BY0000248 Self-restoration of contaminated territories Gerassimos ARAPIS Laboratory of Ecology and Environmental Sciences, Agricultural University of Athens, Iera odos 75, 11855 Athens, Greece Emlen SOBOTOVICH, German BONDARENKO, Igor SADOLKO Department of Environmental Radiogeochemistry, Institute of Geochemistry, Mineralogy and Ore Formation, National Academy of Sciences of Ukraine, 34 Palladin av., Kiev 152680, Ukraine Evgeny PETRAYEV, Galina SOKOLIK Department of Radiochemistry, Belarus State University, Leninski prosp. 4, Minsk 220080, Belarus Abstract This paper illustrates the experience gained in the field of natural restoration of contaminated vast ecosystems. Prior to recommending a large-scale application of any rehabilitation technique, it is important to know the medium- and long- term intensity of self-restoration for most of the affected territories. Three main ways express the process of self-restoration: 1) the natural radioactive decay, 2) the transfer of radionuclides out of natural ecosystems and 3) the ability of some pedological components to fixate the contaminants. The first way is a real decontamination process resulting in the removal from the biosphere of significant quantities of radionuclides. Indeed, during the last years the total activity of short- life-isotopes was decreased by a factor of some thousand and actually, the main contaminants are 137Cs and 90Sr which are decreasing according to their half-life. The two other ways of self-restoration are closely connected with radionuclides migration (vertical or/and horizontal) in soils. The vertical migration velocities of 137Cs and 90Sr in typical soils of contaminated regions in Ukraine and Belarus were evaluated annually during 9 years since the accident. In most of these soils the migration rate of 90Sr seems higher than this of 137Cs and ranges from 0.71 to 1.54 cm/year and 0.10 to 1.16 cm/year respectively. At present time the main part of radionuclides is located in the upper 10 cm layer of soils. The ability of soils components to immobilize the radionuclides was also investigated. From 1989 to 1994 approximately 57% of 137Cs was converted in fixed forms and for the year 2000 it is expected that this percentage will be 80%. Finally, for total contaminated regions, the obtained results on vertical migration velocity of radionuclides as a function of the soil type, are presented under the form of a map in order to help decision makers to determine the feasibility and the methodology for restoration of areas contaminated by 137Cs and 90Sr. 1. Introduction The scale of the territory contaminated from the Chernobyl accident (approximately 140000 Km2) makes difficult the implementation of an extensive restoration strategy on account of its economical, technical and social implications. Before to recommend a large-scale application of any rehabilitation technique, it is important to know the medium- and long- term intensity of self-restoration for the most of the affected territories. This work presents the experience gained during the last decade in the field of natural restoration of contaminated vast ecosystems. Three main ways express the process of self-restoration: 1) the natural radioactive decay, 2) the transfer of radionuclides out of natural ecosystems and 3) the ability of some pedological components to fixate the contaminants for long-term. The first way is a real decontamination process resulting in the removal from the biosphere of significant quantities - 465 - of radionuclides. The two other ways of self-restoration are closely connected with radionuclides migration (vertical or/and horizontal) in soils and their entry to the food-chain. It is obvious that any reduction in time of radionuclide transfer into the surface biota and/or any decrease of the radiation of soils, caused by spontaneous natural processes, brings out a reduction of internal and external irradiation doses. 2. Methodology The natural behaviour of 137Cs and 90Sr in typical soils of contaminated regions in Ukraine and Belarus was studied for 9 consequent years since the accident. However, the study of natural restoration of contaminated areas must be considered in terms of variable landscape-geochemical complexes and thus the study of radionuclide's behaviour in different media such as soil, vegetation, surface and ground waters, became of great interest for the assessment of the efficacy of decontamination techniques applicable in areas affected by the Chernobyl accident. Reasoning from the concept that self-restoration of natural landscapes is any removal of radionuclides from the active geochemical cycle of the affected areas due to the active migration and/or durable fixation into the soils, from 1993 to 1995 we considered two main lines of study