129

Cs outflow from Forest Floor Adjacent to a Residential Area:
Comparison of Decontaminated and Non-decontaminated Forest Floor
137

Tadafumi NIIZATO* and Takayoshi WATANABE
Fukushima Environmental Evaluation Research Division,
Collaborative Laboratories for Advanced Decommissioning Science, Japan Atomic Energy Agency
10-2 Fukasaku, Miharu Town, Tamura-gun, Fukushima, 963-7700, Japan*E-mail: niizato.tadafumi@jaea.go.jp

Abstract
A three-year monitoring of Cs outflow associated with soil erosion from decontaminated and nondecontaminated sites using an experimental plot was conducted in a mountainous forest of Fukushima
Prefecture during the rainy season. The annual 137Cs outflow from the decontaminated site was 9–13 times
larger than that of the non-decontaminated site. However, the outflow from the decontaminated site decreased
from 2.55% to 0.65% corresponding with recovery of the forest floor cover. When the forest floor coverreached 60% or more, the 137Cs outflow from the decontaminated site began to show relatively minor
fluctuations at a similar level to the non-decontaminated site. The decrease in the 137Cs outflow corresponding
to the restoration of the forest floor cover was due to the recovery of the protective effect of the forest floor
against raindrop splashes and a decrease in the ratio of soil components with relatively high 137Cs activity inthe particulate matter.
137

Key words :	decontamination, forest, forest floor cover, radiocesium, residents returning

1. Introduction
Assessing radiocesium outflow from forests
contaminated by radioactive substances from the
Fukushima Daiichi nuclear power plant (FDNPP) accident
is a key issue in promoting the return of evacuatedresidents. An outflow assessment in a decontaminatedforest adjacent to a living area would be particularly
important scientific knowledge to have for estimating thepossibility of recontamination of the decontaminatedliving area, exposing returned residents to radiation
doses.
The transport processes of radiocesium from theforest floor are mainly divided into a soil erosion process
in a particulate-bound state attached to fine soil particles
and a surface runoff process in a dissolved state in surface
running water during rainfall events (Evrard et al., 2015).
The majority of the radiocesium is transported in theparticulate-bound state. Yamashiki et al. (2014)
established a series of nested monitoring stations within
the Abukuma River basin, which was affected byradioactive fallout from the FDNPP accident. They
estimated that 84% to 92% of the total radiocesium wastransported in the particulate form during the period from
August 10, 2011 to May 11, 2012. Iwagami et al. (2017)
137
monitored Cs discharge via suspended sediment, coarse
organic matter, such as leaves and branches, andGlobal Environmental Research
24/2020: 129-136
printed in Japandissolved 137Cs fraction from a forest headwater catchment
in the Yamakiya district, located approximately 35 km
northwest of the FDNPP from August 2012 to September
2013. They reported that the annual proportion of thecontribution to 137Cs discharge from suspended sedimentaccounted for 96% to 99% of the total 137Cs flux.
The amount of particulate and dissolved 137Cs washoff from the forest floor via surface runoff processes was
examined using experimental plots installed on the floorof a mountainous forest in the Abukuma Mountains(Nishikiori, et al., 2015; Yoshimura et al., 2015; Niizato
et al., 2016). Niizato et al. (2016) installed experimental
plots for 137Cs discharge monitoring on a side slope of avalley-head area with a 27°–31° slope angle forested withJapanese konara oak and cedar in the Abukuma
Mountains during the summer monsoons of 2013 and2014. The 137Cs wash-off associated with the outflow ofparticulate matter was calculated at 85% –94% of the total
137
Cs discharge from the forest floor, based on the data in
Niizato et al. (2016). Nishikiori et al. (2015) monitored
the 137Cs wash-off from a forest floor with slopes of 37°–
39° forested with Japanese cypress, red pine and cedar,and a deciduous broadleaved forest with different floor
coverings for 145 days from May to October, 2013. They
revealed the most of the 137Cs wash-off was associatedwith soil particles and the amount of soil loss was
relatively large in forests with little understory and/or©2020 AIRES

