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 is estimated 23,9% of the territory inside the 30-km zone. Similar situation characterizes the I37Cs long-term balance in the divers landscapes. A small differentiation of surfaces is evaluated: 4,1% decrease of the areas showing neutral balance and 4,5% increase of the negative balances or self-restored landscapes. In a similar way a map of balance evaluation of 137Cs in the natural landscapes of Khoiniki district (Gomel region, Belarus) was prepared. Thus, in correspondence with the work done for the 30-Km zone in Ukraine, we created the cartography of 137Cs balance evaluation for Belarus, in order to cover the most important part of the most contaminated territory of these two republics. Our data show that 47.4 % of the territory of Khoiniki district is characterized by negative balance. 19.3 % of this territory shows neutral balance and 33.3 % is characterized by positive balance of I37Cs.
3.2. Vertical Migration of radionuclides
This part is related to the measurements of the radionuclides vertical migration in the soils of Ukraine and Belarus (30-Km zone and Khoiniki district-Gomel region respectively)
- 467 - and is based experimental data collected from 1986 - year of the accident - up to the present time. The results of the vertical distribution of B7Cs in soils profiles, from the Chernobyl area, obtained during 8 years after the accident are presented in table 1 as the annual position of the center of 137Cs-reserve.
Table 1. Dynamic of transfer of the center of I37Cs-reserve in soils of Chernobyl area.
Type of soil Elementary Biocoenosis Depth of Cs reserve center, cm* landscape 1987 1988 1989 1990 1991 1992 1993 1994 sod-semi- eluvial pine forest 0.60 0.78 0.90 1.04 1.18 1.30 1.41 1.40 podzolic-sandy sod-podzolic eluvial pine forest (0.60) (0.85) 1.10 1.10 1.25 1.35 1.42 1.40 dusty-sandy sod-podzolic eluvial long-fellow 0.62 0.82 1.12 1.45 1.75 2.00 2.13 2.20 sandy, dusty- land sandy peat gleic, peat trans- alder marshy (1.15) (2.00) 2.91 3.12 3.75 4.35 5.06 5.58 bog eluvial- forest accumul. primitive alluvial trans-super- pioneer (0.88) 1.25 1.61 (1.95) 2.22 (2.50) 2.69 2.80 sandy aquatic phytocoenosis alluvial semi- super- meadow 0.70 1.24 1.70 1.80 2.15 2.50 3.10 3.20 podzolic semi- aquatic gleic sandy sod-podzolic super - meadow 0.80 1.22 1.44 1.72 2.12 2.35 2.56 2.67 gleic dusty sandy aquatic drained sod-podzolic super- meadow non- 0.72 1.30 1.65 2.20 2.65 3.05 3.56 3.83 gleic loamy- aquatic drained sandy alluvial gleic super- meadow non- (0.90) 1-30 1.86 2.45 2.87 3.42 3.95 4.20 loamy aquatic drained soddy-gleic super- meadow 0.92 1.25 1.77 2.24 2.74 3.30 4.10 4.40 aquatic peatbog super- 1.44 (2.00) 2.73 2.95 3.44 4.52 5.65 6.05 aquatic
Notes: * = error of definition is not more than 10% () = data are obtained on the base of 2-3 profiles
Analysis of the motion of the center of 137Cs-reserve as a function of time, shows that slower speed of the vertical migration of 137Cs (about 0.1-0.2 cm/year) is observed in areas with sod-podzolic sandy, dusty-sandy and sandy-loamy soils of the eluvial landscapes covered by pine and oak-pine forests. Such slow speed is probably due to the presence of litter, where is the main part of radionuclides reserve and thus it plays the role of an organic barrier against the active migration of 137Cs. Areas represented by long-fallow lands and sod- podzolic semi-gleic dusty-sandy, and alluvial dusty-sandy soils, are characterized by higher speed of vertical migration of 137Cs (about 0.2-0.3 cm/year). The velocity of migration is higher (0.3-0.5 cm/year) for areas which represent sod-podzolic gleic loamy-sandy. The same velocity of vertical migration show primitive alluvial sandy soils of river beaches. The highest velocity is observed on areas with soddy gleic and peat-bog eutrophic soils, as well as in alluvial ones. The speed arrives 0.7-1 cm/year. Finally, the velocity of the displacement of the center of 137Cs-reserve for soils of the same type is not constant from year to year. In
