Article published in: Environmental Geology (2004) 46:820–830

DOI 10.1007/s00254-004-1067-3

Published version available at: http://link.springer.com/article/10.1007/s00254-004-1067-3

Sedimentological study of the Chernobyl NPP site to schematise

radionuclide migration conditions

Andrei Matoshko, Dmitry Bugai, Lionel Dewiere, Alexander Skalskyy

Andrei Matoshko

Institute of Geography, National Academy of Sciences of Ukraine, 44, Volodymyrskaya Str., 01034,

Kiev, Ukraine

Dmitry Bugai (corresponding author)

Institute of Geological Sciences, National Academy of Sciences of Ukraine, 55-b Gonchara Str.,

01054, Kiev, Ukraine, e-mail: dmitri.bugay@gmail.com,

tel. +380(44)216-30-23 fax +380 (44) 216-93-34

Lionel Dewiere

Institute for Radioprotection and Nuclear Safety, B.P. 17, 92262 Fontenay aux Roses Cedex, France,

Alexander Skalskyy

Institute of Geological Sciences, National Academy of Sciences of Ukraine, 55-b Gonchara Str.,

01054, Kiev, Ukraine

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Abstract

The approach, methods and results of a sedimentological study of a near-surface

stratum of Late Pleistocene – Holocene deposits in the near-zone (5-10 km radius) of the

Chernobyl Nuclear Power Plant are presented. Sedimentological analyses are carried out at

three levels of detail: regional-, local- and object- scale. The unsaturated zone and unconfined

aquifer at the site are composed of two main genetic types of deposits, aeolian and alluvial,

including several dynamic facies. Consideration of lithological properties leads to following

ranking of the main genetic sediments facies with respect to radionuclide migration

retardation potential: aeolian < alluvial channel < alluvial overbank < alluvial abandoned

channel. Based on sedimentological interpretations, the geological environment is

schematised into a set of typical geological sections, possessing different radionuclide

retardation potential.

Keywords: sedimentology, geo-environment, groundwater, geology barrier, sorption,

strontium-90, Chernobyl accident, alluvium, aeolian deposits

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1. Introduction

The Chernobyl accident has resulted in a significant radioactive contamination of the

groundwater system at the Chernobyl Nuclear Power Plant (ChNPP) site. In the course of

more than one and a half decades since 1986 (date of the accident), radionuclide migration

occurred to the geo-environment from the “Sarcophagus”, the “Red Forest” radioactive waste

dumps, the cooling pond of the ChNPP, and from other sites containing radioactive fallout

materials from the damaged Unit 4 (Bugai et al. 1996, 1997; Dzhepo and Skalskyy 2002).

The Institute of Geological Sciences of the National Academy of Sciences of the

Ukraine (IGS) has been involved since 1986 in purposeful hydrogeological studies in the

Chernobyl exclusion zone, related to development of the groundwater monitoring system at

the site, risk assessment of waste sites, and the groundwater remedial measures. In 1999-

2002, the international Chernobyl Pilot Site Project has been carried out in the Chernobyl

exclusion zone (Bugai et al. 2000, 2002; Dewiere et al. in press). The collaborative project

between the French Institute for Radioprotection and Nuclear Safety (IRSN) and Ukrainian

institutes studied and modelled radionuclide migration from a shallow near surface

radioactive waste burial, containing dispersed nuclear fuel materials.

The hydrogeological studies referred to included comprehensive characterisation of

the near-surface geo-environment at the ChNPP site (i.e., stratum of sandy Late Pleistocene-

Holocene deposits) at different scales with regard to geological structure, lithological

properties, and radionuclide transport (sorption) parameters. Results from these studies have

provided an experimental basis for this paper. Analyses of the data led to establishing links

between deposit genesis and lithology, and deposit structural properties as well as hydraulic

and sorption parameters, which have important implications for the understanding of a barrier

role of deposits of various geneses.

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The objective of this paper is to present data on the structure and lithological

properties of the near-surface stratum of deposits, composing the unsaturated zone and

unconfined aquifer in the near-zone of the ChNPP, and to discuss structural, hydraulic and

sorption properties of sediments of various genesis considering their role, as radionuclide

migration barriers. Sedimentological analyses provide a basis for a schematisation of the

geological environment at the ChNPP site into a set of typical geological sections, possessing

different radionuclide retardation potential.