130

T. NIIZATO and T. WATANABE

organic layers (forest floor cover; 5% –15%), suggestingthat forest cover strongly affected 137Cs wash-off.
The protective effects of the forest floor cover, thatis the presence of a litter layer, against soil erosion havebeen reported based on field soil erosion monitoring in
artificially disturbed and undisturbed forest floors of
evergreen needle-leaved and deciduous broad-leaved
forests (e.g., Murai & Iwasaki, 1975; Miura et al., 2003;
Hu et al., 2012). Miura (2000) proposed a new term,“floor cover percentage (FCP),” to quantify the forest
floor cover from the viewpoint of the protective effects
against raindrop erosion. FCP indicates the percentage offorest floor that is covered with either litter or understory
within 50 cm height above the forest floor (Miura, 2000).
This FCP index enables us to evaluate the total protectivefunction of the forest floor. Miura et al. (2003) measured
sediment transport rates using sediment traps in Japanesecypress, cedar and red pine forests located on steep slopes
under a humid climate, before and after removing theforest floor cover. The rates increased immediately after
removing the floor cover, and decreased corresponding toincreased FCP without delay. This result shows thesediment transport rate is sensitive to changes in FCP(Miura et al., 2003). Moreover, the above results indicate
diminishment of the forest floor cover results in increased
amounts of radiocesium outflow via soil erosion on forest
slopes.
From the perspective of reducing radiation doses in
the living environment of residents near a forest, removalof organic layers such as fallen leaves, etc. up to about
20 m from the edge of the forest is effective and efficient(Ministry of Agriculture, Forestry and Fisheries, 2011;
Kaneko & Tsuboyama, 2012; Ministry of theEnvironment, 2016). On the other hand, lining up
sandbags at the edge of the forest or other measures toprevent soil movement and runoff must be taken after theremoval of fallen leaves and other organic sedimentmatter on steep forest slopes (Ministry of theEnvironment, 2016).
Yamamoto et al (2014) conducted experiments
removing organic layers (L, F, and H layers) and137
monitoring the amount of Cs wash-off from a forest
floor with a 31° slope angle in a deciduous broadleaved
forest in the Yamakiya district about 40 km northwest of
the FDNPP. The monitoring was performed in a plot withorganic layers removed and a non-treated plot for fivemonths from July to December 2013. After the removal
treatment, 9.65% and 0.64% – 0.69% of the 137Cs in theforest floor had been washed off from the plot with the
L-F-H layers removed and the plot with the L-F layers
removed, respectively. The wash-off was 0.08% in theplot with no removal treatment (Yamamoto et al., 2014).
This revealed a protective function of organic layers
against radiocesium outflow from a forest floor, although
their discussion of the effect of the forest floor coverage
recovery process in natural environments was inadequate.

Although decontamination work in forests nearliving areas reduces radiation doses in the residents’living environment, radiocesium outflow via soil erosion
from the forest floor is relatively higher compared to a
non-decontaminated forest floor. It is expected, however,that the radiocesium outflow associated with soil erosion
will gradually decrease with time corresponding torecovery of the forest floor cover due to the deposition oflitter and growth of understory on the floor after
decontamination.
This study presents the results of three years ofmonitoring radiocesium outflow via soil erosion and therecovery process of forest floor cover in a forest floor
decontaminated according to the Decontamination
Guidelines of the Ministry of the Environment, Japan.
The natural resilience of the forest floor shown in thisstudy is expected to be applied to measures for preventing
radiocesium outflow in combination with engineering
measures and chemical treatment.

