Post-deposition early-phase migration and retention behavior of radiocesium in a litter–mineral soil system in a Japanese deciduous forest affected by the Fukushima nuclear accident

Matsunaga

et al. 2016

Self Cite

The Fukushima Daiichi nuclear power plant disaster caused serious radiocesium (137Cs) contamination of forest ecosystems over a wide area. Forest-floor organic layers play a key role in controlling the overall bioavailability of 137Cs in forest ecosystems; however, there is still an insufficient understanding of how forest types influence the retention capability of 137Cs in organic layers in Japanese forest ecosystems. Here we conducted plot-scale investigations on the retention of 137Cs in organic layers at two contrasting forest sites in Fukushima. In a deciduous broad-leaved forest, approximately 80% of the deposited 137Cs migrated to mineral soil located below the organic layers within two years after the accident, with an ecological half-life of approximately one year. Conversely, in an evergreen coniferous forest, more than half of the deposited 137Cs remained in the organic layers, with an ecological half-life of 2.1 years. The observed retention behavior can be well explained by the tree phenology and accumulation of 137Cs associated with litter materials with different degrees of degradation in the organic layers. Spatial and temporal patterns of gamma-ray dose rates depended on the retention capability. Our results demonstrate that enhanced radiation risks last longer in evergreen coniferous forests than in deciduous broad-leaved forests.

Section: Discussionsupporting

confidence: 81%

mentioning

confidence: 99%

Forest type effects on the retention of radiocesium in organic layers of forest ecosystems affected by the Fukushima nuclear accident

Matsunaga

et al. 2016

Self Cite

“…The behavior of 137 Cs in soil after deposition is one of the key factors necessary to evaluate the long-term radiation risks that may be caused by external and internal exposure. It is generally accepted that, once 137 Cs reaches the surface layers of the mineral soil, it is strongly adsorbed (i.e., fixed) by clay minerals 2,3 , rapidly reducing the mobility and bioavailability of 137 Cs in such soils 4–10 . Konopleva et al .…”

Section: Introductionmentioning

confidence: 99%

A new perspective on the 137Cs retention mechanism in surface soils during the early stage after the Fukushima nuclear accident

Nishimura

et al. 2019

Self Cite

The Fukushima Daiichi nuclear power plant accident caused serious radiocesium ( 137 Cs) contamination of the soil in multiple terrestrial ecosystems. Soil is a complex system where minerals, organic matter, and microorganisms interact with each other; therefore, an improved understanding of the interactions of 137 Cs with these soil constituents is key to accurately assessing the environmental consequences of the accident. Soil samples were collected from field, orchard, and forest sites in July 2011, separated into three soil fractions with different mineral–organic interaction characteristics using a density fractionation method, and then analyzed for 137 Cs content, mineral composition, and organic matter content. The results show that 20–71% of the 137 Cs was retained in association with relatively mineral-free, particulate organic matter (POM)-dominant fractions in the orchard and forest surface soil layers. Given the physicochemical and mineralogical properties and the 137 Cs extractability of the soils, 137 Cs incorporation into the complex structure of POM is likely the main mechanism for 137 Cs retention in the surface soil layers. Therefore, our results suggest that a significant fraction of 137 Cs is not immediately immobilized by clay minerals and remains potentially mobile and bioavailable in surface layers of organic-rich soils.

“…The aggregated transfer factor (T ag ) values for 137 Cs in tree leaves were also generally similar across treatments (although estimated to be higher in the decontaminated area than in the untreated area in some cases) ( Table 3). These results seem reasonable given that, prior to decontamination, a large proportion of the deposited 137 Cs was immobilized in the surface mineral soil via its interactions with soil constituents [40][41][42][43][44][45] . This behavior could reduce 137 Cs availability for root uptake by plants 15,46,47 to a similar extent in both the untreated and decontaminated areas.…”

Section: Discussionmentioning

confidence: 61%

“…The T ag values obtained in the present study did not show any time-dependent trends during the three-year observation period ( Table 3), indicating that 137 Cs fluxes in this forest had almost stabilized through natural ecosystem processes by the first sampling occasion (approximately 4.3 years after the accident). Again, this suggests that the mobility and availability of 137 Cs in this forest decreased rapidly owing to the rapid shift in the main reservoir of 137 Cs from the litter layer to the underlying mineral soil, and that, as a result, 137 Cs recycling within the forest (i.e., the uptake of 137 Cs by trees from the litter layer and the redeposition of 137 Cs onto the forest floor via annual leaf fall) was inhibited 43,53 . This is also substantiated by the observation of low 137 Cs activity concentrations in newly emerged leaves ( Fig.…”

Section: Discussionmentioning

confidence: 99%

Effectiveness of decontamination by litter removal in Japanese forest ecosystems affected by the Fukushima nuclear accident

Nishimura

et al. 2020

Self Cite

The Fukushima Daiichi nuclear power plant accident caused serious radiocesium ( 137 Cs) contamination of forest ecosystems over a wide area. The removal of the forest floor litter layer has been considered a potential method for forest decontamination; however, its effectiveness remains largely unknown. We conducted a pilot-scale decontamination study in a deciduous broadleaved forest in Fukushima. The entire forest was decontaminated by removing the litter layer in July 2014, approximately 3.3 years after the accident, with the exception of two untreated plots. For three years after decontamination, we quantified 137 Cs contamination levels in the litter and topsoil layers and in the tree leaves, in the untreated and decontaminated areas. The decreased inventories of litter materials and the litter-associated 137 Cs in the decontaminated areas were observed only in the first year after decontamination. Generally, no decontamination effects were observed on the 137 Cs transfer in tree leaves. The primary reason for this was the rapid shift in the main reservoir of 137 Cs from litter layers to the underlying mineral soil, which differs from the observations in post-Chernobyl studies of European forest ecosystems. The results suggest that litter-removal decontamination can only be successful if it is implemented more quickly (within 1-2 years after the accident) for Japanese forest ecosystems.The accident at the Fukushima Daiichi nuclear power plant (NPP) in March 2011 released large quantities of radionuclides into the atmosphere 1 and consequently caused serious radioactive contamination of terrestrial ecosystems over a wide area of eastern Japan 2 . Among the radionuclides deposited onto the terrestrial ecosystems, radiocesium ( 137 Cs), with a physical half-life of 30.1 years, is the primary cause for concern because 137 Cs-contaminated ecosystems can increase the radiation exposure to the local population for many years, via both an elevated ambient dose rate in the air (external exposure) and the consumption of contaminated food products (internal exposure) 3 .Decontamination is an important post-accident countermeasure to reduce the radiological impact of a nuclear accident on humans and ecosystems 3,4 . Although since the Fukushima NPP accident, decontamination efforts have been progressing in residential and agricultural areas, forested areas have received a low priority in the decontamination decision-making process, as in the case of the Chernobyl NPP accident 3 , and has not been carried out on a large scale 5 , even though forests provide a wide range of social, cultural, and economic values. This is because the forested areas are vast, occupying approximately 70% of the land in the heavily contaminated area 6 , but are generally not where people live permanently or regularly spend much time. Another reason may be that the forested areas in Japan are concentrated in mountainous and hilly regions with steep terrain, which makes them very sensitive areas to soil erosion. Technical, radiological, and econ...