Sanderson, DCW, Cresswell, AJ , Tamura, K., Iwasaka, T. and Matsuzaki, K.

52 Pages·2016·1.49 MB·English

by  Alan Cresswell

Sanderson, D.C.W., Cresswell, A.J. , Tamura, K., Iwasaka, T. and
Matsuzaki, K. (2016) Evaluating remediation of radionuclide contaminated
forest near Iwaki, Japan, using radiometric methods. Journal of
Environmental Radioactivity, 162-63, pp. 118-128.
(doi:10.1016/j.jenvrad.2016.05.019)

This is the author’s final accepted version.

There may be differences between this version and the published version.
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http://eprints.gla.ac.uk/119518/

Deposited on: 23 May 2016

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1 Evaluating remediation of radionuclide contaminated forest near Iwaki, Japan,
2 using radiometric methods.
3
4 D.C.W. Sanderson*, A.J. Cresswell
5 Scottish Universities Environmental Research Centre, East Kilbride, Glasgow G75 0QF
6 K. Tamura
7 Faculty of Life and Environmental Sciences, University of Tsukuba, Japan
8 T. Iwasaka
9 Miraishiko Inc., Kanegaya, Asahi-ku Yokohama, Japan
10 K. Matsuzaki
11 Yunodakesansonai, Iwaki, Japan
12
13 Abstract
14 Radiometric surveys have been conducted in support of a project investigating the
15 potential of biofuel power generation coupled with remediation of forests contaminated
16 with radionuclides following the Fukushima Daiichi accident. Surveys conducted in
17 2013 and 2014 were used to determine the distribution and time dependence of
18 radionuclides in a cedar plantation and adjacent deciduous forestry subject to downslope
19 radionuclide migration, and a test area where litter removal was conducted. The
20 radiocaesium results confirmed enhanced deposition levels in the evergreen areas
21 compared with adjacent areas of deciduous forestry, implying significant differences in
22 depositional processes during the initial interception period in 2011. Surveys were
23 conducted both with and without a collimator on both occasions, which modified the

