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Journal Articles

Spatial variations in radiocesium deposition and litter-soil distribution in a mountainous forest catchment affected by the Fukushima nuclear accident

Atarashi-Andoh, Mariko; Koarashi, Jun; Tsuzuki, Katsunori; Takeuchi, Erina; Nishimura, Shusaku; Muto, Kotomi*; Matsunaga, Takeshi*

Journal of Environmental Radioactivity, 238-239, p.106725_1 - 106725_8, 2021/11

To understand the spatial variation in soil $$^{137}$$Cs inventory in complex mountainous topography, a whole-area investigation of $$^{137}$$Cs deposition in a broad-leaved forest catchment of a mountain stream was conducted using grid sampling. Across the catchment, organic and surface mineral soil layers were collected at 42 locations in 2013 and 6 locations in 2015. $$^{137}$$Cs deposition on the forest floor exhibited high spatial heterogeneity and altitude-dependent distribution over the catchment. The $$^{137}$$Cs retention ratio in the organic layer ranged from 6% to 82% in 2013. The $$^{137}$$Cs retention ratios had positive correlations with the material inventory in the organic layer and the elevation. The $$^{137}$$Cs retention ratios in the organic layer were less than 20% in 2015, even at the locations where the retention ratio was higher than 55% in 2013. Although there was spatial variation in the migration speed, $$^{137}$$Cs migration from the organic layer to mineral soil was almost completed within 4 y of the deposition.

Journal Articles

Characterizing vertical migration of $$^{137}$$Cs in organic layer and mineral soil in Japanese forests; Four-year observation and model analysis

Muto, Kotomi; Atarashi-Andoh, Mariko; Matsunaga, Takeshi*; Koarashi, Jun

Journal of Environmental Radioactivity, 208-209, p.106040_1 - 106040_10, 2019/11

 Times Cited Count:7 Percentile:57.6(Environmental Sciences)

Vertical distributions of $$^{137}$$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 $$^{137}$$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 $$^{137}$$Cs could be suppressed rapidly in Japanese forests. The diffusion coefficients of $$^{137}$$Cs in the mineral soil were estimated to be 0.042-0.55 cm$$^2$$y$$^{-1}$$, which were roughly comparable with those of European forest soils affected by the Chernobyl Nuclear Power Plant accident. Model predictions indicated $$^{137}$$Cs mainly distributed in the surface mineral soil at 10 years after the accident. It suggest that the $$^{137}$$Cs deposited onto Japanese forest ecosystems will be retained in the surface layers of mineral soil for a long time.

Journal Articles

A New perspective on the $$^{137}$$Cs retention mechanism in surface soils during the early stage after the Fukushima nuclear accident

Koarashi, Jun; Nishimura, Shusaku; Atarashi-Andoh, Mariko; Muto, Kotomi; Matsunaga, Takeshi*

Scientific Reports (Internet), 9, p.7034_1 - 7034_10, 2019/05

 Times Cited Count:19 Percentile:84.46(Multidisciplinary Sciences)

The aim of the present study is to explore the retention mechanisms of $$^{137}$$Cs in the surface soil layers of terrestrial ecosystems affected by the Fukushima NPP accident, with a specific focus on the interactions between $$^{137}$$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 $$^{137}$$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 $$^{137}$$Cs extractability of the soils, $$^{137}$$Cs incorporation into the complex structure of particulate organic matter is likely the main mechanism for $$^{137}$$Cs retention in the surface soil layers.

Journal Articles

Radiocesium distribution in aggregate-size fractions of cropland and forest soils affected by the Fukushima nuclear accident

Koarashi, Jun; Nishimura, Shusaku; Atarashi-Andoh, Mariko; Matsunaga, Takeshi*; Sato, Tsutomu*; Nagao, Seiya*

Chemosphere, 205, p.147 - 155, 2018/08

 Times Cited Count:14 Percentile:65.6(Environmental Sciences)

There is little understanding of how soil aggregation can affect the mobility and bioavailability of $$^{137}$$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 $$^{137}$$Cs content and extractability and mineral composition. In forest soils, aggregate formation was significant, and $$^{137}$$Cs was largely associated with large-sized aggregates. In contrast, there was less aggregation in agricultural field soils, and most of $$^{137}$$Cs was in the clay- and silt-sized fractions. Across all sites, the $$^{137}$$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 $$^{137}$$Cs in organic-rich (forest and orchard) soils.

