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The main sources of cesium-137 penetration into Kaniv water storage reservoir are suspensions, phytoplankton and waters of Kiev water storage reservoir.

In Kremenchuk water reservoir, caesium-137 accumulation in bottom sediments reached, about 400 in 2000. In deeper areas of Dniprodzerzhinsk and Kahovka water reservoirs radioactive nuclides accumulation also takes place. However, the radiogeochemical regime is currently in its initial phase and caesium137 reserves are increasing.

Ukraine’s geological surroundings became not only an accumulating capacity for radioactive nuclides originating from the Chernobyl NPP released but also a long-term source of their penetration into crops and vegetables, milk and meat products, surface and inadequately protected groundwater sources.

An anomalous radioactive-ecogeochemical province has been formed on Ukraine's territory [1, 2].

Table 1 is showing the high concentration of caesium-137 in the different types of soils within Ukrainian territory as in moved and as in the fixed forms (total storage).

Table 1. Caesium-137 concentrations in different soil types in Ukraine.

Besides the big quantity of the moving and fixed forms are witnessing about long time potential of the radio geochemical contamination within agricultural and natural landscapes.

4. Geological environment as the main source for complex influence of low radioactive dose and soil contamination with heavy metals and chemical products

After Chernobyl accident and regional radionuclides contamination of the landscapes within its territory Ukraine could be considered as the artificial radionuclide province. But in the same time we can observe the origination of the technogenic landscape geochemical province within IUAs, mine regions, water irrigation systems etc. Only mountain structures of the Carpathian (Western region) and Southern Crimea can be considered as natural geochemical landscapes (natural geochemical backgrounds).Of course, we can find natural geochemical landscapes within Ukrainian territory but these fragments are not able to stabilize the biogeochemical, hydrochemical and even hydrogeochemical parameters.

The main conclusion in this situation is the following - Ukraine shows the

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Table 2. Geological environment (GE) contamination and population illness.

This principally new multi-factors ecological situation is creating lifeimportant and new tasks for the geological and radiobiological sciences:

1)special zoning of the Ukrainian territory and agricultural landscapes in accordance with the radiobiogeochemical risks for the human being and biosphere;

2)reconstruction of the State Natural Environment Monitoring System in accordance with the radiobiogeochemical tasks;

3)investigation and substantiation of the long-term permissible level of the low radioactive doses for the different groups of the population in the conditions of regional landscape geochemical contaminations.

References

1.Chernobyl Accident: Reasons and Consequences. The Expert Conclusions, Acad. Nesterenko V., Dr. Sc. Yakovlev Ye. etc. Minsk “Pravo i Economica”, 1997

2.Chornobyl Catastrophe. Chief Editor Acad. Baryachtar V.G. Kyiv, Editorial House of Annual Issue “Export of Ukraine”, 1997

BEHAVIOUR OF THE RADIONUCLIDES IN PEAT SOILS

G. BROVKA, I. DEDULYA, E. ROVDAN

Institute for Problems of Natural Resources Use & Ecology, National Academy of Sciences of Belarus, 10, Staroborisovsky trakt , 220114 Minsk, BELARUS

1.Introduction

Peatlands occupy 2.9 millions hectares or about 12 % of the territory of the Republic of Belarus. As a result of the Chernobyl Nuclear Power Plant accident most of their areas were subjected to radioactive contamination. From a radiation danger viewpoint long-existing Cs137 and Sr90 are. at the lead. Prior to the Chernobyl catastrophe the peat soils were intensively used in agriculture and great quantity of fertilisers and pesticides had been introduced . Their combined impact with radionuclides complicates considerably the ecological situation. Ameliorated peat soils are subjected to water and wind erosion promoting the radionuclide transfer.

To minimise the negative influence of the radioactive contamination upon the environment forecasting assessments of radionuclide transfer under the impact of natural and anthropogenic factors are needed, as well as basic means of directed impact upon their transportation processes.

2.Mathematical models

The analysis of literature and our own researches showed that the radionuclides transfer processes in soils in the region of positive temperatures are most completely described by means of the mass exchange equations, regarding the kinetics of their sorption in the system solid phase vs. solution [1, 2]:

where C1 and C2 –radionuclide concentration in porous solution and solid phase of the soil respectively; Do - the coefficient of the radionuclide molecular diffusion; V– linear velocity of the moisture connective transfer; Kc– liquid phase communication coefficient; Kd - distribution coefficient; W– material moisture content .

In the development of the theory of mass transfer in natural dispersed systems at negative temperatures on the basis of thermodynamics of irreversible processes and hydrodynamics of thin layers of a liquid with regard to the appearing in these layers of the discharge pressure [3, 4] we have obtained a

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F. Brechignac and G. Desmet (eds.), Equidosimetry, 139–146.

© 2005 Springer. Printed in the Netherlands.