- 468 - fact, the difference of depth of the reserve centers between 1989 and 1990 was 0.1 cm while between 1992 and 1993 it was 0.6 cm. The topsoil of Khoiniki is characteristic of the whole Belarus Polessye which was highly contaminated by the Chernobyl accident. The following dominant soil types were investigated [5,6,7]: 1. Soddy-podzolic soils (sandy, sandy-loam). 2. Soddy-podzolic soils with redundant moisture. 3. Soddy-podzolic-gley soils. 4. Soddy-gley soils. 5. Peaty-marshy soils of a lowland type, as well as alluvial ones. The swift of the center of the radionuclides total reserve is calculated on the base of rate parameters, estimated according to Konstantinov's and Prokhorov's quasi diffusive models, which were worked out for global radionuclides fall-out [8,9]. Table 2 shows the migration speed of 137Cs and of '"Sr in the five different types of soil. 1) Soddy-podzolic soils (sandy, sandy-loam) and 2) soddy-podzolic soils with redundant moisture: The speed of radionuclides migration in soddy-podzolic automorphous soil does not differ from that of those with redundant moisture. Trie average speed of I37Cs in soils of these types seems to be 0.48 cm/year and of '"Sr 0,74 cm/year. In a subgroup of unprocessed soddy-podzolic soils the migration speed of 137Cs is 0.14-0.26 cm/year and of "Sri. 17-5.70 cm/year. 3) Soddy-podzolic-gley soils: In soddy-podzolic-gley soils we observed a clear dependence of migration speed of radionuclides from hydrological conditions. So, the average linear speed of migration of 137Cs is found 1.16 cm/year in soils with constant high moisture (the so-called "wet" soils), whereas in "drier" soils of this type the speed of 137Cs migration is considerably lower: 0.43 cm/year. Similar observations are made with the migration speed of ^Sr. In "wet" soddy- podzolic-gley soils the migration is about 1.30 cm/year and in "dry" soils 0.65 cm/year. 4) Soddy gley soils: In soddy gley soils the speeds of migration for both radionuclides do not seem to differ and were found to be approximately 1 cm/year. 5) Peaty-marshy soils of a lowland type, as well as alluvial ones: Migration characteristics of radionuclides in peaty-marshy soil of a lowland type were variable. The speed of 137Cs transfer in meliorated peat bogs is approximately 0.4 cm/year and in non-meliorated ones about 0.9 cm/year. '"Sr speed in the above mentioned soils was found 0.52 and 1.54 cm/year respectively.
3.3. Classification of migration velocity
Here in below we show the cartographic classification of the above soils as a function of the observed migration velocities of 137Cs. Two maps are prepared separately for the most contaminated areas of Ukraine and Belarus. For the 30-Km zone of Ukraine the observed intensity of 137Cs migration as a function of the type of soils, is classified in five groups and presented on the map of figure 1. This map was prepared on the base of definition of groups of soil with similar velocities of dislocation of the center of 137Cs-reserve. Similar cartographic work was made showing the migration speed of the two radionuclides (137Cs and ^Sr) into the most typical soils of the Khoiniki district.
- 469 - Table 2
Speed of 137Cs and ^Sr vertical migration in soils of different type (cm/year, average speed and standard deviation)
TYPE OF SOIL TYPE OF SOIL TYPE OF SOIL TYPE OF SOIL TYPE OF SOIL TYPE OF SOIL TYPE OF SOIL A 3 C C1 D E E1
137Cs -Sr 137CS «* 137Cs Sr 137CS -Sr 137CS -Sr 137Cs -Sr 137Cs -Sr
Average: 0,45 0,71 0,50 0,76 1,16 1 30 0,43 0,65 1,07 1,03 0,92 1,54 0,39 0,52 St. Dev.: 0,24 0,13 0,14 0,21 0,26 0 19 0,01 0,08 0,39 0,11 0,26 0,72 0,10 0,06
SOIL A = SODDY-PODZOLIC SANDY AND SANDY-LOAM (AUTOMORPHOUS)
SOIL B = SODDY-PODZOLIC SANDY AND SANDY-LOAM WITH MOISTURE
SOILC = SODDY-PODZOLIC-GLEY ("WET')
SOIL C1 = SODDY-PODZOLIC-GLEY ("DRY")
SOIL D = SODDY-GLEY
SOIL E = PEATY-MARSHY SOIL E1 = PEATY-MARSHY (MELIORATED) - 470 - Figure 1 0 2 4 6km Intensity of Cs-137 vertical migration in Chernobyl zone (cm/year) - 0.1-0.2 ISSili - 0.5-0.7 : 2 - 0.2-0.3 %M&z3M ->0.7 ::::::'''::';!f-::::::!::::: :-::'::";:''-3!:::;:;;i: - 0.3-0.5 - removed soils - 471 - 3.4. Migration of Cs on slopes The study of natural migration of radionuclides in sloping soils of 30-km zone began in September 1993. Soils from four new sampling areas were systematically analyzed and measured. These areas differ in slope angle, in distance from Chernobyl NPP, in density and forms of fallout, and in some bio-geochemical characteristics. We also used previous data obtained in 1986-1993 from some other slopes.. Self-restoration was expressed by the intensity (I) and efficiency (E) of the process of 137Cs migration which were calculated by the simple equations: E=(AO-AT)/AO.100% and I=dA/dt, were: Ao= the initial activity of radionuclides and Ar= the