The Great East Japan Earthquake occurred on March 11, 2011, and 4 days later, the Tokyo Electric Power Company (TEPCO) Fukushima No. 1 Nuclear Power Plant accident happened. The accident caused serious nuclear pollution damage for the Fukushima area, and it was reported that the forest area had received especially severe damage. However, its present situation has not been studied yet, nor has reforestation been planned. Therefore, we surveyed the amount of nuclear radiation at the forest of Minamisoma City where the amount of radiation has been reported as extremely high. We set several survey plots in the forest and surveyed the radiation amount of leaves, branches, wood (bark and stem), soil, litter interception, and irrigation water. The surveying results showed nuclear pollution was not spread equally in the Minamisoma forest, but in several “hot spots,” that some litter interception indicated high radiation amounts, the extraction rates from bark to xylem were different between conifers and deciduous trees and between standing living trees and mushroom bed logs, and radioactive cesium was not detected in transpiration water.
13.1 Introduction
The Great East Japan Earthquake occurred on March 11, 2011. It was followed during the next 4 days by the Tokyo Electric Power Company (TEPCO) Fukushima Daiichi Nuclear Power Station accident. The accident caused serious nuclear pollution damage in the Fukushima area, and it has been reported that forested areas sustained especially severe radiation damage (Uehara et al. 2014; Kaneko et al. 2013; Nonaka et al. 2012; Schreurs and Yoshida 2013; Takeuchi 2011). However, no definitive research has yet been undertaken into the consequences of the accident, and no plans exist for rehabilitating the forest. This study therefore surveyed the nuclear radiation levels in the forest at the city of Minamisoma, where radiation levels were reported to be extremely high (Ministry of Education, Culture, Sports, Science and Technology 2011).
13.2 Methods
We identified six survey areas in the Minamisoma forest (Fig. 13.1) and surveyed the radioactive cesium levels of leaves, branches, trunks (bark and xylem), soil, litter layer, and irrigation water. We also surveyed transpiration water from the leaves of trees. We used a radiation surveying system and survey meter from Hitachi Aloka Medical and a germanium detector system from CANBERRA Industries. The survey lasted from April 2012 to July 2013.
Fig. 13.1
Survey areas in Minamisoma
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13.3 Results
13.3.1 Forest Soil
Radiation levels found in the soil around conifers (Chamaecyparis and Cryptomeria) were high, whereas levels around deciduous Quercus were lower. The radiation levels in clear-cut plots were lower than in the stands (Table 13.1).
Table 13.1
Radioactive cesium (Cs) detected at 5-cm depth in forest soil (Bq/kg)
Area
Altitude (m)
134Cs
137Cs
Total Cs
Haramachi-ku
135
13,600
19,100
32,700
Chamaecyparis stand (25 years)
Haramachi-ku
120
5,250
7,460
12,710
Chamaecyparis stand (25 years)
Haramachi-ku
20
3,220
5,550
8,770
Quercus acutissima stand
Haramachi-ku
130
1,380
1,990
3,360
Clear-cut plot of Quercus serrata
Haramachi-ku
120
78
177
255
Clear-cut plot of Cryptomeria
Haramachi-ku
0
68
143
211
Minamiebi seaside
Odaka-ku
20
14,700
22,300
37,000
Cryptomeria stand (40 years)
13.3.2 Litter Layer
Some areas indicated extremely high levels of radiation. The data for Tetsuzan Dam in Haramachi and the Chamaecyparis stand were especially high (Table 13.2). In terms of geographic features, the former is in a valley and the latter is in a flat field without barrier materials (Figs. 13.2 and 13.3). It has already been reported that Cryptomeria and Pinus are sensitive to radiation, and the litter layer is where radioactive cesium is easily accumulated (Yoshida 2012). In addition, it was reported in the same work that evergreen trees absorb more radioactive cesium than deciduous trees. Our survey data corroborated these findings, indicating that radiation levels in evergreen conifer wood were higher than in deciduous wood.