2. Materials and Methods
2.1 Study Area
Radiocesium outflow monitoring was conducted in a
deciduous broadleaved tree forest in the Yamakiya district
within the Abukuma Mountains, 35 km northwest of the
FDNPP (Fig. 1). The level of radioactive contaminationin the district was measured at 497 kBq m−2 (as of April 1,
2013) in 137Cs deposition via the Sixth Airborne
Monitoring results (Ministry of Education, Culture,
Sports, Science & Technology in Japan, 2020). At thismonitoring site, Niizato et al. (2016) carried out
monitoring of the radiocesium outflow associated withsurface wash-off from experimental plots installed in a
non-decontaminated forest floor. The annual 137Cs outflow
was estimated at 0.07% – 0.15% of the total 137Cs deposition
to the site from 2013 to 2014 (Niizato et al., 2016).
The mean annual precipitation in this district overthe past 30 years (1981–2010) has been 1223.8 mm yr−1
at the Yamakiya Meteorological Station, located 2.7 km
northwest of the site (Ministry of Land, Infrastructure,
Transport and Tourism, 2020). This district is mainly
covered by deciduous broad-leaved trees of konara oak(Quercus serrata), mizunara oak (Quercus crispula), andJapanese chestnut (Castanea crenata), except at thesouthern edge of the forest, where Japanese red pine
(Pinus densiflora) trees dominate. The average tree height
and tree density in the canopy layer are 10.9 m and740 ha−1, respectively. Most of the forest floor is covered
with litter composed mainly of fallen branches and leavesof deciduous broadleaved trees. The thickness of the litter
layer is usually 1–3 cm. The undergrowth cover,including mountain azalea (Rhododendron kaempferi)and bamboo grass (Sasa nipponica), ranges from 35% to60%. Brown forest soil is most common in the forest
slopes of the district. The soil down to a 5 cm depth isCs outflow in decontaminated and non-decontaminated forest

137

sandy-loam to loam in conventional textural classes(Niizato et al., 2016).
Removal of understory and organic layers (L, F, andH layers) in decontamination work on the forest floor at
the monitoring site was performed in July–September
2014 and August–September 2015 within 20 m from aliving area by the local government. A mineral soil layer
was exposed on the forest floor after the decontamination
work.
2.2 Monitoring and Sampling
Two experimental plots for monitoring were installed
on a decontaminated forest slope (decontaminated plot)
and a non-decontaminated forest slope (control plot)
facing west with a 27°–30° slope angle (Fig. 1). The
decontaminated plot was located 40 m south of the2
control plot and the plot area was 8 m (2.0 m × 4.0 m).
The control plot had a plot area of 53.4 m2 (6.0 m × 8.9 m)
in 2016 and 26.7 m2 (3.0 m × 8.9 m) in 2017–2018. Bothexperimental plots were located on rectilinear, straightslopes, and had common features of physical geography
except for the forest floor cover. The forest floor in thecontrol plot had an 86% – 94% cover ratio from its litter
layer and understory during the monitoring period, while
the forest floor cover ratio in the decontaminated plot
started from 30% in March 2016 and increased to 83% in
November 2018 through deposition of litter andunderstory growth.
Each experimental plot had a rectangular shape, thelong side of which was oriented along the dip direction of
the slope, and was surrounded with stainless-steel boards

131

for the control plot and with hard vinyl chloride
corrugated sheets for the decontaminated plot to prevent
transportation of particulate matter and surface runoff
water into the plot from the outside (Fig. 1). Particulatematter, composed of soil particles and fine litter
fragments, accumulated in a sediment trap installed at thebottom end of each plot with same width as the plot. The
particulate matter that drained from the plot with its
associated surface runoff water flowed into a 50 L tankfor the decontaminated plot and a 200 L tank for thecontrol plot connected to their respective sediment traps
via a pipe.
The particulate matter samples collected from thesediment traps were passed through a 2-mm sieve, andweighed after drying at 105°C for 24 h. The samples
stored in the tank were mixed well within the tank, thencollected as turbid water using a scoop. The turbid water
samples were passed through a 2-mm sieve, and weighed
after evaporation to dryness at 105°C for more than 24 husing a dry oven. Those samples were homogenized byhand stirring with a dispensing spoon, and then packed
and sealed in a polystyrene container (U-8) for radioassay
per sampling date.
Samples of the particulate matter were collected
within 2- to 8-week intervals in correlation withprecursory rainfall events. Associated field surveys andmonitoring efforts were performed mainly during
Fukushima’s summer monsoon from April to November,in the years 2016–2018. This monthly span was chosen
as most appropriate for research due to rainfall acting as
the principal cause of transportation of particulate matter

Fig. 1 Location of the monitoring site (left) and view of the entire decontaminated plot in May 2017 (right). The topographic data werecreated based on the results of an airborne laser survey conducted by the Geospatial Information Authority of Japan.

