Muto, Kotomi; Atarashi-Andoh, Mariko; Matsunaga, Takeshi*; Koarashi, Jun
Journal of Environmental Radioactivity, 208-209, p.106040_1 - 106040_10, 2019/11
Vertical distributions of Cs in the soil profile were observed at five forest sites with different vegetation types for 4.4 years after the Fukushima Dai-ichi Nuclear Power Plant accident, and Cs migration in the organic layer and mineral soil was analyzed based on a comparison of models and observations. Cesium-137 migration from the organic layer was faster than that observed in European forests, suggesting that the mobility and bioavailability of Cs could be suppressed rapidly in Japanese forests. The diffusion coefficients of Cs in the mineral soil were estimated to be 0.042-0.55 cmy, which were roughly comparable with those of European forest soils affected by the Chernobyl Nuclear Power Plant accident. Model predictions indicated Cs mainly distributed in the surface mineral soil at 10 years after the accident. It suggest that the Cs deposited onto Japanese forest ecosystems will be retained in the surface layers of mineral soil for a long time.
Koarashi, Jun; Nishimura, Shusaku; Atarashi-Andoh, Mariko; Muto, Kotomi; Matsunaga, Takeshi*
Scientific Reports (Internet), 9, p.7034_1 - 7034_10, 2019/05
The aim of the present study is to explore the retention mechanisms of Cs in the surface soil layers of terrestrial ecosystems affected by the Fukushima NPP accident, with a specific focus on the interactions between Cs, soil minerals, and organic matter. Soil samples were collected from field, orchard, and forest sites in July 2011, separated into three soil fractions with different mineral-organic interaction characteristics. The results show that 20-71% of the Cs was retained in association with relatively mineral-free, particulate organic matter-dominant fractions in the orchard and forest surface soil layers. Given the physicochemical and mineralogical properties and the Cs extractability of the soils, Cs incorporation into the complex structure of particulate organic matter is likely the main mechanism for Cs retention in the surface soil layers.
Koarashi, Jun; Atarashi-Andoh, Mariko
Journal of Radioanalytical and Nuclear Chemistry, 320(1), p.179 - 191, 2019/04
To quantify the Cs retention behavior in forest-floor organic layers in relation to the litter dynamics, litter samples were collected at five forest sites in Fukushima in 2011 and 2015, and separated into eight fractions with different precursor species and degrees of degradation; afterward, Cs inventory was determined in each litter fraction. The ecological half-lives of Cs in the litter fractions were estimated to be from 0.7 to 4.4 years and found to depend not on the sampling site but rather on the precursor species of the litter materials. Broadleaf- and pine-needle-originated litter fractions exhibited shorter ecological half-lives of Cs, while those of cedar-originated and finely fragmented litter fractions were longer. The results suggested that the organic layers in Japanese forest ecosystems have a low Cs retention capability, although it differs depending on the forest type.
Koarashi, Jun; Nishimura, Shusaku; Atarashi-Andoh, Mariko; Matsunaga, Takeshi*; Sato, Tsutomu*; Nagao, Seiya*
Chemosphere, 205, p.147 - 155, 2018/08
There is little understanding of how soil aggregation can affect the mobility and bioavailability of Cs in soils. To explore this, soil samples were collected at seven sites under different land-use conditions in Fukushima and were separated into four aggregate-size fractions. The fractions were then analyzed for Cs content and extractability and mineral composition. In forest soils, aggregate formation was significant, and Cs was largely associated with large-sized aggregates. In contrast, there was less aggregation in agricultural field soils, and most of Cs was in the clay- and silt-sized fractions. Across all sites, the Cs extractability was higher in the large-sized aggregate fractions than in the clay-sized fractions. The results demonstrate that large-sized aggregates are a significant reservoir of potentially mobile and bioavailable Cs in organic-rich (forest and orchard) soils.
Fujita, Hiroki; Maehara, Yushi; Nagaoka, Mika; Koarashi, Jun
KEK Proceedings 2017-6, p.35 - 39, 2017/11
no abstracts in English
Muto, Kotomi; Atarashi-Andoh, Mariko; Koarashi, Jun; Takeuchi, Erina; Nishimura, Shusaku; Tsuzuki, Katsunori; Matsunaga, Takeshi*
Journal of Radioanalytical and Nuclear Chemistry, 314(1), p.403 - 411, 2017/10
Fluvial export of particulate and dissolved Cs was investigated to reveal its sources and transfer mechanisms in a broadleaved forest catchment using a continuous collection system. The finest size fraction ( 75m), consisting of decomposed litter and surface mineral soil, was the dominant fraction in the particulate Cs load, although the contribution of coarser size fractions increased during high water discharge in 2014. The dissolved Cs originated from the decomposition of Cs-contaminated litter. Temporal changes in Cs distribution in the litter-mineral soil system indicated that the dissolved Cs load will be moderated in several years, while particulate Cs load has the potential to continue for a long time.
