Science of the Total Environment 872 (2023) 161997

Contents lists available at ScienceDirect

Science of the Total Environment
journal homepage: www.elsevier.com/locate/scitotenv

Evaluating changes in radionuclide concentrations and groundwater levels
before and after the cooling pond drawdown in the Chornobyl Nuclear
Power Plant vicinity⁎

Hikaru Sato a, , Maksym Gusyev b, Dmytro Veremenko c, Gennady Laptev d, Naoaki Shibasaki e, Yuichi Onda f,
Mark Zheleznyak b, Serhii Kirieiev c, Kenji Nanba baFaculty of Life and Environmental Sciences, University of Tsukuba, Ibaraki, Japan
Institute of Environmental Radioactivity, Fukushima University, Fukushima, Japanc
State Specialized Enterprise Ecocentre, SAUEZM, Chornobyl, UkrainedUkrainian Hydrometeorological Institute, Kyiv, UkraineeFaculty of Symbiotic Systems Science, Fukushima University, Fukushima, JapanfCenter for Research in Isotopes and Environmental Dynamics, University of Tsukuba, Ibaraki, Japanb

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T• The Chornobyl cooling pond drawdown
in 2014 changes the radionuclides' transport.
• Average groundwater levels declined
from 0.5 m to 3.9 m at 65 % of shallow
wells.
• 90Sr increased at 21 % of shallow wells
while 137Cs did not.
• The 90Sr concentration in the Pripyat
Floodplain water increases.
• The 90Sr contribution to the Pripyat River
before and after the CP drawdown did notchange signiﬁcantly.

A R T I C L E

I N F O

Editor: Christian Herrera
Keywords:
Surface water
Groundwater
Caesium-137
Strontium-90
Cooling pond drawdown
Chornobyl Nuclear Power Plant

A B S T R A C T

In the vicinity of the Chornobyl Nuclear Power Plant (ChNPP), the cooling pond (CP) was an artiﬁcially maintained
reservoir with water levels regulated to 7 m above the Pripyat River until May 2014, when its pumps stopped operating, resulting in a natural drawdown. To investigate the surface-groundwater system before and after the drawdown,
we evaluated the spatial and temporal changes in 90Sr and 137Cs radionuclide concentrations and groundwater levelsin the shallow unconﬁned aquifer near the ChNPP from 2010 to 2019. Additionally, we compared water levels and90
Sr concentrations in Azbuchin Lake, wetlands inside the CP, and the Pripyat River. Using three-year averages before(2011−2013) and after (2017–2019) the drawdown period, we found that 90Sr concentrations signiﬁcantly increased
up to 102 kBq/m3 in the Pripyat River ﬂoodplain, north of ChNPP, exceeding the WHO drinking water guideline of 10
kBq/m3. In contrast 137Cs concentrations ranged consistently between 10 and 100 Bq/m3. The groundwater levels decreased over 50 cm at approximately 65 % of shallow monitoring wells and up to 6 m near the CP. The 90Sr concentration increases in some wells at the Pripyat River ﬂoodplain were associated with decreased dilution rates fromthe CP due to the reduced CP leakage, causing changes in groundwater ﬂow direction and decreases in groundwatervelocity. From the new ﬁnding of this study that the drawdown increased 90Sr concentrations near the ﬂoodplain, we

Abbreviations: ChEZ, Chornobyl Exclusion Zone; ChNPP, Chornobyl Nuclear Power Plant; CP, cooling pond; RWDS, radioactive waste disposal site; RWTSP, radioactive waste temporarystorage sites.
⁎ Corresponding author at: Faculty of Life and Environmental Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8572, Japan.
E-mail address: sato.hikaru.gf@alumni.tsukuba.ac.jp (H. Sato).

http://dx.doi.org/10.1016/j.scitotenv.2023.161997
Received 29 November 2022; Received in revised form 20 January 2023; Accepted 30 January 2023
Available online 02 February 2023
0048-9697/© 2023 Elsevier B.V. All rights reserved.

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Science of the Total Environment 872 (2023) 161997

estimated the 90Sr ﬂux and contribution to the Pripyat River and the 90Sr contribution did not change signiﬁcantly after
the drawdown. However, radionuclides may accumulate more at the ﬂoodplain in the future; therefore, additional
monitoring is required to verify 90Sr transport from areas of elevated concentrations and its impact on groundwaterin the aquifer.

1. Introduction

due to the surface water decline (Bulgakov et al., 2009). In addition, leakage of water and radionuclides from the bottom of the CP inﬂuenced thegroundwater ﬂow regime and radionuclide concentrations on a large
scale. Numerous previous studies have focused on interactions between
the CP and nearby surface water features and radionuclide migration during CP operation. However, no studies have evaluated the changes insurface-groundwater interactions before and after the drawdown.
Downward migration of 90Sr from the shallow trenches to groundwaterin the Red Forest and ChNPP sites located 2.5 km southwest of the ChNPP
was investigated by ﬁeld and modeling studies (Bugai et al., 2002, 2007,2012a, 2012b; Le Gal La Salle et al., 2012; Matoshko et al., 2004; Vanet al., 2009, 2012; Onishi et al., 2007). According to Bugai et al. (2002),
the 90Sr migration velocity at the pilot site was extremely slow comparedto the groundwater ﬂow velocity. Prior to the drawdown, the 90Sr transport
to the Pripyat River was estimated using ﬁeld data from tracer experiments
at wells in the dike between the CP and Pripyat River (Bugai et al., 2005;
Bugai and Skalskky, 2013, 2018). However, these studies analyzed datathat were collected during CP operation, and only one study by Bugai
et al. (2022) included data on post-operation groundwater levels and radionuclide concentrations, which found only ﬁve out of 73 wells with increasing 90Sr concentrations by investigating long-term trends in annual
radionuclide data and groundwater levels in wells in the unconﬁned andconﬁned aquifers. Therefore, no evaluation exists of the surfacegroundwater ﬂow system and shallow unconﬁned aquifer before andafter the CP drawdown, and an investigation of 90Sr concentration changes
and migration is needed in the vicinity of the CP and ChNPP sites. Furthermore, the ﬂux of 90Sr to the Pripyat river after the drawdown is important inclarifying the impact of the large-scale decline of groundwater level.
Thus, evaluating changes in the groundwater system and radionuclide
concentrations associated with the large-scale drawdown in CP waterlevel is important for long-term management near the CP and for predicting
increases in radionuclide concentrations. This study evaluated changes in
90
Sr and 137Cs concentrations and groundwater levels in wells in the shallow unconﬁned aquifer in the vicinity of ChNPP before and after the CP
drawdown. We selected monitoring wells with monthly measurementsfrom 2010 to 2019 and quantiﬁed spatial changes using a 3-year averageof 90Sr and 137Cs concentrations before (2011–2013) and after drawdown(2017–2019). Temporal changes were evaluated using time-series data ofmonthly 90Sr and 137Cs concentrations, and observed groundwater levels
were compared with the Pripyat River water levels near the CP. We reexamined seasonal changes in the groundwater levels to analyze changes
in the behavior of the hydrological system and compared the groundwatertransport of 90Sr to the Pripyat River before and after the drawdown. We
also compared the 3-year average 90Sr concentrations in shallow groundwater with the CP surface water and Azbuchin Lake (Fig. 1) to understandthe effects of large-scale environmental changes and provide important
baseline data for the monitoring and sustainable management of environmental radioactivity and numerical radionuclide modeling.

