137Cs Trapped by Biomass within 20 km of the Fukushima Daiichi

Jul 26, 2013 - We surveyed forest 20–50 km from the plant in July and September 2011 to evaluate 137Cs trapped in biomass within 20 km of the plant...
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137

Cs Trapped by Biomass within 20 km of the Fukushima Daiichi Nuclear Power Plant Akio Koizumi,*,† Tamon Niisoe,† Kouji H. Harada,† Yukiko Fujii,† Ayumu Adachi,† Toshiaki Hitomi,† and Hirohiko Ishikawa*,‡ †

Department of Health and Environmental Sciences, Graduate School of Medicine, Kyoto University, Kyoto 606-8501, Japan Research Division of Atmospheric and Hydrospheric Disasters, Disaster Prevention Research Institute, Kyoto University, Uji 611-0011, Japan

‡

S Supporting Information *

ABSTRACT: Analysis of 137Cs trapped in biomass in highly contaminated zones is crucial in predicting the long-term fate of 137Cs following the explosion at the Fukushima Daiichi Nuclear Power Plant. We surveyed forest 20−50 km from the plant in July and September 2011 to evaluate 137Cs trapped in biomass within 20 km of the plant. We determined the ambient dose rate and collected forest soils and twigs at 150 sampling points. Removability from the canopy was evaluated by washing leaves and branches with water and organic solvents. The biomass of the forest canopy was then calculated. 137Cs fallout was simulated with an atmospheric transport model. The modeled dose rate agreed with observations (n = 24) (r = 0.62; p < 0.01). Washing experiments demonstrated that unremovable portions accounted for 53.9 ± 6.4% of 137Cs trapped by deciduous canopy (n = 4) and 59.3 ± 13.8% of 137Cs trapped by evergreen canopy (n = 10). In total, it was estimated that 74.5 × 1012 Bq was trapped by canopy in the forest within the no-go zone, with 44.2 × 1012 Bq allocated to unremovable portions, and that 0.86% of the total release was trapped in biomass as of September 2011.

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INTRODUCTION

Following the Tohoku earthquake and tsunami of March 11, 2011, there were explosions at the Fukushima Daiichi Nuclear Power Plant (DNPP) on March 15, which released massive quantities of radionuclides into the atmosphere. The total amount of 137Cs released into the atmosphere has been estimated to be approximately 8.6 × 1015 Bq.1 More than 70% of Fukushima Prefecture (Figure 1) in northern Japan is covered by forest, and the majority of radioactivity fell on this forest. The fates of radionuclides in forest are complicated and not well-understood. Dynamics of 137Cs in forest can be divided into two phases: acute (0−3 years) and late (3+ years) phases.2 In the acute phase, radionuclides are captured by the forest canopy. In the late phase, radionuclides enter the recycling process through absorption from soil, redistribution in the forest ecosystem, and immobilization in tree biomass. Once 137 Cs has entered the recycling process of the forest ecosystem, 137 Cs disappears only at the rate of its radioactive decay with a half-life of 30 years, despite radionuclides in the soil slowly departing the ecological system through heavy precipitation and runoff.2 After the explosions at the DNPP, the Japanese government set up a no-go zone; i.e., an area within a 20 km radius of the DNPP. It has been speculated that massive amounts of radionuclides have fallen in this area and the land will be © 2013 American Chemical Society

Figure 1. Locations of nine sampling sites. The red “×” indicates the Fukushima DNPP. A total of 29 blue boxes and 50 open circles represent model grid cells and monitoring sites by the Japanese government15 inside the 20 km zone. Details of the 9 sampling sites and 50 monitoring sites are listed in Tables S1 and S2 of the Supporting Information.

