105

Role and Effect of a Dam on Migration of Radioactive
Cesium in a River Catchment after the Fukushima Daiichi
Nuclear Power Plant Accident
Seiji HAYASHI* and Hideki TSUJI
Fukushima Branch, National Institute for Environmental Studies
10-2 Fukasaku, Miharu Town, Tamura-gun, Fukushima, 963-7700, Japan*E-mail: shayashi@nies.go.jp

Abstract
Positive and negative roles of dam reservoirs in radiocesium behavior in a river catchment were
examined based on the results of comprehensive, long-term hydrological monitoring from 2013 to 2017 at
Matsugabou Dam on the upstream reaches of the Uda River, which is one of the main rivers in the northerncoastal region of Fukushima Prefecture. An estimation of the annual mass balance of Cs-137 associated withsuspended solids (SS) in the reservoir showed that Matsugabou Dam had the effect of reducing the migrationof radiocesium associated with SS from the dam reservoir catchment to the downstream reaches by more than
85% by storing SS in the reservoir bed. Moreover, proactive discharge control for preventing flood inundationin the downstream region had the ability to make this storage function perform even more effectively. On theother hand, the estimated annual mass balance of dissolved Cs-137 in the reservoir showed that the amount in
the water discharged clearly tended to exceed the amount in the inflow, suggesting that substantial amounts ofdissolved radiocesium were presumably produced by dissolution/elution from the stored bottom sediments inthe dam reservoir. This negative role of a dam producing dissolved radiocesium may contribute to long-term
contamination of freshwater ecosystems in downstream water bodies.
Key words :	bottom sediment, dam reservoir, discharge control, dissolution, runoff

1. Introduction
Most of the seriously radioactively contaminatedareas resulting from the Fukushima Daiichi nuclear
power plant (FDNPP) accident consisted of mountainous
forests (Morino et al., 2011). Few forested areas havebeen decontaminated because decontamination operations
have been limited to inhabited areas such as residential
districts, farmland, roads and marginal forests nearresidential districts (Ministry of the Environment, 2019).
Consequently, the remaining vast forested area is left as apotential source of radiocesium penetration into
downstream regions. Therefore, there are concerns aboutrecontamination of the downstream regions as well as
prolonged contamination of freshwater ecosystems by
runoff from forested areas. Meanwhile there are over 90
water supply dams in Fukushima Prefecture, where
agriculture is well developed as one of the mainindustries. Although radiocesium runoff originating fromthe FDNPP accident has been confirmed to be limited toless than 0.3% annually in forest catchments (Shinomiya
et al., 2014; Iwagami et al., 2017; Tsuji et al., 2016), it is
Global Environmental Research
24/2020: 105-113
printed in Japan

also a fact that a successive inflow of radiocesium indissolved or particulate forms has been occurring fromdam reservoir catchments covered mainly by seriously
contaminated forest. As for behaviors of radiocesium
originating from the FDNPP accident in dam reservoirs,
although it is assumed that radiocesium associated withsuspended solids (SS) flowing into a reservoir tends toaccumulate in the bed through the water storage functionof the dams (Kurikami et al., 2014; Huon et al., 2018), a
quantitative evaluation of that function has yet to be fully
accomplished based on long-term monitoring.
Additionally, research on the effects of the re-suspension
or dissolution of radiocesium from bottom sediment isvery limited (Funaki et al., 2020). There is a need toaccumulate scientific knowledge from the viewpoint of
not only understanding the behavior of radiocesium in adam reservoir but also examining the roles of dam
reservoirs in radiocesium dynamics in river catchments.
An approach of clarifying radiocesium dynamics
comprehensively in a significantly contaminated dam
reservoir catchment would contribute to a detailed
examination of radiation exposure risks via various©2020 AIRES

106

S. HAYASHI and H. TSUJI

pathways in river catchments.
The objective of this paper is thus to presentcharacteristics of radiocesium dynamics in a dam
reservoir catchment mainly covered with forest affectedby the FDNPP accident that have been elucidated on thebasis of the data from our observations since 2012.
Moreover, based on these results, the positive andnegative roles of dam reservoirs in radiocesium behavior
in a river catchment are discussed from the viewpoint ofconsidering various pathways of radiation exposure.