Radioactive materials remaining in the environment due to nuclear accidents and waste contaminate the surrounding environment over the longterm. The Chornobyl and Fukushima Daiichi nuclear power plant accidents
released radioactive plumes into the atmosphere and contaminated water
bodies, soils, and vegetation near their sites with radioactive fallout(Beresford et al., 2016; Onda et al., 2020). Additionally, nuclear fuel processing facilities and landﬁlls for highly radioactive waste related to thedecommissioning of nuclear facilities also contaminate the surrounding
soil (Fredrickson et al., 2004; Tyler, 2020). Radioactive materials migratefrom these contaminated soils into groundwater and are discharged intorivers and seepage areas via groundwater ﬂow (Bixio et al., 2002; Daiet al., 2002; Slater et al., 2010; Zachara et al., 2013). Concerns remainthat the discharge of highly contaminated groundwater may adversely affect drinking water and the surrounding ﬂora and fauna (Lerebours et al.,2018; Mappes et al., 2019).
In April 1986, the Chornobyl Nuclear Power Plant (ChNPP) accident released a large amount of nuclear fuel particles, including 137Cs and 90Sr,
which were deposited on the ground near the ChNPP site. Access to theseareas was limited via the 30-km Chornobyl Exclusion Zone (ChEZ), and radionuclide monitoring was performed from the ChEZ to the downstream
area of Pripyat River to avoid radionuclide migration downstream to
Kyiv. In the ChEZ, 137Cs and 90Sr concentrations as of 1997 reached ahigh of more than 4000 kBq/m2 near the reactor building, and the highly
concentrated area expanded to the west and northwest from the ChNPP(Kashparov et al., 2018). To address this issue, contaminated topsoil, vegetation, and construction debris were buried in trenches near the ChNPP
during clean-up activities in 1987–1988 (International Atomic Energy
Agency [IAEA], 2006). In the ChNPP area, 800 trenches are subdivided
into “sectors,” and these sectors are a potential source of radionuclide migration to groundwater (Dewiere et al., 2004; Roux et al., 2014). Among
the radioactive materials in the soil and trenches, 90Sr poses the greatest
threat to groundwater contamination because of its high mobility and susceptibility to hydrogeological effects (Bugai et al., 1997). For example, thesolid/liquid distribution coefﬁcient (Kd) of 90Sr was the lowest (2–17 mL/g) in those of Cs and Pu (Bugai et al., 2020). Another potential source of radionuclide contamination in the ChNPP area is the cooling pond (CP),which was an artiﬁcial reservoir located on the ﬂoodplain used to cool
the ChNPP reactors (Bugai et al., 1997, 2022).
The CP was heavily contaminated by the fallout from the ChNPP accident. The 90Sr concentrations in the water increased for several years
after the accident due to the breakdown of fuel particle but have been decreasing since 1990, stabilizing during 2000–2010 at 1–2 kBq/m3 (IAEA,
2019; Nasvit, 2002). As an artiﬁcial reservoir, the CP water balance wasmaintained by a continuous inﬂow of water pumped from the Pripyat
River, its outﬂows were evaporation, exﬁltration seepage to drainage
ditches, and leakage to groundwater through bottom sediments (Bugai
et al., 1997; IAEA, 2019). The primary release pathway for 90Sr and 137Cs
was seepage of contaminated water into drainage ditches and subsurface
pathway leakage. Release from the cooling pond in 2010 contributed approximately 10 % of the total 90Sr transport by the Pripyat River (Bugai
and Skalskky, 2013; IAEA, 2019). In May 2014, the pumping was discontinued, initiating an uncontrolled drawdown of the CP and raising a major
concern regarding the radionuclides in the CP bottom sediments that accumulated over the years after the drawdown (Bulgakov et al., 2009; IAEA,2019). For example, the mobilization of contaminants from fuel particles
in the newly exposed areas of the CP is associated with the change of pH

2. Study area
2.1. Cooling pond
The CP was constructed southeast of the ChNPP in 1976 and enlarged in
1981, with a surface area of 22.9 km2 and a water volume of 151 × 106 m3(IAEA, 2019) (Fig. 1). The water balance during operation was determinedby the inﬂow of water pumped from the river, and the outﬂows consisted of
evaporation, seepage into the north and south drainage channels, and2

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Science of the Total Environment 872 (2023) 161997

Fig. 1. Location of monitoring wells in the vicinity of the Chornobyl Nuclear Power Plant (ChNPP) with the colour-coded well depth. Surface water features as of August 13,
2013 are colored light blue and mapped using selected low cloud-cover Landsat8 C2L2 (QA_PIXEL) images downloaded from the United States Geological Survey (USGS)
EarthExplorer (https://earthexplorer.usgs.gov/). Surface water features are shown above the terrain elevation. The cooling pond (CP) is shown by blue line and the area inside of the CP is colored according to its bathymetry based on Buckley et al. (2002). The shaded area indicates the spatial distribution of 137Cs and 90Sr concentrations exceeding 1 × 104 kBq/m2 as of 1986, which were estimated by inverse estimation using half-life based on the database of Kashparov et al. (2018). The black polygon andpink polygon with slant stripe indicate the radioactive waste disposal site (RWDS) and radioactive waste temporary storage sites (RWTSP), respectively. (For interpretationof the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)current water balance, the inﬂow is only precipitation, and the outﬂows
are evapotranspiration and leakage into the subsurface (Fig. 2b IAEA,2019).

leakage to the underlying aquifer (Fig. 2a, Bugai et al., 1997). Water was
pumped from Pripyat River at the BNS site north of the CP (Fig. 1). The
water level during the operation was set at approximately 111 m above Baltic Sea level (a.Bs.l.), which is 6–7 m higher than that of the Pripyat River.
An investigation of the bathymetry in 1999–2001 (Buckley et al., 2002) reported a lowest elevation of 94 m a.Bs.l. with deeper sites in the southern
portions of the CP. The area below 7.0 m in depth accounts for 16.81 km2of the CP, which means that 75 % of the ground is exposed when thewater level is approximately 104 m a.Bs.l. After decommission, the waterlevel declined by 6.2 m from May 2014 at 110.8 m to December 2021 at
104.6 m a.Bs.l. The CP drawdown was signiﬁcant at 1.8 m/y from May
2014 to 2016 and subsequently decreased to 0.4 m/y from 2017 to 2019
(Kanivets et al., 2020). The CP reservoir was divided into three areas
after 2016 when the water level reached 106 m a.Bs.l. The northern andsouthern areas were separated by the former dam installed in the 1ststage of the ChNPP, and the northern area was further divided into east
and west by the Current-Guiding Dike (Fig. 1 IAEA, 2019). The waterlevel subsequently decreased below the bottom of the northern and southern drainage channels, stopping the seepage and drainage ﬂow. In the

2.2. Topography and hydrogeology around the ChNPP site
The terrain below 120 m a.s.l. around the CP consists mainly of alluvial
sandy sediments (Matoshko et al., 2004). The western side is elevated byterraces, with elevations ranging from approximately 130 to 190 m(Fig. 1). Numerous lakes and marshes of various sizes exist in the ﬂoodplainof the Pripyat River, including the Pripyat backwater and Azbuchin Lakes,
which are located 2.5 km and 0.8 km west of the CP, respectively. During
pumping, the water level in the CP was higher than that in Azbuchin
Lake and the Pripyat River. This caused CP water to feed into both water
bodies.
The unconﬁned aquifer near the ChNPP consists of Holocene, Pleistocene, and Pliocene aeolian and alluvial sandy sediments (Matoshko et al.,2004). The Chornobyl site is located on the ﬁrst terrace above the ﬂoodplain on the right bank of the Pripyat River, where the unconﬁned aquifer
3

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Science of the Total Environment 872 (2023) 161997

Fig. 2. Water balance of the CP before and after the drawdown. a: the water balance of the CP before the drawdown. b: the water balance of the CP after the drawdown. Thisﬁgure was made based on Bugai et al. (1997).

is approximately 30 m thick. In the ﬂoodplain, where the CP site is located,
the unconﬁned aquifer is 18–20 m thick. The hydraulic conductivity of theaquifer was estimated to be 5–25 m/day based on pumping tests (Bugai
et al., 1997). Underlying the Pleistocene or Pliocene sandy sediments are
Eocene marine carbonate silts and marls approximately 10–20 m thick,forming a low permeable layer.