Received: Revised: Accepted: Published: 9612

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Princeton, NJ). Aliquots from each sample were weighed and sealed in cylindrical plastic containers (outer diameter of 55 mm and height of 55 mm). Branchwood samples were vertically aligned at the center of a container using filling spacers. Leave samples were folded and placed at the center of a container. To confirm homogeneity of samples, samples were analyzed twice at different positions. Characteristic γ-ray energies were monitored to identify and quantify the radiocesium (134Cs, 604.7 keV; 137Cs, 661.7 keV). The detector was calibrated using a γ-ray reference source from the Japan Radioisotope Association (Tokyo, Japan). Cellulose powder fortified with radiocesium was analyzed in the same manner as branchwood and leave samples to match sample geometry. All radioactivity values were corrected to March 15, 2011, using physical half-lives (134Cs, 2.06 years; 137Cs, 30.1 years). Estimation of Biomass and Simulation. The total biomass of the canopy within the 20 km radius was calculated using biomass equations4 and forest statistics officially reported mainly by the Fukushima prefectural authorities.5 To estimate total biomass, we classified the trees into two groups: deciduous and evergreen. The detail is described in section S1 of the Supporting Information. The fallout of 137Cs was calculated for each 5 km area using a widely established atmospheric transport model, the Models-3 Community Multiscale Air Quality (CMAQ) modeling system.6 The computational domain was 440 km wide from west to east and 540 km wide from south to north and included the Fukushima DNPP. The vertical structure had 40 layers constructed with a pressure-based terrain-following coordinate system (i.e., sigma coordinates). The typical depth of the lowest layer was about 40 m. The meteorological fields were computed using version 3.2 of the Weather Research Forecasting (WRF) model,7 including 3-hly analysis with the mesoscale model of the Japan Meteorological Agency. We employed an emission scenario taken from Terada et al.1 that assumed that there was an explosive daytime release on March 15, 2011, when extremely high dose rates up to 11.9 mSv/h were observed at the main gate of the plant after a large explosive sound and vibration in the morning.8,9 137Cs was emitted exclusively in the lowest layer at the point corresponding to the Fukushima DNPP. The simulation period was March 11 to April 1, 2011. The removal of 137Cs was mediated by dry and wet deposition.6 We observed the size distribution of atmospheric 134Cs and 137Cs in Fukushima city in July 2011 and found that the radionuclides mainly belonged to size bins less than 1 μm.10 The dry deposition velocity depends upon the particle size in general, but size dependency is almost negligible for particles with diameters ranging between 0.05 and 2.0 μm.11 The mass concentration and radioactivity of airborne particles less than 0.05 μm in diameter are typically negligible because of the extremely small size of the particles.12 Wet deposition depends upon the precipitation rate rather than the particle size. Therefore, we can discard the effects of the size distribution of particles in both dry and wet deposition. In this work, the particle size was fixed at 1 μm. The accumulated deposition of 137Cs calculated by the CMAQ modeling system was converted to the air dose rate. We assumed that the radioactivity of 134Cs was equal to that of 137 Cs on March 15, 201113 and used conversion factors of 5.4 × −6 10 (mSv/h)/(kBq/m2) for 134Cs and 2.1 × 10−6 (mSv/h)/ (kBq/m2) for 137Cs.14 Amounts of 137Cs trapped by forest canopy were estimated using the relationships between ambient dose rates and levels in branches or leaves/needles (panels A

contaminated heavily with radionuclides. Because the area is a no-go zone, few data are available for the acute phase following the Fukushima DNPP explosions, although such data are essential to the prediction of the long-term fate of 137Cs.2 In the current work, we estimated the fall of radiocesium over the forest in the no-go zone from data obtained in a field survey of the 20−50 km zone of the DNPP. We here report the radiocesium distribution in forest biomass following the Fukushima DNPP explosions as of September 2011.

