ECORAD Expert and Information System has been developed for studying and predicting the biogeochemical migration of radionuclides. Models for the biogeochemical migration of radiocarbon in the main types of terrestrial ecosystems of European Russia were included in the bank of mathematical models. The comprehensive description of models is presented, and their capabilities in the prediction of the C-14 behavior in ecosystems are illustrated.

The long-term dynamics of radionuclides in soil solutions of various subhorizons of litter and mineral layer of soddypodzolic sandy forest soils at 30-km zone aroudn Chernobyl nuclear power station was studied. One year after the accident the relative contents of 90Sr. 106Ru. 134,137Cs. 144Ce in soil solutions (radionuclide fractions in the liquid phase of soil) for the whole contaminated layer of soil were very low (0.034 – 3.5%) and varied from one plot of zone to another because of unequal physico-chemical properties of nuclear deposition. By 1989 this parameter for radiocesium decreased 5–9 times and then varied slightly. The data indicate that since the accident three processes have taken place: radiocesium transfer from fallout into soluble forms, its migration down the soil profile and fixation of radionuclide in organic-mineral layer of soil under the forest litter.

Radiocesium is normally bound only rather weakly and unspecifically by humic substances, in contrast to the actinides Pu and Am. Recently, however, it was observed that fallout 137Cs in the soil solution from an Of-horizon of a podzol forest soil (slightly decomposed plant material) was associated essentially only with one single size fraction of the humic substances. In deeper soil layers with well humified material (AOh-horizon), radiocesium was associated with all size fractions of the dissolved organic matter (DOM). To examine whether this unexpected behaviour is also observable for DOM isolated from other soils, we determined the association of fallout 137Cs,90Sr,238Pu, 239+240Pu and 241Am with various size fractions of DOM from in situ soil solutions isolated from two layers (0–2 cm and 2–5 cm) of two grassland soils (a soddy podzolic soil and a peat soil) within the 10 km zone of the nuclear reactor at Chernobyl (Ukraine). The four size fractions of DOM as obtained by gel filtration of the soil solution were (mean nominal molecular weight in daltons): fraction I: ≥2000, fraction II: 1300; fraction III: 560, fraction IV: inorganic compounds. The results for the well humified DOM (humus accumulation horizon of podzol, deeper layer of peat soil) showed that Pu and Am are essentially associated with the high molecular weight fractions, while Sr is present only in the `inorganic' fraction. Radiocesium is found in all the size fractions separated. A quite similar pattern was also found for Pu, Am, and Sr in the soil solution from only slightly decomposed plant material (0–2 cm of peat soil), but not for radiocesium. This radionuclide was again essentially only observable in one single low molecular weight fraction of DOM. The above results thus support our recent observations in the different horizons of a forest podzol mentioned above, even though no reason for the different binding of radiocesium by well humified soil organic matter and by only slightly decomposed plant material can be given at present. The data demonstrate, however, that information on only the total amount of a radionuclide in the soil solution will not be sufficient to interpret or predict its fate adequately in the soil.

Dynamics of the 137Cs content in the components of the forests in the 30-km zone around the Chernobyl nuclear power plant (NPP) in 1986–1994 are associated mainly with such factors as the size of radioactive particles in the fallout, ecosystem humidification and soil type, tree age. The influence of particle size was especially noticable between 1986–1987 and was displayed by low biological availability of radionuclides in the near part of the zone (within the 10-km radius circle around the NPP) in comparison with more distant regions (within the 30-km radius circle). Later, the expression of this influence decreased and transfer factor (the ratio of 137Cs content in overground phytomass to the soil contamination density) became approximately the same for all plots with similar ecological and fallout characteristics. Humidity of landseape and soil type determined the velocity of radionuclide vertical migration in the soil and 137Cs biological availability. These parameters were maximum for the hydromorphic soils of wet landscapes enriched in organic substance and poor clayey minerals. Differences of 137Cs accumulation in overground phytomass of trees caused by tree age are displayed in the higher 137Cs concentration in structural parts of young trees as compared with old ones.