deposition of Cs-137 from the FDNPP accident into
the DRC, SFC and WURC was 260 kBq/m2, 170 kBq/m2
and 210 kBq/m2, respectively, based on the third airborne
survey conducted by the Ministry of Education, Culture,
Sports, Science and Technology (2011).
2.2 Sampling and Monitoring
To elucidate temporal changes in radiocesium
activity concentration in the river and water discharged
from the dam and evaluate the mass balance ofradiocesium in the Matsugabou Dam reservoir, automatichydrological observation (2150 Area Velocity Module,
Teledyne Isco, Lincoln, NE, USA) and turbidity
observation (DTS-12, Forest Technology Systems Ltd.,
Victoria, British Columbia, Canada) were conducted in
the stream to measure the flowrate at the outlet of the
SFC (37°48’10.7”N 140°45’28.1”E, #2 in Fig. 1) from
July 2012 with a time interval of 10 min. Turbidity was
automatically measured at the chapter just downstream ofthe dam discharge point (37°47’44.3”N 140°46’23.2”E,#3 in Fig.1) using TC-500 (OPTEX Co., Ltd., Otsu,
Shiga, Japan) from May 2013 and at the observatory at
the outlet of the WURC (37°47’23.6”N 140°54’42.8”E,#1 in Fig. 1) using an INFINITY-Turbi (JFE Advantech
Co., Ltd., Nishinomiya, Hyogo, Japan) from July 2012
with a time interval of 10 min. Measured data with a timeinterval of 10 min. for the dam discharge volume and theflowrate at the downstream observatory were obtained
from Soma City and Fukushima Prefecture, respectively.
Water samples were collected at the hydrologicalobservation point in the SFC from Oct. 2012, theinflow chapter to the dam reservoir (37°47’44.6”N
140°44’43.7”E, #4 in Fig. 1), mainly during low flow
conditions from June 2014, in the chapter justdownstream of the dam discharge point from Oct. 2013,
and at the WURC observatory from Oct.2012. Sixty liters
or more of river water was regularly sampled using a
20 L plastic bucket at two-to-four-week intervals fromthe center of flow during low-flow conditions. During
rain runoff events, an auto water sampler (ISCO

2. Materials and Methods
2.1 Study Area
Matsugabou Dam constructed on the upstream
reaches of the Uda River and completed in 1997 wasselected as the objective of this study (Fig. 1). The Uda
River is one of the main rivers in the northern coastal
region of Fukushima Prefecture, about 50 km northnorthwest of the FDNPP. Matsugabou Dam reservoir,
which has a surface water area of 0.72 km2 and total
storage capacity of 9,710 × 106 m3, is operated mainly tosupply irrigation water to the developed paddy fields inthe middle and lower reaches of the Uda River catchment.
The dam reservoir catchment (DRC, catchment area:
25.4km2) consists mainly of forest (80%), with 7%
paddy fields, 5% farmland and 3% built-up areas. The soilin the DRC is mainly brown soils or red-brown forest
soil. A small forested catchment (SFC, catchment area:
0.34km2) was selected to study the radiocesium
runoff characteristics from a forested area. Additionally,
to estimate the effect of dam storage functions on
prevention of radiocesium migration to downstream
areas, an observation site was set in the downstream
reaches of the Uda River (#1 in Fig. 1) to get an
overview of the radiocesium runoff characteristics of
the whole Uda River catchment (WURC, catchment area:
95.6 km2) consisting of 79% forest, 7% paddy fields, 5%
farmland and 4% built-up areas. The spatially averaged

Matsukawaura
LagoonMatsugabou Dam
reservoir

DRC

Uda Riv.

Fukushima Prefecture

SFC

WURC

Forestcatchment#2#4

Reservoir#1

Uda River catchment
Matsugabou Dam#3: Hydrologic observation & water sampling points: Rainfall observatory by Matsugabou Dam Maintenance Facility: Bed sediment sampling point

Fig. 1 Outline of study site. Data from the third airborne survey (MEXT, 2011) are used for the Cs-137 deposition map.