Additionally, several RWTSP are located in inappropriate hydrogeologicallocations (IAEA, 2019).
2.4. Monitoring wells for groundwater level and radionuclides near the ChNPPThe State Specialized Enterprise Ecocentre measured groundwaterlevels during 1986–2019 and collected groundwater samples during
1993–2019 at 148 monitoring wells, which are in the unconﬁned aquifer.
Well depths are 2–40 m, with an average of 16 m, and 50 % of all wells
are shallower than 10 m deep. Areas with high concentrations were sampled monthly, whereas other areas were sampled once every three months.
Groundwater near the ChNPP was managed by the State Specialized Enterprise Ecocentre located within the ChEZ.

2.3. Radionuclide waste storage site vicinity of the ChNPP
Within a 10 km radius of the ChNPP, there are two radioactive waste
disposal sites (RWDS) (Fig. 1). The “Podlesny” RWDS is located north of
the ChNPP, between the Pripyat backwater and CP (Fig. 1). This repository
was intended for the disposal of high-level waste with dose rates of 0.05 to
2.5 Gy/h or higher at 10 cm below the surface. Approximately 11,000 m3 ofwaste, mainly building materials, metal fragments, sand, concrete, andwood, is contained in two vaults therein and covered with concrete(IAEA, 2006). The total radioactivity of the waste was estimated to be2600 TBq in 1990 (IAEA, 2006). The “3rd Stage” RWDS consists of an underground concrete vault located on a man-made “islet” formed by the CP
and spillway (Fig. 1; IAEA, 2019). This facility contains mainly sand, concrete, metal, construction material, and brick wastes at low and intermediate levels with dose rates of up to 0.01 Gy/h at 10 cm from the surface of thewaste container (IAEA, 2006). More than 26,200 m3 of solid waste, with a
total radioactivity of 4 × 1014 Bq, was stored in containers therein and covered with sand and clay. The groundwater table is high in this area, and thefacility was ﬂooded to 0.5–0.7 m above the bottom before the CP drawdown (IAEA, 2006).
Unlike the RWDS, radioactive waste temporary storage sites (RWTSP)
near the ChNPP did not use storage containers (IAEA, 2006). This iswhere low-level radioactive waste, including vegetation, soil, and debris,from the 1987–1988 clean-up are stored (IAEA, 2006). The waste is stored
in trenches or pits 2.0–2.5 m deep and 6–7 m high (Ledenev et al., 1995),
with a total volume estimated at approximately 1.5 × 106 m3 with a total
radioactivity of 3.5 × 1014 Bq in 2015 (Molitor et al., 2017). There has
been no work to waterproof the bottom of the trench or pit, or to install
screens to prevent inﬁltration. The RWTSP is the greatest groundwater contamination concern and the “Neftevaza” RWTSP, located along the shore of
Pripyat backwater where 39 trenches containing radioactive waste in theﬂoodplain area are ﬂooded with groundwater (Ledenev et al., 1995).

3. Methods
3.1. Sampling of surface water and groundwater
In the ChEZ, gauging sites with water level bars were established at all
surface water ponds and streams, and water sampling was conducted using
polyethylene storage tanks, a water sampling bucket, and a glass
bathometer bottle. The Pripyat River levels were measured at the BNS
site in the northern part of the CP (Fig. 1) and in the Chornobyl City
about 17 km downstream from the BNS site. River water samples were collected at the Pripyat River at Chornobyl City site using either a boat or a 1 L
glass bathometer simultaneously at two river depths, as well as by measuring river velocity. The two river depths were approximately 0.2 m below
the water surface and 0.8 m above the river bottom for at least 10 locations
across the river width to obtain an average for all samples. Low- and nonﬂowing lakes, ponds, and CP were sampled at the maximum distancefrom the bank. The groundwater level was measured before sampling
using a manual water level gauge and well purging. Following initial
pumping to obtain clear groundwater, groundwater was sampled using
the pump and promptly stored in polyethylene containers. Prior to surface
and groundwater sampling, the collection equipment was thoroughly
rinsed with sampled water at each site, which was held in 19 L and 1 L storage tanks for 137Cs and 90Sr analyses, respectively. The analytical procedures for concentrating large volume of water samples for radiometric4

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Science of the Total Environment 872 (2023) 161997

measurements such as using exchange resins for 137Cs and radiochemical
concentration for 90Sr are described in detail by Bugai et al. (2022).
The 137Cs was measured via gamma spectrometry 919 Spectrum Master4-channel analyzer (Ortec, USA) with PGT (Princeton Gamma-Tech) detectors (Canberra) with an efﬁciency of 25–50 % and a Nokia LP-4900B 2channel detector (Nokia, Japan) for X-ray gamma spectrometry with detectors (Canberra; Ortec) with 10 % efﬁciency. The radionuclide 90Sr was measured using an NRR-610 low-level alpha-beta radiometer (Tesla, USA),
which can hold up to 55 samples simultaneously, and an SEB-01–150
beta-spectrometer (Atom Komplex Prylad, Ukraine), which consists of an
intelligent scintillation detection unit at 150 mm, passive low-backgroundprotection, and AKWin software (Atom Komplex Prylad, Ukraine).

Table 1
Speciﬁcations of monitoring wells.

Contour maps and bubble plots were prepared to determine the distributions of water levels and radionuclide concentrations, respectively. The
measurement frequency of the groundwater level and radionuclide concentrations varied depending on the monitoring well locations, which were
sampled more frequently near the ChNPP (Supplementary, Table. S1, S3).
Therefore, long-term observation data for the groundwater levels were correlated with water levels in the Pripyat River or nearby groundwater to interpolate the values for unmeasured periods. The groundwater levels foreach period were determined using this method. The surface water levelsof the Pripyat River measured at the BNS gauge (Fig. 1) and CP were utilized to deﬁne groundwater gradients as groundwater ﬂow discharge
boundaries. Using these data, a contour map of the groundwater level
was created using the kriging method. However, radionuclide concentrations during unmeasured periods could not be estimated by using surrounding radionuclide data due to irregular variations. Therefore, distribution
maps were prepared based on 3-year averages of the concentrations. The
raw data and statistical analyses was shown in Supplementary Table S1for 90Sr and Table S3 for 137Cs. 90Sr data depict the variability in the dataat each well before and after the CP drawdown and conﬁrm the validityof most of the well data that we used for analysis; however, some data
have large standard deviations. 137Cs data was sampled less frequently
about half of 90Sr and showed greater variability than 90Sr. In addition,
multiple depth-speciﬁc strainers were installed at in some locations, andthe shallowest wells were used for distribution maps of groundwater levels
and radioactive concentrations (Fig. 1). Hence, 88 % of all monitoring wells
for analysis were shallower than 20 m.
Six characteristic sites near the CP were selected to measure the groundwater level and radionuclide concentration trends for each well (Table 1).
Wells 3A, 2A, 1A, and PK-14 1B, 2B, and 3B were installed north of the
CP in Pripyat River ﬂoodplain, which contains small ponds. Wells 2/1
and 168 were installed west of the CP near the ChNPP. Wells 193 and PK127 were installed in the southwest and southeast, respectively. The
shallowest well at the 168 location (168/Q1) stopped observations in October 2014 after the CP decommission because no water remained in the well.
Therefore, 168/Q2 was used to create the distribution maps.

