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Головна arrow Екологія arrow Самоочищення природного середовища після чорнобильської катастрофи
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Theoretical generalization of radiogeochemical researches within the area contaminated with artificial radionuclides after the Chernobyl catastrophe lead to a new methodological approach to environmental self-clearing based on synchronization between biogenic migration and geochemical transformation of radionuclides in soils. In contrast to natural attenuation and intrinsic bioremediation, which considered decontamination of abiogenic constituent of landscape; environmental self-clearing has covered all natural processes that lead to reduction of contaminant concentration in trophic cycles, including immobilization of pollutants in soil without their removal outside the landscape.

Formation of radiation dose for rural population of Ukraine above of 80 % is determined by radionuclide oral incorporation, determining the exclusive importance of trophic cycles contamination for ecological safety of radiation contaminated areas. The rate of radiation dose decrease is 10 times faster than radioactive decay of 137Cs and 90Sr.

Self-clearing begins from the time of the contaminant fallout to the earth surface. Therefore, technogenic prerequisites that covered emergency factor and postdamage counter-measures, determine primarily the rate of ecosystems self-remediation. The emergency factor is determined by composition and deposition of contamination, form of fallouts. The main counter-measures during the breakdown elimination were the number of measures for fire extinction, disposal of radioactive soil, wood, metal works, etc, dust-depressing, land rehabilitation and water protection measures utilizing a number of toxic compounds. These measures contributed to transient improvement of radiation situation and to decreasing the personnel irradiation, but in the future years are resulted in prolonged secondary environment contamination with organic and inorganic pollutants.

Besides technogenic prerequisites, physico-geographical factors still determine the spread of radioactive contamination. Meteorological factors (rainfall and its localization, strength and direction of wind) and relief geological structure determine the exportation, infiltration, transit and accumulation of radioactive contamination. Zonal distribution of radioactivity within Europe is described by exponential regularity.

Physico-chemical criteria of self-clearing are defined by nuclear and chemical properties of contaminants, and physico-chemical properties of fallouts. Radioactive decay is the only process resulting in the complete radionuclide removal from the environment. The decay constant is the main criterion of assessment of self-clearing rate.

Radionuclide speciation in soil is described by the kinetic transformation simulator. The main thesis of the concept speciation in geochemistry of artificial radionuclides consists in synchronism of pollutant's biogenic migration and abiogenic speciation. The conjugation between these processes is determined by mobile species dynamics, rate of biogeochemical flux, and landscape-geochemical conditions. The velocity of biogeochemical flux of radionuclides in soil-plant-cow milk system depends on landscape-geochemical conditions. Biogeochemical flux to the first chain of trophic cycle involves from 20 to 100 % of mobile 137Cs and between 4 and 60 % of mobile 90Sr. Biogeochemical flux to higher chains involves from 1 to 6 % of 137Cs mobile species in soil.

Biogeochemical flux of 137Cs increases according to the growth of biomass production from dry meadows at soddy-podzolic soil to over-damped peaty soils within flood-lands. Biogeochemical flux of 90Sr increase in opposite direction, that is determined by the intensity of 90Sr involving to biological circulation and corresponds to regularities of natural Sr isotope bioaccumulation in geochemically conjugated landscapes. Self-clearing ability of the ecosystem increases in the opposite direction to intensity of biogeochemical flux.

Intensity of 137Cs biogeochemical flux in forest ecosystems increases exponentially to radionuclide mobile species dynamics and grows in correspondence with biomass production. The half-time of biological self-clearing of pine-tree wood is between 15 and 250 years depending on landscape-geochemical conditions.

The integral parameter calculated from the ratio between integral rate constant of radionuclide biogeochemical flux and decay constant defines the self-clearing velocity. The rate of meadow ecosystem self-clearing exceeds 3–15 times the decay rate of appropriate radionuclides. The self-clearing of pine-tree wood mainly corresponded to radionuclide decay.

The rate of self-clearing is substantially higher than velocity of natural attenuation and intrinsic bioremediation. The latter two processes depend on the surface run-off and descending migration. The natural attenuation and intrinsic bioremediation of l37Cs are determined by decay rate, and those of 90Sr are twice as fast as the decay, while the intensity of self-clearing about 10 times exceeds the decay rate of 137Cs and 90Sr.