132

T. NIIZATO and T. WATANABE

within the forested area.
The forest floor cover percentage of thedecontaminated plot was calculated from the ratio ofintersection points with litter and/or understory on theforest floor with 25 cm square grids in the plot. Theintersection points with litter and understory werecounted using a digital photograph of the floor takenfrom above 50 cm height from the floor (Watanabe et al.,2019). The percentages of forest floor cover in the control
plot were estimated by direct measurement of the area
without the litter layer or understory in the field
investigation.
Soil samples were collected using a scraper plate(Tsukahara-SS Corporation, Japan) with a metal frame of2
450 cm (15 cm × 30 cm), the sampling area for vertical
distribution of 137Cs near both plots. The samples were
homogenized by hand stirring with a dispensing spoon
after drying at 105°C for 24 h, and then packed andsealed in a polystyrene container (U-8) for radioassay per
sampling date.

2.4 Calculation of 137Cs Outflow
Transportation of particulate matter from theexperimental plots was accompanied by the outflow ofparticulate-bond 137Cs. The volume-weighted mean 137Cs
activities of the particulate matter, litter and soil samples
were computed by summing the amounts of 137Cs inindividual samples during the monitoring period and thendividing this value by the total amount of the sample
weight. The 137Cs outflow via soil erosion (Bq m−2) was
calculated by multiplying the weight of the particulatematter (kg) by its 137Cs activity (Bq kg−1) and thendividing this value by the plot area (m2). The annual 137Cs
outflows were calculated by summing up daily flux forthe number of days in the monsoon season (April to
November). The daily fluxes were calculated by dividing
137
Cs outflow during the monitoring period by the numberof monitoring days. The outflow percentage of theparticulate-bound 137Cs was calculated using 137Cs
inventories obtained from decontaminated and nondecontaminated forest slopes near the experimental plots.

2.3 Radiocesium Measurements
137
Cs activity was determined using an n-type highpurity Ge-detector (GSW275L germanium detector,
CANBERRA Industries Inc., Meriden, CT, USA). A
multichannel analyzer (MCA7600, Seiko EG＆G ORTEC,
Tokyo, Japan) with spectrum analysis software (Gamma
Explorer + (Plus), CANBERRA Industries Inc) wasused for pulse-height analysis employing counts at the661.64 keV peak. An efficiency calibration was
performed with a multiple gamma-ray emitting standard
source (including nine nuclides) packed in 100 mL plastic
U-8 containers (Japan Radioisotope Association, Tokyo,
Japan). The radioactivities reported herein were corrected
for radioactive decay with respect to April 1, 2013. The
137
Cs activities of the particulate matter, litter and soil are
shown in dry weight.

3. Results and Discussions
3.1 Annual 137Cs Outflow
The forest floor cover in the control plot showedslight seasonal variation (86% in early June and 94% inlate November) due to leaf fall from October toNovember during the monitoring period from 2016 to2018. The forest floor cover in the decontaminated plot
showed significant change with time (Table 1). A sparse
distribution of bamboo grass and litter (mostly fallen
twigs) was found in March of the year following
decontamination of the monitoring site and the coverratio had increased to 30%. The floor cover percentage
had been restored to about 50% by understory growth andlitter deposition by early December of that year. After thecover percentage reached 70% in September 2017, two