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MATERIALS AND METHODS Sampling and Determination of Radioactivity. A field survey was conducted in July and September 2011 in forest within the zone 20−50 km from the DNPP, a zone which is referred to as the “emergency evacuation preparation zone” by the Japanese government (Figure 1). We visited nine sampling sites, where we determined the ambient dose rate and collected soil and twigs of deciduous and evergreen trees in the forest (see Table S1 of the Supporting Information). The ambient dose rate was determined about 1 m above the ground at each sampling point using an ionization survey meter (ICS-331, Hitachi Aloka Medical, Ltd., Tokyo, Japan). A soil core (5 cm in diameter and 5 cm deep) was taken from the ground surface at each site by embedding a steel cylinder vertically into the ground. In total, 150 samples were collected, and they are listed in Table S1 of the Supporting Information. Removability was evaluated by washing each sample portion of trees with water and then with a mixture of organic solvents.3 We selected trees with contamination greater than 5.0 × 104 Bq/kg to eliminate the effects of the background level of 137Cs. Each 1 g portion cut out from the point was washed in 10 mL of ultrapure water (≥18 MΩ cm) for 10 min with sonication (40 kHz) in an ultrasonic washing bath (ASU-3 M ASONE, Tokyo, Japan). To remove radionuclides adsorbed by resin, the portion was then washed in 10 mL of a mixture of tetrahydrofuran and toluene (1:1) for 1 min with sonication in the ultrasonic washing bath. Removability was determined as the removed radioactivity divided by the total activity before washing with ultrapure water and the mixture of tetrahydrofuran and toluene. Radiometric analysis of the washing solution was carried out to evaluate the recovery of washing experiments. The time course of removed radioactivity at 1, 3, and 10 min was assessed for three deciduous trees and six evergreen trees. A washing experiment with organic solvent followed by water was also conducted to validate the quantitative removal of radiocesium. Three independent washing experiments were conducted for each sample, and the mean values of the removability were taken for each sample. Autoradiography images of leaves and needles were taken to visualize the radioactivity. An imaging plate (BAS-IP MS 2040, Fuji Film) was exposed for 16 h to leaves and needles at 25 °C. Autoradiography was carried out with a fluorescent image analyzer (FLA2000, Fuji Film, Tokyo, Japan). Images of samples before and after washing were compared, employing the method described below. However, the imaging scanner system automatically adjusted the contrast of images, and the images are therefore only qualitative. Thus, densities of images are not proportional to the radioactivities. Radiometric determinations were performed using a lowbackground high-purity germanium detector with high resolution at the Radioisotope Research Center of Kyoto University and a multichannel analyzer (4096 channels, range of 0−3000 keV, MCA8000, Princeton Gamma Technologies, 9613

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and B of Figure 2), which were determined from our field survey and sampling measurements. Calculation of total

monitoring sites are listed in Table S2 of the Supporting Information. We compared the calculated air dose rates to dose rates observed from May to September 2011, assuming radioactivity of the emitted 131I to be extinct. All data for the monitoring points were binned according to the model resolution into 24 horizontal cells. The fit of the simulated and observed values was evaluated by the fractional difference16 f averaged for all compared grid points f=

Vmdl − Vobs Vmdl + Vobs

where Vmdl and Vobs are the modeled and observed values, respectively. |f | < 0.33 indicates an error factor less than 2, which means Vobs/2 < Vmdl < Vobs × 2.

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RESULTS AND DISCUSSION The ratios of 134Cs/137Cs (mean ± standard deviation) were 0.97 ± 0.06 for soil, 0.97 ± 0.09 for branches, and 1.00 ± 0.11 for leaves/needles (see Table S1 of the Supporting Information). The ambient dose rate was significantly correlated (p < 0.001) with the radioactivities of both branches and leaves/needles (panels A and B of Figure 2). The ambient dose rate ranged from 0.3 to 24.0 μSv/h and was highly correlated with soil radioactivity (Bq/m2) (R2 = 0.543; p < 0.001) (Figure 2C). The correlation suggests that the higher contamination levels lead to greater amounts of 137Cs in forest biomass. The linear and statistically significant (p < 0.05) regression function enabled us to quantitate 137Cs in biomass quasi-quantitatively from the ambient dose rate. Washing experiments were conducted for 4 deciduous trees and 20 evergreen trees (Table 1). Recovery of 137Cs (activity of washed sample and washing solution divided by activity of prewashed sample) in a water washing experiment was 107 ± 14.4% for leaf samples and 106 ± 16.2% for branchwood samples. Recovery in an organic solvent washing experiment was 105 ± 9.4% for leaf samples and 101 ± 11.6% for branchwood samples. The removability of 137 Cs was Table 1. Removability of 137Cs Determined in a Washing Experiment for Leaves/Needles and Branchwood Portions trees leavesa branchwooda canopyb

needlesa

Figure 2. Relationships between ambient dose rates (μSv/h) and radioactivities of biomass/soil: (A) branchwood (Bq/kg), (B) leaves/ needles (Bq/kg) and (C) soil (Bq/m2). Red and blue dots in panels A and B represent evergreen and deciduous trees, respectively.

