [Display omitted] • Soil moisture content and air dose rate were estimated using effective rainfall. • Air dose rate changes were estimated using measured soil moisture content changes. • Magnitude of soil shielding effect was elevated by preceding rainfall. • This method might reveal influences besides rainfall on air dose rate changes. [ABSTRACT FROM AUTHOR]

To analyse the 137Cs distribution and migration under various fractions of organic matter layers, this study investigated easily recognizable, originally shaped organic L-fractions, and not easily recognizable, early fermented and fragmented organic F-fractions, of both oak (Quercus serrata) and cedar (Cryptomeria japonica) sampled from Osawa watershed sites at Nihonmatsu City, Fukushima Prefecture, Japan. The organic materials were put on top of soil columns from Field Museum (FM) Tamakyuryo in Hachioji City, Tokyo. The 137Cs vertical distribution in forest soil profiles was analyzed using the relaxation mass depth, ho (kg m−2). Soil columns with both L and F- organic layer fractions of both oak and cedar, labelled as Oak-L, Oak-F, Cedar-L and Cedar-F with four replications (n = 16), were set up by the laboratory column-based method and kept under five months' incubation period. Soil columns after incubation were sampled at depths of 0–1 cm, 1–2 cm, 2–5 cm and 5–10 cm. Results of 137Cs inventory in the organic fractions showed that 86% (oak and cedar) of the total organic layer fractions 137Cs inventory accumulated within the F-layer, indicating that the transformation of litter is a huge source for potentially mobile 137Cs, especially the oak F-layer (67% 137Cs inventory) and further continuous transfer into the forest soil mineral layers. A higher ho in L treated soils (Oak-L and Cedar-L) compared to the F treatments implied that the low 137Cs amounts penetrated faster and deeper due to their water-soluble nature. Furthermore, Cedar-F showed a higher ho of 24.3 kg m−2 than Oak-F of ho, 14.0 kg m−2, and a significant positive relationship between 137Cs retention and total carbon (TC) (p < 0.05) suggested the influence of soil organic matter on 137Cs penetration and retention. The C/N (carbon nitrogen ratio) results revealed that organic matter fractions of high C/N including 137Cs, as observed in Cedar-F, in which decomposition does not advance, penetrates soil depths while the organic matter fraction of low C/N, observed in Oak-F, showed that decomposition advanced to release 137Cs which was held by adsorption unto the RIP (radiocesium interception potential) of soil surface. In addition, infiltration by water as a transportation process was suggested to largely influence the downward migration and retention of 137Cs at lower depths of Cedar-F. [ABSTRACT FROM AUTHOR]

Approximately 70% of the total land area affected by the fallout from the Fukushima accident is forested, and therefore monitoring of the ambient dose rate in forest environments is essential to ensure that the population and natural habitats of these areas are protected from radiological hazards. However, there are little available data on the ambient dose rate for forest environments. This study investigated temporal changes in the ambient dose rate in different forest environments of Fukushima Prefecture. We conducted repeated measurements of the ambient dose rate in 2014 and 2016 at the same measurement points as those used by the Ministry of Agriculture, Fishery and Forestry of Japan (MAFF) in 2011. The measurements revealed that the decreasing trend in the ambient dose rate varied among the different forest types and time periods. The ambient dose rate in EGC decreased slower than that induced by the physical decay of radiocesium for the period of 2011–2014. However, such slow declining trend of ambient dose rate was likely followed by quick reduction during the following years (2014–2016 and 2011–2016). On the other hand, in MBL and DBF forests, the ambient dose rate decreased 10–20% faster than that induced solely by physical decay of radiocesium for the observation period 2011–2016. • Ambient dose rate in Fukushima forests was repeatedly measured. • Ambient dose rates in mixed broad-leaved and deciduous forests were higher than evergreen conifers. • Ambient dose rate in evergreen conifer decreased slower than mixed broad-leaved and deciduous broad-leaved forests for the period of 2011–2014. • Decline of dose in evergreen conifer during the period of 2011–2014 was slower than that induced solely by physical decay of radiocesium. • Decline of dose in mixed broad-leaved and deciduous broad-leaved forest was 10–20% faster than the physical decay of radiocesium. [ABSTRACT FROM AUTHOR]

The deposition of Fukushima-derived radiocesium via falling litter in a coniferous forest 180 km downwind immediately following the nuclear power plant accident was investigated. The litterfall contribution to the transfer of radiocesium from the forest canopy to the forest floor was determined, and this pathway was compared with hydrological pathways. The results demonstrated that during the observation period, a total of approximately 5.5 kBq m − 2 of Fukushima-derived radiocesium was deposited on the forest floor through throughfall (53%), stemflow (2.3%) and litterfall (45%) routes. The data revealed that the contributions of hydrological pathways became less important as time passed. However, the litterfall route, which transferred approximately 31% (2.5 ± 0.6 kBq m − 2 ) of the local fallout within the observation period, continued depositing radiocesium onto the forest floor. [ABSTRACT FROM AUTHOR]