Journal Articles

Sources of $$^{137}$$Cs fluvial export from a forest catchment evaluated by stable carbon and nitrogen isotopic characterization of organic matter

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

 Times Cited Count:12 Percentile:86.92(Chemistry, Analytical)

Fluvial export of particulate and dissolved $$^{137}$$Cs was investigated to reveal its sources and transfer mechanisms in a broadleaved forest catchment using a continuous collection system. The finest size fraction ($$<$$ 75$$mu$$m), consisting of decomposed litter and surface mineral soil, was the dominant fraction in the particulate $$^{137}$$Cs load, although the contribution of coarser size fractions increased during high water discharge in 2014. The dissolved $$^{137}$$Cs originated from the decomposition of $$^{137}$$Cs-contaminated litter. Temporal changes in $$^{137}$$Cs distribution in the litter-mineral soil system indicated that the dissolved $$^{137}$$Cs load will be moderated in several years, while particulate $$^{137}$$Cs load has the potential to continue for a long time.

Journal Articles

Vertical distributions of global fallout $$^{137}$$Cs and $$^{14}$$C in a Japanese forest soil profile and their implications for the fate and migration processes of Fukushima-derived $$^{137}$$Cs

Koarashi, Jun; Atarashi-Andoh, Mariko; Amano, Hikaru*; Matsunaga, Takeshi

Journal of Radioanalytical and Nuclear Chemistry, 311(1), p.473 - 481, 2017/01

 Times Cited Count:14 Percentile:88.74(Chemistry, Analytical)

Vertical distributions of global fallout $$^{137}$$Cs and $$^{14}$$C were investigated in a Japanese forest soil in 2001. Even 38 years after the fallout, $$^{137}$$Cs was still observed mostly in the uppermost 5 cm. A preferential accumulation of $$^{137}$$Cs was found in a 1-cm-thick transition layer between organic-rich A and underlying B horizons. This unique observation indicated that $$^{137}$$Cs migrated through the A horizon at a rate of 0.20% y$$^{-1}$$ and the transition layer acted as a barrier for $$^{137}$$Cs migration to deeper layers. The vertical distributions of $$^{137}$$Cs and $$^{14}$$C were significantly correlated, suggesting a coupled downward migration of $$^{137}$$Cs and organic matter on a time scale of decades, along the same physical pathways.

Journal Articles

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

Koarashi, Jun; Atarashi-Andoh, Mariko; Matsunaga, Takeshi; Sanada, Yukihisa

Scientific Reports (Internet), 6, p.38591_1 - 38591_11, 2016/12

 Times Cited Count:35 Percentile:83.58(Multidisciplinary Sciences)

Forest-floor organic layers play a key role in controlling the overall bioavailability of $$^{137}$$Cs in forest ecosystems; however, there is still an insufficient understanding of how forest types influence the retention capability of $$^{137}$$Cs in organic layers in Japanese forest ecosystems. Here we conducted plot-scale investigations on the retention of $$^{137}$$Cs in organic layers at two contrasting forest sites in Fukushima. In a deciduous broad-leaved forest, approximately 80% of the deposited $$^{137}$$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 $$^{137}$$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 $$^{137}$$Cs associated with litter materials with different degrees of degradation in the organic layers.