Role & effect of a dam on radiocesium migration
6712, Teledyne Isco, Lincoln, NE, USA) was used tocollect 3 L of river water for every sample in addition tothat taken in the same manner during low flow conditions
to sample the stream water at 30 – 60 min intervals.
Bottom sediment samples were collected using an
undisturbed sediment core sampler (HR type, RIGO Co. Ltd,Tokyo, Japan) at a site near the dam every late autumn
after the typhoon season from 2012 to 2016. In particular,
three core samples were collected at almost the same
point to get an understanding of the spatial variation inradiocesium accumulation properties in 2015 and 2016.
2.3 Measurement
Particulate and dissolved radiocesium in water
sampled during low flow conditions were collected
separately using two types of cartridge filters (Tsuji et al.,2014; Yasutaka et al., 2015) immediately after sampling.
As for the samples collected during rain runoff events,
either centrifuge separation (15,760×g, 25min) to extract
suspended matter or the cartridge filters described above
were adapted depending on the SS concentration after
being brought back to the laboratory to measureparticulate radiocesium concentration. After centrifuge
separation, the extracted suspended matter was placed in
100-mL plastic containers, dried at 60°C, weighed andanalyzed for Cs-134 and Cs-137. The SS concentration of
all of the water samples was measured by a filtering
method using glass fiber filter papers (pore size, 0.7 µm,
GF/F; GE Healthcare, Tokyo, Japan) to develop an
equation to estimate the respective SS concentration fromturbidity data measured at each observatory site as wellas to evaluate its association with the particulate
radiocesium concentration.
The collected reservoir bottom sediment coresamples were sliced at 1–2 cm intervals from the surface,
and the sliced samples were placed in 100-mL plastic
containers, dried at 60°C, weighed and analyzed for
Cs-134 and Cs-137.
Cs-134 and Cs-137 activities in samples were
measured using a coaxial or a well-type germanium
detector (GC2518 or GCW7023; Canberra, Meriden,
CT). A Spectrum Explorer (Canberra Japan, Tokyo,
Japan) was used to analyze the γ-ray spectra.
MX033U8PP (The Japan Radioisotope Association,
Tokyo, Japan) was used as the standard source for theefficiency calibration. Estimates of the activities ofsamples trapped in the cartridge filters were geometricallycorrected using the formula of (Tsuji et al., 2014;
Yasutaka et al., 2015). The activities were decaycorrected to the sampling date, and then the activity
concentration was determined on a dry weight basis.
Since it was confirmed that the proportion of activity
concentrations of Cs-134 and Cs-137 for each sample
was almost 1.0 after decay correction to Mar. 11, 2011,
only Cs-137 was used, as described in the Results andDiscussion below, from the viewpoint of evaluating longterm radioactive contamination.

107

3. Results and Discussion
3.1 Particulate Radiocesium Runoff Properties from
Forests and Entire River Catchments
The activity concentrations of dissolved Cs-137 werevery low (less than 0.02 Bq/L) at the outlets of both the
SFC and the WURC at the beginning of our observation
period and have also been decreasing since then.
Therefore, only particulate Cs-137 was considered in
evaluating the amount of radiocesium migration at theriver-catchment scale in this study because dissolved
radiocesium contributes little to radiocesium flux during
the rain runoff events that play an important role intransport and dispersion of radiocesium in riverwatersheds and coastal marine environments, as pointed
out by Nagao et al. (2013). Figure 2 shows the temporalchange in the activity concentration of Cs-137 associatedwith SS in the outlets of the SFC and WURC (#2 and #1
in Fig. 1). The temporal change of each respective decline
in concentration was fitted using a single component
exponential model represented by the following equation:
Cs_SS = A ∙ e – λ ∙ t

137

SS

…… (Eq.1)

where 137Cs_SS (Bq/kg-SS) ) represents the activity
concentration of Cs-137 associated with SS, A (Bq/kgSS) and the effective decay constant, λSS (1/y) areempirically determined constants, and t (y) is time
elapsed since deposition. The statistical analysis used todetermine these parameters was performed using Origin
version 2020 data analysis and graphing software(OriginLab Corporation, Northampton, MA, USA). The
concentrations at both sites fluctuated greatly until a hugerain event, tropical storm Etau, occurred in September
2015 with a large amount of rainfall over two days as issaid only to occur every 50 to 100 years. It is especiallycurious that the concentrations at the SFC were relatively

Tropical storm Etau occurrence

137Cs_SS = 15.2e-0.20t

137Cs_SS = 12.2e-0.26t

Fig. 2 
Change over time in activity concentrations of Cs-137
associated with SS in the small forest catchment (SFC) andthe whole Uda River catchment (WURC) after the FDNPP
accident.