Table 1 Forest floor cover and 137Cs activity of particulate matter in the decontaminated plot during different sampling periods.
abPeriodGross precipitation (mm) Forest floor cover (%) Particulate matter (g m-2 ) 137Cs activity (kBq kg-1 )
15
30.0
9.1
27.7
Mar. 16-Apr. 4, 2016
173
47.5
39.3
23.6
Apr. 4-May 29, 2016
152
50.0
134.5
26.4
May 29-July 11, 2016
436
52.5
416.0
24.4
July 12-Sept. 1, 2016306
no data
52.0
20.9
Sept. 2-Nov. 22, 2016
12
47.5
4.5
28.5
Nov. 22-Dec. 8, 2016
66
42.5
22.3
25.4
Apr. 16-May 14, 2017
60
50.0
58.8
22.2
May 14-June 14, 201781
no data
37.2
38.8
June 14-July 13, 2017
264
52.5
88.3
25.4
July 13-Aug. 10, 2017
132
70.0
51.1
23.1
Aug. 10-Sept. 15, 201783
no data
12.2
18.8
Sept. 15-Oct. 13, 2017
291
80.0
17.2
26.9
Oct. 13-Nov. 30, 2017
91
70.0
34.6
20.0
Apr. 28-May 21, 2018
150
62.5
19.0
16.6
May 21-July 29, 2018142
no data
50.5
23.4
July 29-Aug. 27, 2018
126
77.5
22.1
26.1
Aug. 27-Sept.24, 2018
105
82.5
12.8
17.2
Sept. 24-Oct. 28, 2018
4
82.5
3.0
18.4
Oct. 28-Dec. 5, 2018
c: Decay corrected to April 1, 2013.
a: Data from MLIT (2020).
b: Counting error ranges from 112 to 599 Bq kg-1.

c

Cs outflow (kBq m-2)
0.25
0.93
3.56
10.15
1.09
0.13
0.57
1.31
1.44
2.25
1.18
0.23
0.46
0.69
0.32
1.19
0.58
0.22
0.06

137

Cs outflow in decontaminated and non-decontaminated forest

133

137

Table 2 137Cs activity of particulate matter in the control plot during different sampling periods.
a

b

c

137
137
PeriodGross precipitation (mm)Particulate matter (g m-2)
Cs activity (kBq kg-1)
Cs outflow (Bq m-2)
118
3.8
34.5
132
Apr. 12-May 19, 2016
4
2.8
15.9
44
May 19-May 30, 2016
54
5.0
26.2
132
May 30-June 15, 2016
75
0.7
29.2
22
June 15-June 29, 2016
25
0.2
26.6
6
June 29-July 13, 2016
10
0.4
28.2
10
July 13-July 29, 2016
426
13.0
37.4
488
July 29-Sept. 1, 2016
56
2.3
45.0
104
Sept. 1-Sept. 16, 2016
154
1.2
29.4
34
Sept. 16-Sept. 30, 2016
35
1.7
29.4
49
Sept. 30-Oct. 14, 2016
2
0.4
31.9
14
Oct. 14-Oct. 28, 2016
49
0.1
33.2
4
Oct. 28-Nov. 11, 2016
15
0.2
29.3
6
Nov. 11-Nov. 29, 2016
7
0.5
44.8
24
Nov. 29-Dec. 9, 2016
49
1.2
39.5
48
Mar. 24-Apr. 14, 2017
26
2.2
36.0
78
Apr. 14-Apr. 28, 2017
4
0.7
28.3
21
Apr. 28-May 12, 2017
43
2.0
32.2
64
May 12-May 22, 2017
12
1.4
13.6
19
May 22-May 26, 2017
May 26-June 9, 2017
38
2.1
21.5
44
June 9-June 30, 2017
19
1.0
21.5
22
June 30-July 13, 2017
50
1.2
18.2
22
July 13-July 28, 2017
168
7.5
20.8
157
July 28-Sept. 1, 2017
190
6.0
39.4
236
Sept. 1-Sept. 15, 2017
38
1.3
40.8
51
Sept. 15-Sept. 29, 2017
58
1.8
32.2
57
Sept. 29-Oct. 13, 2017
24
1.3
30.3
39
Oct. 13-Oct. 26, 2017
199
4.4
23.9
104
Oct. 26-Nov. 17, 2017
68
1.6
22.2
36
Nov. 17-Nov. 30, 2017
24
0.7
23.2
16
Nov. 30-Dec. 15, 2017
12
0.4
21.1
9
Apr. 13-Apr. 26, 2018
44
1.0
22.0
21
Apr. 26-May 11, 2018
73
1.5
20.1
31
May 11-June 1, 2018
39
1.3
21.5
29
June 1-June 28, 2018
30
1.0
18.8
19
June 28-July 30, 2018
99
2.9
25.5
74
July 30-Aug. 28, 2018
142
3.6
31.7
115
Aug. 28-Sept. 25, 2018
126
1.3
16.9
22
Sept. 25-Oct. 29, 2018
105
1.8
19.5
35
Oct. 29-Dec. 6, 2018
4
0.2
9.6
2
-1
-1
-1
a: Data from MLIT (2020). b: Counting errors are 210-2124 Bq kg , 150-553 Bq kg , and 122-306 Bq kg in 2016, 2017 and 2018, respectively.
Decay corrected to April 1, 2013. c: Decay corrected to April 1, 2013.