Koarashi, Jun; Atarashi-Andoh, Mariko; Amano, Hikaru*; Matsunaga, Takeshi
Journal of Radioanalytical and Nuclear Chemistry, 311(1), p.473 - 481, 2017/01
Vertical distributions of global fallout Cs and C were investigated in a Japanese forest soil in 2001. Even 38 years after the fallout, Cs was still observed mostly in the uppermost 5 cm. A preferential accumulation of Cs was found in a 1-cm-thick transition layer between organic-rich A and underlying B horizons. This unique observation indicated that Cs migrated through the A horizon at a rate of 0.20% y and the transition layer acted as a barrier for Cs migration to deeper layers. The vertical distributions of Cs and C were significantly correlated, suggesting a coupled downward migration of Cs and organic matter on a time scale of decades, along the same physical pathways.
Koarashi, Jun; Nishimura, Shusaku; Nakanishi, Takahiro; Atarashi-Andoh, Mariko; Takeuchi, Erina; Muto, Kotomi
Chemosphere, 165, p.335 - 341, 2016/12
We established field lysimeters in a Japanese deciduous broad-leaved forest soon after the Fukushima nuclear accident to continuously monitor the downward transfer of Cs at three depths: the litter-mineral soil boundary and depths of 5 cm and 10 cm in the mineral soil. Observations were conducted at two sites within the forest from May 2011 to May 2015. Results revealed similar temporal and depth-wise variations in Cs downward fluxes for both sites. The Cs downward fluxes generally decreased year by year at all depths, indicating that Cs was rapidly leached from the forest-floor litter layer and was then immobilized in the upper (0-5 cm) mineral soil layer through its interaction with clay minerals. The decreased inventory of mobile (or bioavailable) Cs observed during early stages after deposition indicates that the litter-soil system in the Japanese deciduous forest provides only a temporary source for Cs recycling in plants.
Koarashi, Jun; Atarashi-Andoh, Mariko; Matsunaga, Takeshi; Sanada, Yukihisa
Scientific Reports (Internet), 6, p.38591_1 - 38591_11, 2016/12
Forest-floor organic layers play a key role in controlling the overall bioavailability of Cs in forest ecosystems; however, there is still an insufficient understanding of how forest types influence the retention capability of Cs in organic layers in Japanese forest ecosystems. Here we conducted plot-scale investigations on the retention of Cs in organic layers at two contrasting forest sites in Fukushima. In a deciduous broad-leaved forest, approximately 80% of the deposited Cs 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 Cs 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 Cs associated with litter materials with different degrees of degradation in the organic layers.
Matsunaga, Takeshi; Nakanishi, Takahiro; Atarashi-Andoh, Mariko; Takeuchi, Erina; Muto, Kotomi; Tsuzuki, Katsunori; Nishimura, Shusaku; Koarashi, Jun; Otosaka, Shigeyoshi; Sato, Tsutomu*; et al.
Journal of Radioanalytical and Nuclear Chemistry, 310(2), p.679 - 693, 2016/11
Particulate Cs in stream water was collected continuously for two years in order to assess the long-term trend of the Cs discharge from the forest environment. Sampling was conducted from December 2011 to December 2013 in a mountainous stream, which received the Cs from the Fukushima Daiichi Nuclear Power Plant accident. A seasonal increase in fluvial transport load of particulate Cs associated with suspended solids (SS) was observed in August and September when rainfall was abundant. The particulate Cs concentration decreased at a faster rate than the rate due to radioactive decay. This decrease might be resulted from redistribution of the easily eroded and polluted soil surface due to heavy rain events such as typhoons. These findings indicate that the particulate Cs load was subject to the inter-annual variations in rainfalls, and decreased gradually over a long period of time due to a decrease in Cs concentration in SS.
Ota, Masakazu; Nagai, Haruyasu; Koarashi, Jun
Science of the Total Environment, 551-552, p.590 - 604, 2016/05
A model for Cs transfer in forest was developed to assess behavior of Fukushima-derived Cs. The model simulation well reproduced observed 3 year migration of Cs in organic layer and mineral soil. Long-term predictions indicated that more than 90% of the deposited Cs remains in the top 5 cm soil till 30 years, suggesting that forest acts as a long-term reservoir of Cs with limited loss via groundwater pathway. Impacts of soil organic matter (SOM) on Cs dynamics were investigated by modifying parameters of Cs turnover. The results showed that SOM-induced reduction of Cs adsorption, slower fixation of Cs by clay and enhanced mobilization of the fixed Cs elevate soil-to-plant transfer of Cs by increasing fraction of dissolved Cs. A substantial proportion (27% 73%) of Cs in these soils was delivered to horizons deeper than 5 cm decades later. These results suggested that SOM significantly influences behavior of Cs over long-term.