108

S. HAYASHI and H. TSUJI

higher than those at the WURC, contrary to differences in
the initial amount of Cs-137 deposited between thecatchments. This may be due to the fact that
decontamination operations in farmland and inhabitedareas in the upper reaches of the WURC began in theautumn of 2012. Additionally, no significant decreasing
trend in the activity concentration of Cs-137 associatedwith SS could be confirmed in the forest stream (Hayashi
et al., 2016). However, the concentrations decreased withrelatively small fluctuation after tropical storm Etau,
which consequently brought about a significant
decreasing trend exceeding natural decay at the both sites.
The effective decay constant (λSS) in the SFC wasestimated to be 0.20 (y-1) which is in the range of thevalues (0.15 – 0.44 (y-1)) estimated by Iwagami et al.
(2019) based on monitoring from Apr. 2012 to Nov. 2016
in several small forest catchments in Yamakiya District,
Kawamata Town, located around 35 km northwest of the
FDNPP. Although no detailed analysis has been
conducted to explain these decreases that are much fasterthan natural decay in the activity concentration ofcesium-137 associated with SS in a forest stream, theextreme rain event of 2015 is assumed to have eroded
fine surface mineral soils with high concentrations ofradiocesium or washed highly contaminated forest
organic matter out from the river banks and riparianzones which play a dominant role in sediment production(Sakuma et al., 2018). This might bring about thedecreases shown in Fig. 2 in the concentration andfluctuation of the activity concentration of Cs-137
associated with SS since then. Also for the effective
decay constant (λSS) in the WURC, the estimated value(0.26 (y-1)) corresponds approximately to the reported
values in other major rivers in the northern coastal region(0.06 – 0.41(y-1); Taniguchi et al., 2019). Although theeffects of land use or decontamination operations areassociated with SS (Bq/g)

137Cs activity concentration

25.0
SFC
WURC

20.0
15.0

Table 1 Ratio of annual Cs-137 runoff associated with SS to theamount of Cs-137 deposited, estimated from the third
airborne survey data (MEXT, 2011) in the small forest
catchment (SFC) and the whole Uda River catchment(WURC).

10.0
y = ‒ 1.2ln(x) + 14
R² = 0.26

5.0
0.0

indicated as a cause of the concentration clearlydecreasing (Taniguchi et al. 2019), a detailed mechanism
remains to be clarified by applying an SS source analysis.
Figure 3 shows the relationship between SS
concentration and Cs-137 activity concentration
associated with SS in the SFC and WURC. The plottedCs-137 concentration values are decay-corrected to Mar.
11, 2011 to remove the effect of physical decay on therelationship. While increases in SS concentration easily
exceeding 1,000 mg/L would obviously contribute to
decreases in the activity concentration of Cs-137
associated with SS from huge rain events, resulting in
sediment runoffs with larger amounts of sandy
components, variations in SS concentration were notconfirmed to have a significant effect on Cs-137
concentrations associated with SS at either site whenrelatively high rain events happened. Ratios of annual Cs137 runoff associated with SS to the amount of Cs-137
deposited in the catchments are presented with annual
rainfall amounts observed at the Matsugabou Dam
maintenance facility (Fig.1; 37°47’46.5”N 140°46’13.7”E)in Table 1. Data from the third airborne survey (Ministryof Education, Culture, Sports, Science and Technology,2011) were used to calculate the amount of Cs-137
deposited in each catchment. The annual amount of Cs137 runoff was calculated using sequential data on
flowrates and SS concentrations and by applying theestimated Cs-137 concentration decay equation at eachsite (Fig. 2). The annual runoff ratio varies considerably
whether huge rain events occur or not. In fact, in 2015
when tropical storm Etau hit in September, 90% of theannual Cs-137 runoff amount was produced during thisrain event. Decay in Cs-137 concentrations associatedwith SS is also an important factor influencing the runoff
ratio, because the ratio in 2016 was less than that in 2013by half despite the annual rainfall amount in 2016 being
larger than in 2013.
3.2 Role of Dam Reservoirs in Radiocesium Storage
Functions
As a result of applying Eq. 1 to the time series dataon Cs-137 radioactivity concentration associated with SS

0

1000

2000

3000

SS (mg/L)

4000

5000

Fig. 3 Relationship between SS and activity concentrations of Cs137 associated with SS in the small forest catchment (SFC)
and the whole Uda River catchment (WURC).