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differential equation, describing the radionuclides migration in frozen soils:

where Ci -concentration of i water soluble compounds is accounting for the mass unit of the rock skeleton; R- specific water gas constant; Mi and Mw - molecular masses of water soluble compound and water respectively; Qi - quantity of ions, on which the molecule of water soluble compound dissociates; Ui , Usk – the densities of ice and the soil skeleton respectively; Wu - amount of unfrozen water in the soil; Kf- – filtration coefficient of the frozen soil; Dio – diffusion coefficient of the water soluble compound in porous solution; P–hydrostatic pressure.

According to (3), the variation of the concentration of radionuclides in frozen soils is defined by the radionuclide diffusion processes at the molecular level, by processes of convective transfer along the unfreezing layers under the impact of hydrostatic pressure, the temperature and general concentration of water soluble compounds and divergence of moisture flow due to the non-linearity of the distribution of the moving forces of transfer and phase transfers of water into ice.

In a homogeneous temperature field the velocity of diffusional transfer of radionuclides is determined by the effective diffusion coefficient:

where W corresponds to the natural dispersed systems moisture content in the region of positive temperatures or to the non-frozen amount of water in that of negative ones.

3. Methods

The models suggested, describing the radionuclides migration (1-3) carrying their phenomenological character, and containing transfer coefficients such as effective diffusion Def, distribution Kd, diffusions in porous solution Do, mass exchange D, communication of water ways Kc , filtration of frozen soils Kf should be determined by experiments [ 5].

The distribution coefficient Kd characterises the radionuclides distribution between the liquid and solid phases of soils and is numerically equal to the ratio of

141 the balanced radionuclide concentrations in solid and liquid phases (5). To

where Cwmequalsthe specific activity, accounting the unit of the humid material of the solid residue, Cs- porous solution specific activity of the centrifuged porous solution, W – the moisture content of the solid residue after centrifugation.

The experiments conducted showed that radionuclides distribution coefficients in soils in the sphere of positive temperatures actually do not depend on moisture content and temperature. This makes it possible to consider Kd in soils in negative and positive temperatures spheres, having close values.

The coefficient Dps characterises the radionuclide molecular diffusion in porous solution in soils in thawing state through the unfreezing water layers in negative temperatures spheres:

To determine through tests the effective diffusion coefficients Def and the diffusion coefficients in porous solution Dps, the integral method has been applied based on the solution of diffusion equation for the two plate-shaped samples, contacting each other at different initial transfer potentials [6] and corresponding to a previously proposed method [7]. For the practical realisation of the integral method of the identification of diffusion coefficients, cassettes have been used made from hydrophobic material, formed as a flat disc of 90 mm diameter and 10 mm height. When studying Def and Dps in positive temperatures sphere the first cassette was filled with the material containing the salt of the ion studied at a well defined concentration, and the other one with the material of the same moisture content without salt.

While defining experimentally Def and Dps in the negative temperatures sphere it is mandatory to account for the specific nature of the mass exchange in the frozen soils namely that the convective ingredient of the radionuclide transfer can to a great deal exceed the diffusional one. In order to exclude the convective transfer of radionuclides and to enhance the exactness of Dps determination it is necessary to either conduct the test at a very low radionuclide concentration or additionally definite compound should be inserted in each sample. The compounds then, being in a variety of samples, should be differentiated by chemical or physical ways, and should have close values of the coefficients of molecular diffusion and initial molecular concentrations. The ideal means for that purpose can be compoundsincluding stable and radioactive isotopes.

To obtain the dependence Def upon the total concentration of the salt of the investigated element in a series of tests the salt concentration of the stable isotope of the element given was varying.

The mass exchange coefficient D characterises the velocity of the fixation of the radionuclides balance in the system solid phase vs. porous solution according to (2). Experimentally D is defined through the kinetics of the change of

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the specific activity of the porous solution, squeezed by the centrifuge.

The coefficient of the liquid phase communication Kc is regarded as the parameter accounting for all heterogeneities of the porous medium system – windingness and corrugation of pores, availability of blind and closed pores, etc. Numerically, Kc is equal to the ratio of the coefficient of diffusion of unsorbed material versus water-soluble compound in the volumetric solution (6). Kc is mainly the function of moisture content, density and temperature and does not depend on the specific properties of water-soluble compound and its concentration. Having determined Kc of the system by means of already defined compound, the values obtained can be used for other compounds. Preliminary estimation of Kc can be obtained through a more simple conductometrical method.

Considering the labour-intensity and the duration of the tests on the direct determination of effective diffusion coefficients, the main information on the dependence of Def on the temperature, moisture content and concentration of the radionuclide can be obtained by counting., (4); Experimental data about Do, Kc,dvK coefficients, and some other limited data, being experimentally obtained , should be used to control the results.