9.15
9.88
6.18
9.2
10.7
7.0
15.0
12.9
19.2
30.0
9.0
19.6
31.0
18.8

89
89
89
89
89
89
122
114
110
110
114
110
110
89

6.7
7.4
3.6
7.1
8.7
5.8
2.1
7.3
16.8
27.3
4
17.2
26.5
17

8.2
8.9
5.1
8.6
9.7
6.8
14.1
11.3
18.2
28.8
8
18.6
28
18

105.80

16.8

89

15

16

105.80

6.8

89

5

6

30.12437
30.12146
30.11920
30.12457
30.12529
30.13028
30.08568
30.09771

15
1630.1320430.20979

From to51.40720
51.40626
51.40592
51.40189
51.40266
51.40432
51.37986
51.39603

where Vreal (m/d) is the actual velocity, n (unitless) is the porosity, L (m) is
the length of the ﬂoodplain between the CP and Pripyat River used for the
groundwater ﬁltration calculation, and h (m) is the aquifer thickness orwater table depth in the unconﬁned aquifer. Vreal was estimated by Bugai
et al. (2005) as follows, as the 90Sr migration velocity, VSr (m/d), in the
groundwater:
V Sr ¼

V real
R

R¼1þ

ρK d
n

(3)

(4)

where R (unitless) is the 90Sr retardation factor estimated from the 90Sr
sorption distribution coefﬁcient (Kd [m3/g]), soil density (ρ[g/m3]), andporosity. The Δt was estimated from the distance (D [m]) and the 90Sr migration velocity (VSr), as follows:
Δt ¼ D=V Sr(5)90
Sr concentrations and groundwater levels of wells PK-14 3 B, PK-32
64, PK-64 92/2, and PK-113 151 3 k were used for ﬂux calculations, similar
to Bugai and Skalskky (2013). In this study, 90Sr transport was also calculated for well 1A, located near Azbuchin Lake, which showed high 90Sr concentrations in the lake water. Bugai et al. (2005) and Bugai and Skalskky(2013, 2018) used the groundwater velocities measured by single-hole
tracer dilution tests (Bugai et al., 2005) as Vreal in the aquifer. In thisstudy, the Darcy velocity using the hydraulic gradient between the welland the river was used as Vreal × n, because the velocities were not obtained at the wells and changed after the CP drawdown. The well information and parameters used for the 90Sr transport calculation are listed in
Supplementary Table S5. Our calculations used representative wells near
the river and assumed that the concentrations in these wells were representative of the concentrations throughout the unconﬁned aquifer. Therefore,
the results potentially overestimated the risk considerations.

3.3. 90Sr transport analysis
The 90Sr ﬂux (TBq/y) to the Pripyat River was calculated using the following equation, which was described by Bugai and Skalskky (2013):(1)

where Asub(t) (Bq) is the 90Sr ﬂux through the subsurface, Qsub (m3/d) is thegroundwater discharge, Cave (Bq/m3) is the yearly average 90Sr concentration in wells, and t (d) and Δt (d) are the time of measurement and time required for 90Sr migration from the well to the Pripyat River, respectively.
Qsub is calculated as
Qsub ¼ V real  n  L  h106.42110.43107.72108.43108.10106.30113.97115.22115.26115.3551.36175 113.29
113.23113.3351.32231 105.85

1
2
3
4
5
6
7
8
9
10
11
12
13
14

Wellheadelevation(m a.s.l.)Longitude Latitude

3.2. Water level and radionuclide concentrations used for analysis

Asub ðtÞ ¼ Qsub  C ave ðt  Δt Þ

Screeninner
diameter(mm)

Coordinate (degree)

1A
2A
3A
PK-14 1B
PK-14 2B
PK-14 3B
2/1
168/Q1
168/Q2
168/Q3
193/Q1
193/Q2
193/Q3PK-127
127/1
PK-127
127/2
PK-127
127/3

Screeninterval (mbelow
ground)

Well
Depth
(m)

No. Well ID

4. Results
4.1. 90Sr concentrations change in monitoring wells
The average 90Sr concentrations in the monthly wellbore samples during 2011–2013 and 2017–2019 are shown in Fig. 3a and b, respectively.

(2)
5

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Science of the Total Environment 872 (2023) 161997

Fig. 3. Average 90Sr concentrations in monthly wellbore samples during (a) 2011–2013 and (b) 2017–2019, (c) the difference between them, and (d) the change ratio. The
size and colour of the bubbles indicate the magnitude of the concentration or ratio. Surface water features are shown by blue areas. The water areas are colored light blue (a:as of August 13, 2013; b, c and d: as of May 19, 2019) and were mapped using selected low cloud-cover Landsat8 C2L2 (QA_PIXEL) images downloaded from the United States
Geological Survey (USGS) EarthExplorer (https://earthexplorer.usgs.gov/). The inﬁltration area of the ChNPP wastewater treatment plant is indicated by blue polygon at
north of the ChNPP. The black polygon and pink polygon with slant stripe indicate the radioactive waste disposal site (RWDS) and radioactive waste temporary storagesites (RWTSP), respectively. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)6

H. Sato et al.

Science of the Total Environment 872 (2023) 161997(Supplementary Fig. S1 a–c). However, total three wells showed increase
the change rate signiﬁcantly greater than 300 % in the unconﬁned aquifer.
It can be seen at well 4H at the Podlesny RWDS, well 13 at the 3rd Stage
RWDS, and well 171/1 located inside the RWTSP (Supplementary
Table S3 and Fig. S1 (d)). Other 5 lower wells except for well 201 also increased the change ratio and they are located inside the RWTSP near thewell 171/1. The wells with signiﬁcantly increase in the change ratio are located in the vicinity of the radioactive waste facilities. 137Cs in well 201 on
the left bank of the Pripyat River also increased, however the relationship to
the radionuclide waste site is nothing.

The difference between them is shown in Fig. 3c, and the rate of change isshown in Fig. 3d. The 90Sr concentration data used for Fig. 3 are presented
in Supplementary Table S1.
Prior to the CP drawdown (2011–2013), the average concentration of90
Sr was highest (1.5 × 105 Bq/m3) at monitoring well 2/1, located southwest of the ChNPP (Fig. 3a). Monitoring well 1/2, located 1.6 km north ofwell 2/1, had the second highest concentration, at 1.2 × 105 Bq/m3(Fig. 3a). Additionally, the average 90Sr concentrations in groundwatershow an order of magnitude difference from the southern area at the
ChNPP to the northern area near the Pripyat River (Fig. 3a). These distributions are consistent with the estimated surface deposition in 1986 (Fig. 1),
indicating that the high-concentration distribution area did not extend to
the west or east. In addition, an inﬁltration area of the ChNPP wastewater
treatment plant was present north of the ChNPP, and the average 90Sr concentrations in this area were low (Fig. 3a). Moreover, concentrations at the
Pripyat River ranged from 32 to 590 Bq/m3. The average 90Sr concentrations in the wells between the CP and Pripyat River were two orders of magnitude smaller than those southwest of ChNPP and ranged from 1.0 × 102
to 5.0 × 103 Bq/m3. The concentrations on the ﬂoodplain between the CP
and Pripyat River were approximately an order of magnitude higher thanthose upstream of the CP.
Fig. 3b shows the average 90Sr concentrations during 2017–2019, when
the highest concentration was 1.4 × 104 Bq/m3 at well 2A. In Fig. 3b, well
1/2 had the second highest concentration of 1.2 × 104 Bq/m3. Moreover,
the average 90Sr concentrations in groundwater in the north near the
Pripyat River were an order of magnitude higher than those in the southwest (Fig. 3b). For the monitoring wells near Azbuchin Lake, the average
90
Sr concentrations were high, whereas the average 90Sr concentrations
near the inﬁltration area north of the ChNPP were low (Fig. 3b). Additionally, the Pripyat River concentrations ranged from 24 to 260 Bq/m3. The
concentrations on the dike between the CP and Pripyat River were approximately an order of magnitude higher than those in the southwestern of theCP.
Fig. 3c shows the difference between the 3-year averages of 90Sr concentrations before and after the CP drawdown; they ranged between −1.3 ×
105 Bq/m3 at well 2/1 to 8.5 × 104 Bq/m3 at well 2A. Increases in 90Sr concentrations above 1.0 × 104 Bq/m3 were found near the small lakes near
the Pripyat River. Using data on the unconﬁned aquifer from 148 monitoring wells, the average 90Sr concentrations increased in 31 wells, and somenested wells showed increases in both shallower and deeper screens, suchas wells 2A and 2 near Azbuchin Lake. Speciﬁcally, the average 90Sr concentration increased by 8.5 × 104 Bq/m3, from 5.2 ± 1.0 × 104 Bq/m3
to 13.8 ± 5.3 × 104 Bq/m3, at well 2A (9.88 m deep) and 1.5 × 104
Bq/m3, from 1.6 ± 0.4 × 104 Bq/m3 to 3.1 ± 0.2 × 104 Bq/m3, at well
2 (14.29 m deep). However, well 1/2 showed a small change of 2.3 ×
103 Bq/m3; the average 90Sr concentrations remained high before andafter the drawdown. Thus, this small change at well 1/2 led to the minorchange ratio shown in Fig. 3d.
Fig. 3d shows the rate of change in the average concentrations before
and after the CP drawdown to identify new “hot spots with increasing90
Sr concentrations. As shown in Fig. 3d, the average 90Sr concentrationin groundwater decreased southwest of the ChNPP, whereas the concentrations increased signiﬁcantly on the north side, near the Pripyat River. In
particular, the ﬁgure indicates that the two hotspots were near the Pripyat
River, which had up to 600 % increased 90Sr concentrations in water.
Hence other than these hotspots most of the area showed decreased 90Sr
concentrations. Moreover, most of the wells near the RWDS, where radioactive waste was stored in containers, showed decreasing concentration
trends.
The same analysis was performed for 137Cs as for 90Sr (Supplementary
Table S3, Fig. S1). However, evaluating the changes in 137Cs was more difﬁcult that for 90Sr, because the 137Cs data had larger standard deviations
and fewer measurements than those of 90Sr. When only shallow wells areused for analyzing as shown in Fig. S1, more than 90 % of the 137Cs concentrations were lower than 102 Bq/m3, with no signiﬁcant difference between
before and after the drawdown compared to 90Sr concentrations