Cs-137 runoff ratio (%)
SFC

WURC

Annual rainfall*
(mm)

2013

0.081

0.25

1152

2014

0.084

0.066

1700

2015

0.15

0.47

1732

Year

2016

0.037

0.092

1432

2017

0.027

0.023

1346

*Observed at the Matsugabou Dam maintenance facility.

Role & effect of a dam on radiocesium migrationin the water discharged from Matsugabou Dam, theradioactivity concentration was found to decreasesignificantly faster than the physical decay rate (Fig. 4;
λSS: 0.22 (1/y)), although it showed large variations
among samples. Just as in the cases of the forest stream
or downstream reaches, no SS concentration effects on
Cs-137 associated with SS could be clearly distinguished.
The Cs-137 concentration fluctuated considerably under
low discharge conditions with less than 5 mg/L of SS
concentration (R2=0.03, p=21, N=63). Additionally, it
was not clear whether Cs-137 concentrations associatedwith SS were influenced by increased SS concentrations
in discharged water from pronounced sediment inflows to
the dam reservoir during rain runoff events because of
insufficient sample numbers.
Figure 5 shows the vertical distribution of Cs-137
radioactivity concentration in undisturbed bottom
sediment core samples collected around the deepest point
near the dam itself each year in late autumn from 2012 to2016. The samples for 2015 were collected in November
after tropical storm Etau passed. Since the bottom
sediment in this area with a water depth of more than
30 m is rarely disturbed by water flow, the vertical
distribution of radiocesium in bottom sediment is formed
only through settling and accumulation of inflow
sediment. The most notable point is that the sedimentlayer with the highest concentration of Cs-137 had
obviously migrated downward to deeper levels from 2014
to 2015. Hayashi (2017) suggested that the sedimentlayer with the highest peak in the Cs-137 concentration
be assumed to have been influenced by direct deposition
to the reservoir surface or the initial inflow to thereservoir after the accident. Those layers were located alittle deeper from the surface of the bottom sediments inanalyses using undisturbed bottom sediment core samples
in the Matsugabou Dam reservoir in 2012 and 2013. They109

also suggested that the layer with the highest
concentration apparently continues migrating downward
due to intermittent sediment inflow from the dam
reservoir catchment. From that observation, the settlingand deposition of huge amounts of sediment inflow into
the dam reservoir when tropical storm Etau hit in
September 2015, are considered to be the main reason thelayer with the highest concentration of Cs-137 moved
downward remarkably in the bottom sediment. Also, oneof the core samples collected in 2016 showed the highest
concentrations to be in a deeper layer of the bottom
sediment. This may have been caused by spatial
heterogeneity in the process of sedimentation anddeposition of the inflowing sediment due to complex
topography of the reservoir bed. The huge amounts of
sediment inflow and their accumulation at the bottom arealso confirmed by annual measurements of dam reservoir
sedimentation conducted by the Soma Land Improvement
District (not published). The measurement data show that3
the amount of sand accretion in Jan. 2016 (114,000 m )increased by twice as much as that in Jan. 2015
(53,000m3). Although the amount of Cs-137 accumulatingin the bottom sediment samples showed an annual
increasing trend, it was not significant (R2=0.37, p=0.27).
In addition to insufficient sample numbers, this variation
among the sampling points might be influenced by spatial
heterogeneity in accumulation of Cs-137 on the reservoir
bed.
Since the annual changes in the observed
sedimentation and vertical distribution of Cs-137 activity
concentrations in the bottom sediment showed that thedam acted to store radiocesium associated with SS
pouring in from the reservoir catchment, an attempt toquantify the function was made by annually comparing
137Cs activity concentration

0.0

0

20

(kBq/kg-dry soil)
40

60

80

100

137Cs_SS = 13.4e-0.22t

Mass depth (g/cm2)

2.0
4.0
6.0
8.0
10.0
12.0
14.0

Fig. 4 
Change over time in activity concentrations of Cs-137
associated with SS in the water discharged from Matsugabou
Dam after the FDNPP accident.

2016-a2016-b
2016-c2015-a2015-b2015-c
2014
2013
2012

Fig. 5 
Annual changes in vertical distribution of Cs-137activity
concentrations in bottom sediment collected in theneighborhood of Matsugabou Dam itself. All of the samples
were collected each late autumn after the typhoon season.