branchwooda canopyb

radioactivity of 137Cs trapped by forest canopy is described further in section S2 of the Supporting Information. Validation of the Simulation Method. The air dose rate at 50 points within 20 km of the plant was monitored once a week by the Japanese government15 (Figure 1). The

relative weight (%)

fraction

Deciduous Trees (n = 4) 9.1 removable unremovable 90.9 removable unremovable 100.0 removable defoliation unremovable Evergreen Trees (n = 20) 44.5 removable unremovable 55.5 removable unremovable 100.0 removable unremovable

percentage (%) 22.2 77.8 40.7 59.3 39.0 7.1 53.9

± ± ± ± ± ± ±

9.0 9.0 3.6 3.6 5.7 1.1 6.4

45.1 54.9 37.1 62.9 40.7 59.34

± ± ± ± ± ±

18.8 18.8 18.1 18.1 13.8 13.8

a Averages ± standard deviations (SDs) were calculated for means of the samples. The mean for each sample is taken over three independent washing experiments. bThe value for each sample was obtained by adding values for leaves and branches using relative weights (%).

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Figure 3. Autoradiographs of leaves/needles. Films were exposed to leaves/needles for 16 h. Samples were randomly selected. Before washing, a small debris of leaves was scattered on an imaging plate, which showed radioactivity outside the leaves. At the left corner of the image of the washed Japanese black pine, there was one needle apart from other needles (see Figure S1 of the Supporting Information). The imaging scanner system automatically adjusted the contrast of the images, and the images are only a qualitative representation. Thus, sensitivities for individual images are different, and thus, densities cannot be compared to each other.

Figure 4. (A) Distribution of the calculated air dose rate (μSv/h). A total of 24 model grid cells used for comparison to observations are indicated by asterisks. (B) Comparison of observations and calculations averaged from May to September 2011 at monitoring points within 20 km of the Fukushima DNPP. The dose rate is corrected to March 15. Agreement is indicated by a correlation coefficient r2 of 0.78 (p = 1.4 × 10−8) and an averaged fractional difference f of 0.06. The solid line indicates a perfect fit between observations and calculations, and the two dashed lines and two dotted lines indicate error factors of 2 and 3, respectively.

and 10 min were compatible (deciduous trees, 9.0 ± 5.7, 14.6 ± 8.6, and 16.2 ± 5.2%; evergreen trees, 17.8 ± 10.4, 21.6 ± 12.8, and 22.8 ± 14.5%, respectively). Therefore, easily detachable 137 Cs seemed to be removed within 10 min. Unremovable portions of the total canopy were estimated using relative weights. Unremovable portions accounted for 53.9% of all fractions for deciduous trees, assuming all leaves to be

determined in a washing experiment for leaves/needles and branchwood portions (Table 1). More than 60% of the trapped radioactivity was associated with unremovable fractions. These values were comparable to those in a washing experiment with organic solvent followed by water (deciduous trees, 32.1 ± 8.7 and 21.9 ± 8.5% for branchwood and leaf, respectively; evergreen trees, 36.3 ± 15.5 and 33.7 ± 19.0% for branchwood and leaf, respectively). Removal proportions of 137Cs at 1, 3, 9615

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1.2 × 1013 1.3 × 1013

2.0 × 1013 4.2 × 1013 6.2 × 1013 7.5 × 1013

3.3 × 108

4.1 × 108

5.9 × 108

7.1 × 108

1.3 × 109

1.7 × 109

branch wood

canopy

needles

branch wood

canopy

total canopy

a

9.0 × 1011

7.5 × 107

leaves

PI = prediction interval.

biomass

estimated Bq in the fraction

removable unremovable removable unremovable removable unremovable removable + defoliation unremovable

removable unremovable removable unremovable removable + defoliation unremovable

fraction

Cs Radioactivity Trapped by Twigs and Estimation of

137

estimated volume (kg)

Table 2.