4.2. Monthly time-series data of groundwater levels and 90Sr and 137Csconcentrations
Figs. 4 and 5 show the time-series data of monthly precipitation,
groundwater levels, and concentrations at representative wells with increasing and decreasing average 90Sr concentrations. The locations ofthese 10 wells are shown in Fig. 3c and their 16 screens are listed inTable 1. At 168 and 193Q, three monitoring wells were nested with a screen
at different depths in each well. In Figs. 4 and 5, the three panels show theobserved time-series data from 2010 to 2019 for each screen, and the vertical pink line shows the start of the CP drawdown. This panel setup allows us
to compare the CP inﬂuence before and after the drawdown. In Fig. 4,
monthly Pripyat River water levels measured at the BNS site after the drawdown were used for wells situated 1–1.5 km upstream of the BNS site
(Fig. 4a and b), and monthly river water levels measured at the city ofChornobyl were used for the nested well situated 10 km downstream of
the BNS site (Fig. 4c). In addition, the Azbuchin Lake data are shown in
Fig. 4a with a focus on 2016–2019.
In the top panels of Fig. 4a–c, groundwater levels were affected by thewater levels of the Pripyat River, which were highest during the snowmelt
season from March to May and heavy rainfall events. Annual precipitation
at the ChNPP from 1990 to 2019 was 612.6 ± 99.5 mm, the largest amountwas 953.8 mm in 2012. The wet- (May to August) and dry- (January to
April) season precipitation was 50–70 mm/month and 30–40 mm/
month, respectively. The monthly precipitation exceeded 150 mm in July
2011, August 2012, May 2014, and October 2016. As shown in Fig. 4a,
the groundwater levels at 3A, 2A, and 1A before the CP drawdown were approximately 104–107 m a.Bs.l. and ﬂuctuated approximately 1–2 m with
the Pripyat River levels. The groundwater levels declined by approximately
1–2 m after the CP drawdown and approached the level of the Pripyat Riverduring ﬂooding. The water levels in the wells near Azbuchin Lake weremaintained at a higher level than that of the downstream groundwaterlevels before the CP drawdown. However, the groundwater and Azbuchin
Lake levels were reversed during the rainy season after the CP drawdown.
As shown in Fig. 4b, wells PK-14 1B and 2B, located near the CP,
showed no response to the ﬂuctuations of the Pripyat River levels; in contrast well 3B, situated near the river, responded to the Pripyat River before
the CP drawdown. The groundwater levels in these wells were approximately 107–108 m and 104–105 m, respectively with a difference of approximately 3 m. After the CP drawdown, the groundwater levels in both
1B and 2B declined to 105 m and responded to ﬂuctuations in the Pripyat
River. The groundwater levels of well 3B were similar to the level of Pripyat
River, at approximately 104 m, indicating surface-groundwater connectivity. A similar pattern was also observed in nested well PK-127 (Fig. 4c),which was situated south of the CP. The groundwater levels, which were
nearly constant at approximately 105 m, declined by 2 m after the CP
was decommission and began to respond to the Pripyat River. The patternof ﬂuctuations in groundwater levels at wells 2/1, 168, and 193, which
are located at a distance from the Pripyat River and shown in Fig. 5a, b,
and c, respectively, remained the same after the CP drawdown, with agroundwater level drop of 1–2 m.
From the middle panels in Fig. 4, we identiﬁed increasing and consistent seasonal trends in 90Sr concentrations after the CP drawdown. As
shown in Fig. 4a, 90Sr concentrations in wells 3A, 2A, and 1A were between
104 and 105 Bq/m3 during 2010–2013; after the drawdown, values at 3A7

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Fig. 4. Time series of precipitation, groundwater levels (m a.Bs.l), and 90Sr and 137Cs concentrations in selected wells north and east of the CP before and after the drawdown,
which is shown by the vertical pink line. The blue bands indicate the “wet season” due to snow melt, when river levels were high due to snow melt. The locations of these wells
are indicated in Fig. 3c. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)and c, 90Sr concentrations in wells 168 and 193 showed no signiﬁcantchanges before or after the CP drawdown.
The 137Cs concentrations were between 101 and 102 Bq/m3 at all selected wells (Figs. 4 and 5), which is 1–3 orders of magnitude lower than
the 90Sr concentrations. In addition, 137Cs concentrations did not signiﬁcantly change after the decommission of the CP.

and 2A increased by approximately 10 times (Fig. 4a). This corresponds tothe timing and magnitude of the increase in 90Sr concentrations in
Azbuchin Lake. As shown in Fig. 4b, at PK-14 wells, the 90Sr concentrations
were approximately 104 Bq/m3 at well 3B and 103 Bq/m3 at wells 1B and2B from 2010 to 2013. However, all three PK-14 wells showed an increasein 90Sr concentrations after the CP drawdown, especially 1B, which is theclosest to the CP. As shown in Fig. 4c, 90Sr concentrations in the nested
well PK-127 were approximately 103 Bq/m3 without signiﬁcant changes
owing to the CP drawdown.
From the middle panels in Fig. 5, we identiﬁed decreasing or consistentseasonal trends in 90Sr concentrations after the CP drawdown. As shown in
Fig. 5a, 90Sr concentrations at well 2/1 increased before 2012, with a temporary further increase from 105 to 106 Bq/m3 at the end of 2013 and 2014
due to heavy precipitation in 2012–2013. However, 90Sr concentrations decreased to 104 Bq/m3 after the CP drawdown (Fig. 5a). As shown in Fig. 5b4.3. Changes in groundwater levels and direction
Changes in the groundwater regime due to the CP drawdown were evaluated at the monitoring wells and are illustrated by groundwater-level contours during the dry season (Fig. 6). In Fig. 6a, the groundwater-level
contours are shown for January 2014, when the CP was in operation, andFig. 6b shows the groundwater-level contours for January 2019, which
was after the CP drawdown. The 3-year average precipitation at the8

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Fig. 5. Time series of precipitation, groundwater levels (m a.Bs.l), and 90Sr and 137Cs concentrations in the selected wells west and south of the CP before and after the
drawdown, which is shown by the vertical pink line. The locations of these wells are indicated in Fig. 3c. (For interpretation of the references to colour in this ﬁgurelegend, the reader is referred to the web version of this article.)