110

S. HAYASHI and H. TSUJI

the amount of Cs-137 associated with SS between the
inflow into the dam reservoir and the discharge from thedam. Each annual amount of Cs-137 inflow associatedwith SS was calculated by multiplying the estimatedamount of Cs-137 deposition by each annual runoff ratioof Cs-137 associated with SS in the dam reservoir
catchment. The annual runoff rate in the DRC wasassumed equal to that of the watershed, which is the
WURC minus the DRC. This is because the spatial
distribution of land use in the catchment is similar to thatof the DRC. Specifically, it was estimated by dividing thedifference calculated by subtracting the annual amount of
Cs-137 associated with SS discharged from the dam fromthe annual amount of runoff from the WURC, by theamount of Cs-137 deposited in the area, subtracting the
DRC area from the WURC area. Table 2 presents theannually calculated runoff ratios, inflows and amounts of
Cs-137 discharged associated with SS, and the effects thedam had on storing Cs-137 associated with SS andpreventing its migration to the coastal area. Although the
inflow and amount of Cs-137 discharged varied fromyear to year corresponding to the annual rainfall amount,i.e., the amount of sediment inflow from the dam
reservoir catchment, the runoff ratio was confirmed toremain at more than 84% every year. The contribution ofinternal production to the concentration of Cs-137
associated with SS and its mass balance in the dam
reservoir is not clear but may not be significant, as shown
by Tsuji et al. (2020) in the Yokokawa Dam reservoir on
the Ohta River in the northern coastal region of
Fukushima Prefecture. It is especially curious that theretention ratio in 2015 (93%) when the huge storm event
happened was higher than that in 2017 (84%), which wasa typical dry year. Since the effective decay rate in theconcentration of Cs-137 associated with SS in the WURC
(λSS: 0.26), which was faster than that in the SFC (λSS:0.20) or the dam discharged water (λSS: 0.22), was appliedin estimating the runoff ratio of Cs-137 associated with
SS from the DRC, the runoff ratio of 2017 may have been
underestimated. Moreover, as pointed out by Hayashi
et al. (2016), active water storage management for
preventing flood inundation downstream may haveresulted in a larger retention ratio by promoting thesettling and deposition of the amount of Cs-137 inflow
associated with SS during the huge rain event, accounted

for about 95% of the annual total inflow. When comparedwith the case without a dam, these high ratios also
indicate that the dam plays a role in reducing the amountof Cs-137 inflow associated with SS into the coastal area
(Matsukawaura Lagoon; Fig. 1) by about 30% every year
through its storage function.
While huge amounts of sediment flowed into thereservoir, Hayashi et al. (2016) suggested that proactive
dam discharge control for flood prevention downstream
had accelerated the accumulation of sediment on thereservoir bed. This is also supported by the sedimentation
survey results described above. Additionally, in terms ofusefulness of water storage management for storing
radiocesium associated with SS in dam reservoirs, in asimulation of the Ogaki Dam reservoir Yamada et al.
(2015) found that the amount of clay discharged,
including large amounts of radiocesium from thereservoir, could be reduced by a factor of three by raising
the height of the dam exit. These results clearly show that
not only does a dam function to prevent migration anddiffusion of radiocesium associated with SS by storing itin the reservoir, but also that proactive discharge control
has the potential to help fulfill this function more
effectively.
3.3 Effect of Dam Reservoirs as Sources of
Dissolved Cs
Figure 6 shows the annual changes in dissolved
Cs-137 activity concentration measured every two weeks
or monthly in the dam reservoir inflow stretch and thedam discharge channel. Unlike Cs-137 associated withSS, the activity concentration of dissolved Cs-137
showed annual periodic variations at both sites.
Moreover, since the amplitude as well as the activity
concentration of the dissolved Cs-137 have been
declining with time, the effective decay constant andefficiency half-life (Teff) of the Cs-137 concentration
could be estimated by fitting the data to the following
exponential function equation consisting of the decay
component and the periodical component using Origin
version 2020 (OriginLab Corporation, Northampton, MA,
USA). The curves fitted by the equation have also beendrawn in their respective graphs.
Dissolved 137Cs = exp ( – λdis∙ t + b + e ∙ sin( 2� (cd+ t) ))Table 2 Annual mass balance of Cs-137 associated with SS in the Matsugabou Dam reservoir and the effects of thedam on storing it in the reservoir, preventing its migration to the coastal area.
Year

Ratio of runoff
from the DRC(%)