9.2 1.1 1.5 2.6 2.5 3.7 3.0

× × × × × × × 1012 1013 1013 1013 1013 1013 1013 8.1 × 1012

2.8 × 1014

Deciduous Trees 1.9 × 1012 2.1 × 1010 12 6.7 × 10 7.2 × 1010 6.3 × 1013 3.5 × 1011 13 9.2 × 10 5.2 × 1011 13 7.2 × 10 4.5 × 1011

4.4 × 1013

1011 1011 1012 1012 1012 9.2 × 1013 5.2 × 1011 Evergreen Trees 4.7 × 1013 1.8 × 1012 13 5.7 × 10 2.2 × 1012 7.5 × 1013 3.2 × 1012 1.3 × 1014 5.4 × 1012 14 1.2 × 10 5.0 × 1012 14 1.8 × 10 7.6 × 1012 1.9 × 1014 5.5 × 1012

× × × × ×

95% PI lower

a

6.9 × 1012

2.0 7.0 4.7 6.9 5.6

95% PI upper

a

× × × × × 1011 1012 1013 1013 1013

× × × × × × × 1013 1013 1013 1013 1013 1014 1013

1.3 × 1014

2.8 3.4 4.6 7.9 7.4 1.1 9.1

2.1 × 1013

6.0 2.1 1.4 2.1 1.7

upper deposition case

Cs Trapped by Biomass within 20 km

estimate

137 137

× × × × × 1010 1011 1012 1012 1012

× × × × × × × 1012 1012 1012 1012 1012 1013 1013 1.5 × 1013

3.1 3.7 5.1 8.7 8.2 1.2 1.0

2.3 × 1012

6.6 2.3 1.6 2.3 1.9

lower deposition case

Cs (Bq)

× × × × × 1011 1011 1012 1012 1012

× × × × × × ×

1012 1013 1012 1013 1013 1013 1013 5.9 × 1013

4.6 1.6 7.7 3.4 1.2 5.0 1.6

9.3 × 1012

1.0 8.0 2.4 9.3 3.3

lower removable case × × × × ×

1012 1013 1013 1014 1014

× × × × × × ×

1013 1014 1014 1014 1014 1014 1014 1.1 × 1015

7.0 2.4 1.1 4.9 1.8 7.3 3.0

3.7 × 1014

2.9 2.3 9.5 3.7 1.2

total upper limit

× × × × ×

109 1010 1011 1011 1011

× × × × × × ×

1011 1011 1012 1012 1012 1012 1012 2.7 × 1012

6.1 7.4 1.1 1.8 1.7 2.5 1.8

1.7 × 1011

6.9 2.4 1.2 1.7 1.5

total lower limit

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There are several sources of uncertainties in this study: (1) uncertainty because of variability of the samples collected in the field survey, (2) model uncertainty, (3) uncertainty in the relationship between the ambient dose rates and the radioactivity of biomass, and (4) uncertainty because of discrepancies between washing experiments and the real-time situation of washing in the forest canopy. We presented uncertainty in the 137Cs amounts because of variability of the samples collected in the field survey and the relationship between ambient dose rates and radioactivities in biomass as a 95% prediction interval (see Figure S2 of the Supporting Information). According to prediction intervals, we incorporated uncertainty in 137Cs radioactivity trapped by canopy in Table 2. We evaluated uncertainty in the modeled dose rates, which was due to uncertainty in the modeled deposition flux, as an error factor less than 3 through comparison to observations (Figure 4B). The fourth uncertainty is estimated to be an error of factor 2, as stated above. To calculate the total uncertainty, the 95% upper and lower predicted intervals for uncertainties 1 and 3 were multiplied by 3 and 1/3 to adjust for uncertainty 2 and obtain upper and lower depositions, 50 or 100% of the removable portion of data obtained in washing experiments was used to calculate uncertainty 4, and the final upper and lower limits were then calculated. The estimates had uncertainties of 3 orders of magnitude. Our results thus suggest that 0.86% (0.1−16.3%) of all 137Cs released by the Fukushima DNPP has been trapped by biomass. This fraction accounts for 10.7% (1.9−67.2%) of the accumulated amount of 137Cs deposited within the forest in the no-go zone (see Table S3 of the Supporting Information) and is considered to be recycled within the forest in the no-go zone. In the Chernobyl nuclear power plant accident (NPPA), 8.5 × 1016 Bq of 137Cs was estimated to have been released to the environment,17 which is 9.9 times the amount released from the Fukushima DNPP. Although there is no estimate of the 137 Cs fallout on the forest in the Chernobyl NPPA, disappearance rates because of radioactive decay and transport off site for 137Cs in the forest are known to be extremely low and have been estimated to be less than 1% per year.19 Thus, recycling 137Cs within the forest ecosystem of the no-go zone will occur over the long term in Fukushima, and the zone will be a large 137Cs reservoir. It is difficult to implement effective countermeasures or technology-based decontamination in the forest as in the Chernobyl NPPA. Thus, primitive countermeasures, such as the restriction of activities and the gathering of wild food or firewood, should be implemented for the next 20 years within the no-go zone area in Fukushima. The current study involves several assumptions and further uncertainties. First, we cannot provide data on the relationships between ambient dose rates and radiation levels in branches or leaves/needles at sites where the air dose rates are greater than 24 μSv/h, which was the maximum value among observed air dose rate levels. Thus, 137Cs trapped by branches and leaves/ needles at model grid points within the no-go zone may be underestimated because calculated air dose rates at 2 of the 29 model grid points in this zone were greater than 24 μSv/h. Nevertheless, the unremovable fraction did not increase significantly, only from 0.51 to 0.61%, when the upper limit was not set. Second, 137Cs in forest was assumed to be in an acute phase and not accompanied by recycling in September 2011. These assumptions may again result in the underestimation of 137Cs trapped in biomass.