137Cs activity (kBq kg–1)

Depth (cm)Litter layer
0-1
1-2
2-3
3-4
4-5
5-6
6-7
7-8
8-9
9-10

50

100

150

0

Litter layer
0-1
1-2
2-3
3-4
137Cs inventory4-5
5-6
110.8 kBq m–2 6-7
Decontaminated plot 7-8
8-9(Dec. 8, 2015)
9-10

14.5
26.3
12.4
3.2
1.1

Depth (cm)

0

50

100

150

9.6
14.3
6.3
5.0
2.1

162.8

38.1

137Cs inventory575.2 kBq m–2

Control plot
(Oct. 21, 2016)

Fig. 2 Vertical distribution of 137Cs to the depth of 10 cm near the decontaminated and control plots. The numbers on the right side of thebar chart indicate the 137Cs activities of litter and soil layers. The counting error was less than 10%.

years after decontamination, the percentage stayed over
70%, except for May−July, 2018 (Watanabe et al., 2019;
Table 1).
The weight of the particulate matter from thedecontaminated plot was 10 –20 times larger than that of
the control plot (Tables 1 and 2). On the other hand, the137
Cs activities of the particulate matter from thedecontaminated plot each year were comparatively lower
than those from the control plot due to decontaminationof the monitoring site (Tables 1 and 2). The 137Cs activityof the soil layer at a depth of 0 –1 cm near thedecontaminated plot was one sixth that of a similar
measurement near the control plot (Fig. 2). The vertical
distribution of 137Cs near the decontaminated plot about

134

T. NIIZATO and T. WATANABE

Table 3 Annual 137Cs outflow accompanied by the transportation of particulate matter.

Grossa Forest floor
Plot
Periodprecipitation
cover (%)(mm)
Decontaminated plot Mar. 16-Dec. 8, 2016
1094
30.0-52.5
Apr. 16-Nov. 30, 2017977
42.5-80.0
Apr. 28-Dec. 5, 2018
618
62.5-82.5
Control plot
Apr. 12-Dec. 9, 2016
1030
86.0-94.0
Mar. 24-Dec. 15, 20171022
86.0-94.0
Apr. 13-Dec. 6, 2018
662
86.0-94.0

Particulatematter(g m-2)
655
287
142
32
37
15

Cs activity(kBq kg-1)

137

b

24.5
25.9
21.4
33.0
27.8
23.7

Annual 137Cs outflow(kBq m-2)
14.7
7.9
3.7
1.1
0.9
0.4

(%)

c

2.55 (13.23)
1.38 ( 7.16)
0.65 ( 3.38)
0.19
0.16
0.06

a: Data from MLIT (2020). b: Volume-weighted mean as of April 1 2013. The counting error ranges from 79 to 128 Bq kg-1.
c: Percentage of annual 137Cs outflow was calculated using the inventory of the decontaminated and non-decontaminated forest
slopes shown in Fig. 2. See text for details of that calculation.

Table 4 Estimation of the weight percentage of litter and soil in the particulate matter.