ﬂow on the western side of the CP, creating a groundwater stagnation area
south of the ChNPP (Fig. 6a). Therefore, contaminated surface water in the
CP was a major source of groundwater recharge and inﬂuenced the local
and regional groundwater gradients.
The water level in the CP decreased to 105.6 m a.Bs.l. in January 2019.
The area north of the CP no longer ﬂows from the CP and has been replacedby direct ﬂow from the ChNPP to the Pripyat River. In addition, the distance between the water-level contours increased and the ﬂow velocity decreased. On the east and north sides of the CP, the contours demonstratemuch smaller gradients, as shown by the wide separation in Fig. 6b. In contrast, on the west side of the CP, the contours resulted from the regional gradient, and the decline in water levels in the CP resulted in faster velocities
than before the CP was decommissioned.
Estimated changes in groundwater levels between January 2014 andJanuary 2019 are shown in Fig. 6c. A 50 cm change in groundwater levels
was identiﬁed at approximately 65 % of the monitoring wells. Changes of 5
m occurred near the CP, as indicated by the extended concentric circles(Fig. 6c). Contour lines indicating 1 m of change in groundwater levels

ChNPP was 1.6 times higher during 2011–2013 (773 ± 44 mm) than thatduring 2017–2019 (496 ± 29 mm); the 2014–2016 period had a 3-year average precipitation of 603 ± 43 mm. The winter season is dry, withmonthly average air temperatures signiﬁcantly below 0 °C (January 2014:−5.7 °C, January 2019: −4.7 °C). January was selected because it was minimally affected by rainfall. The average precipitation values for the threemonths immediately prior to January 2014 and 2019 were 36.6 ±20.8 mm and 26.7 ± 21.9 mm, respectively, and the monthly averagewater levels of the Pripyat River measured at the BNS site were 103.95 ±0.07 m and 103.10 ± 0.14 m a.Bs.l, respectively.
Prior to the CP drawdown, the groundwater gradient was inﬂuenced byinﬁltration from the CP. Local groundwater ﬂowed north from the CP to
Azbuchin Lake (Fig. 6a). In January–April 2014, when the CP was still operating, the surface water level in the CP was approximately 111 m a.Bs.l;
thus, the difference between the CP and Pripyat River was approximately
7 m. In Fig. 6a, a steep groundwater gradient can be seen on the east andnorth sides of the CP, which had the fastest groundwater velocity in thisarea. In addition, the CP water levels inﬂuenced the regional groundwater
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Fig. 6. Groundwater level contours (m a.Bs.l.) of the unconﬁned aquifer monitoring wells on (a) January 2014 and (b) January 2019 and (c) the difference between them (m).
Enlarged views of the north and southeast of the CP were shown on the right side of panel (a). The water areas were mapped by selected low cloud-cover Landsat8 C2L2
(QA_PIXEL) images (a: as of March 9, 2014; b and c: as of May 19, 2019) downloaded from the USGS EarthExplorer (https://earthexplorer.usgs.gov/).

and southern drains stopped ﬂowing in 2017 due to the decline in waterlevel below their elevation. The current surface water area of the CP has
been reduced to three lacustrine wetlands formed above the lowpermeability CP bottom, and groundwater leakage via the CP bottom
sediments is the only pathway for radionuclide migration. The changes
in the CP leakage had most inﬂuence during the dry season, extending as
far as 3 km away. The magnitude of this change was up to 6 m at theedge of the former CP (Fig. 6c). Based on the characteristic leakage
length λ (Gusyev and Haitjema, 2011) and the CP leakage extent demonstrated in Fig. 6c, the northern area of the CP had two times higher
leakage length to the northern and eastern sides than that of the southern area of the CP. Moreover, this leakage factor difference indicatesfour times higher hydraulic conductivity of the CP bottom sediments
between northern and southern CP areas. This is an important consideration, as the northern area of the CP had the most nuclear fuel particles
deposited in 1986, and the Pripyat ﬂoodplain, with several small lakes,
had RWTSP containing buried highly radioactive waste (Ledenev et al.,
1995; IAEA, 2019).

