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Overview of Environmental Impact Assessment Studies onRadioactive Contamination after the Fukushima Daiichi
Nuclear Power Plant Accident
Kazuki IIJIMA1, Seiji HAYASHI 2* and Masanori TAMAOKI 2
Fukushima Environmental Evaluation Research Division, Japan Atomic Energy Agency
10-2 Fukasaku, Miharu Town, Tamura-gun, Fukushima, 963-7700, Japan
2
Fukushima Branch, National Institute for Environmental Studies
10-2 Fukasaku, Miharu Town, Tamura-gun, Fukushima, 963-7700, Japan*E-mail: shayashi@nies.go.jp

1

Abstract
The radioactive environmental contamination that resulted from the Fukushima Daiichi nuclear power
plant accident has clearly been declining during the nearly 10 years that have passed since the accident. Many
studies, however, have indicated prolongation of radioactive contamination in natural ecosystems, probably
arising from non-decontaminated forested areas, so a more detailed examination is needed on theenvironmental dynamics of bioavailable radiocesium and its transfer to ecosystems. Also, regarding theinfluences of radiation on organisms, the effects on wild organisms of the evacuation of humans have been
more pronounced than the direct effects from radiation, especially in the Fukushima evacuation zones.
Key words :	dissolved Cs, radiation effect, transfer to ecosystem, wild organism

1. Introduction
As shown by the time change in radiation air dose
rates measured by airborne surveys (Nuclear Regulation
Authority, 2020), serious environmental radioactive
contamination occurred widely as a result of the
Fukushima Daiichi nuclear power plant (FDNPP)
accident. The air dose rate has clearly been declining
during the nearly 10 years that have passed since theaccident thanks to decontamination operations in theliving sphere of the affected areas as well as the natural
decay of Cs-134, the half-life of which is 2.0 years, andweathering of radiocesium.
On the other hand, the forested areas that cover over
80% of the evacuated zones remain mostly nondecontaminated. Considering the reported behavioral
properties of radiocesium in forests such as its highadsorption in clay particles in the soil (Takahashi et al.,2019) and limited runoff ratios even during rain events(Tsuji et al., 2016; Iwagami et al., 2017a), it is assumed
that large amounts of the deposited radiocesium will be
retained in the surface soil layer over a long period oftime. Serious radioactive contamination of forested areasand subsequent long-term contamination of natural
ecosystems were reported after the Chernobyl nuclear
power plant (CNPP) accident in 1986 in the former
Soviet Union (Belli & Tikhomirov, 1996; Bulgakov et al.,Global Environmental Research
24/2020: 085-093
printed in Japan

2002). The same situation is of increasing concern in thearea affected by the FDNPP accident. Additionally, since
long-term retention of radiocesium in the forest floor
means prolonged conditions of high dose rates in the partof the forest environment near the ground, we cannot
overlook the effects of radiation doses on organisms overthe mid to long term.
In this paper, we focus especially on radiocesiumtransfer to natural ecosystems and the various effects ofradiation on organisms in an attempt to assess theenvironmental effects of the FDNPP accident through theaccumulated scientific knowledge from previous research
papers. Concretely, we first give an overview of the
behavioral properties of dissolved radiocesium with highbio-availability, which plays an important role in its
transfer to natural ecosystems in forests and river
systems. Then we describe the state of radiocesiumtransference to forest trees, freshwater fish and wild
animals. In addition, the actual state of radiation doses’
influence on organisms and the influence on biota of theabsence of humans due to evacuation are described
respectively as direct and indirect effects of serious
environmental radioactive contamination.

©2020 AIRES

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K. IIJIMA et al.