activity at the time of sampling. We took also into account the processes of physical decay of 137Cs. During the last 9 years the efficiency of self-decontamination for the studied sites ranged from 7,5-8% (for the site of Kopachy) to about 20% (for the site of Novoselki), or from 1 to 2.5-3% per year. The distribution of 137Cs in the soil of a sloping burnt forest needed a separate examination. Analysis of our data shows that before the fire of 1991, the efficiency (E) of self-restoration during the first 5 years following the accident was estimated 5.2-5.7 % (±1.1 %) or 1.1-1.2 % (±0.25 %) per year. The intensity (I) was found to be 600 Bq/kg per year. Even an increase was detected from the top to the foot of the slope. In two years following the forest fire the above parameters of self-restoration were changed and E increased to 23 % (±4.5 %) - for the top of the slope rose to 36 % (±7%) or 11.5-15 % per year - and I also increased to 3-4.5 KBq/kg per year. In all cases it was observed a tendency of redistribution of 137Cs on the slopes and at present time the maximum activity seems to be concentrated in the middle part of the slope. In table 3 is presented an example of the described above behaviour. Table 3. Distribution of 137Cs in soil profiles from a sloping site in Savichi (1994) Sampling Upper layer Total activity Aiot / Aup point on the activity (Aup) (A**) slope Ci/km2 Ci/km2 1 (upper) 28.7 32.1 1.12 2 24.0 24.0 1.00 3 35.7 40.5 1.13 4 29.6 35.6 1.20 5 25.7 31.0 1.21 6 16.6 24.1 1.45 7 18.4 26.0 1.41 g 25.3 29.1 1.15 9 19.4 24.4 1.26 10 24.8 31.2 1.26 11 (down) 14.6 20.9 1.43 3.5. Existence form of radionuclides In order to determine the form of radionuclides in soils, we investigated the site which is situated on the landscape of Shepelichi, at the first over-valley terrace of Prypiat river. Sampling point was situated on a shallow-wavy and relatively dry terrace. Sandy soddy-podzolic soils are lain on aged-alluvial sands. A humus layer of soils reaches 25-31 sm. This site was ploughed before 1986. A selective leaching of elements was achieved by consecutive treatment of soils using solutions with different composition. For the watersoluble form we used distilled water, for the exchanging ones 1M ammonium acetate solution and for the acidsoluble ones 1M nitric - 472 acid solution. The leaching was made at the ratio of solid and liquid phases equal to 1:5, at 200° C temperature and without intensive mixing. The distribution of radionuclides' forms of existence (occurrence) in soil's profile is presented in table 4. Table 4. Content (%) of mobile form of radionuclides in vertical profile of soil from the sampling site of Chystogalov. Layer Form ""Ce 106Ru 134Cs 137Cs "Sr % % % % % 0-1 water-soluble 0.40 0.61 0.22 0.15 2.19 exchangeable 1.39 4.5 3.86 6.25 1-2 water-soluble 1.09 0.18 0.13 3.23 exchangeable 1.65 1.21 6.07 4.38 41.3 2-3 water-soluble 1.2 0.31 0.11 1.46 exchangeable 2.8 3.6 5.0 5.56 27.7 3-4 water-soluble 1.79 0.31 0.14 1.73 exchangeable 1.95 1.27 5.6 4.0 30.0 4-5 water-soluble 4.24 0.96 0.24 3.02 exchangeable 3.05 2.42 12.5 9.04 53.5 5-6 water-soluble 5.71 0.29 1.67 exchangeable 5.35 4.43 8.92 5.11 45.9 6-7 water-soluble 7.19 10.0 1.11 0.38 7.22 exchangeable 5.16 13.3 8.22 5.64 58.3 8-9 water-soluble 23 18.9 7.80 4.67 1.19 exchangeable 45.7 : 71.2 70.8 50.0 10-11 water-soluble 17.4 8.89 2.13 exchangeable 75 : 17.8 16.0 87.0 12-14 water-soluble 6.30 28.9 exchangeable ; 9.26 45.6 In land biomass 137Cs and ^Sr contents vary from 10 to 200 Bq/Kg. 137Cs and 90Sr mobile forms contents (sum of water-soluble and exchangeable forms) in soils and relation of this nuclides in grass presented in table 5. Table 5. Mobile forms of 137Cs and '"Sr in soddy-podzolic soil and ratios of these radionuclides in grass. Year Mobile forms in soil I37Cs,% MSr, % Cs-137/Sr-90 in grass* 1987 8 15 0,35 0,95 1988 7,2 24 0,3 - 1989 8,1 30 0,27 0,39 1991 4,9 37 0,13 0,35 1992 3,5 40 0,09 0,12 1993 - - . 