within the EC/CIS Experimental Collaboration Project n°4 (ECP-4). The first one was based on the evaluation of the intensity of radionuclides migration. A study of vertical redistribution of radionuclides in different soil types was conducted in Ukraine and Belarus. An important surface ablation of soils may be occurred in sloping landscapes thus, they can considered as a good topographical model which may give indications of long-term behaviour of radionuclides soon, we studied the efficiency of self- restoration on four sloping sites with different incline, soil types, vegetation character, density of contamination and form of radionuclides. The second investigation line was based on the dynamics of distribution of 90Sr and 137Cs and their forms of occurrence in different types of soils and the concurrent changes of phytomass contamination. The main processes of self-restoration resulting to the reduction of external irradiation are the radioactive decay and the vertical or/and lateral migration of radionuclides. The processes leading to the decrease of internal irradiation are the radioactive decay, the vertical or/and lateral migration and the immobilization of radionuclides. It is necessary to know the main parameters of self-restoration of different types of soils for the recovery of large contaminated areas. This knowledge forms the background for any project and decision associated to the application of countermeasures. Indeed the following information can be delivered: 1. Predict the decontamination of soils as a function of time and thus forecast of natural rehabilitation of contaminated areas. The affected areas in Ukraine, Belarus and Russia being vast, it is essential to know the intensity of their natural self-restoration. 2. Classify the contaminated territories and select the areas and sites for the implementation of countermeasures. 3. Define the secondary effect of the rehabilitation actions and introduce them in the cost- benefit analysis of the countermeasures. 4. Qualify the contaminated areas according to their rate of self-restoration in order to plan their reintegration into the normal uses. Within the period of our involvement in the ECP-4, studies were conducted related mainly to the two first of the above items. Data referring to the properties of migration - 466 - (horizontal and vertical) and of immobilization of radionuclides into the contaminated soils of different type are presented below. 3. Results 3.1. Evaluation of the radioecological balances of contaminated territories The evaluation of the radioecological balance of affected areas in Ukraine and Belarus was made in the base of general quantitative estimations of radionuclides horizontal migration. The balance of radionuclides in the landscapes reflects the ability of the natural systems to "evacuate" the pollutants. It can be negative or positive, depending of the direction and the intensity of the natural processes of migration of radionuclides. In addition to the classic geochemical parameters, terrestrial balance of radionuclides depends on the forms of relief, the lithology of soil forming deposits and the vegetation cover. These factors are to be taken into account in order to evaluate the ability of different elements of landscapes to evacuate or to accumulate 137Cs [1,2,3]. Based on the balance of this radionuclide, the landscape elements of 30-km zone (Ukraine) or of Khoiniki region (Belarus) can be classified on three main groups 1) areas with negative balance (where the process of radionuclides evacuation is dominant), 2) areas with neutral balance and 3) areas with positive balance (and accumulation of radioactivity) [4], We prepared short-term and long-term maps of balance evaluation of 137Cs surface migration in natural landscapes of Chernobyl zone. Short-term balance reflects the evaluation from the present situation up to 20-30 years in future. Long-term balance is a forecast situation 60-80 years from now, when some stable forest succession will arrive, and shows globally small differences compared to the short-term one. The landscape of extreme-morainic ridge of Chistogalovka, shows both in short-term and in long-term aspects, a clear negative balance of I37Cs. Generally landscapes with negative short-term balance of the radionuclide occupy 36,9% of the estimated territory. Neutral short-term balance is expected for sandy territories of the river terraces and fluvioglacial plains and together with water surfaces and industrial areas cover approximately 38,9% of the territory. Zones of positive short-term balance of !37Cs are identified with closed depressions, watershed catchments, elements of erosional network and rear lowered parts of river terraces and flood plains. Positive balance of the radionuclide