2. Migration Behavior of Dissolved Cs in
Terrestrial Areas
In a previous review (Hayashi, 2016), the following
points were indicated as future issues regarding the
behavior of Cs in the environment around Fukushima:- Organization of scientific knowledge on managing
radiocesium runoff from forests based on many casestudies,- Detailed investigations of dissolved radiocesium inriver systems in terms of generation mechanisms,
particularly the contribution of highly contaminatedorganic matter (leaf litter) and seasonal fluctuations
in concentrations,- Investigations on the behavior of sediments with highconcentrations of radiocesium through direct
deposition and initial inflow to clarify the actual
state of bottom sediment contamination in dam
reservoirs, and- Investigations on the possibility of dam reservoirs
acting as a source of dissolved radiocesium.
		 In this chapter, we review recent investigations
on the behavior of dissolved radiocesium in river
systems and forests, which are considered to be twosources.
2.1 Sources of Dissolved Cs in River Systems
Highly contaminated organic litter, such as leaf litter,is considered one of the possible sources of dissolved
radiocesium in river systems. This possibility is supported
by both seasonal fluctuation, with high levels in summer
and low in winter (Tsuji et al., 2016; Nakanishi &Sakuma, 2019), and a rapid but extremely temporary
increase in concentration immediately after rainfall starts(Tsuji et al., 2016; Iwagami et al., 2017b; Sakuma et al.,2019). According to phenomenological model
calculations, these trends in dissolved radiocesium
concentration cannot be expressed by traditional
adsorption-desorption distribution models, so it is thoughtthat the kinetically controlled dissolution of radiocesium
during the decomposition of organic matter might
contribute (Sakuma et al., 2019). Dissolved radiocesium
concentrations are correlated with water temperature(Nakanishi & Sakuma, 2019), dissolved organic carbon+(DOC) and potassium (K ) concentrations (Tsuji et al.,2016), also suggesting contribution of organic matterdecomposition. However, since the width of fluctuation
decreases with time (Nakanishi & Sakuma, 2019), thecontribution of this process might be decreasing.
Desorption from forest soil or river sediments is
another possible source. In the case of forest streams, thedeclining trend of dissolved radiocesium in stream water
is similar to that in groundwater (Iwagami et al., 2017b).
The level of radiocesium dissolved in water contained insoil layers was found to be quite high in July 2011 anddeclined after that, and could be fit with a two-component

exponential model (Iwagami et al., 2017c). The faster
decline rate showed a good correlation with theradiocesium interception potential (RIP), suggesting that
desorption from clay minerals contributed to generationof dissolved radiocesium in forest soil. In the case of river
sediments, the cation exchange capacity (CEC) is
influenced by both clay minerals and organic matter
content (Tachi et al., 2020a). The adsorption behavior at-9
10 M of the initial concentration of radiocesium can be
expressed by a first-order reaction, but the rate constantdecreases with an increase in organic matter content,
indicating that organic matter inhibits radiocesium accessto frayed edge sites (FESs) (Tachi et al., 2020b). In
particular, the dissolved radiocesium concentration in
output water from one of the dam reservoirs inFukushima has shown slightly higher concentrations andslower decline than that of the input water (Funaki et al.,2020). It is thought that radiocesium that has accumulated
in the reservoir sediments may partly be desorbed byammonium ions, dissolving into the reservoir’s water.
The conditions in the reservoir sediment are likely toremain in a reducing condition under which ammonium
ions are easily generated and can desorb cesium ions.
2.2 Behavior of Cs in the Forest Floor
Most of the radiocesium deposited in forests has
moved from the canopy to the forest floor. Throughfall isthe dominant pathway of radiocesium transfer in the firstyear, and the contribution of litterfall gradually increases
with time (Kato et al., 2017). The trend of radiocesium
decreasing in the canopy is expressed by a double
exponential field-loss model. In the six months after theaccident, 82% of the initially deposited radiocesium waspresent on the forest floor in mixed forests and broadleaved forests, and 78-56% in the case of cedar forests(Komatsu et al., 2016). The ratio of radiocesium on thecedar forest floors to the initially deposited amount wasalso reported to be 80% three years after the accident(Gonze et al., 2017) and 94% four years after it(Yoschenko, 2017).
On the forest floor, radiocesium in the litter layer
migrates to the lower mineral layer, and is strongly
adsorbed by the minerals. It has been reported that theradiocesium concentration in the mineral layer located a
few centimeters below the surface gradually increased
and became higher than that in the litter layer after two or
three years (Takahashi et al., 2019). The exchangeable
fraction of radiocesium was of 10% in the organic layer
and 6% in mineral layer after five months, and constant at
2 – 4% in two to four years in both layers, showing thatradiocesium could not easily move to deeper parts(Manaka et al., 2019). Throughfall dominates as thesource of radiocesium migrating downward from thelitter layer in the case of cedar forests, while
decomposition of litter dominates in the case of broadleaved forests (Kurihara et al., 2018).