defoliated, and 59.3% for evergreen trees. It should be noted that washing by sonication could damage vegetation and the removable fraction could include radionuclides absorbed inside the organ. It should also be noted that there are discrepancies between the washing experiment and real-time situation of washing in the forest canopy. The current washing method has been validated in a previous study,17 and a previous investigation conducted in the acute phase following Chernobyl reported the removal portion as 25%18 for evergreen trees, while we found a value of 45%. Taken together, we estimated total uncertainty in the removal portion as 80%; i.e., the uncertainty inherent in our method is a factor of 2. Absorption of radioactivity by trees was qualitatively confirmed by autoradiography (Figure 3). Leaf and needle shapes were clearly observed. In a control measurement using the same species in Kyoto, no radioactive image was observed (data not shown). Therefore, the radioactivity could result from radionuclides emitted from the DNPP. Major emitted radionuclides with short half-lives had disappeared in this study period, and radiocesium was considered a predominant radionuclide. Radioactivity was observed in the leaf blade and veins of a Japanese beech leaf even after washing, suggesting that radioactivity was absorbed by trees. There were many small spots on the needles of Japanese black pine, while the origin remains unclear. We next estimated the radioactivity trapped by the forest canopy within the 20 km radius. Ambient dose rates were calculated using WRF/CMAQ models, and the trapped amounts were estimated using the relationships between the ambient dose rate and radioactivity in branches or leaves/ needles, which were established from observations between radii of 20 and 50 km (panels A and B of Figure 2). The modeled dose rate generally agreed with observed values, as indicated by a correlation coefficient r2 of 0.78 and an averaged fractional difference f of 0.06 (panels A and B of Figure 4). At 17 of 24 grid points, the air dose rate was reproduced with an error factor less than 2. The error factors at seven other grid points were still less than about 3. The model estimates the fallout on land within the 20 km radius as 10.5 × 1014 Bq, which accounts for 12.2% of the total emissions (8.6 × 1015 Bq) 1 from the crippled plant (see Table S3 of the Supporting Information). We next estimated the amounts trapped by biomass within the 20 km radius of the Fukushima DNPP. It is estimated that 74.5 × 1012 Bq or 0.86% of the total released has been captured by forest canopy (Table 2). Evergreen trees trapped 62.0 × 1012 Bq of that total. In autumn 2011, 0.7 × 1012 Bq of 137Cs, corresponding to the unremovable portion trapped by leaves of deciduous trees, had fallen onto the forest floor (Table 2). The major portion of the radioactivity, 44.2 × 1012 Bq, is estimated to have already been absorbed by trees and accounts for 0.51% of the total emission (sum of unremovable 137Cs amounts in branchwood of deciduous trees and needles/branchwood of evergreen trees in Table 2). Because 137Cs has no chemical production or degradation process, except radioactive decay in the atmosphere, the deposition flux calculated by the WRF/CMAQ model is proportional to the emission. The calculated 137Cs amount trapped by biomass is nearly proportional to the air dose rate, which was derived from the modeled accumulated deposition, and consequently, also to the emission. Although the amounts were affected by uncertainty of the emission, the fractional values were not. 9617

