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4. Experimental site description

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Polissky district, which is proposed as an experimental site, situated in the Western periphery of the Chornobyl zone.

Climatic conditions. Climate of Polesie is temperate semi-continental under influence of cyclonic activity. It is characterised by moderately cold winter and moderately warm summer. Average temperature of January, which is a coldest month, -5-6°C. Average temperature of February is higher, but this is a month of the temperature extremes (-22-25°C, even -33-35°C). July is characterised with both maximal average temperature (17-19°C), and highest temperature extremes (35-36°C).

The season of vegetation begins in the mid April and continues to the late October. Its duration is about 220 days. Sum of the active temperatures (above 10°C) is about 2660°.

Average annual precipitation is about 580 mm, with variations between 400-850 mm. Up to 70% of the annual precipitation is related to the warm period of year.

Landscapes and ecosystems. The low and plain relief (110-145 m above sea level) of the fluvioglacial sandy plain was formed at the North-Eastern slope of the Ukrainian Crystalline Shield within the area of the Dnieper stage of an ancient quaternary glaciation, which corresponds to the Zaal stage of the Western Europe.

Among the most typical landscapes of the region there are: river flood plains and terraces, extreme-moraine ridges, moraine-fluvioglacial and limnoglacial plains. This part of Ukraine is notable for the considerable swampiness. The bogs account for 22,5% of the district.

Its northern part presents landscape of low, ripple moraine-fluvioglacial plain, which lies 15-35 m lower, then extreme-moraine ridge. This level of the relief is composed by fluvioglacial sand, bedded by morainic loam at depth 0,8-1,0 m. The concentration of the dust particles less 0,05 mm is very low (4-8%). This explains poorness of soddy-podzolic sandy soil formed here (pH 4,2-4,5, humus 0,8-1,5%). The landscape of low moraine-fluvioglacial plain is covered by pine forests.

A number of isolated depressions of 0,3-0,8 km diameter scattered on the surface of the landscape. Swamps with alder forests or sedge-reedous bog coenoses occupy most of them.

Landscapes of the river terraces are very typical for Chernobyl zone. Flat surfaces of terraces are composed of alluvial sands with deficiency of the clay and dust particles (4-6%). Thus sod-podzolic sandy soil formed here is dry and poor (0,4-0,6% of humus). The terraces are cowered mainly by different-age pine forests of artificial origin. Back, lowered parts of the river terraces, which are situated close to terrace joint, are occupied by eutrophic bogs with peat bog soil (thickness of the peat is about 0,5-2,5 m) and forests of Alnus glutinosa (L.) Gaertn. or by sedge-reedous bog coenoses.

Mostly typical forest ecosystems of the region are Pinetum cladinosum and

Pinetum phodococco-dicranosum, with the soil cover of Koeleria glauca (Sorend.) DC, Festuca sulcata (Hack.) Num.p.p., Antennaria dioica (L.) Gaertn., Phodococcum vitis-idaea (L.) Avror., Dicranum scoparium Hedv., Cladonia

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silvatica (L.) Hoffm. and C. rangiferina (L.) Webb. Under influence of the regular forest farming, the wood stands of the territory belong mainly to the young and average age groups.

The landscapes of flood plains of Uzh river and its tributaries represent the lowest level of relief of the test site. Flat and segment-crest surfaces of the frontal and middle parts of the flood plains with a number of the meander lakes and bypasses are composed by layered alluvial sands and loams and covered by the shrubs of Salix acutifolia Willd. with grassyxerophytous soil cover, forb-grassy and sedgehygrophytous meadows on alluvial soils. Flat and segment-crest surface of the frontal and middle parts of the flood plains with a number of the meander lakes and bypasses are composed by layered alluvial sands and loams and covered by the xerophytous shrubs, forb-grassy and sedge-hygrophytous meadows on the alluvial soils.