2. Materials and methods

2.1. Methodological basis of the sedimentological studies

The methodological approach of the presented studies is based on a knowledge of

genetic types, suites and dynamic facies of continental formations (Shantser 1982). The near-

surface part of the continental sedimentary cover is considered. It is represented by deposits

which form definite accumulative bodies of different origin, age and scale. Most considerable

changes in lithological properties are usually associated with boundaries of accumulative

bodies, which are discriminated by origin. Such accumulative bodies, called suites (Goretsky

1970; Matoshko et al. 2002), reflect elementary rhythms of sedimentation in different forms

(fluvial, glacial, aeolian, etc.). Suites are composed of facies that are lithologically uniform

members, which have originated in similar dynamic conditions of sedimentation. Individual

facies are characterised by definite lithological properties (sedimentary structures, particle-

size distribution, density, porosity, cation exchange capacity, etc.) and/or by regularities of

changeability in space of these properties. Within our study the deposit properties described

above were analysed as those, which have direct relation to hydraulic and geo-chemical

parameters governing groundwater flow and radioactive contaminant transport in the

subsurface.

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2.2. Geological data

Sedimentological analyses are carried out at three levels (scales): regional, local and

object-scale in accordance with levels of relevant hydrogeological studies.

The regional level refers to the territory in a radius of 5-10 km around the ChNPP

(Fig. 1). Regional-scale analyses are based on materials of several tens of relatively deep (e.g.,

30-40 m) exploratory boreholes penetrating the whole thickness of the unconfined aquifer in

Quaternary deposits, which is of prime interest from the perspective of subsurface

radionuclide migration. The core material of some of these boreholes has been subjected to

detailed lithological analyses. This scale gives an overview of a general geological structure

and lithology of the near-surface stratum of Late Pleistocene – Holocene deposits, which host

the unconfined aquifer.

Local-scale analyses utilize data of several thousands of shallow (e.g., 5-15 m)

geotechnical boreholes within the Industrial Zone of the ChNPP. The dense net of boreholes

(the distance between boreholes is tens of metres) allows reconstructing at a respective scale

the mode of occurrence of subsurface suites of deposits and of their dynamic facies.

The object-scale is represented by experimental sites: Pilot Site and the Prypiat Zaton

exposure, which were studied in the French-Ukrainian Chernobyl Pilot Site Project. The Pilot

Site is located at the first terrace of the Prypiat River within the area occupied by the trenches

with waste from the post-accident site clean-up operations (see Fig.1). The Prypiat Zaton

exposure represents a 8-12 m high natural erosion scarp of the first terrace above the Prypiat

River flood plain, stretching 100-200 m along the bank of the Prypiat River inlet (an

artificially widened abandoned channel of the river). Here the geological structure of the near-

surface deposit stratum was studied at two vertical wall clearings (no.1 and no.2; see Fig.1).

The above two sites, where the most detailed geology characterization studies were carried

out, are representative of the study area on a whole (Bugai et al. 2000). To describe the

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object-scale geology data, we use also the geology information obtained during excavation of

foundation pits for the ChNPP main building (see Fig.1).

2.3. Sampling methods

As a rule, an auger drill was used to collect deposit samples for lithological studies. At

the Prypiat Zaton exposure, samples were taken by making continuous furrows on the wall of

a clearing according to sampling intervals, and then by mixing up the collected deposit.

Deposit sampling intervals depended upon a scale of research. The 1.0 m sampling interval in

vertical direction was normally used in deep boreholes for analyses at a regional scale. At the

Prypiat Zaton exposure (Clearing no.1) and for shallow boreholes within the Pilot Site the 0.5

m and 0.25 m vertical sampling intervals were utilized. At the Clearing no.2 of the Prypiat

Zaton exposure, where one of the research objectives was geo-statistical analysis of the

sediment hydraulic conductivity distribution (Dewiere et al. in press), the sampling interval in

vertical and horizontal direction was as small as 0.1 m.

2.4. Analytical methods

Particle-size distribution. Deposit samples were processed first by solution of

dispersant (Na4P2O7) for destruction of clay and fine silt coatings at sand particle surfaces.

Separation of coarse and sand fractions was carried out by means of sieves with subsequent

weighing (State standard… 1967). The original device for sounding of suspension density

(inventor's certificate of the USSR no. 1395996 by N.A.Partsevsky) was used for

differentiation and estimation of the percentage of a silt-clay fraction. Granulometric classes

and fractions of deposits were defined according to the classification generally adopted in the

former USSR and Ukraine (Table 1).

Soil porosity. First, the bulk density of dry soil was determined by the method of

volumetric weighting (State standard… 1978). Soil porosity was calculated using table values

of specific gravity of soil particles for different fractions.

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Cation exchange capacity (CEC). The CEC was determined by measuring adsorption

of methylen blue dye from a water solution according to Kulchitsky (1960).