Journal Articles

Year-round variations in the fluvial transport load of particulate $$^{137}$$Cs in a forested catchment affected by the Fukushima Daiichi Nuclear Power Plant accident

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


 Times Cited Count:5 Percentile:54.39(Chemistry, Analytical)

Particulate $$^{137}$$Cs in stream water was collected continuously for two years in order to assess the long-term trend of the $$^{137}$$Cs discharge from the forest environment. Sampling was conducted from December 2011 to December 2013 in a mountainous stream, which received the $$^{137}$$Cs from the Fukushima Daiichi Nuclear Power Plant accident. A seasonal increase in fluvial transport load of particulate $$^{137}$$Cs associated with suspended solids (SS) was observed in August and September when rainfall was abundant. The particulate $$^{137}$$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 $$^{137}$$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 $$^{137}$$Cs concentration in SS.

Journal Articles

Fluvial discharges of particulate and dissolved radiocesium from a forest and its monthly trend

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 $$mu$$m, 500-2000 $$mu$$m, 75-500 $$mu$$m, and $$<$$75 $$mu$$m fractions. The $$^{137}$$Cs concentrations in the samples were measured using $$gamma$$-ray spectrometry with Ge semiconductor detectors. The $$^{137}$$Cs discharge increased with the river water discharge. The particulate $$^{137}$$Cs discharge was dominant in both 2013 and 2014. The $$^{137}$$Cs discharge rate of the dissolved component increased in winter, when the river water discharge decreased.

Journal Articles

Catchment-scale distribution of radiocesium air dose rate in a mountainous deciduous forest and its relation to topography

Atarashi-Andoh, Mariko; Koarashi, Jun; Takeuchi, Erina; Tsuzuki, Katsunori; Nishimura, Shusaku; Matsunaga, Takeshi

Journal of Environmental Radioactivity, 147, p.1 - 7, 2015/09


 Times Cited Count:19 Percentile:61.83(Environmental Sciences)

We collected a large amount of radiocesium air dose rate data by mountain-walking with a small $$gamma$$-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$$^{2}$$ 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.

Journal Articles

Increase in rare earth element concentrations controlled by dissolved organic matter in river water during rainfall events in a temperate, small forested catchment

Matsunaga, Takeshi; Tsuzuki, Katsunori; Yanase, Nobuyuki; Kritsananuwat, R.*; Hanzawa, Yukiko; Naganawa, Hirochika

Journal of Nuclear Science and Technology, 52(4), p.514 - 529, 2015/04

 Times Cited Count:3 Percentile:32.14(Nuclear Science & Technology)

In a temperate, forested river catchment, distribution of dissolved rare earth elements (REE) during rainfall events was studied as analogues elements for transuranic nuclides. Concentrations of dissolved REE showed almost concurrent increases and decreases with the river water discharge during rainfall events. The concentration variations were tightly coupled with those of optical properties related to humic substances of dissolved organic matter (DOM). An ultrafiltration analysis revealed that colloidal REE present in 10 k - 30 kDa size fraction was the chief component for dissolved REE increase in high water flow condition. Shale-normalized concentration patterns of REE suggest an involvement of humic substances of DOM. A high correlation between size fractioned REE concentrations and specific ultraviolet absorbance suggests that aromaticity would be an essential property of DOM in regard to its complexation with dissolved REE in the studied river water.

Journal Articles

A Passive collection system for whole size fractions in river suspended solids

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

 Times Cited Count:3 Percentile:32.14(Chemistry, Analytical)

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.

Journal Articles

$$^{137}$$Cs vertical migration in a deciduous forest soil following the Fukushima Dai-ichi Nuclear Power Plant accident

Nakanishi, Takahiro; Matsunaga, Takeshi; Koarashi, Jun; Atarashi-Andoh, Mariko

Journal of Environmental Radioactivity, 128, p.9 - 14, 2014/02

 Times Cited Count:74 Percentile:93.41(Environmental Sciences)

To investigate $$^{137}$$Cs mobility deposited on the forest floor because of the Fukushima nuclear accident, we investigated the vertical migration of $$^{137}$$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 $$^{137}$$Cs in the litter layer moved into the mineral soil within one year after the accident. The topsoil prevented migration of $$^{137}$$Cs and the annual migration below a 10 cm depth accounted for 0.1% of the total $$^{137}$$Cs inventory. The migration of $$^{137}$$Cs by seepage water was undetectable from the vertical distribution of $$^{137}$$Cs inventory in the soil profile. In the present and immediate future, most of the $$^{137}$$Cs deposited on the forest floor will probably remain in the topsoil successively, although a small but certain amount of bioavailable $$^{137}$$Cs exists in forest surface soil.