Koarashi, Jun; Fujita, Hiroki; Nagaoka, Mika
Journal of Nuclear Science and Technology, 53(4), p.546 - 553, 2016/04
Quantitative evaluation of the atmospheric discharge of radiocarbon (C) is of primary importance for accurately assessing the radioecological impact of the operation of the Tokai reprocessing plant (TRP). However, monitoring the atmospheric C discharge was not conducted at the TRP prior to October 1991. The main purpose of this study was to establish the chronology of atmospheric C discharges for the entire operation period (1977-2014). We found strong correlation between the C discharge and spent fuel reprocessing data obtained after October 1991; we used this correlation to estimate the monthly C discharges in the 1977-1991 period. The total amount of atmospheric C discharge was estimated at 7741 217 GBq for the entire period. Tree-ring C analysis and model calculations using the established discharges were conducted to assess the excess C concentrations around the TRP in the past. The results indicated no significant radioecological impact of atmospheric C discharges from the TRP.
Muto, Kotomi; Atarashi-Andoh, Mariko; Takeuchi, Erina; Nishimura, Shusaku; Koarashi, Jun; Tsuzuki, Katsunori; Nakanishi, Takahiro; Matsunaga, Takeshi
KEK Proceedings 2015-4, p.252 - 257, 2015/11
As a result of the Fukushima Daiichi Nuclear Power Plant accident, a large amount of radiocesium released into the atmosphere was deposited in forests. This study estimated the monthly trend in the fluvial discharges of radiocesium from a forest. The study site was a forested catchment in Kitaibaraki City. Radiocesium in river water was collected with a filtration system as both particulate and dissolved components. Filters and columns including dissolved Cs absorbent were replaced every month. The collected suspended solids were sieved into 2000-3000 m, 500-2000 m, 75-500 m, and 75 m fractions. The Cs concentrations in the samples were measured using -ray spectrometry with Ge semiconductor detectors. The Cs discharge increased with the river water discharge. The particulate Cs discharge was dominant in both 2013 and 2014. The Cs discharge rate of the dissolved component increased in winter, when the river water discharge decreased.
Atarashi-Andoh, Mariko; Koarashi, Jun; Takeuchi, Erina; Tsuzuki, Katsunori; Nishimura, Shusaku; Matsunaga, Takeshi
Journal of Environmental Radioactivity, 147, p.1 - 7, 2015/09
We collected a large amount of radiocesium air dose rate data by mountain-walking with a small -ray survey system, KURAMA-II, to create an air dose rate map of a mountainous deciduous forest that received radiocesium from the Fukushima Dai-ichi Nuclear Power Plant accident. Measurements were conducted in a small stream catchment 0.6 km in size in August and September 2013, and the relationship between air dose rates and the mountainous topography was examined. Air dose rates increased with elevation, suggesting that more radiocesium was deposited on ridges, and that it had remained there for 2.5 years with no significant migration due to soil erosion or water drainage. Slope aspect also strongly affected air dose rates. By the continuous measurement using KURAMA-II, we describe the variation in air dose rates in a mountainous area and suggest that it is important to consider topography when selecting sampling points to estimating dose rates or contaminant deposition.
Matsunaga, Takeshi; Nakanishi, Takahiro; Atarashi-Andoh, Mariko; Takeuchi, Erina; Tsuzuki, Katsunori; Nishimura, Shusaku; Koarashi, Jun; Otosaka, Shigeyoshi; Sato, Tsutomu*; Nagao, Seiya*
Journal of Radioanalytical and Nuclear Chemistry, 303(2), p.1291 - 1295, 2015/02
An innovative, yet simple method for the passive collection of radioactive materials in river water has been developed and validated. The method employes large filter vessels, containing multiple cartridge filters. River water is led to the system naturally using a drop of the riverbed by hose from upstream. This method makes long-term, unmanned monitoring possible. In addition to regular radioactivity analyses, this method provides an opportunity for the characterization of suspended materials based on its ample collection quantities (more than several tens of grams). This method may also be applicable to sediment-bound chemicals.
Koarashi, Jun; Atarashi-Andoh, Mariko; Takeuchi, Erina; Nishimura, Shusaku
Scientific Reports (Internet), 4, p.6853_1 - 6853_7, 2014/10
The accident at the Fukushima Daiichi Nuclear Power Plant caused serious radiocesium (Cs) contamination of forest ecosystems located in mountainous and hilly regions with steep terrain. To understand topographic effects on the redistribution and accumulation of Cs on forest floor, we investigated the distribution of Fukushima-derived Cs in forest-floor litter layers on a steep hillslope in a Japanese deciduous forest in August 2013. Both leaf-litter materials and litter-associated Cs were accumulated in large amounts at the bottom of the hillslope. At the bottom, a significant fraction (65%) of the Cs inventory was observed to be associated with newly shed and less degraded leaf-litter materials, with estimated mean ages of 0.5-1.5 years, added via litterfall after the accident. Newly emerged leaves at the site were contaminated with Fukushima-derived Cs in May 2011 (two months after the accident) and Cs concentration in them decreased with time. However, the concentrations were still two orders of magnitude higher than the pre-accident level in 2013 and 2014. These observations are the first to show that Cs redistribution on a forested hillslope is strongly controlled by biologically mediated processes and continues to supply Cs to the bottom via litterfall at a reduced rate.