Polissia is notable for the considerable swampiness of the river flood plains and terraces. In the accident zone bogs account for 22,5% of the territory. Mostly typical swamps occupy rear lowered flat parts of the river terraces and flood plains, composed by a eutrophic peat (thickness 0,5-2,5 m), with a peat bog soil (pH 5,0-5,5, organic matter up to 75%), covered by alder forests and sedge-reedous bog coenoses. Two thirds of the bogs and wetlands were drained and ameliorated during last decades before the accident, to increase acreage of the arable areas. After the accident the drainage systems are degrade, and natural humidity of the drained wetlands is restore.

5. Conclusion

The landscape based GIS is an effective tool for the structuration and classification of the ecosystems, for the field experimental data collection, analysis and extrapolation, as well as the ecotoxicological parameterisation of the ecosystems and solution the regional and local ecotoxicological problems.

6. References

1.Kutlakhmedov Y., Polikarpov G., Kutlakhmedova-Vyshnyakova V.Yu. Radiocapacity of Different Types of Natural Ecosystems and their Ecological Standardization Principles // J. Radioecol. – 1997. – V.6 (2). – p. 15-21.

MAPPING TRANSFER PARAMETERS OF RADIONUCLIDES IN TERRESTRIAL ENVIRONMENTS

S. DENYS, G. ECHEVARRIA, J.L. MOREL

Laboratoire Sols et Environnement, UMR 1120 ENSAIA-INPL/INRA, 2 av. de la Forêt de Haye, BP 172, F- 54 505 VANDOEUVRE les NANCY, FRANCE

E. LECLERC-CESSAC

Agence nationale pour la gestion des déchets radioactifs (Andra), 1-7 rue Jean MONNET, - 92 298 CHATENAY MALABRY cedex, FRANCE

Abstract

Safety assessment models for potential sites selected for underground repositories of high level and long-lived radioactive waste requires the prediction of phytoavailability of such radionuclides at the regional scale. In this context, the areas in which the radionuclides may be highly mobile or accumulate have to be well known, as they will contribute to a maximal dose for Man. The parameters controlling the phytoavailability of the radionuclides are mostly defined by experiments based on sieved-soil sample and a methodology is needed to extrapolate these parameters to the regional scale by taking into account the variability of the soil properties within the landscape. A mapping of three radionuclides phytoavailability (63Ni, 99Tc and 238U) was conducted here using the MapInfo mapping software. The distribution frequency of the phytoavailability parameters was represented over the 186 km2 area of the French laboratory for the study of deep underground nuclear waste disposal, in Bure. Isotopically exchangeable pool of Ni, Et and pH of the soil were the two phytoavailability parameters chosen respectively for 63Ni and for 238U and were both measured on sieved samples coming from the soil units defined at 1 : 50 000. The redox potential, Eh was the parameter used for 99Tc and was measured in the field. For each radionuclide, class of soils were built according to their properties in term of radionuclides phytoavailability and maps of phytoavailability were drawn at 1: 25 000.

Results allowed a prediction of the phytoavailability of the 63Ni, 99Tc and 238U at a regional scale, based on the superimposition of the laboratory measurement of the parameters significantly controlling the mobility and the phytoavailability of the three radionuclides and knowledge of soils over the area. Critical areas were also determined where either accumulation in soils may be highest or accumulation in plants may be highest. They also allowed defining the most likely transfer parameters for the three radionuclides in the area of Bure.

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F. Brechignac and G. Desmet (eds.), Equidosimetry, 119–129.

© 2005 Springer. Printed in the Netherlands.