Overview of environmental impact assessment studies
Litter from forests and elution from the litter/organic
soil layer are likely contributors to the dissolved
radiocesium concentrations in river systems. Based on acompartment model analysis Kurikami et al. (2019)
reported that the origination of dissolved radiocesium
concentration in river water and freshwater fish could be
explained by a combination of these three sources.
Further investigations to clarify the dissolution
mechanism and behavior of radiocesium in forests aredesired, which would enable the establishment of models
for estimating future concentrations.

3. Radiocesium Transfer to Natural Ecosystems
Most of the radiocesium emitted in large quantities
into the environment by the FDNPP accident was
deposited in the mountainous forested areas of
Fukushima Prefecture and its surroundings. Many studies
after the accident suggest that although most of thedeposited radiocesium remains in the surface soil layer
because it is strongly bound with clay particles in the soil,
the runoff ratio from forest catchments is limited to lessthan 1.0% annually even when large storms occur (Tsuji
et al., 2016; Iwagami et al., 2017a). Therefore, forested
areas would not be involved with re-contamination in thedownstream catchment area as the source of radiocesium
migration. However, this situation implies thatradiocesium deposited on the forest floor, especially Cs137 with a long half-life (30.1 yrs.) stays nearly retainable
in the surface soil layer, which is biologically active(Takahashi et al., 2019). Therefore, understanding
radiocesium transfer in natural ecosystems is essential for
inferring future trends.
In this chapter, we outline the current state ofradiocesium distribution and properties of its transfer in
the natural ecosystems of forested areas, freshwater
bodies and wild animals, based on reports from previous
studies after the FDNPP accident.
3.1 Forest Ecosystems
As for the state of radiocesium transfer into forest
products, which is of most concern regarding radioactive
contamination of forest ecosystems, the main researchthat has been conducted has targeted evergreen needleleaved trees like Japanese cedar and cypress which havebeen utilized as timber products (Kanasashi et al., 2015;
Kajimoto et al,. 2015; Kato et al., 2017; Nishikiori et al.,2019). Since uptake followed by translocation from theabove-ground parts, mainly foliage, where it was
deposited soon after the accident, and root uptake areassumed as the transfer processes of radiocesium towood, this is of most interest. Several attempts have been
made to quantitatively evaluate the respective
contributions (Mahara et al., 2014; Nishikiori et al., 2015;
Yoschenko et al., 2018). Imamura et al. (2020) conducteda source analysis utilizing the ratio of Cs-137 activity

