Two cores of bottom sediments were collected in 2018 to a depth of ∼200 cm in the deepest part of the Scheckino reservoir on the
Upa River (9500 km2), Tula region, Russia. This area was severely
contaminated by radiocesium (r-Cs) after the Chernobyl accident in 1986. The fact that 137Cs activity concentrations in a specific horizon of the bottom sediments correspond to 137Cs concentrations associated with
suspended matter delivered to the reservoir, provides a basis for
constructing the dynamics of particulate 137Cs activity concentrations
in the Upa River catchment from 1986 to 2017. Over the time since the
Chernobyl accident, the particulate 137Cs concentrations have decreased
by more than an order of magnitude, with only minor changes occurring during
the last 15 years. Using a typical value for the distribution coefficient
Kd for the rivers of the Chernobyl contamination zone, dissolved
137Cs activity concentrations in the Upa River have been estimated and
their changes over the past 30 years since the accident have been studied.
The resulting estimates of dissolved 137Cs concentrations in the Upa
River have been found to be in good agreement with measured data over the
period 1987–1991. The proposed and tested method provides a basis for
reconstructing the long-term dependence of radionuclide concentrations in
rivers and reservoirs based on their vertical distribution in bottom
sediments. Reconstructed time dependencies of particulate and dissolved
137Cs activity concentrations in the Upa River were found to be
described well by the proposed semi-empirical “diffusion” model based on
an assumption that the time dependency of particulate r-Cs in the river
corresponds to the time dependency of its concentration in top soil layers
across the catchment which can be approximated by a dispersion-convection
equation with physically meaningful parameters.
Introduction
Large areas of land were contaminated with radionuclides from the Chernobyl
fallout (Izrael, 1998). These areas became long-term sources of
radionuclides to natural waters and aquatic ecosystems in the Central
European Russian landscape (Ivanov et al., 2017). Wash-off by surface runoff
is the primary pathway for contamination of water bodies after initial
fallout deposition from the atmosphere (Konoplev et al., 1992). Initial
radioactive contamination of water bodies after the Chernobyl accident was
relatively high as a result of direct fallout onto river and reservoir
surfaces.
Over longer time periods after fallout, radionuclides bound in catchment
surface soils are slowly transferred to rivers by the erosion of soil
particles (particulate phase) and by desorption from soil to solution
(dissolved phase). The rates of radionuclide transfer are influenced by the
extent of soil erosion, the strength of radionuclide binding to catchment
soils and the potential for migration down the soil profile (Golosov et al.,
2018). Detailed analysis of Chernobyl data over an extended period provides
a basis for long-term prediction of changes in environmental radioactive
contamination as a result of the Fukushima accident, or any other emergency
involving radioactive emissions to the surrounding environment.
Unfortunately, long-term data are not always available regarding changes in
radioactive contamination, and this is the case for 137Cs in the
Chernobyl zone rivers. Against this background, this study attempted to
reconstruct, from 1986 to the present day, changes in 137Cs
concentrations in the Upa River as a result of the Tula-Oryol contamination
zone, based on the current vertical distribution of 137Cs (as of 2018)
in bottom sediments of the Scheckino reservoir. These changes are described
by a semi-empirical diffusion model.
Materials and methodsSampling and processing of bottom sediments
The Upa River basin lies in the northern part of the Central Russian Upland
within the Tula region (Fig. 1). Mean annual precipitation in the basin is
∼540 mm yr-1. The southern part of the basin, with a high
proportion of arable land, was heavily contaminated by the Chernobyl fallout
in April–May 1986. The northern forested part appeared to be less
contaminated. Tula city is located in the north-east part of the Upa basin.
The upper part of the Upa river (1362 km2) is restricted by the
Scheckino reservoir.
Location of the Upa river basin (a) and map of Chernobyl-derived 137Cs deposition in the Upa River catchment upstream the Scheckino reservoir (Izrael, 1998) decay corrected for 1 May 1986 (b).
The reservoir has an area of 6 km2 and is elongated upwards of the
dam following the drowned valley of the Upa River. For the investigation of
radionuclide concentrations in bottom sediments, the upper part of the
reservoir was selected since local sources of sediment such as bank erosion
and inputs resulting from the transport of sediment by small streams were
judged to be minor contributors.