Journal Articles

Temporal variations in metal enrichment in suspended particulate matter during rainfall events in a rural stream

Matsunaga, Takeshi; Tsuzuki, Katsunori; Yanase, Nobuyuki; Kritsananuwat, R.*; Ueno, Takashi; Hanzawa, Yukiko; Naganawa, Hirochika

Limnology, 15(1), p.13 - 25, 2014/01

 Times Cited Count:5 Percentile:21.94(Limnology)

We studied a discharge of heavy metals/metalloids in rainfall events to a rural stream in Kuji River basin of central Japan. In terms of elemental enrichment with respect to the crustal composition, we found similarity between atmospheric deposits and suspended particulate matter (SPM) at a rural stream. Both exhibited distinctive enrichment in several metals (Ni, Cu, Pb, Sb, and Cd). Elemental analysis of SPM in the stream in rainfall events at short intervals revealed that the metal enrichment factors in SPM were progressively decreased with an increase of flow rate. Judging from features of SPM, this phenomenon was probably caused by alternative change of SPM matrices from upper soil constituents to more lithologic material with increasing stream flow rate. In quantification of respective contributions of metals of different origins to fluvial discharge, change of SPM matrices during a rainfall event and involvement of dissolved fraction need to be taken into consideration.

Journal Articles

Comparison of the vertical distributions of Fukushima nuclear accident radiocesium in soil before and after the first rainy season, with physicochemical and mineralogical interpretations

Matsunaga, Takeshi; Koarashi, Jun; Atarashi-Andoh, Mariko; Nagao, Seiya*; Sato, Tsutomu*; Nagai, Haruyasu

Science of the Total Environment, 447, p.301 - 314, 2013/03

 Times Cited Count:108 Percentile:96.09(Environmental Sciences)

The effect of intensive rainfall in the on distribution of Fukushima accident-derived $$^{137}$$Cs in soil was examined in Fukushima-city in post-rainy season and were compared with those in pre-rainy season. It is concluded that rainfall during the rainy season had a limited effect on $$^{137}$$Cs distribution in soil, indicating its overall immobility. Chemical extraction of $$^{137}$$Cs from selected soil samples indicated that $$^{137}$$Cs in soil was hardly water-soluble and extractable with 1M ammonium acetate. This supports the overall immobility of $$^{137}$$Cs in our soils. No direct relationship was obtained between the mineral composition and the $$^{137}$$Cs retention in upper soil layer. In contrast, positive correlations were found between the $$^{137}$$Cs extractability and soil properties such as pH, organic matter content, and finer-sized particle contents in the soils. These results suggest that the mineralogical effect may be masked by non-specific adsorption offered by physicochemical properties of the soils.

Journal Articles

Retention of potentially mobile radiocesium in forest surface soils affected by the Fukushima nuclear accident

Koarashi, Jun; Moriya, Koichi*; Atarashi-Andoh, Mariko; Matsunaga, Takeshi; Fujita, Hiroki; Nagaoka, Mika

Scientific Reports (Internet), 2, p.1005_1 - 1005_5, 2012/12

 Times Cited Count:34 Percentile:71.91(Multidisciplinary Sciences)