4. Results

The Chernobyl radionuclides behaviour was studied by the use of an ameliorated peat-mire soil (peat deposit “Pogonyanskoye”, 21 km off the ChNPP). It is peat of a high decomposition degree (38-38%). Prior to the Chernobyl catastrophe the whole deposit area was drained and used for perennial grass production. The ground water level varies depending on the season from 0 to 1 m.

The characteristics of the migration and sorption of radionuclides Cs137, Sr in fen peat (2,5 kg/kg), quartz sand (0,2 kg/kg), bentonite (1,25 kg/kg), kaolin (kg/kg), have been experimentally investigated.

It was stated that for Sr90 in all systems studied, few values of distribution coefficients are specific and due to this, greater values of the effective diffusion coefficients were found if compared to Cs137. With regard to the sorption capacity of the Cs137 ion, the systems studied can be arranged as follows: fen peat >

in fen peat Def= 0.9 10-13 m2/c, quartz sand Def= 1.8 10-13 m2/c, bentonite D 2.9 10-13 m2/c and kaolin Def= 3.2 10-14 m2/c. At the same time the effective diffusion coefficients of Sr90 are equal in the fen peat Def= 3.2 10-12 m2/c, in quartz sand Def= 1.5 10-11 m2/c, bentonite Def= 1.1 10-11 m2/c, kaolin Def= 7.0 10-12 m2/c.

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The analysis of the results showed that the maximum of the radionuclides activity on the salted site shifted into the deep peat deposits and by October 1997 it was at the depth of about 5 cm, whilst on the checked site as well as in the years proceeding, it was found to be at the depth of 2 cm. Though in the test given no practical results were achieved, it reveals the main possibility of controlling the radionuclides mobility with the help of electrolytes solutions. At present, further endeavours are being tested using more effective leaching materials.

The results of the application of subsurface brine water (Gomel region, Belarus Polesye) to the peat soil (peat deposit "Savichi", Gomel district) on the Sr90 distribution are given at Fig. 2.

Fig. 2. Distribution of Sr90 along peat soil profile under the impact of subsurface brine water.

The subsurface brine water has been added at the soil surface in the quantity of 5 kg m-2 in May 1998. The effect of subsurface brine water on Cs137 migration is small. At the same time the effect of the subsurface brine water on Sr90 distribution is considerable. Thus, on the control plot about 80-90% of the radionuclides are concentrated at a depth of 12 cm; at the same time the subsurface brine water addition leads to a migration of Sr90 in deeper soil layers and in this case only 40-50% is concentrated at 12 cm .

Chernobyl NPP revealed the vertical migration velocity of radionuclides to be

small. On the peat soils, which are not influenced by anthropogenic effects, the depth of migration 137Cs has not exceeded 16 cm, and for 90Sr, 20 cm. The main

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quantity (about 90 % ) 137Cs is in the top 5 cm layer, and for 90Sr in the 9 cm layer. On tillage soils the radionuclides are evenly distributed along the depth of tillage level [ 8, 9].

The mathematical simulation of 137Cs and 90Sr vertical migration in the ameliorated peat soil (peat deposit “Pogonyanskoye”) and comparison with the data of natural observation showed rather a good reproduction by the model used of basic radionuclides transfer processes in a soil. If compared to 137Cs binding with a soil adsorbing complex a rather quick establishment of a sorption balance and a greater migration mobility of 90Sris being stipulated.

The forecast assessment testifies to a slow radionuclide migration in natural conditions: in 20 years in the layer of 0-5 cm about 90% of the summarised reserve in a soil layer of 137Cs and about 53 % of 90Sr will be concentrated.

The moisture increase, the availability of the moisture filtering flow, the

coefficient to 50fold in the layer of 0-5 cm, there will be 5% of 90Sr left, the distribution maximum will shift into the 15-16 cm layer , and beneath 20 cm there will be 15% of 90Sr.

5. Conclusion

On the basis of theoretical and experimental studies a method of assessment and prognosis of the radionuclides migration soils has been created.

Experimental methods for the determination of transfer characteristics and their assessment, and also the methods for prediction of radionuclide migration in frozen grounds should provide a clear distinction between driving forces and fluxes of the convection and diffusion transfer.

The proposed mathematical models are phenomenological and the transfer coefficients involved in these models are recommended to be determined experimentally.

The methods were developed for investigation of diffusion and convection transfer characteristics of radionuclides in thawing and frozen grounds with the allowance for the sorption by solid phase of the soils. On the basis of the experimental data obtained the information constraints for the mathematical models of the radionuclides migration in soils have been set.

On the basis of researches in the field, lab test and mathematical modelling the impact of physical-chemical factors and the soil component composition changes upon the radionuclides Cs137, Sr90 migration and sorption in natural dispersed systems (peat, sand, bentonite, kaolin, sapropel has been investigated.

The impact of electrolytes, including compounds of Cs and Sr stable isotopes upon the radionuclides behaviour in peat soils has been made known,