* Corresponding author. email: [email protected]
24 angular response of the detector to separate radiation signals from above and below the
25 detector. The combined data have been used to define the influence of radionuclides in
26 the forest canopy on dose rate at 1 m, indicating that, in evergreen areas, the activity
27 retained within the canopy even by 2013 contributed less than 5% of ground level dose
28 rate. The time dependent changes observed allow the effect of remediation by litter
29 removal in reducing radionuclide inventories and dose rates to be appraised relative
30 natural redistribution processes on adjacent control areas. A 15x45 m area of cedar
31 forest was remediated in September 2013. The work involved five people in a total of
32 160 person hours. It incurred a total dose of 40-50 µSv, and generated 2.1 t of waste
33 comprising forest litter and understory. Average dose rates were reduced from 0.31 µSv
34 h-1 to 0.22 µSv h-1, with nuclide specific analyses indicating removal of 30 ± 3% of the
35 local radiocaesium inventory. This compares with annual removal rates of 10-15%
36 where radionuclide migration down-slope over ranges of 10-50 m could be observed
37 within adjacent areas. Local increases were also observed in areas identified as sinks.
38 The results confirm the utility of time-series, collimated, radiometric survey methods to
39 account for the distribution and changes in radionuclide inventory within contaminated
40 forests. The data on litter removal imply that significant activity transfer from canopy to
41 soil had taken place, and provide benchmark results against which such remediation
42 actions can be appraised.
43
44 Keywords Fukushima nuclear accident, collimator, radioactivity, radiocaesium, gamma
45 ray spectrometry
46
47
48 Highlights
49 • Radiometric measurement of the distribution of radioactivity contamination in
50 Japanese cedar forest
51 • Use of collimator to evaluate forest canopy contributions
52 • Evaluation of remediation factors following forest litter removal
53 • Quantification of self remediation in control area
54
55
56
57
58 1. Introduction
59 Forests are known to intercept radionuclides following atmospheric release and
60 dispersion from nuclear sites. With activities including maintenance, commercial
61 logging, exploitation for wild food collection and recreational activities, radioactivity in
62 forests presents a range of radiological issues relating to external exposure, and
63 contamination of forest products and wild foods. There are also non-radiological issues,
64 including those associated with perceived environmental quality, cultural, ecological
65 and social value systems. In both cases there is a need for careful assessment of the
66 distribution of radioactive contaminants and for management systems based on an
67 understanding of radionuclide distribution and behaviour.
68
69 The work presented here forms part of a pilot project supported by the UK Foreign and
70 Commonwealth Office to investigate the potential of coupling forest decontamination
71 with biomass energy production (Dutton, 2013). As part of this project investigations
72 were initiated to characterise the distribution of radionuclides within a forest near Iwaki,
73 resulting in a radiometric survey in early 2013 prior to litter removal operations in a
74 small test area within the survey zone. Dose rate measurements were conducted in this
75 area immediately before and after the litter clearance operations. A repeat radiometric
76 survey was then conducted one year after the initial work to characterise the
77 environmental change, both in the remediated area and in adjacent areas as a result of
78 redistribution processes.
79
80 Prior to the Fukushima accident significant areas of forest have been contaminated by
81 nuclear weapons’ testing and following nuclear accidents. The processes that govern
82 radionuclide translocation between different compartments within forest ecosystems,
83 and removal of radionuclides from the forest, are complex and involve multiple
84 pathways. These processes are difficult and time consuming to measure using sampling
85 methods, and the number of studies reported in the literature is limited. Reviews of
86 behavioural and ecological studies (Ipatyev et.al. 1999, Nimis et.al. 1996), including
87 transfer to and within plants (IAEA 2010, Calmon et.al. 2009), and remediation options
88 (Tikhomirov et.al. 1993, Fesenko et.al. 2005, Guillitte & Willdrocht 1993, Guillitte
89 et.al. 1993, 1994, Nisbet et.al. 2009) summarise knowledge of the general behaviour of
90 radionuclides within forest ecosystems. Specific studies are nonetheless needed to
91 assess the behaviour in new areas, such as those affected by the Fukushima accident.
92
93 Initial behaviour has been related primarily to canopy interception followed by
94 translocation and redistribution within the living parts of trees and their associated
95 forest litter and soil. In the first five years following the Chernobyl accident similar
96 levels of contamination were reported in forested and adjacent pasture areas subject to
97 wet deposition (Tikhomirov & Shcheglov 1994, Bunzl et.al. 1989), with some
98 differences where dry deposition mechanisms are implicated. Initial interception of up
99 to 70-80% of activity by coniferous (predominately spruce and pine) forest canopies has
100 been reported, with substantial transfer from canopy to litter and soil observed in
101 Ukraine, and in Nordic Countries in the first year after deposition (Tikhomirov &
102 Shcheglov 1994, Ipatyev et.al. 1999). Longer term behaviour is expected to be
103 determined by nutrient recycling and exchange processes between soil, litter and rooting
104 systems, with considerable variability on local and regional scales.
105
106 In Japan, approximately 70% of the contaminated area in Fukushima Prefecture is
107 forested (Hashimoto et.al. 2013), in areas of considerably topographic relief, with high
108 seasonal rainfall and snow-run-off. The accident occurred in early March 2011, at a
109 time when few deciduous species were in leaf, limiting early leaf interception and
110 immediate translocation to evergreen species. Canopy interception factors in coniferous
111 forests (Japanese cypress, Chamaecyparis obtuse, and Japanese cedar, Cryptomeria
112 japonica) in Japan, determined by the comparison between activity in rainwater
113 collected in open terrain and throughfall and stemflow over the period 11th-28th March
114 2011, of 92% for radiocaesium have been reported (Kato et.al. 2012, 2015), whereas for
115 deciduous broadleaf forests the majority of activity has been reported to have been