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concentration to stable cesium (Cs-133) concentration inseveral components of Japanese cedar and soil porewater. They estimated that root uptake had contributed toabout 50% of the increment in Cs-137 in the stemwoodof a 40-year-old tree from Jul. 2011 to Aug. 2017 at a
maximum, although the size of this contribution was
highly uncertain. Thiry et al. (2020) applied of theircomprehensive modeling of radiocesium cycling inforests to pine trees in Belarus and Ukraine after the
CNPP accident using the observed data. They proved thatroot uptake contributed to the transfer of Cs-137 to wood
more strongly than uptake from foliage since 10 years
after the accident. Even in Fukushima, continuous
observation will be needed to see if the contribution ofroot uptake to the Cs -137 transfer to the wood ofJapanese cedar and Japanese cypress increases in the
future, although it will depend on how much bioavailable Cs-137 is generated in the soil, corresponding
to forest environment conditions.
Based on a five-year study starting from Aug. 2011,
five months after the FDNPP accident, Imamura et al.
(2017) estimated the initial deposition of Cs-137 onto theabove-ground parts of konara oak to be less than onethird of those of Japanese cedar or Japanese cypress, due
to lack of foliage at the time of the accident in the case of
deciduous broad-leaved trees. Surprisingly, Cs-137
activity concentrations in deciduous konara oak leaves
did not change over the five years, although the Cs-137
activity concentrations in the wood significantly increased
year by year. Kanasashi et al. (2020) showed that Cs-137
activity concentrations in growing coppiced shoots ofkonara oak have had a significant positive correlation
with exchangeable Cs-137 in soil in Fukushima after the
FDNPP accident. It may be necessary to examine thecontribution of root uptake as one of the main controlling
factors in determining Cs-137 activity concentrations in
the leaves and stemwood of deciduous broad-leaved trees
as well as translocation from the deposited bark andbranches. Generally, many deciduous broad-leaved trees
have shallower root systems than Japanese cedars andJapanese cypresses, suggesting that they may be able totake up exchangeable Cs-137 in soil supplied by leachingfrom litter more easily.
Since radiocesium cycling caused by transfer todeciduous broad-leaved trees via root uptake will be astrong concern regarding long-term radioactive
contamination of forest ecosystems, countermeasures tothis issue should be considered in future research.
3.2 Freshwater Ecosystems
Since radioactive contamination of fish is a big
concern from the viewpoint of inland water fisheries andthe leisure industry in freshwater ecosystems, theMinistry of the Environment; Ministry of Agriculture,
Forestry and Fisheries; and Fukushima Prefecture havebeen conducting respective monitoring projects to88