Table 13.2
Radioactive cesium detected in litter layer (Bq/kg)
Area
Altitude (m)
134Cs
137Cs
Total Cs
Haramachi-ku
280
78,300
143,000
221,300
Tetsusan Dam
Haramachi-ku
120
14,180
32,400
46,580
Cryptomeria stand (40 years)
Haramachi-ku
80
5,340
10,800
16,140
City museum: Abies densiflora stand
Haramachi-ku
120
3,830
6,680
10,510
Chamaecyparis stand (20 years)
Odaka-ku
50
113,000
177,000
290,000
Chamaecyparis stand (15 years)
Odaka-ku
120
53,200
93,600
146,800
Cryptomeria stand (40 years)
Odaka-ku
100
11,300
19,200
30,500
Cryptomeria stand (40 years)
Odaka-ku
120
7,380
12,300
19,680
Meeting house
Odaka-ku
110
697
1,100
1,797
Horse Park: Prunus, Zelkoba
Fig. 13.2
A 15-year Chamaecyparis stand in Odaka-ku
Fig. 13.3
Air radiation is 11–13 μSv at 15-year Chamaecyparis stand in Odaka-ku
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13.3.3 Branches and Leaves
The levels of radioactive cesium varied considerably, but the levels in conifers and evergreen trees were higher than in deciduous trees (Table 13.3).
Table 13.3
Radioactive cesium detected in branches and leaves (Bq/kg)
Area
Altitude (m)
134Cs
137Cs
Total Cs
Haramachi-ku
120
1,930
3,280
5,210
Cryptomeria (40 years)
Haramachi-ku
120
2,130
3,010
5,140
Cephalotaxus harringtonia
Haramachi-ku
120
1,080
2,450
3,530
Quercus myrsinifolia
Haramachi-ku
120
1,270
1,690
2,960
Carpinus tschonoskii
Haramachi-ku
120
1,030
1,560
2,590
Zanthoxylum piperitum
Haramachi-ku
120
337
814
1,151
Orixa japonica
Haramachi-ku
120
ND
171
171
Padus grayana
Odaka-ku
50
14,500
21,500
36,000
Chamaecyparis (15 years)
Odaka-ku
120
8,030
19,000
27,030
Cryptomeria (40 years)
Odaka-ku
50
4,180
6,030
10,210
Callicarpa japonica
Odaka-ku
50
1,984
3,419
5,403
Acer palmatum
13.3.4 Bark and Wood of Standing Trees
The levels of radiation in conifers were higher than in deciduous trees (Table 13.4).
Table 13.4
Radioactive cesium detected in the bark and wood of standing trees (Bq/kg)
Altitude (m)
Bark
Xylem
Xylem/bark
134Cs
137Cs
Total Cs
134Cs
137Cs
Total Cs
Haramachi-ku
60
2,187
2,275
4,462
1,358
1,654
3,012
67.5 %
Quercus serrata
Odaka-ku
20
233
249
482
144
150
294
61.0 %
Quercus serrata no. 1
Odaka-ku
20
362
341
703
157
184
341
48.5 %
Quercus serrata no. 2
Odaka-ku
20
719
1,213
1,932
4,258
762
5,020
63.1 %
Quercus serrata no. 3
Odaka-ku
50
13,418
15,418
28,836
7,619
9,418
17,037
59.0 %
Quercus serrata
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13.3.5 Quercus serrata Mushroom Bed Logs Outdoors
Fukushima Prefecture, and the city of Minamisoma in particular, are key locations for production of mushroom bed logs and are home to many mushroom farmers. After the nuclear accident, however, many bed logs were abandoned outdoors (Fig. 13.4). The levels of radiation found in these bed logs varied greatly, but the levels found in xylem were approximately around 60 % of the levels found in bark (Table 13.5).
Fig. 13.4
Abandoned Quercus serrata mushroom bed logs in Odaka-ku
Table 13.5
Radioactive cesium detected in Quercus serrata mushroom bed logs outdoors (Bq/kg)
Altitude (m)
Bark
Xylem
134Cs
137Cs
Total Cs
134Cs
137Cs
Total Cs
Xylem/bark
Haramachi-ku
60
2,187
2,275
4,462
1,358
1,654
3,012
67.5 %
Quercus serrata
Odaka-ku
20
233
249
482
144
150
294
61.0 %
Quercus serrata no. 1
Odaka-ku
20
362
341
703
157
184
341
48.5 %
Quercus serrata no. 2
Odaka-ku
20
719
1,213
1,932
4,258
762
5,020
63.1 %
Quercus serrata no. 3
Odaka-ku
50
13,418
15,418
28,836
7,619
9,418
17,037
59.0 %
Quercus serrata
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13.3.6 Herbaceous Vegetation and Sprouts in Clear-Cut Plots
We also studied herbaceous plants and sprouts that germinated in the spring of 2012. With the exception of Lamiaceae, these data show lower radiation levels than those for trees (Table 13.6).