3. Results and discussion

3.1. Geological structure

In tectonic respect, the studied territory is located within the North-western slope of

the Ukrainian Shield. The indirect data indicate that a crystalline basement occurs at 20 to 80

m a.s.l., and the thickness of the sedimentary cover varies from 130 to 190 m. The

sedimentary cover consists of marine and continental rocks of Mesozoic and Cenozoic

erathems.

The Chernobyl NPP site occupies a part of the first terrace above the flood plain at the

right bank of the Prypiat River (Fig.1). The surface is mostly flat with an elevation ranging

from 111.5 to 115 m above sea level (a.s.l.). The unsaturated zone and the first from the

surface unconfined aquifer are composed of Pliocene and Pleistocene – Holocene deposits

(Matoshko 1995, 1996; Matoshko et al. 2002). Upper Pleistocene – Holocene deposits are

exposed at the surface. Below is located a non-stratified continental horizon of the Pliocene –

Middle Pleistocene age (Fig.2). The total thickness of Pliocene and Pleistocene – Holocene

deposits is about 30 m. The above sediments are sub-divided into aeolian and alluvial suites.

Sandy Pleistocene – Pliocene deposits are underlain by the aquitard layer of a regional extent

composed of dense marine carbonate silts and marls of the Kiev Suite of Eocene.

The thickness of the Upper Pleistocene alluvial suite is judged to be 9-12 m (Fig. 3).

Its lower boundary is not clearly seen in exposures and borehole sections. It consists of two

members: the channel facies, and the over-bank - abandoned-channel facies. The first member

is represented by homogeneous fine-medium sands, with inter-laminas of silt. There are often

up to 0.5 m thick lenses of peat in the uppermost part of the member. In many cases channel

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deposits form the whole section of the alluvial suite, fixing the last location of channel before

withdrawal of the terrace from a river influence.

More often channel facies are overlain by the abandoned-channel and over-bank facies

with a distinct contact. The above facies can be distinguished only for locations, where they

are exposed or the core was analysed by lithological methods. On a whole, they are

represented by inter-bedded layers of sands and silts (Fig. 4). Thickness of lenses varies from

several centimetres to 1-2 m, and their extension changes from several to hundred and more

meters. In some places within the medium and lower part of the member there are lenses of

peaty sands.

In the Clearing no.2 of Prypiat Zaton exposure (Fig. 5) the upper part of section is

composed of wavy laminated sandy silts with few sub-layers of sands. These deposits are

interpreted as over-bank facies. There is a distinct ochre ferrite bed at the top of the member,

which is supposedly associated with a former active cryogenic layer. In the centre of the

clearing a narrow sub-vertical relic ice vein can be seen. Characteristic rims border the upper

wedge-shaped edge of the vein. The lower part of the vein is filled up by alluvial deposits,

and the upper widened part is in-filled by aeolian sands. Such structures are considered as a

reliable evidence of permafrost or at least of seasonally deep frost penetration in the Late

Pleistocene in conditions of mean annual air temperature between 0 and – 6oC (Murton et al.

2000). Several similar veins were found within the Clearing 2 at the 3.5-4.0 m intervals. All

of them have the same horizontal orientation (25-300 azimuth) which indicates the existence

of a relic polygonal net. As a rule, such nets occupy vast areas (Washborne 1979).

Aeolian deposits rest upon the alluvial suite (Fig. 3, 4). Their thickness varies from

several centimetres up to 5-7 m in dunes. They are homogeneous, fine sub-horizontally and

cross-bedded laminated sands. They include two facies (sheet and dune) distinguished by the

form of sedimentary structures and by vertical changes of particle-size distribution (Matoshko

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1985). In some places immediately at the land surface the “man-made” deposits are

accumulated (the removed soil, vegetation remnants, concrete and asphalt debris, etc.). These

deposits have appeared during construction of the ChNPP and after the accident in 1986.

3.2. Main features of the Upper Pleistocene – Holocene sedimentation

To better understand structural and lithological features of deposits, it is important to

discuss their origin within the background of a general sedimentary evolution of the studied

territory. The Late Pleistocene Prypiat River valley has inherited the valleys of previous

epochs. Its formation began with the phase of incision in alluvium of the second terrace and in

more ancient alluvium lying below, followed by the lateral shift of the river and erosion,

which resulted in accumulation of a channel alluvium. The two-three-member structure of

alluvial suite (see section 3.1, Fig.3) is an evidence of a meandering type of riverbed

evolution. Wide spreading of abandoned channel facies and a very gentle shape of the line of

contact of the first terrace to the plateau is indicative of a very large (possibly > 1 km) width

of channels, and of a large radius of meanders. It can be hypothesised that the Industrial Zone

of the ChNPP is situated within one of the described above large meanders of abandoned

channels. Meandering of the river was accompanied by an intensive over-bank sheet

accumulation from suspension flows during floods.