116 deposited directly onto the ground surface (Koarashi et al., 2014). These are
117 comparable to interception factors reported for similar forests in Europe following the
118 Chernobyl accident; studies of Norway spruce (Picea abies) and beech (Fagus
119 sylvatica) forests at Höglwald near Munich report interception factors of 70% and 20%
120 respectively (Bunzl et al 1989, Schimmack et al 1991), data from forests near Kiev
121 report retention coefficients of 10-50% for deciduous forests and upto 100% for pine
122 forests (Prister et.al. 1994), and Melin et al (1994) reports interception factors for spruce
123 (Picea abies) and unfoliated deciduous forests in Sweden of approximately 90% and
124 <35% respectively.
125
126 Litterfall has been reported to be a significant process in the transfer of radiocaesium
127 from the canopy to the ground in forests in Fukushima. Teramage et al (2014) reports
128 that over a 200 d period, litterfall accounted for 30% of activity transferred to the
129 ground from the canopy of a cypress (Chamaecyparis obtuse) forest. Over an 18 month
130 period, Kato et.al. (2015) report that litterfall accounted for 40% of activity transfer for
131 both young and old cedar (Cryptomeria japonica) stands, and 64% of activity transfer
132 for broadleaf stands. Endo et al (2015) also reports litterfall accounting for ~50% of
133 transfer for deciduous forests, and 69% for cedar. These studies show a significantly
134 greater contribution from litterfall compared to the experience in Europe. Bunzl et al
135 (1989) reported 7% (4.6% per year) of transfer by litter fall for Norway spruce. Bonnet
136 & Anderson (1993) reported 13-17% transfer per year by litterfall for Sitka (Picea
137 sitchensis) and Norway spruce in mid Wales.
138
139 The transfer of activity from the canopy may be expressed as a double exponential
140 decay with decay constants, which may be expressed as ecological half lives, for fast
141 and slow components. Studies of Japanese cedar, cypress and broad leaf forests (Kato et
142 al 2012, 2015, Teramage et al 2014) have reported a fast component with decay
143 constants equivalent to a half life of 87 d, with slow components with equivalent half
144 lives of 390 d (broad leaf), 550 d (mature cedar), and 780 d (young cedar). Bunzl et.al.
145 (1989) reported fast and slow components with effective ecological half lives of 90 d
146 and 230 d for spruce forests. Prister et.al. (1994) reported effective half lives for a fast
147 component of 2-5 d for several different species of tree in Kiev, with slow components
148 characterised by half lives of 25-100 d. An experimental contamination of spruce trees
149 (Sombré et al., 1994) resulted in fast and slow components with effective half lives of 6
150 d and 120 d. Conversely, other studies reported no significant long term decline in
151 activity (with effective half lives greater than 1 y) or even a slight increase in activity
152 (Tobler et al 1988, Raitio & Rantavaara, 1994). These are more similar to the
153 observations in Japanese forests than the studies with slow components with decay
154 constants equivalent to half lives of 200 d or less.
155
156 Studies of the rate of transfer from the organic soil layers to mineral soils in Japan have
157 reported significant differences at different locations. Mahara et.al. (2014) report that
158 soil cores collected at the Fukushima Forestry Research Centre, Koriyama, in 2013
159 showed that more than 99% of radiocaesium activity deposited on the ground was in the
160 litter layer and top 2.5 cm of the soil column. In contrast, Hashimoto et.al. (2013)
161 reports that for four other sites in Fukushima Prefecture the majority of the
162 radiocaesium had migrated to the mineral soils by 2012. For most studies in European
163 forests, transfer from the organic to mineral soil layers was slow. In Italy, Belli et al
164 (1994) reported less than 2% of radiocaesium in the mineral layers, in Sweden Fawaris
165 & Johanson (1994) report <5% of activity in mineral soils in 1991, Melin et al. (1994)
166 reports 7% of activity in mineral soils in 1990 and McGee et al. (2000) reports 77% of
167 activity in top, mostly organic, 10 cm of soil layers in 1992. In Switzerland, however,
168 Tobler et.al. 1988 reported that only 56% of radiocaesium activity was in the litter
169 layers by October 1986, and on sandy soils in Denmark, Strandberg (1994) reported
170 20% of radiocaesium in litter layers in 1991. Despite these exceptions, it appears that in
171 general radiocaesium has been transferred to mineral soils more rapidly in Japan than in
172 Europe. Hashimoto et.al. (2013) hypothesise that this “is a result of the relatively warm
173 climate and heavy rainfall which lead to more rapid litter decomposition and
174 substantially thinner organic soil layers than in many European forests”.
175
176 Rapid translocation of intercepted activity into cedar and red pine sapwood and
177 heartwood has been observed (Kuroda et.al. 2013). Lower activity concentrations in a
178 range of deciduous broadleaf trees compared to evergreen trees have been observed,
179 (Yoshihara et.al. 2013), as have direct interception by bark and translocation in
180 deciduous fruit trees (Sato et.al. 2015). These differences in the rates of processes in
181 Japan compared to Europe following the Chernobyl and Kyshtym accidents have been
182 attributed to differences in climate, environment, timing of the Fukushima accident and
183 potential differences in the chemical and physical forms of radioactive releases.
184
185 Potential countermeasures for forest systems range from clear felling and ploughing to
186 access restrictions, with a corresponding range of economic and ecological effects
187 (Tikhomirov et.al. 1993, Fesenko et.al. 2005, Guillitte & Willdrocht 1993, Guillitte
188 et.al. 1993, 1994). Another approach to forest remediation is the removal of leaf litter
189 and surface soil layers. This is labour intensive, exposes workers to radiation dose,
190 generates significant volumes of waste, and may also have adverse effects such as loss
191 of habitat for wildlife, reduced soil fertility, and potentially increased soil erosion.
192 Nonetheless it has the potential of reducing dose rates in areas of high utilisation, and
193 potentially of intercepting forest run-off.
194
195 The removal of litter and understory has been widely adopted in Japan to remediate the
196 edges of forests, to a distance of 20 m from roads and buildings, although examinations
197 of the effectiveness of this approach have been limited. Within the JAEA
198 Decontamination Pilot Project, litter removal from 11 forest sites showed reductions in