Plot

Year

Decontaminated plot

2016
2017
2018
2016
2017
2018

Control plota

Cs activity (kBq kg-1)

137

Particulate matter
24.5
25.9
21.4
33.0
27.8
23.7

Weight percentage (%)
LitterSoil (0-1cm depth)

14.5

26.3

9.6

162.8

litter
15
3
42
85
88
91

Soil
85
97
58
15
12
9

a: 137Cs activity shown in Table 3 and Fig. 2.

three months after decontamination indicates that thesurface soil layer was also slightly removed from up to 1
cm depth at maximum in the course of removing the litter
layer and understory during the decontamination work.
The annual 137Cs outflow from the decontaminatedplot was 9–13 times larger than that from the control plot,reflecting of the weight of the particulate matter and its
137
Cs activity in both plots (Table 3). Moreover, theoutflow from the decontaminated plot decreased
corresponding with recovery of the forest floor cover
during the monitoring period from 2016 to 2018. The
outflow in 2018 from the decontaminated plot was one
fourth that in 2016, an outflow percentage of less than
1%. This is a similar level of 137Cs outflow percentage aswith the control plot. When we calculated the 137Cs
outflows of the decontaminated plot based on the 137Cs
inventory in the decontaminated forest floor, they were
estimated to be 13.2%, 7.2%, and 3.4% in 2016, 2017,and 2018, respectively (Table 3).
The particulate matter containing 137Cs wascomposed of litter and soil components on the forest
floor. The weight percentages of the litter and soil were
estimated from an end-member mixing analysis using the137
Cs activities of the particulate matter, litter and soil
layers in the forest floor (Table 4). The soil component of
the particulate matter in the decontaminated plot ranged
from 60% to 90%, though it was less than 20% in thecontrol plot. The weight percentage of the soil component
in the decontaminated plot had decreased to 58% by
2018.
A significant amount of splashed soil was attached tothe 30-cm-high hard vinyl chloride corrugated sheets
bordering the decontaminated plot (Fig. 3, left). There
were many 5–25-mm-height pedestals and soil pillars in
the decontaminated forest floor (Fig. 3, right).
Destruction of these pedestals or soil pillars was observed
but no rills developed on the floor. These forest floor
conditions indicate that the main factor in thetransportation of the particulate matter on thedecontaminated forest floor was raindrop splashing ofparticulate matter on the forest floor.
137
The above results indicate the decrease in Cs
outflow corresponding to restoration of the forest floor
cover is the result of recovery of the protective effect of
the forest floor against raindrop splashes and a decreasein the amount of soil components with relatively high137
Cs activity in the particulate matter.
3.2 137Cs Outflow and Forest Floor Cover
The monitoring and sampling periods varied byexperimental plot (Tables 3 and 4). For comparing thesame unit interval, the 137Cs outflows from the forest floor
described above were converted to daily flux (Bq m−2
day−1). In addition, high rainfall intensity events tended to
produce a greater geomorphological impact since rainfall
intensity determines the size frequency distribution ofraindrops that fall (e.g., Anderson & Anderson, 2010). To
normalize by rainfall intensity per hour, daily fluxes of137
Cs outflow were divided by the sum of squares ofrainfall intensity per 1 h during the monitoring periods(Bq m−2 day−1 mm−2).
The daily flux of 137Cs outflow normalized by rainfall

Cs outflow in decontaminated and non-decontaminated forest

137

135

Fig. 3 Soil splashed by raindrops attached to the 30-cm-heigh hard vinyl chloride corrugated sheets (left) and pedestals (right) on theforest floor in the decontaminated plot (July 14, 2017).

Fig. 4 Daily flux of 137Cs outflow normalized by rainfall intensity
per 1 h in the decontaminated and control plots (upper).
Normalization by rainfall intensity was calculated using thedata from MLIT (2020). The forest floor cover percentages
in the decontaminated plot are shown in the lower graph.