During the final stages of formation of alluvial suite, the territory had gotten into

conditions of permafrost caused by a maximum fall of temperatures in Quaternary (Velichko

and Nechaev (1994) refer the maximum of the periglacial zone expansion to 20-16 ka BP).

This period was denoted by an evolution of ice veins, and by active layer deformations.

Gradual transition of riverbed to the subaerial conditions in the subsequent period has

given rise to aeolian processes. The layer of aeolian sands is the result of wind erosion of

channel deposits (the least coarse uppermost sands), with following transportation to short

distances. The beginning of aeolian processes probably took place in a permafrost conditions,

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which is confirmed by in-filling of upper part of ice veins by aeolian sands (Fig.5).

Degradation of the permafrost has happened probably around 12-14 ka (Sidorchuk et al.

2001). The peak of aeolian activity after melting of the permafrost was marked by formation

of dunes. During the Holocene epoch the structure of alluvial and aeolian suites within the

first terrace probably has not undergone significant changes.

3.3. Lithological properties of deposits

Lithological investigations were aimed at obtaining systematic data on sedimentary

structures (see section 3.1), particle-size distribution, porosity, mineral composition, and

cation exchange capacity (CEC) of the two subsurface suites of deposits composing an

uppermost part of geologic section that is aeolian and alluvial deposits.

3.3.1. Particle-size distribution

Results of the reported studies, as well as conclusions from the previous work

(Matoshko 1995) have shown that different genetic types and facies of Quaternary deposits

within the study area can be distinguished by section-averaged data on particle-size

distribution of sediments, and/or by data on a definite regular changes (or stability) of

particle-size distribution along the geology section. Characteristic “lithological fingerprints”

of main genetic types and facies of deposits were developed.

Deposits of aeolian suite are characterised (based on particle-size analysis of 209

samples from Pilot Site and Prypiat Zaton) by predominance of fine and medium sand

fractions, by a lowest content of the clay fraction (less than 1%), and by best sorting, in

comparison with other deposit. Mean values of fine sand fraction content vary from 54 to

64%. Shapes of section-averaged particle-size distribution diagrams across the study area

(e.g., Pilot Site vs. Prypiat Zaton) are identical or very similar (Fig. 6, 7).

The composition of overbank facies of the alluvial suite (64 samples; Prypiat Zaton) is

characterised by predominant stable content of silt (62-64%) and clay (about 16%). At the

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same time, a much higher variability of lithology of sediments is observed in the vertical

section on a small-scale. It is associated with separate few centimetres thick laminas, which

could differ even by a granulometric class. The particle-size distribution of abandoned

channel facies (15 samples; Prypiat Zaton) changes widely from a clayey silt to silty sands.

The relevant profile diagram has irregular pattern (Fig. 6). Both these facies are characterised

by unimodal flattened particle-size distribution curves (Fig. 7).

Channel facies (18 samples; Prypiat Zaton) differ from previous ones by a much better

sorting and by an essentially less content of silt and clay fractions (Fig. 6).

3.3.2. Porosity

Porosity of deposits composing the unsaturated zone and unconfined aquifer varies

from 33 to 42%. The over-bank and abandoned-channel facies have the highest values of

porosity, while aeolian deposits have the lowest (see Fig.6).

3.3.3. Mineralogy

According to literature data (Lukashev 1961; Kuznetsov 1973), which are supported

by results of chemical analyses carried out within the present study, alluvial sands of the

Prypiat basin are predominantly quartz with admixtures of the K-Na feldspar (5-9 %), and of

accessory heavy minerals (less than 0.5%). In aeolian sands the content of quartz reaches 98-

99% and the content of heavy minerals is 1-2%. The clay fraction (i.e. particles of a less than

0.01 mm size) of alluvial deposits is composed of hydromica with montmorillonite admixture,

as well as fine-dispersed calcite, quartz, and amorphous ferrous oxides. The clay fraction of

aeolian deposits is almost totally represented by a fine-dispersed quartz.

3.3.4. Cation-exchange capacity

Aeolian deposits have the least cation-exchange capacity (CEC) values, which are

usually less than 1 meq/100 g. This is due to a particle-size distribution with a low clay

content, and due to predominantly quartz mineralogy of clay fraction (see above).

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The overbank and abandoned channel alluvial deposits have the CEC values of about

5-10 meq/100 g (see Fig.6). A strong correlation is observed between the sediment CEC and

the content of a clay fraction (Fig. 6). Clay-size particles are represented by weathered

minerals (hydromica, montmorillonite, see above), with a higher specific surface area, which

have a significant influence on the exchange capacity.