The effect of forests on the radionuclide primary distribution in different components of the contaminated ecosystems is considered by the example of Chernobyl accident. A basic mathematical model is developed describing 137Cs biogeochemical cycling under conditions of quasi-steady state radionuclide redistribution in the ecosystem. The radionuclide fluxes between different ecosystem components are estimated. Forest ecosystems are proved to diminish radionuclide migration in the environment, and forest should be regarded as an important sanitary factor. The contribution of contaminated forests and forest products to the total irradiation dose to local population is estimated. Special countermeasures are elaborated in order to diminish unfavorable consequences of forest radioactive contamination. A long-term dynamics of radioactive situation in the forest ecosystems is forecasted and further studies on the subject are drafted.

The 9-years dynamic of Chernobyl-derived radionuclides in the vegetation and soil covers of the forest ecosystems of the European part of the CIS is considered. The quantitative estimation has been done for main fluxes of Cs-137 in the forests of automorphic landscapes: influx to the vegetation, return with the litterfall, stem flow and throughfall, vertical migration in the soil profile (including intrasoil flow), and redistribution within the system of geochemically connected landscapes.

This paper reports on research on the migration of radionuclides, resulting from post Chernobyl fallout, in various types of natural soil. Experimental sites were established in 1987 at distances of 5-27 km from the site of the accident, corresponding to a deposition range of 0.2-300 MBq/m(2). Radionuclide distribution of (134,) (CS)-C-137, Sr-90,Ce-144 and Ru-106 in the soil profiles were studied in order to determine their migration characteristics. Today, eight years after the accident, forest litter still remains a major repository of radionuclides released into the forest ecosystems in the vicinity of the Chernobyl nuclear power plant.Radionuclide migration occurs mostly in the wet landscapes with boggy-peat soils, and especially in high-water landscapes with specific alluvial soils. Minimal migration activity is observed in the meadow-dry soils of watersheds. (CS)-C-134, 137 and Sr-90 are found to be equally mobile in the dry sandy soils of watersheds, while Sr-90 is distinctly more mobile in agrillaceous alluvial soils of the Pripyat river valley. Distribution of radionuclide composition down the profile of alluvial soils suggests that lessivage-like processes are taking place, with direct migration of the fallout particles occurring within the soil profile. The so-called hydromorphic (wet) landscapes, such as pear bogs and river valleys, are characterized by maximum radionuclide migration activity, while the minimum is observed in dry (automorphic) sandy soils.

Samples of sandy forest soils, meadow sandy-peat soil and meadow sandy-loam soil were taken at different sites within a 30-km zone around the Chernobyl NPP (ChNPP). The samples were extracted with water and a 0.1-N solution of ammonium acetate. The extracts were measured for gamma-radionuclides and stable cation content. The content of all mobile forms of the radionuclides present in the 0-10-cm soil layer accounts for 0.5-5% of the total radionuclide content in this layer, depending on the type of radionuclide and soil. Water soluble forms of the radionuclides were found in the 0-5-cm layer only. Exchangeable radionuclide forms were represented, as a rule, by radiocaesium in both the 0-5- and 5-10-cm layers. Content of Cs-137 exchangeable forms in the organic-mineral horizon were roughly inversely proportional to the sum of stable exchangeable cations and organic matter content. Forest vegetation takes up a significant share of the mobile forms of radiocaesium from the soils.

The relative content (α) of 137Cs (1987–1991), 106Ru, 134Cs, 144Ce (1987) and the chemical speciation of radionuclides in the liquid phase of forest soils at different plots in the 30-km Chernobyl zone were studied. One year after the accident, substantial variations in the α-value between the different plots and a variation in α along the soil profile were observed due to unequal physico-chemical properties of nuclear fallout and soils. The α-value calculated for the total contaminated layer (αav) at different plots varied within relatively narrow limits. Between 77 and 97% of 137Cs in soil solution was shown to be included in organic compounds (MMw, 102–104 Da), the latter making a major contribution to the radionuclide transport from soil to plant. The α-value for 137Cs for all plots in the Chernobyl 30-km zone and the differences in α value between plots had a tendency to decrease with time. These data indicate that the process of 137Cs fixation by the soil solid phase was prevalent. Now, the ‘soil-soil solution’ system is approaching equilibrium for the radionuclide distribution between solid and liquid phases.

According to results of research in the 30-km zone around the accident at the Chernobyl nuclear power plant in 1986-1990, the authors analyzed the development of processes of migration of radionuclides and revealed factors determining the direction and rate of redistribution of Cs-137 in different components of forest biogeocenoses in the zone of the accident. Data are given that quantitatively characterize the dynamics of cesium in the vegetation and soil.