Ueno, Yumi; Koarashi, Jun; Iwai, Yasunori; Sato, Junya; Takahashi, Teruhiko; Sawahata, Katsunori; Sekita, Tsutomu; Kobayashi, Makoto; Tsunoda, Masahiko; Kikuchi, Masamitsu
Hoken Butsuri, 49(1), p.39 - 44, 2014/03
The Japan Atomic Energy Agency has conducted a monthly monitoring of airborne C discharge at the forth research building (RI facility) of the Tokai Research and Development Center. In the current monitoring, C, which exists in various chemical forms in airborne effluent, is converted into CO with CuO catalyst and then collected using monoethanolamine (MEA) as CO absorbent. However, this collection method has some issues on safety management because the CuO catalyst requires a high heating temperature (600C) to ensure a high oxidation efficiency and the MEA is specified as a poisonous and deleterious substance. To establish a safer, manageable and reliable method for monitoring airborne C discharge, we examined collection methods that use different CO absorbents (MEA and Carbo-Sorb E) and oxidation catalysts (CuO, Pt/Alumina and Pd/ZrO). The results showed 100% CO collection efficiency of MEA during a 30-day sampling period under the condition tested. In contrast, Carbo-Sorb E was found to be unsuitable for the monthly-long CO collection because of its high volatile nature. Among the oxidation catalysts, the Pd/ZrO showed the highest oxidation efficiency for CH at a lower temperature.
Nakanishi, Takahiro; Matsunaga, Takeshi; Koarashi, Jun; Atarashi-Andoh, Mariko
Journal of Environmental Radioactivity, 128, p.9 - 14, 2014/02
To investigate Cs mobility deposited on the forest floor because of the Fukushima nuclear accident, we investigated the vertical migration of Cs through seepage water, using a lysimetric method. The study was conducted in a deciduous forest soil over a period spanning two months to two years after the accident. Our observations demonstrated that the major part of Cs in the litter layer moved into the mineral soil within one year after the accident. The topsoil prevented migration of Cs and the annual migration below a 10 cm depth accounted for 0.1% of the total Cs inventory. The migration of Cs by seepage water was undetectable from the vertical distribution of Cs inventory in the soil profile. In the present and immediate future, most of the Cs deposited on the forest floor will probably remain in the topsoil successively, although a small but certain amount of bioavailable Cs exists in forest surface soil.
Nakanishi, Takahiro; Atarashi-Andoh, Mariko; Koarashi, Jun; Kokubu, Yoko; Hirai, Keizo*
Journal of Environmental Radioactivity, 128, p.27 - 32, 2014/02
Although considerable research has been conducted on the importance of recent litter versus older soil organic matter as DOC sources in forest soil, a more thorough evaluation of the temporal patterns of DOC and WEOC is necessary. We investigated the seasonal variation in water-extractable organic carbon in a Japanese cool-temperate beech forest soil by using the carbon isotopic composition (C and C) of WEOC as a tracer for the carbon sources. Fresh leaf litter DOC significantly contributed to WEOC in May when the snowmelt occurred. In the rainy season, increases in the concentration of WEOC and the proportion of hydrophobic compounds were caused by high microbial activity under wetter conditions. From summer to autumn, the WEOC in the mineral horizons was also dominated by microbial release from the soil organic matter. These results indicate that the origin and dynamics of WEOC are strongly controlled by seasonal changes in the environmental conditions.
Ota, Masakazu; Nagai, Haruyasu; Koarashi, Jun
Journal of Geophysical Research; Biogeosciences, 118(4), p.1646 - 1659, 2013/12
We investigated the role of root litter input and dissolved organic carbon (DOC) transport in controlling subsurface SOC dynamics by a soil C model. The model involves C turnover by decomposition, interaction between SOC and DOC, and DOC transport along water flows, for three C pools (characterized by turnover times of years, decades and millennia). Model simulations with a range of rooting profiles demonstrated a large proportion (36-78%) of SOC is apportioned to the subsurface (below the first 30 cm) soils and a significant part (39-73%) of the subsurface SOC turns over decadally. DOC transport appeared to be dominant for distributing the C to the deeper horizons. Our results suggest soil C studies focusing on the surface alone significantly underestimate the stock of decadally cycling C and underpredict the responses of soil C to global changes.