K. IIJIMA et al.

elucidate the state of radiocesium concentrations in fish.
By utilizing these monitoring data, Ishii et al. (2020)
showed that the state of Cs-137 transfer to fish clearly
differed by habitat (river or lake) or feeding habit of fishusing the concentration factor (CF: CF = Cs-137 activity
concentration in fish (Bq/kg-fw)/Cs-137 in water (Bq/L)),
which serves as an indicator of the present state ofradiocesium transfer from the aqueous environment toaquatic organisms. It has also been confirmed that larger
fish show higher radiocesium concentrations than smaller
ones even in the same habitat or among the same species(Wada et al., 2019; Ishii et al., 2020). Freshwater fish
usually supplement cations including radiocesium
through feeding and have a lower excretion rate duringosmoregulation compared to marine fish (Wada et al.,2019). Consequently, since their radiocesium activity
concentration varies depending largely on what they eat,clarifying the flow of radiocesium via the food-web isclearly essential to understanding the mechanism oftransfer to fish.
Generally, radiocesium activity concentrations inomnivorous or carnivorous fish are higher than that in
herbivorous or planktotrophic fish (Wada et al., 2016;
Ishii et al., 2020), so how radiocesium is transferred tofish by the detritus food chain via insects feeding onforest litter is being examined. As an example, it has been
confirmed that differences in radiocesium concentrations
in litter, which is the basal food of forest and stream
ecosystems, are reflected in radiocesium concentrations
in communities of aquatic organism (Sakai et al., 2016).
Factors inhibiting lower of radiocesium activity
concentrations in fish need to be investigated andeffective countermeasures to prevent radiocesium fromtransferring to fish need to be examined as much as
possible based on scientific knowledge obtained from anevaluation of transfer properties through the food web inorder to promote the recovery of industries related to
freshwater fish.
3.3 Wild Animals
As for the transfer of radiocesium originating fromthe FDNPP accident to wild animals, the current andtransition states have been examined mainly with regard
to game animals from the viewpoint of food-safety. From
the reported Cs-137 activity concentrations in game meatdata from Asian black bears (Ursus thibetanus), wildboars (Sus scrofa), sika deer (Cervus nippon), green
pheasants (Phasianus versicolor), copper pheasants(Syrmaticus soemmerringii), and wild ducks (Anaspoecilorhynch and Anas platyrhynchos) in more than 10
prefectures after the FDNPP accident between 2011 and2015, Tagami et al. (2016) confirmed that theconcentrations in wild boars were the highest of all, butwere lower than those reported after the CNPP accident.
Also from a comparison between Cs-137 activity
concentrations in the meat of Asian black bears or wildboars between 2011 and 2016 and Cs-137 inventories in
the areas where they had been trapped, estimated byairborne surveys (Nuclear Regulation Authority 2020),
Nemoto et al. (2018) reported that each concentration
was proportional to the inventory and the concentrationin wild boars was higher than in Asian black bears if they
had been trapped in the same area.
As for the aggregated Cs-137 transfer factor (Tag =activity concentration in meat (Bq/kg-fw)/inventory in2soil (Bq/m )), which is an indicator of transfer properties
from radioactive contaminated land to game animals, thegeometric means (GM) for green pheasant and wild duckremained on the order of 10-4 during the monitoring
period, while those for Asian black bear, wild boar, sika
deer and copper pheasant were one order higher (Tagami
et al., 2016; Nemoto et al., 2018). Compared with the
Tag after the CNPP accident, directly comparable dataobtained for wild boars were in the same range of the GM
values (4 × 10−3 to 6.7 × 10−2) collected within five years
after the CNPP accident by International Atomic Energy
Agency (IAEA; Howard et al., 1996).
The annual change in Tag depends on the species.
While it decreases slightly in some species (wild boar,
green pheasant and wild duck), no significant change has
been found in other species (Asian black bear, sika deerand copper pheasant). Although clear seasonal changes in
Cs-137 activity concentration were found in wild animals
after the CNPP accident (Semizhon et al., 2009), a
uniform opinion has yet to be formed in the case of the
FDNPP accident. While Tagami et al. (2016) concluded
that no seasonal trends could be found in any species
from the collected data, Nemoto et al. (2018) stated thatmuscle Cs-137 varied seasonally and that this seasonalvariation also differed between Asian black bears andwild boars. It might be necessary to conduct more
successive sample collections and detailed analyses
considering differences in feeding among species ordifferences in environment between the areas affected by
the FDNPP and CNPP accidents to gain a clearer
understanding of the properties. Concretely, progress will
be needed in studies focused on gastric contents of wild
animals to elucidate increases in concentrations according
to when and what they eat (Saito et al., 2020). This
pertains especially to the most contaminated wild boars
whose numbers are confirmed to have increased due tothe effects of evacuation orders affecting seriously
contaminated areas (Lyons et al. 2020). More detailed
research on ranging behavior, dietary habits and the longterm behavior of radiocesium in forest ecosystems will beneeded in Fukushima Prefecture and surrounding areas(Hinton et al., 2015).