0,15 1994 2,2 49 0,036 0,047 * calculated on data from Arhipov A.N., Arhipov N.P., Gorodetzkiy D.V., Meshalkin G.S. "Dyinamika radioekologicheskoy obstanovky naselhosugodjah 30-km zone". Preprint NPO "Pripyat", 1994, Zelony mys (Russian). - 473 - 4. Conclusions and discussion Related to the radioecological balances of the contaminated territories, the 30-km zone of Chernobyl NPP can be divided in three parts that are approximately equal, one of them is self-restored. The self-restored part which has negative balance seems to increase in time, extended to landscapes with neutral balance, and it contaminates the second part which is obviously characterized by a positive balance. The percentage of the surfaces with positive balance seems not to differentiate in time. The last part of the territory is characterized by a neutral balance and seems to decrease in time transformed slowly in self-restored territory. In Khoiniki region approximately the half of the territory is self-restored, 33% shows positive balance and only 19% has a neutral character. Related to the vertical migration and distribution of radionuclides into soil profiles our study showed that at present time the main part (up to 90%) of radionuclides is in the upper 10 cm layer. The rate of vertical migration essentially depends on type of soil and natural landscape. The rate of penetration of the 137Cs-reserve is within 0.1-0.15 cm per year for grass-podsol sandy and sandy loam soils covered by pine forests, and 0.7 cm (or more) per year for grass-gley and peaty-boggy soils. The obtained data allowed us to classify the soils of some highly contaminated areas in Ukraine and Belarus according to intensity of vertical migration. Our calculations on external dose reduction due to the 10-year natural vertical migration of 137Cs, results in a decrease arising up to 30%. For the sloping areas, the intensity of self-restoration depends on type of soils, character of the cover vegetation and inclination of slope. For soils of the same type, the factor of acceleration of self-restoration is estimated to be 1.5-3 when the slope incline increases from 8-10° to 30-40°. However, in future self-restoration intensity and efficacy will decrease due to the increasing long-term fixation of radionuclides by different soil components. Nevertheless, the damages of the surface layer caused by erosion or removal of vegetation will become of great importance. Since the accident more than 57% of 137Cs was converted in fixed forms and for the year 2000 it is expected that this percentage will be 80%. The distribution of the activity of mobile forms of 137Cs and '"Sr in vertical section of soil coincides with that of activity of radionuclides' reserve. 94-97% of the total 137Cs activity is accumulated in the upper 5-cm soil layer. From 1987 till 1994 the steady increasing of'"Sr mobile form content in soils and contrary the decreasing of that of 137Cs were observed. Such effect can be explained by the immobilization of 137Cs in soil. Dynamics of the ratios of 137Cs to ^Sr mobile forms reflects the rate of 137Cs immobilization. The time of transformation of 50% of mobile I37Cs into immobilized form is evaluated to be 3-5 years for soils of 30-Km zone. Finally, the trends of ratios of I37Cs to 90Sr concentrations in over ground parts of plants are conform to that of mobile forms of that radionuclides in soil. Intensive vertical migration results in a decrease of radionuclide transfer into the phytomass because of their wash-out the root layer. ACKNOWLEDGEMENTS The authors wish to thank Dr. Vassili Davydchuk, Ukrainian Institute of Geography, for definition of radioecological balances and for cartographic assistance, and the European Commission for financial support (contract N° COSU-CT94-0080). - 474 - REFERENCES [1] Arapis G., Davydchuk V., Koutlahmedov Y., Sadolko I. (1994): Estimation of self- decontamination of ecosystems to optimize the strategy of decontamination. Fourth International Scientific and Technical Conference on the Problems of Chernobyl Accident Clean-Up, Chernobyl-Zeleny Mys, 24-28 October (in press). [2] Davydchuk, V. (1992): Landscape approach to the estimation of radionuclides migration conditions in the Chernobyl accident zone. Seminar on the Radioecology and Counter- Measures. Kiev, 27 April-2 May, Proceedings of I.U.R. Soviet branch, p. 117-123. 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Forms of occurrence and migration of Chernobyl radionuclides in Belarussian soils, Proceeding of SPECTRUM 94, Nuclear and Hazardous Waste Management International Topical Meeting, August 14-18, Atlanta, Georgia, USA. [8] Konstantinov I.E. Skotnikova O.G., Soldayeva L.S. (1974). The migration forecast of Cs-137, Pochvovedeniye, 5, 54-58 (in Russian). [9] Prokhorov V.M. (1981). Migration of radioactive contamination in the soils, Moscow: Energyizdat, (in Russian). - 475 -