The bottom sediments were collected in February 2018 using a pistol sampler
in the deepest parts of the Scheckino reservoir in which higher rates of
sedimentation were expected. The cores were wrapped in cellophane film and
transported to laboratory. In laboratory, the cores were sliced into 2 cm
sections, weighed, dried at 105 ∘C for 8 h and weighed again for
determining moisture content and calculating dry sediment mass. The samples
were then ground, sieved through 2 mm mesh and placed in a container of
defined geometry for measurement of 137Cs activity using a
semi-conductor γ-spectrometer with high purity Ge detector.
Semi-empirical “diffusional” model of <inline-formula><mml:math id="M24" display="inline"><mml:mrow class="chem"><mml:msup><mml:mi/><mml:mn mathvariant="normal">137</mml:mn></mml:msup><mml:mi mathvariant="normal">Cs</mml:mi></mml:mrow></mml:math></inline-formula> dynamics in river waterParticulate <inline-formula><mml:math id="M25" display="inline"><mml:mrow class="chem"><mml:msup><mml:mi/><mml:mn mathvariant="normal">137</mml:mn></mml:msup><mml:mi mathvariant="normal">Cs</mml:mi></mml:mrow></mml:math></inline-formula> in surface runoff and rivers
Over the long term, it can be assumed that a major source of suspended
matter in surface runoff and rivers is the upper layer of catchment soils.
Radionuclide concentrations in top soil decrease over time due to vertical
migration to deeper layers. Hence vertical migration in catchment soils is a
critical process responsible for the decline in particulate-associated
radionuclide concentrations in surface waters. According to the
convection-diffusion model (Crank, 1975; Prokhorov, 1981; Bulgakov et al.,
2002) a change in radionuclide concentration in the top soil layer Cs
with time is given by the following equation:
Cst=σπDeffte-v24Deff+λt
where Deff is the effective dispersion coefficient; v is the effective
velocity of convective transport; λ is the radioactive
decay constant; σ is the radionuclide deposition density, and t is
time.
The time dependence of particulate radionuclide concentrations Cp in
surface runoff and river water can be described by this equation using
averaged values of Deff and v over the catchment area.
Studies of r-Cs vertical migration in soils of river catchments show that,
as a rule, its transport due to dispersion prevails over its convective
transport (Konshin, 1992; Ivanov et al., 1997; Konoplev et al., 2016).
Therefore,
v24Deff≪λ
Eq. (1) can be simplified as follows:
Cpt=σπDeffte-λt∼e-λtt
Vertical distribution of 137Cs, decay-corrected for 1986, in the reservoir cores. The total inventory of 137Cs in core C1 is 1030 kBq m-2 compared with 1160 kBq m-2 in core C2.
Dissolved <inline-formula><mml:math id="M42" display="inline"><mml:mrow class="chem"><mml:msup><mml:mi/><mml:mn mathvariant="normal">137</mml:mn></mml:msup><mml:mi mathvariant="normal">Cs</mml:mi></mml:mrow></mml:math></inline-formula> in surface runoff and rivers
Dissolved radionuclides in surface runoff and rivers are transferred from
soil to water due to cation exchange. As this takes place, only the
exchangeable fraction of radionuclides is involved in exchanges with the
dissolved phase. Therefore, dissolved radionuclide concentrations in water
Cd relate to particulate concentrations as follows (Konoplev and
Bulgakov, 2000):
Cd(t)=Cp(t)⋅αexKdex=Cp(t)Kdtot
where αex is the exchangeable fraction of the radionuclide, and Kdtot and Kdex are, respectively, the total and exchangeable
distribution coefficients of the radionuclide.
Over longer time periods, when equilibrium between exchangeable and
nonexchangeable forms of r-Cs is reached and when condition (2) is valid,
the equation for radionuclide dissolved concentrations can be presented as
follows:
Cdt=αex∞σKdexπDeffte-λt=σKdtotπDeffte-λt∼e-λtt
The advantage of this semi-empirical “diffusion-based approach” is that
all phases after the accident can be described by the same equation using
the same physically-based parameters which can be estimated or determined in
the field or using laboratory studies. Generally speaking, the
decay-corrected particulate and dissolved 137Cs activity concentrations
in both surface runoff and rivers should follow the inverse square root of
the time function.