4. Effects of Radiation and Human Evacuation on
Wild Organisms and Ecosystems in Fukushima
As a result of the FDNPP accident, all of the wildOverview of environmental impact assessment studies
animals and plants have been left behind in highradiation-dose areas, so they are still exposed to radiationin Fukushima. It is, therefore, of high concern whether
any adverse effects have been found in wild organisms in
the Fukushima area resulting from long-term, low-dose
exposures to radiation. In the first half of this chapter, we
focus on radiation effects in wild organisms found in
Fukushima, where small but serious adverse effects ofradiation have been detected. However, the risks fromradiation on wild organisms will diminish becauseradiation doses in Fukushima have been decreasing
through natural decay, weathering and efforts fordecontamination. The area covered by the evacuation2
zones has been steadily decreasing, from 1,150 km in
2
August 2013 to 340 km in April 2019, as air dose rates
have decreased. With the lifting of evacuation orders,
about 58,000 former residents of the evacuation zones,
have been permitted to return to their hometowns, but thearea’s population is only 28% of its total before theaccident. Thus, changes in land use with the cessation ofdaily life activities, such as agricultural and horticulturalwork, continue, and these significantly affect the wildorganisms and ecosystems in the evacuation zones. In the
Fukushima evacuation zones, because the air dose rates
are gradually decreasing with time, it will be necessary to
pay more attention to the effects of the evacuation on
wild organisms and ecosystem rather than direct effects
from radiation hereafter. Therefore, the impacts of rapidchanges in human activities on wild organisms andecosystems are also described in the second half of thischapter.
4.1 Estimated Levels of Exposure of Wild Organisms
to Radiation
Levels of contamination with radioactive materials
and estimated radiation exposure doses in marine andterrestrial organisms living in Fukushima were initiallypredicted to exceed the derived consideration reference
levels (DCRLs) determined by ICRP and reported during
first month after the accident (ICRP, 2008; GarnierLaplace et al., 2011; Strand et al., 2014). According to theprediction, marine organisms including seaweed, benthic
invertebrates and benthic flatfish were expected to havesuffered severe effects from radiation categorized as“reduction in life span” in ICRP’s DCRLs (Tamaoki,2016). However, very few adverse effects were found inmarine organisms living in Fukushima.
The only exception was reported by Horiguchi et al.
(2015), in which the abundance and diversity of intertidal
species decreased significantly with decreasing distancefrom the FDNPP in 2012, and no rock shells (Thais
clavigera) were observed within a radius of 20 km of the
FDNPP. Unfortunately, whether the decrease ordisappearance of these species was caused directly by theincrease of radiation doses or not remains unclear at thismoment.

89

4.2 R adiation Effects in Wild Organisms in Fukushima
In contrast to marine organisms, only rodents were
expected to suffer from radiation at the level categorized
as “reduced production” in ICRP’s DCRLs for terrestrialorganisms (Tamaoki, 2016). Onuma et al. (2020)
calculated exposure dose rates for large Japanese fieldmice (Apodemus speciosus) captured in the “difficult-toreturn zones” in Fukushima Prefecture from 2012 to2016, and they found that the total dose rate (sum ofinternal and external dose rates) was estimated at lessthan 1.0 mGy/day. Exposure exceeding 1.0 mGy/day
could reduce fertility in male and female field miceaccording to ICRP’s DCRLs, indicating that the present
dose rates of the field mice living in the capture sites
were lower than the dose rates that would reduce fertility.
Accordingly, no significant differences in apoptotic cell
frequencies or frequencies of morphologically abnormal
sperm were observed in field mice captured in Fukushima
as compared to the results from two control sites, in
Aomori and Toyama prefectures, in 2013 and 2014
(Okano et al., 2016).
Among organisms living in Fukushima,morphological abnormalities were detected in somespecies such as the pale grass butterfly (Pseudozizeeriamaha) (reviewed by Hiyama et al., 2015), gall-forming
aphids (Tetraneura sorini and T. nigriabdominalis)(Akimoto 2014) and conifer tree species (Japanese firtree; Watanabe et al., 2015, Japanese red pine; Yoschenko
et al., 2016). It is noteworthy that no abnormal
morphologies in the butterfly and aphid have been
observed since 2013 (Akimoto, 2014; Hiyama et al.,2015). This indicates that the abnormal phenotypes inthese small insects only occurred within two years after
the FDNPP accident, and severely malformed individuals
were eliminated by natural selection.
In addition, there is no evidence of genetic mutations
causing abnormal morphologies. Further studies areneeded to obtain positive proof that these abnormal
phenotypes occurred due to radiation, and reproductive
experiments such as irradiation of butterflies, aphids andconifer tree species for long periods at low doses would
also be effective for advancing our knowledge onradiation effects in wild organisms.
4.3 
Effects of Radiation on Abundance of Wild
Populations in Fukushima
Radiation effects on population sizes in insect andbirds were also investigated in the Fukushima evacuation
zones. Møller et al. (2013) took a census of spiders,grasshoppers, dragonflies, butterflies, bumblebees andcicadas at 300 sites in forested areas of the evacuation
zones. They found that the abundance of butterflies andcicadas declined significantly with increasing levels ofradiation doses. Interestingly, the abundance of spiders
showed a positive correlation to the radiation doses at thesites where they were collected. Such positive