The fate of $$^{137}$$Cs derived from the Fukushima nuclear accident fallout and associated radiological hazards are largely dependent on its mobility in the surface soils of forest ecosystems. Thus, we quantified microbial and adsorptive retentions of $$^{137}$$Cs in forest surface (0-3 cm) soils. The K$$_{2}$$SO$$_{4}$$ extraction process liberated 2.1%-12.8% of the total $$^{137}$$Cs from the soils. Two soils with a higher content of clay- and silt-sized particles, organic carbon content, and cation exchange capacity showed higher $$^{137}$$Cs extractability. Microbial biomass was observed in all of the soils. However, the $$^{137}$$Cs extractability did not increase after destruction of the microbial biomass by chloroform fumigation, providing no evidence for microbial retention of the Fukushima-fallout $$^{137}$$Cs. The results indicate that uptake of $$^{137}$$Cs by soil microorganisms is less important for retention of potentially mobile $$^{137}$$Cs in the forest surface soils compared to ion-exchange adsorption on non-specific sites provided by abiotic components.

Journal Articles

Factors affecting vertical distribution of Fukushima accident-derived radiocesium in soil under different land-use conditions

Koarashi, Jun; Atarashi-Andoh, Mariko; Matsunaga, Takeshi; Sato, Tsutomu*; Nagao, Seiya*; Nagai, Haruyasu

Science of the Total Environment, 431, p.392 - 401, 2012/08

 Times Cited Count:129 Percentile:96.32(Environmental Sciences)

no abstracts in English

Journal Articles

Migration of atmospherically derived radionuclides in river systems; Findings from researches related to the Chernobyl accidents and nuclear weapon tests

Matsunaga, Takeshi; Tkachenko, Y.*

Nihon Genshiryoku Gakkai-Shi ATOMO$$Sigma$$, 53(10), p.684 - 688, 2011/10

Features of radionuclides migration behavior in river systems have been summarized. Referred researches are those related to the global fallout due to weapon tests and the fallout due the Chernobyl accident. This summary aims to serve to provide basic findings on an occasion of the Fukushima nuclear accident. Migration of radionuclides in river systems is a persistent process including various time scale events. Differences by radionuclides appear clearly in their input fractions from the ground, and also in their long-range transport. For river systems' contamination, it is considered that countermeasures at stages of water treatment and aquatic food intake. Several decision support systems, which compare effectiveness and adverse effects of various countermeasures, must be very useful to examine that applicability comprehensively.

JAEA Reports

Proceedings of the Meeting on Computational and Experimental Studies for Modeling of Radionuclide Migration in Complex Aquatic Ecosystems

Matsunaga, Takeshi; H${aa}$kanson, L.*

JAEA-Conf 2010-003, 143 Pages, 2010/09


The Research Group for Environmental Science of JAEA held a meeting on computational and experimental studies for modeling of radionuclide migration in complex aquatic ecosystems during November 16-20 of 2009. The aim was to discuss the relevance of various computational and experimental approaches to that modeling. The meeting was attended by a Swedish researcher, Prof. Dr. Lars H${aa}$kanson of Uppsala University. The meeting demonstrated that it is crucial (1) to make a model structure be strictly relevant to the target objectives of a study and (2) to account for inherent fluctuations in target values in nature in a manner of qualitative parameterization. Moreover, it was confirmed that there can be multiple different approaches of modeling with relevance for the objectives of a study. These discussions should be considered in model integration for complex aquatic ecosystems consisting catchments, rivers, lakes and coastal oceans which can interact with the atmosphere.

Journal Articles

Environmental behavior of plutonium isotopes studied in the area affected by the Chernobyl accident

Matsunaga, Takeshi; Nagao, Seiya*

Humic Substances Research, 5/6(1), p.19 - 33, 2009/00

The environmental behavior of Chernobyl-derived plutonium has been reviewed in the soil and the aquatic environment in Ukrainian affected areas. Dissolution of released fuel particles was an initiation for migration in the environment for the Chernobyl-derived plutonium. Once dissolved, the plutonium distributed persistently over surface soil layer with a limited infiltration ability. This relative immobility of plutonium in the soil environment can be attributed to its affinity for soil organics, based on chemical partitioning investigations. The relative immobility of plutonium was also seen in its fluvial transport along the Dnieper River. Dissolved organic matter, most probably humic substances, helped stabilize dissolved plutonium in the lake waters.

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