Geochemical criteria of abiogenic speciation and biogenic migration for radionuclides and heavy metals depend on the properties of the soil absorbing complex. Hydrolytic acidity, sum of exchangeable bases, organic carbon content, content of water-soluble and exchangeable species of typomorphic elements are the corresponding mobilization factors. The increase of these parameters can result in the growth of radionuclide and heavy metal migration ability. Exchangeable Mg content, pH of salt extract, ratio between exchangeable К and its bulk content in soil are the corresponding immobilization factors. Effect of the first factor is determined by the property of Mg2+ to activate the sorption centre of soil mineral contributing to their disintegration. Fixation factors prevent radionuclide and heavy metal transfer to the trophic cycles.

The self-clearing of freshwater ecosystem, contaminated after the Chernobyl catastrophe, is defined by the sedimentation of radioactive particles, radionuclide speciation and migration within contaminated columbines and bottom deposits and radionuclide transportation with stream. Dynamics of water contamination within the Dnieper reservoir system is described by exponential regularity that is in accordance with the temporal changing of radionuclide speciation in soils of columbines. The rate constants of river water self-clearing is in good agreement with the rate constant of radionuclide immobilization in soil. The Dnieper reservoir system self-clearing velocity exceeds one order of magnitude the decay rate.

Substantial redistribution of radionuclides between migration species has been observed during 18 years. The part of radionuclides in a form of suspended particulate matter (SPM) is decreased and part of conditionally soluble form is increased. Radionuclide transformation from SPM to conditionally dissolved form is described by exponential regularity. This transformation velocity is comparable with the rate of radionuclide mobilization in soil. Annual transport of dissolved radionuclides from contaminated columbines of the Chernobyl Exclusion Zone to the Black Sea is assessed as 6 • 1011 Bq of 137Cs and 7•1012 Bq of 90Sr.

Specific activity of SPM in river water reaches 15 000 Bq•kg-1 of 137Cs and 700 000 Bq kg-1 of 90Sr. These values exceed several orders magnitude of the contamination of soil and bottom deposits that testifies to the great role of aquatic life in radionuclide secondary redistribution in freshwater ecosystem.

The main form of radionuclide migration in river water is cationic (conditionally dissolved inorganic matter). Up to 60 % of 137Cs and more than 90 % of 90Sr migration are corresponded to this form. The rest of radionuclides is almost equally distributed between dissolved organic matter (DOM) and SPM. The correlation of radionuclide migration forms is subjected to seasonal variations and depends on waterway discharge.

Radionuclide transformation in river water is a synchronous reflection of mobile species dynamics in soils of columbines. Correspondence of radionuclide transformation rate in aqueous and ground ecosystems testifies to common geochemical mechanism of contaminants migration in natural environment.

Bottom deposits of the Dnieper river reservoir system play the role of virtually inexchangeable depot of 137Cs. Transformation of 137Cs from the solid phase of bottom sediment to water soluble form is described by logarithmical normal regularity. The leaching of 90Sr from sediments occurs according to the first order kinetic law. Ecological safety of bottom deposits as the source of river water secondary contamination with radionuclides is determined by complete removal of 90Sr and fast immobilization of 137Cs by the solid phase of sediments. Total radionuclide transport into the Black Sea basin during 15 years is assessed as 2•1014 Bq of 90Sr, and 2•1013 Bq of 137Cs.

The conjugation between abiogenic transformation and biogenic migration of contaminant is the main principle of the concept of speciation in the geochemistry of artificial radionuclide that underlies the forecasting of ecosystem radioeco

logical state and ecological-geochemical mapping of contaminated area. Rate constants of radionuclide transformation in soil are considered to be the criteria of biogenic and abiogenic aqueous migration and self-clearing of radiation- contaminated terrestrial and aqueous ecosystem.

Application of experimental research concerning radionuclide speciation and migration in terrestrial and aqueous ecosystems offers the prospects for utilizing the abandoned area with 137Cs contamination, which 10 times exceeds current standards in the national economy.

The criteria of ecosystem self-clearing serve as the basis for development of hierarchic levels of semi-natural ecosystem, balance assessment of environmental radioactive contamination, forecasting of radioecological ecosystem state, development of rehabilitation measures, and optimization of the system of radioecological monitoring.

Objectivity of the obtained data, possibility of its generalization in the field of geochemistry of technogenesis are determined by fixed date of contamination; absence of natural background caused by exclusively artificial origin of the contamination; high sensibility of radioactive measuring; wide diversity of natural landscape-geochemical conditions within the Ukrainian Woodlands. Availability of common parameters influence on transformation and migration ability of radionuclides, heavy metals and non-metals determines the possibility for generalization of the geochemical concept of radionuclide speciation and ecosystem selfclearing for wide spectrum of technogenic contaminants.

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