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K. IIJIMA et al.

correlations observed in spiders were not observed inparticular in the case of Chernobyl, where all of thearthropods showed negative correlations between theirabundance and radiation dose rates.
The effects of radiation on the abundance ofcommon birds were also studied in Fukushima and theresults obtained were compared to those in Chernobyl(Møller et al., 2012). Fourteen bird species common toboth Fukushima and Chernobyl showed abundance
negatively correlated to radiation doses at the two sites,
but the tendency toward negative effects differed between
Fukushima and Chernobyl. Interestingly, the negative
correlation between the abundance of these 14 common
bird species and radiation doses was more pronounced in
Fukushima than in Chernobyl. The reasons for thesedifferences in radiation-dose-dependent change inabundance of spiders and bird species between
Fukushima and Chernobyl are unclear. Some research
evidence indicates that wild organisms living in
Chernobyl have already finished undergoing adaptation
to and/or selection for high radiation environments
because the investigations in the area started 20 years
after the Chernobyl accident. Thus, it will be very
important to study whether adaptation and/or selection is
occurring in the Fukushima evacuation zones.
4.4 Effects of Human Non-residence by Evacuation on
Population Size in Wild Organisms
To clarify the effect of evacuation on wildorganisms, biota monitoring targeting mammals, birds,
frogs and flying insects inside and outside the evacuation
zones in Fukushima has been undertaken since 2014
(Yoshioka et al., 2016). As reviewed previously, theabundance of carpenter bees (Xylocopa appendiculata)
was lower in the evacuation zone than outside it(Yoshioka et al., 2015; Tamaoki 2016). Moreover, somesmall bees, wasps and beetles were more common inside
the evacuation zone. The causes of these population
changes in some insect species are unclear, but changes
in human activities such as abandonment of cultivation
and/or gardening may affect them. Among frogs, eightspecies have been observed during the monitoring, but
the frequency of observation of each frog species has not
yet been compared inside and outside of the evacuation
zones. The two-year data sets (2014–2015) obtained were
published as a data paper for enabling its use by otherresearchers (Yoshioka et al., 2020).
Mammals have been monitored at 46 sites in ninemunicipalities inside and outside the evacuation zones(locations can be seen at http://www.nies.go.jp/biowm/
map/en_mafu.html). Infrared camera traps were
employed, which automatically respond to infrared
radiation from homothermic animals. Sixteen different
mammal species have been observed through camera
trapping (Fukasawa et al., 2016). Among these, wildboars (Sus scrofa) were caught on camera most often andat the largest number of sites (78% of the monitoredsites). A statistical analysis showed that wild boars weremore abundant in the evacuation zone than outside it,indicating that decreased human activity may help thewild boar population increase. The same tendency was
also found to a lesser extent in other species such as
badgers (Meles meles), Japanese macaques (Macaca
fuscata) and Japanese hares (Lepus brachyurus). The
same result was also reported by Lyons et al. (2020) in
which they found no evidence of impact on population
size in mammals or gallinaceous birds, and showed thatseveral species were most abundant in the evacuation
zones, despite the presence of high radiation doses.
Among them, increased wild boar populations inFukushima have resulted in damage to agriculturaloperations and require control by state-funded hunting
programs. Radiocesium levels in wild boar meat often
exceed government guidelines for food consumption, notonly in and around the evacuation zones but also in the
Aizu area in western Fukushima Prefecture, more than
100 km from the FDNPP. Moreover, wild boars are
considered a pest animal in farmland, and are a major
mediator of classical swine fever which can seriously
damage the pork industry. Indeed, about half of theanimal damage to crop yields in Fukushima is due to wildboars, and wild boars infected by swine fever virus havebeen found in Aizu-Wakamatsu City as of September
2020. Thus, it is important to clarify the population
dynamics and migratory movements of wild boars inFukushima to develop an efficient plan for helping
residents return to the evacuation zones.