The content of organic matter (humic substances) may also substantially influence the

CEC. The humus content can be indirectly estimated from data on organic carbon (OC)

content in sediments. In alluvial deposits the OC varies from 0.02 to 0.05%. An essential

increase of OC content could be expected in lenses of peaty sands. On a whole, the

occurrence of organic matter in alluvium is very irregular, and therefore it is difficult to take

into account the possible content of OC during an a-priory assessment of adsorption capacity

of alluvial deposits.

3.4. Hydraulic and sorption properties of deposits

3.4.1. Hydraulic conductivity

Based on analyses of particle-size data and slug tests, hydraulic conductivity of

aeolian sands in Pilot Site and Prypiat Zaton is estimated at K=3-5 m/day. This estimate was

confirmed in the course of a 1 m-scale natural-gradient tracer tests conducted in the aquifer at

Pilot Site (Bugai et al. 2000).

Alluvial deposits of overbank - abandoned channel facies have the lowest hydraulic

conductivity among the considered deposit types. The specific feature is anisotropic filtration

properties of these deposits with on average a much lower permeability in vertical direction

(z) compared with the horizontal direction (x): Kx≈1 m/day, Kz≈0.02-0.05 m/day (Bugai et al.

2002). The filtration anisotropy of deposits of overbank - abandoned channel facies is

obviously related to the layered structure of these deposits: higher permeability horizontal

sub-layers in the overall relatively low permeability strata result in a higher horizontal Kx

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compared with Kz. For example, hydraulic conductivity values of individual sub-layers at the

Clearing no.2 of the Pripyat Zaton differ by 2 orders of magnitude (in the range from n×0.01

to n×1 m/day) (Bugai et al. 2000).

The alluvial sediments of channel facies, comprising a large part of the unconfined

aquifer section below the elevation of 106-107 m a.s.l. (see Fig.3), are characterized by

highest values of hydraulic conductivity coefficients. Based on particle size analyses, pump-

tests and inverse modeling, section-averaged hydraulic conductivity for these deposits is

estimated at 5-15 m/day (Skalskyy and Kubko 2000).

3.4.2. Sorption distribution coefficients for strontium-90

The most mobile radioactive contaminant in subsurface environment of the Chernobyl

exclusion zone is strontium-90 (Bugai et al. 1996; Dzhepo and Skalskyy 2002). Therefore we

will consider strontium-90 (90

Sr) as an indicator radionuclide, when analyzing sorption barrier

properties of deposits. The main process governing the 90

Sr sorption on deposits is ion-

exchange, and the commonly used parameter for describing radiostrontium sorption on

deposits is distribution coefficient (Kd). As a consequence of the ion-exchange sorption

mechanism, the 90

Sr Kd is highly sensitive to concentration of competing cations (in

particular Ca) in groundwater solution (Patterson and Spoel 1981; Lefevre et al. 1996).

Laboratory batch experiment studies, in-situ field “groundwater – sediment” partition

tests and analyses of retardation of 90

Sr plume emerging from the trench with wastes (Bugai et

al. 2000, 2002), produce coherent estimates of 90

Sr distribution coefficients for aeolian sands

from the Pilot Site and Prypiat Zaton of 2 to 3 ml/g (these estimates correspond to Ca

concentration in test solution of about 25 ml/g).

The 90

Sr sorption on sediments of alluvial overbank and abandoned channel facies

from the Clearing no.2 of Pripyat Zaton was studied in laboratory batch experiments (Bugai et

al. 2000). While the majority of samples from the strata of silty sediments had rather high Kd

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values of 20-50 ml/g, one sample from the inter-bedded sandy lens had a much lower Kd of 6

ml/g. The Ca concentration in the above tests was 25 ml/g. Series of “groundwater –

sediment” partition tests using alluvial sediments of same facies from Pilot Site have resulted

in generally lower Kd values in the range of 3 to 10 ml/g, with a mean value of 5-7 ml/g

(unpublished data of Sergey Zvaritch, Ukrainian Institute of Agricultural Radiology; the

chemistry of test solution in the last series of tests has not been characterized).

Data on direct determinations of 90

Sr sorption parameters for sediments of alluvial

channel facies are not available. However, 90

Sr Kd values (ml/g) for these sediments can be

inferred from data on cation-exchange capacity (Patterson and Spoel 1981):

1000][

][

Ca

CECKK

Sr

Cad =

where KSr

Ca is selectivity coefficient for Ca ↔ Sr exchange on sediment matrix (close to 1),

[Ca] is calcium concentration in solution (meq/l), [CEC] is cation exchange capacity of

sediment matrix (meq/g), and the numerical factor (1000) converts liters to milliliters. The

CEC of alluvial sediments of channel facies usually fall into the range of 0.5 –4 meq/100 g

with a mean value of about 1 meq/100 g (see Fig.6; Fig.2.3 in Dzhepo and Skalskyy 2002).