Amount of inflow
from the DRC(109 Bq)

Amount discharged
from the dam(109 Bq)

Ratio of storage
in the dam
reservoir (%)

Ratio of prevented
migration to the outletof the entire catchment
(%)

2014

0.097

6.4

0.33

95

31

2015

0.68

44

2.8

94

31

2016

0.13

8.6

0.89

90

29

2017

0.031

2.1

0.32

84

28

Role & effect of a dam on radiocesium migration

111

λ: 0.10
Teff: 6.8
λ: 0.28
Teff: 2.5

6.5

Elapsed time from the FDNPP accident (yr)(b) Discharge from the dam reservoir(a) Inflow into the dam reservoir

Fig. 6 Change over time in activity concentrations of dissolved Cs-137 in the inflow (a) into and the discharge(b) from the Matsugabou Dam reservoir. The dark gray lines indicate fitting curves developed by Eq. 1.
The values of the effective decay constant (λ) and effective environmental half-lives (Teff) were estimatedfrom the fitting analyses.

…… (Eq.2)

Where Dissolved 137Cs represents dissolved Cs-137
activity concentration (Bq/L) at the elapsed time t (y)after the FDNPP accident and λdis represents the effective
decay constant (y-1). The constant b representing theinitial value, e representing the amplitude (Bq/L), c
representing the phase difference and d representing theperiod (y-1) are empirically determined. The analyticalresult shows that the tendency of dissolved cesium-137
activity concentration in water discharged from the dam
to decrease (λdis: 0.10 (y-1) ; Teff: 6.8 (y) ) was slower thanthat in the reservoir inflow river ((λdis: 0.28 (y-1); Teff:
2.5 (y)). Moreover, the fitted curves suggest that thetemporal difference in the peaks of concentration, in
which the peak in the discharged water emerges after adelay of about 100 days compared to that in the inflow
water, was generated between them. By applying theequation respectively to the time-series data for the total
inflow volume into the dam reservoir or the amount ofwater discharged from the dam, the annual balance ofdissolved Cs-137 in the dam reservoir was calculated andthe results are organized in Table 3. The trend of
dissolved Cs-137 activity concentration decreasing (Fig.
6) resulted in a tendency of the annual flux of the
dissolved Cs-137 to increase at both the inflow andoutflow of the dam reservoir. Moreover, by reflecting
differences in the rate of decrease in dissolved Cs-137
concentration, the annual amount discharged has
exceeded the annual amount of inflow since 2015, as
markedly shown in 2017 with little annual precipitation(Table 1). Based on long-term monitoring of dissolved
radiocesium in the Ogaki Dam reservoir, Funaki et al.
(2020) showed that the annual dissolved amount of
Cs-137 in the dam discharge water was about 1.6 times
higher than the annual amount of inflow into the dam

Table 3 Annual mass balance of dissolved Cs-137 in the
Matsugabou Dam reservoir.
Amount of dissolved Cs-137 (106 Bq)
Year

A. Inflow
into the dam

B. Discharge
from the dam

B/A

2014

564

537

0.95

2015

481

545

1.13

2016

324

349

1.08

2017

169

319

1.89

reservoir and that the effective environmental half-life of
dissolved Cs-137 in the output water was longer than in
the main input water. He concluded from the measured
vertical distribution of dissolved Cs-137 concentration in
the dam reservoir that dissolution from the reservoir bed
sediment played an important role as an internal source inincreasing the amount of dissolved radiocesium in thewater discharged from the dam. This result strongly
suggests that a substantial amount of dissolved
radiocesium is generated through the contribution ofdissolution from the bottom sediment even in the
Matsugabou Dam reservoir. How the dissolved
radiocesium is produced in the bottom sediment andsupplied to reservoir water, however, should be clarified
in future work, considering that the radioactivity
concentration of Cs-137 has been decreasing in thesurface layer of the bottom sediment as a result ofintermittent sediment inflow from the DRC.
Since the dissolved Cs-137 activity concentration
has remained at less than 0.03 Bq/L for the last few years
in water discharged from the Matsugabou Dam, a
slowdown in the rate of dissolved Cs-137 decrease
through internal production in the dam reservoir would
have very small effects on water uses like drinking or
irrigation. Additionally, no radiocesium activity