extended westward from the CP to the western area of the ChNPP. Only
small changes of several tens of centimeters before and after the CP drawdown were observed surrounding the Pripyat backwater, which is located
2-km away from the northern tip of the CP.
5. Discussion
5.1. Evaluating changes before and after CP drawdown
The CP drawdown stopped artiﬁcial recharge to the unconﬁned aquifer,resulting in the reduction of groundwater levels and velocities, changes ingroundwater ﬂow direction, and altered groundwater-surface water interactions. During the CP operation, high water levels of 111 m a.Bs.l. weremaintained in the CP by the continuous pumping via four pumps at approximately 3.17 m3/s each from the Pripyat River (IAEA, 2019). Hence, water
seeped from the CP via the north and south drainage canals and leaked into
the shallow unconﬁned aquifer. Since November 2014, the CP water levelhas been gradually declining, reducing the seepage; moreover, the northern
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3.5 × 105 Bq/m3 which is above the WHO drinking water guideline of 10
kBq/m3 (WHO, 2017) (Fig. 7a and c). In Fig. 1, this area had a high initialdeposition (Kashparov et al., 2018), and the difference of 90Sr concentrations could be related to the amount of the initial deposition (Fig. 7b andc). In particular, the 90Sr concentrations in Azbuchin Lake and wells 3Aand 2A were affected by reduced ﬂow from the CP with lower 90Sr concentrations and chemical reactions of fuel particles due to the lowering ofwater levels in the surrounding ponds (Bulgakov et al., 2009). In addition,
it is possible that contaminants generated from unidentiﬁed waste trenches
in the past may also contribute as the source. The additional increase in 90Sr
is also noteworthy because the ﬂow direction is now the original ﬂow direction along the topography from the ChNPP, RWDS and RWTSP to the
Pripyat River and Azbuchin Lake. However, the present results show no increase in 90Sr in the vicinity of the “Podlesny” RWDS. This is thought to be
due to the groundwater table decline from the highly contaminated layer
due to the CP drawdown.
Water levels at Azbuchin Lake and Pripyat backwater changed by approximately 2 m and 0.5 m between the two periods, respectively. Water
volume changes could be a factor in the increased 90Sr concentrations in
Azbuchin Lake; hence, the concentration of 137Cs should also have increased by the same ratio. However, the changes in the 137Cs concentrations in Azbuchin Lake and Pripyat backwater were smaller (65 % and 6%, respectively) than those of the 90Sr concentrations (653 % and −28%, respectively) as shown in Supplementary Table S2 and S4. Therefore,
the 90Sr concentrations are likely increased because of transport rather
than volume changes.
The shallower well 2A and deeper well 2 located near Azbuchin Lakeshow increasing of 90Sr concentrations that is likely due to the transport
from the Azbuchin Lake bottom due to approximately the same groundwater levels in both wells. In previous study, Bugai et al. (2022) found “notrend” in the 90Sr concentrations in wells 2A and 2 using long-term annual
data, whereas our results suggest the importance of interannual variability
before and after the CP drawdown. The concentrations at K-7 and K-6 near
the Pripyat River were also elevated by 300 % and 600 %, respectively, despite a small decrease in groundwater level (Figs. 3d and 6c). Nevertheless,this ﬁnding is consistent with the increasing long-term trend observed by
Bugai et al. (2022). The wells with increasing 90Sr concentrations unrelated
to groundwater decline are located near Pripyat backwater, which is highly
contaminated by radioactive fallout on its water surface and surrounded by
nuclear waste trenches at the “Neftevaza” RWTSP, as mentioned in
Section 2.3. The high radionuclide concentrations in this area could havebeen caused by the dissolution of fuel particles in the trenches and Pripyat
backwater (Bulgakov et al., 2009) and their subsequent transport togroundwater (Bugai et al., 2022). This mechanism is supported by thelack of increases in 137Cs concentrations, and presence of 90Sr, due to differences in mobilities (Kd value) (Bugai et al., 2020). Additionally, this result
indicates that the increase in 90Sr concentrations in groundwater near
Azbuchin Lake cannot be explained solely by leaching from the trenches,
because trenches are absent in the vicinity of Azbuchin Lake.
Increase of 137Cs radionuclide has a completely different mechanism
than 90Sr mobilization due to the difference in Kd values (Bugai et al.,2020). Indeed, the location of the wells with increased 137Cs change ratios(Supplementary Fig. S1d) was different from those with increased 90Sr. In
addition, 137Cs concentrations did not change at the sites where 90Sr was
signiﬁcantly increased (Figs. 4 and 5). It is very interesting that the only
three sites with high change ratios above 300 % were in the vicinity of radioactive waste facilities, despite the large and rapid groundwater drawdown due to CP decommissioning. In Fig. 7b, 137Cs concentrations in CP
water have increased 1.5 to 2 times since 2015 after the drawdown andthe concentrations are higher than in the groundwater, about 1 × 103
Bq/m3 in 2015–2019. The 137Cs increase in the CP water was explainedby different mechanisms, such as reduced self-puriﬁcation, increased biomass density, and oxidized fuel particles due to lowered water levels(Bulgakov et al., 2009; IAEA, 2019). In this study, the sites where signiﬁcant rates of change occurred are identiﬁed around RWDS. In addition, increased 137Cs concentrations are also observed in groundwater at depths ofIn the current regime, surface water level of the lacustrine wetlands
within the CP ﬂuctuates due to wet- and dry- season precipitation levels, becoming a driving process in surface water–groundwater interactions. During the wet season, rainfall accumulates in small ponds, resulting in an
increase in surface water levels, which increases seasonal leakage to theshallow unconﬁned aquifer, thereby increasing groundwater velocity compared to the regional groundwater ﬂow. In addition, wet season precipitation may create groundwater mounding that locally affects groundwaterﬂow direction, such as groundwater mounding at the Pripyat River ﬂoodplain with two opposite groundwater ﬂow directions to the Pripyat Riverand Azbuchin Lake. For example, groundwater levels can ﬂuctuate up to
2 m between the wet and dry seasons as observed in well 3A (Fig. 4a)and could potentially cause the vertical and horizontal inﬂow of 90Sr and90
Sr mobilization from sediments to water due to the change in waterhydrochemistry. The high water levels of the Pripyat River, which has
higher concentrations of dissolved 90Sr and 137Cs during the high ﬂow season (Bugai et al., 2022), are above groundwater levels, such as well 3A in
Fig. 4a, indicating a reversal of the hydraulic gradient from the Pripyat
River to nearby wells during the wet season. As a result of these seasonalchanges in the groundwater ﬂow direction and gradients, the dissolved inﬂow of 90Sr may move much slower than groundwater of the unconﬁned
aquifer, explaining the increasing 90Sr concentrations in these wells(Fig. 4). This activated recharge continues for several weeks, with declining
surface area and water levels in the ponds due to seepage and evapotranspiration.
For the dry months, the inﬂuence of the CP inﬁltration on the local
groundwater system has nearly stopped before and after the CP drawdown
due to the reduced leakage from the CP bottom. Prior to the drawdown, thegroundwater ﬂow direction in the shallow aquifer was from the CP to
Azbuchin Lake and Pripyat River and the CP leakage was creating a hydraulic barrier of groundwater with low radionuclide concentrations. This hydraulic barrier was preventing groundwater and radionuclide movementfrom the area north of ChNPP to Azbuchin Lake and Pripyat River. Afterthe CP drawdown, the artiﬁcial leakage of CP surface water stopped making
the regional groundwater ﬂow a dominant process of radionuclide transport from southwest to northeast. In the northern area of the CP reservoir,
the natural water levels of the lacustrine wetlands are only up to 2 m higher
than Pripyat water levels at BNS river gauge suggesting that a direct hydraulic connection still exists between the CP and Pripyat River withmuch slower groundwater velocities. These seasonal ﬂuctuations of CP,
Azbuchin Lake, and Pripyat River water levels could be the driving processes responsible for the increase of 90Sr radionuclide concentrations inmonitoring wells at the Pripyat ﬂoodplain. This process is also supported
that 90Sr concentrations of Azbuchin Lake have a much larger increase compared to the CP water after the CP drawdown as discussed in the next section. Since groundwater ﬂow is the main transport of 90Sr radionuclides
to the Pripyat River in dry months, the continuous monitoring of the radionuclide hazard based on actual groundwater levels is needed for understanding the groundwater inﬂuence on the Pripyat River concentrationsin the future.
5.2. Radionuclide concentration changes in the CP, lake, and groundwater
The decrease in water level in the CP signiﬁcantly affected the surrounding groundwater level and ﬂow (Fig. 6). High 90Sr concentrations in
the groundwater west of the ChNPP before the CP drawdown decreased afterwards. A 1-m decrease in the groundwater table resulted in lower concentrations presumably due to groundwater decreases to below theshallow highly contaminated layer (Bugai et al., 2022). In the area between
the CP and Pripyat River, the changes in 90Sr were very small, approximately −100 %, except for PK-14 1B, which caused the decrease in the hydraulic gradient and the small increase in 90Sr in CP water (Fig. 7).
Therefore, we focused on the factors contributing to the more pronounced
increase in concentration north of the CP.
North of the CP (wells north of 3A and Azbuchin Lake), the 90Sr concentrations were the highest near the Pripyat River, ranging from 5.0 × 104 to11

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Fig. 7. Changes in 90Sr concentrations in groundwater and surface water of the CP and lakes. (a) 90Sr concentrations were analyzed in the groundwater, lakes, and CP at a totalof 12 sites. (b) Changes in the average 90Sr concentration for three years before (2011–2013) and after (2017–2019) the CP decommission, which were related to 90Sr
deposition (kBq/m2) near the CP (c). The deposition was estimated by inverse estimation using half-life based on the database of Kashparov et al. (2018). In panel (b),the box-and-whisker plots were colored by the same colour coding at the same points, with the concentrations on the left before the drawdown and on the right after the
drawdown. The water areas in the panel (a) is mapped using selected low cloud-cover Landsat8 C2L2 (QA_PIXEL) images (as of May 19, 2019) downloaded from the
USGS Earth Explorer (https://earthexplorer.usgs.gov/). Sample name of AZB indicates Azbuchin Lake. (For interpretation of the references to colour in this ﬁgure legend,
the reader is referred to the web version of this article.)comparison of ﬂuxes over this period showed a 3 % decrease. This is evidence that the reduction in the hydrodynamic gradient contributed signiﬁcantly to the reduction in 90Sr ﬂux. In contrast, in well 1A near Azbuchin
Lake, the concentrations decreased by approximately 20 % and ﬂuxes decreased by 30 % from before to after the CP drawdown.
When the delay due to the retardation factor was considered, the 90Sr
ﬂux calculated at wells near the CP and Azbuchin Lake decreased slowly(Fig. 8a). Transport time from wells near the CP to the Pripyat Riverchanged from 5 to 8 y in the north (PK-14) and from 0.5–9 y to 1.5–217 y
in the south (PK-32, 64, and 113) (Supplementary Table S7). In Fig. 8b,
the contribution of the CP area based on four wells has approximately 10% in 2015 and 2–6 % from 2014 to 2019 of the 90Sr ﬂux in the Pripyar
River (Supplementary Table S8). This reﬂects the past 90Sr ﬂux, which
was due in part to the effect of the delay; therefore, there are were signiﬁcant changes between the periods before and after the CP drawdown. In
PK-14, where the concentration increased by approximately 3 % after the
CP drawdown, the transport time changed from 5 to 8 y. Considering thehalf-life of 90Sr (28.8 years), the concentration would decrease by 17 %over 8 y; therefore, the delay further decreased the concentration. Thus,
the wells near the CP show reduced discharge and retardation effects
owing to the lowered hydraulic gradient; moreover, the 90Sr ﬂux is expected to decrease in the future. If the additional 90Sr from the CP ﬂowed
into Pripyat, the effect of 90Sr transport from the CP would be small because
the ﬂux from CP to the river, as estimated by Bugai and Skalskky (2018),
should not exceed 0.7 GBq/y. Transport time at 1A, near Azbuchin Lake, increased from 0.4 to 0.6 y after the CP discharge and immediately
discharged to the river; contributions from 1A to the river were 1–3 %after the CP water level drop, with an increase of about 30 % comparedto 3 years before and after the CP drawdown. This may be because theﬂux at 1A did not change signiﬁcantly, whereas the ﬂux in the river itself
decreased (IAEA, 2019; Igarashi et al., 2020, Supplementary Table S8).