Therefore, we could expect that the 90

Sr Kd value for these sediments is relatively low, and is

similar to the Kd value of aeolian sands (i.e., 1-3 ml/g). The last estimate is in agreement with

data on 90

Sr retardation at the ChNPP cooling pond, where the aquifer is presumably

composed of alluvial sediments of channel facies, and calibrations of the radionuclide

transport model resulted in 90

Sr Kd values of 1-2 ml/g (Bugai et al. 1997). The summary

information on lithologic, hydraulic and sorption properties of the main genetic types of

deposits at the Chernobyl NPP site is summarized in Table 2.

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3.5. Role of deposits of different genesis as barriers to radionuclide migration

Considering data of section 3.3 and 3.4, the following ranking scheme can be

proposed for main genetic types and facies of near-surface deposits at the Chernobyl NPP site

with respect to radionuclide migration retardation potential:

Aeolian < alluvial channel < alluvial over-bank < alluvial abandoned channel.

Field observations of radionuclide migration at Chernobyl site agree with the proposed

sediment ranking. For example, at Pilot Site a significant horizontal spreading in the aquifer

of the 90

Sr plume (emerging from the trench with waste) is observed in the aeolian sand layer,

while the further radionuclide migration in the alluvial sediments of over-bank / abandoned

channel facies is effectively retarded (Bugai et al. 2002). At the contaminated cooling pond of

the ChNPP alluvial sediments of channel facies, which compose local geo-environment, were

able to provide only minor retardation (retardation factor R≈5), and significant releases of 90

Sr

has occurred from the pond to the adjacent Prypiat River (Bugai et al. 1997).

Aeolian sediments appear to possess essentially similar lithological properties at a

scale of kilometers (e.g., features of aeolian sediments in the Pilot Site and Prypiat Zaton are

almost identical). On the contrary, alluvial sediment facies are characterized by much higher

spatial variability in lithological properties. For example, the stratum of alluvial sediments of

overbank and abandoned channel facies in the Pilot Site is essentially different compared with

similar sediment member in the Prypiat Zaton. Therefore care should be taken when

extrapolating specific hydraulic and transport parameters for these sediments from one

location to another location (though general features of sediments are similar on a qualitative

level). Sediment features of different facies discussed above agree with the conceptual

understanding of the sedimentation of the study area (see section 3.2).

To estimate the protective features of the geo-environment, the sedimentary cover at

the ChNPP site could be schematised into four main types of geological sections (Fig. 8)

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which characterise two main zones of the first terrace of the Prypiat River: central and

peripheral (see Fig. 1). The section of the type “A” is typical for the central part of the ChNPP

site, including “Sarcophagus”, Industrial Zone, engineered waste repositories “Podlesny” and

“Kompleksny”, and “Red Forest” site. In many locations of the central part of the site the “B”

section (former channels overlain by aeolian sands) replaces the “A”, while the section “D”

occurs there locally. Sections of type “C” occupy the peripheral part of the first terrace, while

sections of type “D” are associated here with dunes.

The type of a geology section together with data on location of radioactive

contamination sources, on position of groundwater level, and on direction of groundwater

flow determine the principal hydrogeology conditions of radionuclide migration. Typical

geology sections at Fig.8 have different potentials for retardation of radionuclide migration

due to different hydraulic properties and adsorption capacities of composing sediment layers.

As discussed above, alluvial deposit of overbank and abandoned channel facies, deposits with

a higher content of organic matter and peat, as well as peaty layers within alluvial channel

facies could be considered as a medium, whereas aeolian sandy sediments and channel

alluvial deposits – as a low retardants of radionuclide migration. Accordingly, section types

“A” and “B” generally have higher retardation capacity compared with types “C” and “D”.

When considering protective features of alluvial sediments of overbank / abandoned

channel facies underlying aeolian sands (section “A”, Fig.8) consideration should be given to

possible presence of a relict cryogenic vein network in the upper strata of these deposits. Such

vertical veins with possible penetration depth of 1.5 – 3 m in-filled with a higher permeability

and lower adsorption capacity aeolian sand could provide a facilitated migration pathway to

deeper layers of geo-environment.