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1. Introduction

The transfer of radionuclides in the biosphere is a major issue in the calculation of the final dose to man in safety assessments of underground nuclear waste disposals. These safety assessments rely on a method which often selects critical groups, which are the populations located in places where the dose is maximal. This critical group is defined by a specific scenario (e.g. self-sustainable populations) and is situated in those areas in which radionuclides may accumulate or be highly mobile. The soil is the medium that supports terrestrial ecosystems and exerts a significant control on both the transfer of radionuclides to the food chain and the radiation exposure to living organisms. Soil properties may vary considerably across the landscape according to topography, geology, climate and history of land use. Therefore, the variability of soil characteristics at the local scale may subsequently introduce a large variability in transfer of radionuclides in the food chain and the ecosystems. Mobility and transfer of radionuclides in the soil-plant system are specific to soil type and land use (crop, farming practices,…). The distribution of radionuclides transfer parameters in soils is required to determine both the extreme values and their frequency in specific sites. The Bure site (French laboratory for the study of deep underground nuclear waste disposal) displays a wide variety of soil types and land uses. The soil and landscape approach is therefore essential to select best estimates of transfer parameters values of radionuclides.

Among the mechanisms governing plant uptake of radionuclides, the ability of the soils solid phase to supply the soil solution is one of the main processes involved. This ability can be estimated through the Kd, i.e. the soil: solution distribution coefficient of the radionuclide in the soil. Phytoavailability of radionuclides is primarily based on this property and is commonly estimated at the sieved-soil sample level by measuring the uptake of the radionuclide by plants after a homogeneous contamination of the soil sample. Transfer Factor, TF (i.e. soil-to-plant concentration ratio of the radionuclide) or Effective Uptake, EU (i.e. fraction of the radionuclide transferred to the plant) are two parameters used to assess the soil-to-plant transfer of radionuclides and thus their phytoavailability (1, 2, 3, 4). However, those measurements do not take into account the high heterogeneity of soil profiles and their actual functioning including hydrological properties and vertical migration processes. Therefore, a methodology is needed to extrapolate the prediction of the phytoavailability of the radionuclide obtained at the soil sample level to a regional scale, in agreement with the site specificity of soil physical and chemical properties.

The objective of this work was to assess the spatial distribution of transfer properties of radionuclides in soils at the Bure site following a three-step approach. The first step was to measure the mobility of the three radionuclides in soil samples taken from the site and from other situations and their transfer to plants in pot experiments. This allowed for the identification of soil characteristics that controlled these parameters. The second step was to verify that the mobility and the leaching of radionuclides did not strongly influence their transfer to plants in soil cores. Finally, the third step was to extend transfer values to the regional scale according to the spatial distribution of soils and their properties in the Bure site

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after having measured the parameters that control radionuclides mobility and transfer in each of the soil units present. Three radionuclides were chosen to illustrate this approach: nickel-63, uranium-238 and technetium-99. They were chosen as model radionuclides in terms of mobility, presence in natural environments, and importance for the biosphere safety assessment of nuclear fuel disposal.

2. Materials and Methods

2.1 SITE SELECTED FOR THE STUDY

The site for this study is a 186 km2 area next to the underground Andra research laboratory around the Bure municipality in Meuse, and bordering the Meuse and Haute Marne departments (France). It is located on the eastern side of the Sedimentary Basin of Paris and is dominated by a Jurassic limestone plateau and carbonated marl outcrops in the slopes. This area is also affected by iron-rich sandy deposits of the early Cretaceous period.

The pedological map of the area was previously established at the 1: 50 000 scale. The pedological survey allowed the identification of 9 typological soil units (TSU) over the entire area (table 1). The most widespread units (FAO classification) on the plateau were Rendzic Leptosols; alcaric Cambisols and Calcisols. In lowlands, hydromorphic soils were found (Fluvic Gleyic Cambisols). Dystric Cambisol and Luvisol were located on sandy and loamy deposits and were mostly located in forested areas. These soils need to be considered as pH is a significant factor influencing Ni mobility in soils through dissolution of iron and manganese hydroxides (5, 6). pH also influences strongly U mobility (7). Variation of redox potential between soils will affect also 99Tc mobility.