intensity in the decontaminated and control plots areshown in Fig. 4. The daily flux of 137Cs outflow from thedecontaminated plot decreased from 2016 to 2018, while
no obvious change occurred in the control plot except for
May 11, 2017. In detail, there were major fluctuations in
the daily flux of the outflow (about 0.5−9.0 × 10−4 Bq m−2
day−1 mm−2) from the decontaminated plot in the period
from March 2016 to August 2017. After September 2017,
they turned into relatively minor fluctuations (about
0.5−3.7 × 10−4 Bq m−2 day−1 mm−2) corresponding withrecovery of the forest floor cover to 60% or more (Fig. 4,
lower).
Our three-year monitoring results from the year
following decontamination work in 2016 showed 137Cs
outflow decreasing to one fourth at the monitoring site inthis study. The annual 137Cs outflow percentage decreased
to less than 1% of the 137Cs inventory compared to before
decontamination. Though the forest floor cover in thedecontaminated plot reached 60% or more, the amount of137
Cs outflow and its fluctuation range in thedecontaminated plot remained larger than those of thecontrol plot installed in a non-decontaminated location(Tables 1 and 2). There were seasonal variations in forest
floor cover percentages in the decontaminated site from
60% in May−June to 80% or more in October−November
after August 2017 (Fig. 4, Table 1). This suggests thefallen leaves and twigs did not remain on the forest floor
after deposition but were transported from the floor, thenprobably deposited at the bottom of the forest slope. In
the non-decontaminated forest floor, protective effects of137
the forest floor against Cs outflow via soil erosion
appeared to be maintained by the pre-existing litter layer
when fallen leaves were transported immediately after
their deposition on the forest floor due to the effects ofwind or raindrop impacts. Thus, for restoration of theprotective effects of the forest floor to a predecontamination condition, it is important to promote
growth of understory, to prevent the transport of fallen
leaves and twigs away and to prepare conditions for litter
layer formation on the decontaminated forest floor.

4. Conclusion
We conducted monitoring of 137Cs outflow associatedwith soil erosion at decontaminated and nondecontaminated sites using experimental plots from April
2016 to November 2018 during the rainy season in
Fukushima. Our three-year monitoring results showedthat the 137Cs outflow from the decontaminated site was
9–13 times larger than that of non-decontaminated site.
The outflow from the decontaminated site, however,decreased from 2.55% to 0.65% corresponding to therecovery of the forest floor cover during the monitoring
period from 2016 to 2018. The outflow percentage in2018 was at a similar level to that of the nondecontaminated site. An end-member mixing analysis of137
Cs activities between litter and soil on the forest floor
and particulate matter flowing out from the experimental
plot showed that the decrease in the 137Cs outflow
corresponding to the restoration of the forest floor cover
was the result of recovery of the protective effect of theforest floor against raindrop splashes and a decrease in
the soil component with relatively high 137Cs activity inthe particulate matter. When the forest floor cover

136

T. NIIZATO and T. WATANABE

reached 60% or more, the 137Cs outflow from thedecontaminated site began showing relatively minor
fluctuations at a level of outflow percentage similar tothat of the non-decontaminated site. To restore theprotective effects of the forest floor against radiocesium
outflow to a pre-decontamination condition, it will be
important to promote understory growth resulting in litter
layer formation.
Acknowledgement
The authors thank the local landowner who provided
us his hill for our monitoring site. We are grateful tomembers of the Fukushima Environmental Evaluation
Research Division and Fukushima Environmental
Monitoring Division of Japan Atomic Energy Agency for
their helpful suggestions and technical support during thecourse of this work. Our thanks also go to Messrs. Okino
Ryo, Ikeda Makoto and Kawamura Makoto of Mitsubishi
Materials Techno Co. for their technical support in137
Cs outflow in the noncompiling data on
decontaminated area.
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Tadafumi NIIZATO
Dr. Tadafumi Niizato heads the Environmental
Research Group of the Sector of Fukushima
Research and Development, which conducts
environmental dynamics research on the effectsof radiocesium from the Fukushima Dai-ichi
nuclear power plant accident. He enjoys potted
plant cultivation with his wife on weekends in
Miharu Town, which is famous for a very old weeping cherry tree, Takizakura.

Takayoshi WATANABE
Mr. Takayoshi Watanabe is an engineer at theEnvironmental Research Group studying theenvironmental dynamics of radiocesium inforested environments in Fukushima. He enjoys
motor scooter touring, feeling the turn of theseasons through his touring.
(Received 4 September, Accepted 21 December, 2020)