Schematisations based on sedimentological analyses of a type presented in Fig.8 could

provide useful information for prioritisation of historic contaminated sites for remediation

17

purposes, or for the site screening/selection process for new waste facilities. Other possible

applications, where described above information could be of essential value include:

development of preliminary conceptual groundwater flow and radionuclide migration models,

design of groundwater monitoring systems, and/or planning of site characterization programs

aimed at determination of hydraulic and geo-chemical properties of geo- environment.

4. Conclusions

The analysis of geological structure and lithological studies show that there are two

main genetic types of deposits composing the upper part of sedimentary cover at the

Chernobyl NPP site: aeolian and alluvial, including several dynamic facies (aeolian sheet and

dune, alluvial over-bank, abandoned-channel and channel). These depositional facies differ

essentially by lithological properties and respectively by hydraulic and sorption parameters.

Consideration of lithological properties leads to following ranking of the main genetic

sediments facies with respect to radionuclide migration retardation potential:

aeolian < alluvial channel < alluvial overbank < alluvial abandoned channel. The protective

features of the layer of sediments of alluvial overbank / abandoned channel facies underlying

aeolian sands could be reduced due to presence of relict cryogenic vein network providing

facilitated migration pathway to the deeper strata. The described above sedimentological

interpretations allow predicting at qualitative level of lithological and structural properties of

specific geology sediment layers and/or typical geologic sections at different scales, and could

provide useful information for understanding and assessment of the barrier role of deposits of

various genesis for the radioactive contaminant migration in the geo-environment.

18

Acknowledgements

The presented studies were supported by funding through the Chernobyl Pilot Site

Project, a collaborative French-Ukrainian program of the Institute for Radioprotection and

Safety (IRSN, France), as well as by research contracts no.4/6-95 and no.13/132-155H-98

from the Ministry of Emergency Situations and Chernobyl Affairs (Ukraine). We gratefully

acknowledge laboratory particle-size distribution analyses by Nikolai Partsevskyi, laboratory

sorption studies by Sergey Zvarich, and field hydrogeology works by Svyatoslav Kurilo and

Danyl Olegov. Comments and English editing work on this manuscript by an anonymous

reviewer is much appreciated.

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21

Table 1. Main granulometric fractions and classes of deposits (according to classification

adopted in the former USSR and Ukraine).

Definition of granulometric fractions by size

Coarse material > 1.0 mm sand coarse 0.5-1.0 mm

Sand 0.1-1.0 mm medium 0.25-0.5 mm

Fines silt (aleurite) 0.01-0.1 mm fine 0.1-0.25 mm

clay <0.01 mm silt coarse 0.05-0.1 mm

fine 0.01-0.05 mm

Definition of granulometric classes of deposits by percentage of particle-size fractions

Sand Sand fraction is more than 50%

Silty sand Sand fraction is more than 50%, the silt fraction is

predominant among others

Silt Silt fraction is more than 50%

Sandy silt Silt fraction is more than 50%, the sand fraction is

predominant among others

Clayey silt Total content of clay and silt fractions is more than 50%, but

the content of silt is higher

Table 2. Representative values of lithological, hydraulic and sorption parameters for different genetic types and dynamic facies of deposits (Pilot

Site; Prypiat Zaton, Clearings 1 and 2).

Deposits, type, facies

(samples number)

Bulk density,

kg/dm3

Porosity,

%

Clay fraction (<0.01

mm) content, %

Hydraulic

conductivity (K),

m/day

CEC,

meq/100 g

90Sr Kd ,

ml/g

Aeoilan 1.72 34-36 1-2 3-5

(isotropic)

0.5-1.2 2-3

Alluvial, channel facies

1.67 37 2-8 5-15

(isotropic)

0.5-4 1-6

Alluvial ,overbank and

abandoned channel

facies

1.73

35

10-20 ≈ 1 (Kx)

≈ 0.01 (Kz)

(anisotropic)

5-10 10-50

FIGURE CAPTIONS

Figure1. Map of the study site with elements of geologic and geomorphologic structure,

hydrogeology conditions and radioactive waste sites.

Legend: 1 - geologic and geomorphologic boundaries, 2 –geologic and geomorphologic

regions and main landforms (numbered), 3 – boreholes (numbered) and geologic cross-section

line, 4 – clearing of an exposure and its number, 5 – sand pit, 6 – area covered by radioactive

waste dumps, 7 - near-surface engineered radioactive waste repositories,

8 - regional directions of groundwater flow, 9 – main buildings of the ChNPP, 10 – boundary

between two main zones of the different types of geology section (for the upper part of

sedimentary cover within the first terrace above flood plain), 11 – types of geology section.

Numbers in circles: 1 – Quaternary Plateau, 2 – first terrace above the flood plain, 3 – flood

plain, 4 – slope-wash apron, 5 – dunes.