2.2. SOIL SAMPLING

For each soil unit from the 1:50 000 survey, the Ap horizon of a typical soil profile was sampled to measure the phytoavailability of the three radionuclides on a 5 mm sieved soil sample. The variability of soil properties was taken into account by study at a finest scale of a plot of the plateau area (“Glandenoie plot”).

2.3. 63Ni PHYTOAVAILABILITY MAPPING

Isotopic exchange is the main mechanisms that will influence the behaviour of 63Ni in the environment (1, 8). Stable Ni is present worldwide at varying concentrations. However its bioavailability depends on total concentration and on soil physico-chemical conditions. There is a negative significant relationship between E.U. of Ni by plants and the labile pool of stable Ni measure by isotopic exchange kinetics (IEK). Measuring the labile pool of Ni will provide information on the probability of transfer of 63Ni to plants and will therefore permit the classification of soils relative to this parameter (1). Moreover, experiments in undisturbed cores on the three major soil units of the Bure area showed that this approach was also verified when the water regime of the soils

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(evapotranspiration and drainage) on which plants were grown was close to real conditions (9). This confirmed that the approach at the level of the soil sample was fully appropriate to assess the potential of 63Ni availability in soils.

The phytoavailability of stable Ni in the soil samples was estimated by E90d after performing isotopic exchange kinetics (IEK) on five replicates of each sample. The method was carefully described in detail in previous works (1, 8). The E90d pool represents the reservoir in which 63Ni is isotopically diluted during an average period corresponding roughly to the time of a growing season (8).

Prior to mapping, classes of 63Ni phytoavailability were defined from E90d measurement by ANOVA test ( =5%) using the STATBOX Pro software. Definition of classes was based on the homogeneous group of the Neuwman-Keuls test. In addition, a subjective classification was possibly adopted to isolate the E90d values which were found in more than one group. According to the dilution of 63Ni in the isotopically exchangeable pool of Ni, the higher the E90d the lower the phytoavailability of 63Ni. The Map-Info 4.0 mapping software was used to give a polygonal representation of each class of 63Ni phytoavailability superimposed on the pedological maps of either the area either the plots. Each topographic point of the maps was identified by its Lambert II coordinates and a layer was associated to each class of phytoavailability. Each layer corresponded to a polygon, outlining the zones within which the E90d values ranged from the lower to the higher value of the associated class.

2.4. 99Tc PHYTOAVAILABILITY MAPPING

Literature regarding 99Tc mobility and phytoavailability in the environment points to the determining role of redox potential in soils on the fate of 99Tc in soils (2, 4, 10, 11, 12). Reduction of 99TcO4- seems irreversible (12) and therefore its mobility over the long term might be significantly affected even by occasional reducing conditions in soils. If 99TcO4- is to be expected in soils then Kd values will be comprised between 0 and 0.25 and TF will depend on the dilution level of the radionuclide in soil pore-water and the transpiration capacity of crops. Experiments of 99Tc addition on undisturbed soil cores sampled on the three main soil units of the Bure area, showed that in the case of contamination of soils through irrigation water (i.e. during the growing season), evapotranspiration was high enough to retain the water and practically inhibit downward transfers of mobile 99TcO4- in the soil profile (13). Under such conditions, TF was only influenced by the dilution of the radionuclide in the pool of easily available water in the soil. When comparing TFs to crops (wheat, maize) soils were ranked according to their water holding capacity. Reduction of 99TcO4- in soil reducing sites might have a much higher influence on 99Tc transfer parameters than leaching in such a scenario. Transfer factors in soil cores are in general 10 times lower than in pot experiments as a consequence of this dilution effect (13).

Therefore, the map of mobility and transfer of 99Tc in the area is based on the redox potential map. A portable Platinum electrode (Schott Gerätte PT 737 A) coupled with a millivoltmeter (WTW pH 330/SET) was used to measure in situ Eh values in the month of May 2001. For each of the 9 soil units, three different measurements were realised to determine Eh. Again, mapping of 99Tc potential