Results and discussion
Figure 2 presents the vertical distribution of 137Cs activity
concentrations in the two bottom sediment cores collected from the study
reservoir (C1 and C2). Accumulation of sediments entering the reservoir is
associated with an increase in the total inventory of 137Cs. In both
cores, the 137Cs inventory is more than twice the corresponding maximum
inventory for the Upa River basin (Fig. 2)
Regretfully, no systematic long-term monitoring of radioactive contamination
has been conducted on the Upa River. Data are only available for the first
few years after the accident (1987–1991) for total activity concentrations
of 137Cs in water (dissolved + particulate) on the Upa River
(Vakulovsky et al., 1994). The data obtained by the present study regarding
the vertical distribution of 137Cs in bottom sediments in deep-water
zones, where sediments tend to accumulate, can provide a basis for
reconstructing temporal changes in both particulate and dissolved 137Cs
concentrations in the Upa River water following the Chernobyl accident.
Here, we make an assumption that annual sediment accumulation on the bottom
of the reservoir was not changing over the years since the accident. The
shape of the 137Cs vertical distribution in the cored sediment
accumulation zones (Fig. 2) indicates that depositing sediments do not mix
vertically and that the 137Cs incorporated in them does not migrate,
since the 137Cs peak is well defined. Considering the mean rate of
sediment accumulation is about 5 cm yr-1, based on the position of the peak
radionuclide concentration (Fig. 2), the bottom sediment layers can be
attributed to a certain period of sediment accumulation. If 137Cs
activity concentration in a respective layer matches its concentration on
suspended matter in this time period, changes in 137Cs concentrations
on suspended matter in the Upa River from 1986 to 2017 can be easily
derived.
Figure 3 shows a reconstructed time dependence of particulate 137Cs
concentration in the Upa river, based on the data for the two bottom
sediment cores collected in February 2018. As can be seen, over the time
since the accident, 137Cs concentrations have decreased by more than an
order of magnitude, with only minor changes during the last 15 years.
Reconstructed time dependencies of particulate 137Cs activity concentrations (decay-corrected for 1986) in the Upa River based on
137Cs vertical distributions in cores C1 and C2 collected in February 2018.
Figure 4 shows the results of calculating temporal changes in 137Cs
concentrations in suspended material in the Upa River by the described model
(line) with concentrations reconstructed based on the sediment core C1. As
can be seen, the “diffusion” model provides a good representation of the
time dependency of particulate 137Cs concentrations in the Upa River
reconstructed using the depth profile in the bottom sediments.
Diffusion based semi-empirical modelling of reconstructed
particulate 137Cs in the Upa River (line) based on the depth profile
for the bottom sediments in core C1 (points).
Further, using the typical value of the distribution coefficient Kd for
the rivers of the Chernobyl contaminated zone, it is possible to estimate
the concentration of dissolved 137Cs in the Upa River and its variation
over the 30 years following the accident. A Kd of 20 000 L kg-1 was taken
as being a representative value on the basis of analysis of data for a
number of rivers in Chernobyl contaminated areas (Konoplev, 2015). Using
this value, and reconstructed data for particulate 137Cs activity
concentrations, it is possible to estimate the concentrations of dissolved
137Cs in the Upa River for the time window from 1986 to 2017. The
results of these estimations are presented in Fig. 5. The published
measurement data for 1987–1991 (Vakulovsky et al., 1994) is also given for
comparison. It can be seen that the predicted activity concentrations of
dissolved 137Cs in the Upa River and their temporal trend are
consistent with the measured data.
Comparison of the predicted time dependence of dissolved 137Cs activity concentrations in the Upa River based on reconstructed particulate 137Cs from sediments in core C1 with published measurements for 1987–1991 (Vakulovsky et al., 1994). To calculate dissolved 137Cs activity concentrations, the total distribution coefficient Kdtot=20000 L kg-1 (Konoplev, 2015) was used.
Data availability
The data can be accessed by request to authors.
Author contributions
AVK suggested the idea and semi-empirical model, MMI, VNG and EAK designed the sampling and provided radiocesium analysis, AVK analyzed the data and prepared the manuscript with contributions from all co-authors.
Competing interests
The authors declare that they have no conflict of
interest.
Special issue statement
This article is part of the special issue “Land use and climate change impacts on erosion and sediment transport”. It is a result of the ICCE Symposium 2018 – Climate Change Impacts on Sediment Dynamics: Measurement, Modelling and Management, Moscow, Russia, 27–31 August 2018.
Financial support
This research has been supported by Russian Foundation for Basic Research (RFBR
project no. 18-55-50002) and the Japan Society for the Promotion of Science
(JSPS) within the framework of the bilateral project “Assessment and
prediction of sediment and radionuclide fluxes in a river basin affected by
a severe accident”.
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