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S. HAYASHI and H. TSUJI

concentrations exceeding the shipping regulatory value(100 Bq/kg wet weight) have been detected in water
bodies downstream since 2015 (Ministry of theEnvironment, 2020). If it is assumed, however, that
dissolution of radiocesium occurs naturally from dam
reservoir bed sediments, there may be considerable
concern about the effects of dam reservoirs on water use
or contamination of freshwater ecosystems in moreheavily contaminated areas. As for the transfer ofradiocesium into fresh water ecosystems in particular, theconcentration ratio (CR=activity concentration inorganisms (Bq/kg fresh weight/activity concentration inwater (Bq/L)), which is an indicator useful for gaining an
understanding of the state of transference of radiocesium
from water to aquatic organisms, shows values on theorder of several hundreds to several thousands in manykinds of freshwater fish (Ishii et al., 2020). Therefore,
radiocesium activity concentrations in commercial fish
could possibly exceed the specified regulatory value forshipment even if the radiocesium activity concentration isaround 0.1 Bq/L in the water. Storing huge amounts ofradiocesium in a dam reservoir bed for a long period
happens to slow down the decrease in dissolved
radiocesium activity concentrations in the water
discharged through dissolution from the bottom sediment.
This might enhance long-term contamination of aquatic
ecosystem in downstream water bodies.
The function of effectively storing radiocesium
associated with SS in a dam reservoir should be actively
utilized to prevent its migration into downstream water
bodies, particularly, the diffusion to urban areas in thedownstream region by flood inundation during extreme
storm events in the future. On the other hand, it may benecessary to pay close attention to changes in
sedimentation storage capacity resulting fromaccumulation of highly radioactively contaminated inflow
sediment. Frequent occurrences of extreme storm rainfall
events due to climate change (Intergovernmental Panelon Climate Change, 2013) will make long-term stablestorage of contaminated bottom sediment difficult in dam
reservoir beds (Mouri et al., 2014). Also for dissolutionof radiocesium from dam reservoir beds, an appropriate
response needs to be examined based on evaluations fromvarious viewpoints such as evaluating economic losses
while also considering ecosystem services at present or in
the future and performing cost-benefit analyses ofcountermeasures to prevent dissolution.

4. Conclusions
Positive and negative roles of dam reservoirs inradiocesium behavior in a river catchment were examinedbased on the results of comprehensive, long-termhydrological monitoring at Matsugabou Dam, which ismanaged on the upstream reaches of the Uda River, oneof main rivers in the northern coastal region of

Fukushima Prefecture. The main results are as follows.
・The activity concentration of Cs-137 associatedwith SS decreased much faster than natural decay in boththe forest stream and the downstream reaches of the Uda
River. Although the runoff ratio of Cs-137 associatedwith SS depends on rainfall characteristics, it tends todecrease due to decreasing activity concentration.
・Matugabou Dam has the function of reducing
migration of radiocesium associated with SS from thedam reservoir catchment to the downstream reaches bymore than 85% by storing SS in the reservoir. Moreover,proactive discharge control has the potential to help fulfill
this function more effectively, decreasing the risk ofexternal exposure through re-contamination downstreamof the dam.
・An estimation of the annual mass balance ofdissolved Cs-137 in the dam reservoir showed that theamount in the discharged water clearly tended to exceed
the amount in the inflow, with substantial amounts ofdissolved radiocesium presumably produced bydissolution from stores in the bottom sediment. Throughits dissolution, long-term storage of radiocesium in dam
reservoir beds may slow down the decrease in dissolved
radiocesium activity concentration in the discharged
water.
Acknowledgement
Hydrological data on the dam reservoir were
provided by the Soma Land Improvement District andMatsugabou Dam maintenance facility. We thank Drs.
Ito, and Nishikiori, and Messrs. Takita and Tsuchiya for
their help with the field survey, and Dr. Nishina for his
advice on statistical analysis of the dissolved Cs-137 timeseries data. This study was partially supported by a Grantin-Aid for Scientific Research (KAKENHI 16H01791)
from the Japan Society for the Promotion of Science.
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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.

Hideki TSUJI
Hideki Tsuji is a senior researcher at the National
Institute for Environmental Studies. He is
conducting field research on the transport andaccumulation of radiocesium in rivers and lakes.
He is also carrying out chemical analysis,
constructing a numerical model and developing
equipment to be used for environmental
measurements of bioavailable radiocesium.

(Received 24 September 2020, Accepted 28 December 2020)