about 30 m. This may be caused by artiﬁcial factors (e.g., crosscontamination from surface, ﬂow along the well, etc.) and several natural
factors including colloids transport or complexes in relatively deep groundwater, as noted by Bugai et al. (2022). The mechanisms causing presence ofrelatively immobile Cs in monitoring wells are not fully understood and thisstudy only suggests that increased 137Cs concentrations are associated withthe location of radioactive waste.
5.3. 90Sr transport to the Pripyat River before and after the CP drawdown
Data regarding the 90Sr concentrations transported from the CP to the
Pripyat River are needed for downstream impact assessments; therefore,
we analyzed the 90Sr ﬂuxes for 1999–2019 using data collected from themonitoring wells at the ﬂoodplain between the CP and the Pripyat Riverand near Azbuchin Lake. Using the approach of Bugai and Skalskky(2013) as described in Section 3.3, the calculated area is as shown in Supplementary Fig. S2, and the changes in the groundwater ﬂow regime withdecreased discharge and increased traveling times are listed in Supplementary Table S7.
Considering groundwater discharge without the delay due to the retardation factor, the 90Sr ﬂuxes from wells installed near the CP and Azbuchin
Lake from 1999 to 2019 changed from 0.078 ± 0.024 to 0.020 ± 0.012
TBq/m3 and from 0.053 ± 0.061 to 0.014 ± 0.003 TBq/m3, respectively.
After the CP drawdown, they decreased by approximately 70 % comparedto those at the beginning of the measurement (Supplementary Table S6).
The estimated 90Sr ﬂux in the CP area from 1999 to 2010 was 1010–1011
Bq/m3, which is similar to the values published by Bugai and Skalskky(2013) calculated before drawdown, although the velocity calculation differed. Well PK-14 3B, which is north of the CP, has experienced increasing90
Sr concentrations since 2016; the 3-year average after the CP drawdown
was double that before, as shown in Fig. 3 and Fig. 4B. However, a
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Fig. 8. The 90Sr annual ﬂux (a) and contribution (b) at Chornobyl City monitoring site of the Pripyat River from Azbuchin Lake and cooling pond (CP) areas from 2010 to2019.

hypothesis that the increase in 90Sr concentrations in the Pripyat River
ﬂoodplain can be attributed to the decrease in dilution rate from the CP
due to the decreased leakage and the change in groundwater ﬂow direction
as well as the decrease in groundwater velocity. Our study also revealedchanges in 90Sr transport from 2010 to 2019 by groundwater near the CP
and Azbuchin Lake, which were feared to become highly contaminatedsources of the radioactivity, to the Pripyat River. From the new ﬁnding of
this study that the drawdown increased 90Sr concentrations near the ﬂoodplain, we estimated the 90Sr ﬂux and contribution to the Pripyat River andthe 90Sr contribution did not change signiﬁcantly after the drawdown. Although increased concentrations were observed in the ﬂoodplain, they
are not expected to have a signiﬁcant impact on the Pripyat River due tothe delay in transport caused by the lowering of the hydrodynamic gradient. However, north of the ChNPP in the Pripyat River ﬂoodplain, 90Sr concentrations in groundwater may increase gradually due to the vertical andhorizontal inﬂow of 90Sr as well as the 90Sr mobilization in the future. The
total 90Sr ﬂux in the area between the CP and the Pripyat River and near
Azbuchin Lake could still contribute up to 10 % of the 90Sr ﬂux in the
Pripyat River. Therefore, it is necessary to continuously monitor 90Sr concentrations and groundwater levels in the wells and lakes, and establish
monitoring wells within the CP area to measure leakage rates from the lacustrine wetlands.

The contributions of estimate area at ChEZ up to 2019 were below 10 %with little change. This is because 90Sr ﬂux in the Pripyat River and the tributary river also decreased with 90Sr ﬂux in groundwater (IAEA, 2019;
Igarashi et al., 2020). The remaining approximately 90 % is considered tobe contributed by 90Sr transport from surface water and groundwaterother than the estimated areas shown in Supplementary Fig. S2. The State
Specialized Enterprise Ecocentre measured 90Sr concentrations in the
Pripyat River at the ChEZ inlet and outlet, located at the Ukraine–Belarus
border and the city of Chornobyl, respectively (Kireev et al., 2013; Bugai
et al., 2022). According to this data, the 90Sr ﬂuxes received by the Pripyat
River inside the ChEZ have decreased since 2015, while the groundwater
contributions from the CP and Azbuchin areas to the river did not change,
rising only about 2 % each after the drawdown (Supplementary
Table S9). Bugai et al. (2022) indicates that 80–90 % of the 137Cs ﬂux transports from upstream of the ChEZ and 50–70 % of the 90Sr ﬂux transports
within the ChEZ, and describes the 90Sr concentration is controlled by thedischarge from surface waters. Our results also support the ﬁnding of reduced radionuclide transport to the Pripyat River after the CP drawdown
predicted by Buckley et al. (2002). Based on these results, we consider
that the impact from groundwater to the river is small even after the drawdown. However, the 90Sr ﬂux in groundwater may increase slightly in the
future because of additional transport as mentioned in Section 5.2. Therefore, monitoring should focus on the highly concentrated areas near thesmall lakes in the northern area of the CP and the 90Sr ﬂux should be validated by groundwater ﬂow models for accurate ﬂux estimations.

CRediT authorship contribution statement
Hikaru Sato: Conceptualization, Methodology, Validation, Investigation, Writing - Original Draft Visualization. Maksym Gusyev: Conceptualization, Methodology, Validation, Writing - Original Draft, Review &Editing. Dmytro Veremenko: Investigation, Data Curation. Gennady
Laptev: Investigation, Data Curation, Writing - Review & Editing. Naoaki
Shibasaki: Writing - Review & Editing, Supervision. Yuichi Onda: Writing- Review & Editing. Mark Zheleznyak: Supervision. Serhii Kirieiev:
Resources, Supervision. Kenji Nanba: Supervision, Funding acquisition.

6. Conclusions
This is the ﬁrst study to quantitatively evaluate the spatial distributions
and time-series trends in groundwater levels and 137Cs and 90Sr radionuclide concentrations in the shallow unconﬁned aquifer before and after
the CP drawdown. The analyses were performed on observational datafrom 2010 to 2019 in the vicinity of the ChNPP, and we observed thatthe natural drawdown of the CP had been affecting the surrounding
groundwater system since May 2014. This study conﬁrmed that 137Cs concentrations did not signiﬁcantly change before and after the drawdown.
The new ﬁnding of this study is that the average 90Sr concentrations increased north of the ChNPP in the Pripyat River ﬂoodplain including
Azbuchin Lake and decreased east and west of the CP between
2011–2013 and 2017–2019 time periods. This ﬁnding was counterintuitivesince the CP drawdown resulted in a decline in groundwater levels, changes
in groundwater velocity, and a shift in the groundwater ﬂow direction, as
the CP water levels dropped by 6 m. In addition, the current trends of increasing 90Sr concentrations that were established from the available data
are useful for future ﬁeld investigations of groundwater and lakes in the vicinity of the CP.
Although CP drawdown was considered expedient for preventing
groundwater ﬂow contamination by radionuclides, our results support theData availability
Data will be made available on request.
Declaration of competing interest
The authors declare that they have no known competing ﬁnancial interests or personal relationships that could have appeared to inﬂuence thework reported in this paper.
Acknowledgments
This work was supported by the Japanese government (JST/JICA) program for international joint research into global issues, called the Science13

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Science of the Total Environment 872 (2023) 161997
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and Technology Research Partnership for Sustainable Development JSTJICA, Japan (SATREPS project, PI. Kenji Nanba; grant number
JPMJSA1603). We thank Dr. Haruko Murakami for the useful discussions
about the migration of radionuclides and Dr. Dmitri Bugai for providing
materails on Chernbyl site and cooling pond hydrogeology. We are grateful
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Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.scitotenv.2023.161997.
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