Table 13.6
Radioactive cesium detected in herbaceous vegetation, flowers, and sprouts in clear-cut plots (Bq/kg)
134Cs
137Cs
Total Cs
Follopia japonica
ND
ND
ND
Macleoya cordata
ND
ND
ND
Lamiaceae
3,320
4,720
8,040
Male flower of Castanea crenata
306
553
859
Sprout of Quercus serrata no. 1
663
1,000
1,663
Sprout of Quercus serrata no. 2
266
281
547
Sprout of Quercus serrata no. 3
257
295
552
13.3.7 Transpiration Water and Irrigation Water
We also surveyed transpiration water from the leaves of trees (Fig. 13.5). The leaves of 8-year-old and 40-year-old Cryptomeria trees were wrapped in plastic bags (27 cm2) for 24 h on June 13, 2013. Samples of 20 ml were analyzed, but radioactive cesium was not detected. This result suggests that the possibility of secondary radioactive contamination from living standing trees may be low.
Fig. 13.5
Trapping transpiration water from the leaves of young Cryptomeria japonica using plastic bags in June 2013
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We also surveyed irrigation water at some locations in the Haramachi and Odaka areas, but radioactive cesium was not detected.
13.4 Discussion
The foregoing results suggest these points.
1.
Levels of nuclear radiation in Minamisoma were influenced mainly by the city’s location to the northwest of the Fukushima Daiichi Nuclear Power Station and the prevailing wind direction on March 14, 2011.
2.
High levels of radiation were distributed unevenly over valleys or fields without barriers.
3.
In general, the nuclear pollution was not evenly distributed, but was in the form of “hot spots.”
4.
The litter layer in some areas showed extremely high levels of radiation.
5.
Abandoned forests and multi-layered forests indicated high levels of radiation; levels were lower in clear-cut plots.
6.
The extraction rates from bark to xylem were different between conifers and deciduous trees. They were also different between standing living trees and mushroom bed logs (Quercus serrata).
7.
Levels of radiation found in mushroom bed logs abandoned in the outdoors varied greatly.
8.
Radiation levels of herbaceous plants and Quercus serrata sprouts that germinated in the spring of 2012 were clearly lower than those of trees in stands.
9.
No radioactive cesium was detected in transpiration water, suggesting that the probability of secondary radioactive contamination from living standing trees may be low.
Some attempts need to be made to decontaminate forest areas in Fukushima Prefecture, but the prefecture’s forests are extensive and comprise geographically complex terrain, so it is not practically possible to decontaminate them all. However, we propose creating small clear-cut examination plots (Fig. 13.6) in locations where soil erosion cannot occur within the forests. Our survey data showed that the radiation levels of herbaceous vegetation and Quercus serrata sprouts germinated in the clear-cut plot in spring 2012 were clearly lower than those of trees in stands. One practical method, therefore, would be to promote growth of the newly germinated plants and sprouts in such plots while continuing to survey radioactive cesium levels.
Fig. 13.6
Clear-cut plot within a Cryptomeria japonica stand in Haramachi, Minamisoma
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13.5 Conclusion
The results of this study showed that there were still forests in which radioactive cesium levels were high. The half-life of cesium is more than 30 years, and forest regeneration also takes a long time. We should therefore continue to collect data on radioactive cesium and the dynamics of vegetative regeneration.
Acknowledgments
We wish to express our appreciation to Eihachi Horiuchi, chief of Soma’s regional forestry cooperative, who helped with our survey, and Yoichi Takeyama, who provided us with experimental forest plots and guided us around Minamisoma’s forests.
Open Access This chapter is distributed under the terms of the Creative Commons Attribution Noncommercial License, which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.