5. Summary
In this paper, we have provided an overview ofrecent progress in investigations of the environmental
impact induced by the FDNPP accident, focusing
especially on radiocesium transfer to natural ecosystems
and the effects of deposited radioactive materials onorganisms.
Litter from forests and desorption from the litter/organic soil layer are likely contributors to dissolved
radiocesium concentrations in river systems. The originsof the dissolved radiocesium will evolve with time inaccordance with the source of radiocesium migrating
downward from the litter layer to the mineral layer. In
forest ecosystems, radiocesium is thought to migrate intowood by root uptake and/or uptake from deposits onabove-ground parts followed by translocation. The
contribution of root uptake, however, has not been
evaluated as of this time. As a source of dissolved
radiocesium affecting river systems and forest
ecosystems, the behavior of litter decomposing andleading to the dissolution of radiocesium in forests is a
key issue to be investigated in the future.
The transfer of Cs-137 to freshwater fish is affectedOverview of environmental impact assessment studies
by habitat (river or lake), feeding habit, body size and soon. It has also been pointed out that the concentration ofradiocesium in litter is related to that in communities ofaquatic organism. Among wild animals, wild boars
showed relatively higher radiocesium concentrations, but
the tendencies of concentrations to evolve, such as
through seasonal variation, are still under discussion. To
predict future concentrations in these ecosystems, itwould be desirable to clarify dominant pathways oftransfer of radiocesium via the food-web throughcontinuous systematic sample collection and detailed
analysis.
Direct effects of increased radiation doses have notbeen clearly observed among marine organisms, whilemorphological abnormalities have been detected in somespecies of terrestrial organisms, such as small insects andconiferous trees. However, further investigation will beneeded to obtain evidence as to whether or not genetic
mutations caused these abnormalities. Negative effects ofradiation on population size in insects and birds have also
been observed in areas affected by the FDNPP and CNPP
accidents, respectively, though the tendencies differed insome cases. Conversely, the decrease in human activities
caused by conditions of high radiation are thought to leadto increased wild boar populations. In future work, it will
be important to investigate whether adaptation and/or
selection occur in the Fukushima evacuation zones and to
elucidate the population dynamics and migratory
movements of wild boars.
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Kazuki IIJIMA
Kazuki Iijima heads the Fukushima Environmental
Evaluation Research Division at the Collaborative
Laboratories for Advanced Decommissioning
Science, Japan Atomic Energy Agency (JAEA).
He is interested in how radionuclides behave inthe environment, especially the interactions oftrace amounts of radionuclides with minerals andorganic matter of natural origins. Previously, he investigated the behaviorof radionuclides under conditions deep underground (stable and static forlong periods). After the Fukushima Daiichi nuclear power plant accident,
he started studying their behavior at the earth’s surface. (He says it is so
unstable and too dynamic!)

Seiji HAYASHI
Seiji Hayashi is a research group manager at the
Fukushima Branch of the National Institute forEnvironmental Studies. He serves as project
leader for research on radioactive substancebehavior in multimedia environments. His
energetic research activities are contributing to
the environmental recovery of Fukushima
Prefecture and its surrounding region.

Masanori TAMAOKI
Masanori Tamaoki is principal researcher at the
Fukushima Branch of the National Institute forEnvironmental Studies (NIES). He has also been
appointed associate professor at the University ofTsukuba. He has studied plant responses tooxidative stress, especially to air-pollutant ozone.
Ozone is known to generate reactive oxygen
species (ROS) in leaves, harming vegetation. ROS is also produced inorganisms exposed to radiation; hence he launched an investigation into theeffects of radiation on plants in 2011. He transferred to the Fukushima
Branch of NIES from June 2016 and is investigating the impacts ofradiation on wild organisms in Fukushima with his colleagues as project
leader.
(Received 17 December 2020, Accepted 28 December 2020)