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(9) Fukushima Nuclear Accident Analysis Report; Tokyo Electric Power Company: Tokyo, Japan, 2012; www.tepco.co.jp/en/press/ corp-com/release/betu12_e/images/120620e0102.pdf (accessed Feb 15, 2013) (10) Koizumi, A.; Harada, K. H.; Niisoe, T.; Adachi, A.; Fujii, Y.; Hitomi, T.; Kobayashi, H.; Wada, Y.; Watanabe, T.; Ishikawa, H. Preliminary assessment of ecological exposure of adult residents in Fukushima Prefecture to radioactive cesium through ingestion and inhalation. Environ. Health Prev. Med. 2012, 17, 292−298. (11) Slinn, W. G. N.; Hasse, L.; Hicks, B. B.; Hogan, A. W.; Lai, D.; Liss, P. S.; Munnich, K. O.; Sehmel, G. A.; Vittori, O. Some aspects of the transfer of atmospheric trace constituents past the air−sea interface. Atmos. Environ. 1978, 12, 2055−2087. (12) Seinfeld, J. H.; Pandis, S. N. Atmospheric Chemistry and Physics; John Wiley: Hoboken, NJ, 1998. (13) Chino, M.; Nakayama, H.; Nagai, H.; Terada, H.; Katata, G.; Yamazawa, H. Preliminary estimation of release amounts of 131I and 137 Cs accidentally discharged from the Fukushima Daiichi Nuclear Power Plant into the atmosphere. J. Nucl. Sci. Technol. 2011, 48, 1129−1134. (14) Generic Procedures for Assessment and Response during a Radiological Emergency; International Atomic Energy Agency (IAEA): Vienna, Austria, 2000; IAEA-TECDOC-1162, www-pub.iaea.org/ mtcd/publications/pdf/te_1162_prn.pdf (accessed June 21, 2012). (15) Readings of Radioactivity Level Inside of the 20 km Zone of Fukushima Dai-ichi NPP; Nuclear Regulation Authority: Tokyo, Japan, 2011; http://radioactivity.mext.go.jp/en/list/197/list-1.html (accessed June 21, 2012). (16) Kasibhatla, P.; Chameides, W. L.; St. John, J. A threedimensional global model investigation of seasonal variations in the atmospheric burden of anthropogenic sulfate aerosols. J. Geophys. Res. 1997, 102, 3737−3760. (17) Wyttenbach, A.; Bajo, S.; Tobler, L. Spruce needles: Standards and real samples. Fresenius’ J. Anal. Chem. 1993, 345, 294−297. (18) Tobler, L.; Bajo, S.; Wyttenbach, A. Deposition of 134,137Cs from Chernobyl fallout on Norway spruce and forest soil and its incorporation into spruce twigs. J. Environ. Radioact. 1988, 6, 225− 245. (19) United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR). Health effects due to radiation from the Chernobyl accident. In Sources and Effects of Ionizing Radiation: UNSCEAR 2008 Report to the General Assembly with Scientific Annexes; United Nations: New York, 2011; Vol. 2, Annex D; www.unscear.org/ docs/reports/2008/11-80076_Report_2008_Annex_D.pdf (accessed June 21, 2012).

Human communities in Fukushima are not ecologically separated from the forest, and thus, the fate of 137Cs in the forest will profoundly affect contamination levels in these communities. Long-term monitoring within the forest is needed to elucidate the complex dynamics of radionuclides in biomass, to construct rational decontamination strategies, and to evaluate the effects on human health, agriculture, and the timber industry.

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ASSOCIATED CONTENT

* Supporting Information S

Detailed information on calculations, collected samples, the monitoring data set, and additional results. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: 81-75-753-4456 (A.K.); 81-774-38-4159 (H.I.). Fax: 81-75-753-4458 (A.K.); 81-774-38-4158 (H.I.). E-mail: [email protected] (A.K.); [email protected]. kyoto-u.ac.jp (H.I.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by an urgent collaborative research grant from the Disaster Prevention Research Institute, Kyoto University (23U-01), the Environment Research and Technology Development Fund of the Ministry of the Environment (ZB-1202), and the Tokyo Kenbikyoin Foundation.

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REFERENCES

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dx.doi.org/10.1021/es401422g | Environ. Sci. Technol. 2013, 47, 9612−9618