Dirvožemio ilgalaikės užtaršos radionuklidais kaitos tyrimas ir prognozė pagal jų kaitą medžių segmentuose

General Characteristic of the Disertation The problem The development of mankind is inevitably related to the ever increasing impact on nature. During recent decades due to scientific and technical advances, human possibilities of influencing the nature began to substantially exceed our ability to understand, evaluate and forecast consequences of this activity, quite often unexpected and unacceptable. With the nuclear technology development, along with other problems there occurred the problem of the environment contamination with radioactive substances. Their consequences are felt over many years, and radionuclides having penetrated into the environment take part in the exchange processes between various natural media, migrate over long distances, get into biological objects and accumulate in them. Previously, especially after the Chernobyl nuclear power plant (NPP) accident, and currently most attention both in the world and in Lithuania is paid to the investigations of specific activity of radioactive substances in the environment. However, there are a lot of problems of evaluating their biological danger, accumulation, migration and withdrawal in the environment from different environmental objects as well as of forecasting these processes. From the atmosphere radioactive substances are withdrawn with dust, they can be washed out with precipitation and decrease during radioactive decay. Radionuclides deposited to the ground surface are absorbed by soil and vegetation. Topicality of the subject After some time following the global depositions and pollution after the Chernobyl NPP accident, Lithuanian soils are most contaminated with 137Cs and 90Sr. In the work the 137Cs accumulation as well as vertical and horizontal migration in soil have been evaluated. The specific activity of the artificial origin 137Cs in tree segments has been determined, coefficients of its transfer from soil to various parts of the woody plant have been evaluated and compared with the transfer of the natural origin radionuclide 40K. The radionuclide accumulation both in soil and plants, wash–out from the environment and radioactive decay last for years, therefore it is expedient to choose proper methods and evaluate tendencies of the long–term contamination variation. Radionuclides having penetrated into the environment are accumulated in the vegetative ground cover and upper soil layer. Their accumulation in plants occurs directly from the atmosphere through their above–ground parts, while from leaves, branches and sprays as well as root surface radionuclides get into plant tissues and join the biological metabolism. These processes have not been evaluated sufficiently yet, though it is important from both biological and environmental point of view. After evaluating the radionuclide penetration into trees through their above–ground part and roots, radionuclide accumulation and distribution in a tree can be modeled and forecasted as well as a reverse task can be solved – their volume activity at any soil depth can be determined according to the radionuclide specific activity in the tree ring of the corresponding year. Objectives of the work: To determine the accumulation of the artificial radionuclide 137Cs in soil and a tree, to define coefficients of transfer from soil to various woody parts of plants. To forecast the variation of the long–term soil contamination with radionuclides according to their accumulation in tree parts. To develop the model of radionuclide transfer in the system “soil–tree”. Tasks of the work: · To apply the methods of sample collection and their preparation for radiometric measurements and implement the intended equipment. · To master and apply the methodology of determining the 137Cs specific activity in wood. · To determine the dependence of radionuclide specific activities in tree rings on the pollution density at different soil depths, the type of soil, chemical composition, precipitation amount and other factors. · To determine the distribution of the 137Cs specific activity in the vertical soil profile and tree segments. · To define more precisely the distribution of the radionuclide volume activity in soil determined by the method of “tree rings” by modeling their migration in the system „soil–tree”. The novelty of the work · The radionuclide penetration into trees not through their external above–ground part but also through the roots has been evaluated. According to the investigation results the radionuclide accumulation in trees can be determined, and the absorbed dose, by which trees were irradiated not only externally but also in their segments during separate periods, can be forecasted. · According to the radionuclide accumulation in tree rings the soil contamination during separate years has been modeled and forecasted as well as the surface density of radioactive depositions has been determined. · The accumulation of natural and artificial origin radionuclides in soil in the tree influence zone has been determined. · The model enabling evaluation of the radionuclide migration in soil under the tree crfwn has been developed, and the characteristic of the pine (Pinus sylvestris L.) and a birch (Betula pendula L.) to accumulate radionuclides has been evaluated. · The method of the tree ring separation using chisels–choppers has been improved. Practical significance of the work According to the obtained results the absorbed radiation doses in woody plants of the investigated region during different pollution periods can be evaluated. Referring to the research results, the radionuclide exchange in the system “soil–tree” can be determined. The interruption of this exchange and the distribution of radionuclides appearing due to human activity using wood in construction and furniture industry can be evaluated. Data on the radionuclide accumulation in various trees can be useful in investigating the tree upgrowth and spreading of tree diseases. Moreover, in future referring to the results of the work, human exposure using wood in domestic and industrial premises can be evaluated. This work is also significant because after determining coefficients of its transfer from soil of various types to woody plants the total amount of artificial origin radionuclide 137Cs accumulated by the forest system of Lithuania and its regions can be evaluated. Approbation of the work The dissertation results were reported at four international and six republican scientific conferences. On the subject of the dissertation 2 scientific papers were published in foreign peer–reviewed journals (one in ISI system journal), 3 papers in Lithuanian technical peer–reviewed journals, 1 publication in proceedings of international conferences and 7 publications in proceedings of republican conferences, 1 claim for invention is presented.