Letters in rectangles: types of geologic sections - see Fig. 8 and explanation in section 3.5.

The letter in brackets means that given type of section occurs locally within the zone.

Figure 2. Regional geologic cross-section of the investigated territory.

Legend: 1 – sands, 2- silts, 3 – basal till, 4 – clay, 5 – marl, 6 – inter-bedding of sands and

silts, 7 – peat and peaty sand, 8 – boreholes (numbered), 9 – inferred fault, 10 – boundaries

between suites: supposed (upper) and established (lower), 11 - boundaries between

depositional facies, 12 – facial replacement, 13 – groundwater level (generalized).

Indices: Q4 - Holocene, Q3-4 - Upper Pleistocene - Holocene unstratified, Q3 - Upper

Pleistocene, Q1-2 - Lower Pleistocene - Middle Pleistocene unstratified, N2 - Pliocene, P2 –

Eocene.

24

Genetic types of deposits: a - alluvial, mw – melt-water, eol - aeolian, e - presumably waste

mantle, sw – slopewash.

Facies: ob – overbank, ch – channel, a-ch – abandoned channel. Position of the profile is

shown in Fig. 1. (Legend also applies to the Fig. 3)

Figure 3. Local geologic profile within the ChNPP Units 1-4 main buildings (before the plant

construction), including Sarcophagus site. (Legend is explained in Fig. 2. Position of the

ChNPP is shown at Fig. 1).

Figure 4. Rough inter-bedded sedimentary structures of the uppermost aeolian and alluvial

suits according to drawings of the ChNPP main buildings foundation pit in two dimensions.

(Unpublished data of “Atomenergoproect” institute, 1968).

Legend: 1 – sands, 2- silts, 3 - peaty sand. Indices: Q3-4 - Upper Pleistocene – Holocene

unstratified; Q3 - Upper Pleistocene;

Genetic type of deposits: a - alluvial, eol - aeolian; facies: sh - sheet, ob - a-ch - overbank –

abandoned-channel, ch - channel.

Figure 5. Composite cryogenic wedge in overbank alluvial deposits overlain by aeolian cover

sand at the Clearing 2 of the Prypiat Zaton.Two parts of the clearing (upper and lower) are

divided by a 1.5 m wide ledge. Delineation of sedimentary structures was made from a

photograph.

Legend and explanations: 1 - modern sod-podzolic soil (sedimentary structures are invisible),

2 - aeolian sand deposits, 3 - horizon of aeolian deposits transformed by cryogenic processes

(massive structure and local deformations), 4 – alluvial overbank silt deposits (sub-horizontal

and wavy lamination with small load casts); a – sheet facies (sub-horizontal lamination), b –

25

dune facies (inclined envelope and sub-horizontal lamination ), c - polygon rims, d - aeolian

deposits infill in the wedge (sand wedge), e - horizon of alluvial deposits transformed by

cryogenic processes - massive structure and gentle deformations (supposedly associated with

active layer), f - ferrite horizon, g - ice wedge (vein) with sub-vertical ‘lamination’, h – sand

lens.

Figure 6. Profile diagrams of lithological properties of sediment for the Clearing no.1 of

Prypiat Zaton.

Figure 7. Section-averaged particle-size distribution curves for different sediment facies:

(a) aeolian; (b) alluvial: ob –over-bank; ab – abandoned-channel; ch – channel.

Figure 8. Main types of geology sections at the ChNPP site (for the upper part of sedimentary

cover).

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

0% 20% 40% 60% 80% 100%

0.25

0.75

1.25

1.75

2.25

2.75

3.25

3.75

4.25

4.75

5.25

5.75

6.25

6.75

7.25

7.75

8.25

8.75

>0.5 mm 0.5-0.250.25-0.1 0.1-0.050.05-0.01 0.01-0.001< 0.001 mm

m 051015

CEC, meq/100 g

30354045

Porosity, %Grain-size distribution, %

Figure 6

0

10

20

30

40

50

60

70

%

0

10

20

30

40

50

60

%

(a)

0>0.5

mm

0.5-

0.25

0.25-

0.1

0.1-

0.05

0.05-

0.01

0.01-

0.001

< 0.001

mm

eol (Pr Zaton Clear 1) eol (Pr Zaton Clear 2)

eol (Pilot Site)

(b)

>0.5

mm

0.5-

0.25

0.25-

0.1

0.1-

0.05

0.05-

0.01

0.01-

0.001

< 0.001

mm

ob (Pr Zaton Clear 1) ob (Pr Zaton Clear 2)

ab (Pr Zaton Clear 1) ch (Pr Zaton Clear 1)

Figure 7

Figure 8