Objective
Very few data are available on the deposition of radionuclides onto tree canopies and their subsequent biogeochemical pathways within forest ecosystems. However, the Chernobyl accident has highlighted the importance of such ecosystems and their possible contribution to the ultimate dose to man. The objectives of the programme are to produce data by field and laboratory investigations on the deposition and uptake of aerially deposited radionuclides into forest ecosystems and their subsequent fate, with a view to validating and refining models which are currently being developed on transfer within such ecosystems and their dependency on geographical location. This will lead to a substantially greater understanding of the mechanisms of wet and dry deposition of radionuclides in tree canopies and provide information on the amounts of radioactivity which will reach the soil in long term, as a result of processes such as leaching, resuspension, weathering of leaf surfaces, translocation and leaf fall.
Work has been carried out to characterize the processes involved in the dry deposition of caesium labelled aerosols to a canopy of spruce saplings arranged in a wind tunnel. Analyses were by instrumental neutron activation analysis (INAA). 8 categories of tissue were defined in the sampling of the saplings. Deposition velocities (Vg) were calculated for each tissue category, defined as flux to plant surface over air concentration. The values of Vg for deposition to the soil surface, at positions either directly under trees or between trees, were also determined and are significantly different, indicating that deposition was greatest underneath trees. A series of interception fractions (r) were also calculated for each tissue category to measure the efficiency of capture of the aerosol by the canopy. The total interception fraction, calculated for the canopy as a whole, indicates that 99.8% of the depositing aerosol was intercepted by the canopy with 0.2% reaching the soil surface. The derived values of Vg and r correlated well with the density of tissues within upper and lower regions of the canopy although the deposition of aerosol to each of these regions seems to be distinct from the other. The correlations obtained were also negative which is the inverse of the relationship between aerosol capture and herbage density commonly seen in herbaceous canopies.
An experiment to determine the loss of radioactive contamination from the leaf surfaces of spruce trees under field conditions has also been carried out. Loss of both caesium-137 and cobalt-60 followed an approximately exponential pattern with time. Biological half lives were calculated as 21.8 days for caesium-137 and 12.4 days for cobalt-60. Analysis of variance suggests that the rate of loss of these 2 radionuclides is significantly different with caesium-137 being lost more slowly than cobalt-60.
A study is under way to determine, in a wind tunnel, the deposition rates of dye laden particles on twigs of spruce trees as a function of particle size and wind speed, these data being necessary to determine, by means of a model, the depositionrates of these particles on forest stands of given architecture. As deposition rates on spruce twigs formerly obtained were high compared with other obstacles, attention has been focused on the conditions of the experiments which might induce electrical charges differing from thos encountered in the field which in their turn possibly influence the deposition rate. An ultrasonic generator was used. The emphasis in the experiments was to determine the effects of tracer particle diameter on deposition rates by exposing spruce twigs to aerosls of different aerodynamic mass median diameter (MMD). The deposition rates refer to the developed surface area of leaves and were obtained under standard conditions for a wind speed of 5 m/s. They show that the foliar depostion rate (in the size subrange of interest) does not change very much with the MMD of the aerosol. Its average value is about 3E-2 cm/s. The value obtained is 4 times lower than that formerly obtained without taking care of electrical charges. Other experiments have shown that, in this size subrange, the foliar deposition rate was nearly proportional to the square root of the wind speed.
A resistance model for deposition of particles to a forest canopy of given architecture is under development. The resistances either represent resistance turbulent diffusion of particles between layers in the canopy or the boundary layer resistance to transfer of particles in a given stratum to the canopy components. The latter are directly related to the foliar deposition rates measured in the experimental part of the work.
Data is being derived on the deposition and recycling of aerosols within instrumented catchments situated in the Prades mountains of northeast Spain. Methodologies have been established for: leaf aerosols; sampling; field experimentation on the role of litter decomposition and faunal activity in the radionuclides migration in the forest floor and mineral layers; and the measurement of gamma activity. The results of the study of aerosols on leaf surfaces show that desposited particles on the leaves are nonspherical and mainly consist of soil material (silicon, aluminium, potassium and iron). Size distribution data indicate that most of the deposited particles have a diameter larger than 10 um. Particles are located in the adaxial surface of the leaf and their distribution is not regular. If not there, they are retained in the trychomes of the basal part of the leaf near the mid vain.
Tree canopies had no detectable radioactivity or radioactivity coming mainly from Chernobyl. The activity detected in leaves sprouting after the Chernobyl accident was attributed to retranslocation from older ones. Total caesium-137 activity in the litter layer increased from litter to humus and was related to the degree of litter decomposition. Radiocaesium activity in the upper 15 cm of mineral soil came mostly from weapon fallout deposition.
In the migration studies, the radionuclide percentages in the forest soil layers, after 15 and 45 days of polluted leaves field incubation, show that there is an evident evolution with time and that radionuclide migration decreased according to the sequence of strontium approximately the same as caesium and much greater than silver.
The physicochemical characteristics of thermo generated caesium uranium oxide aerosols are under study. Binding energy values of care and valence electrons have been determined in synthetic caesium uranates by X-ray photoemission spectroscopy (XPS). A study of thermo generated aerosols by XPS and X-ray diffraction (XRD) allowed determination of the chemical nature of the aerosol surface. The core of the aerosols appears to be constituted mainly of caesium uranates, which are insoluble, while the surface of the particles is greatly enriched in soluble caesium.
The migration ability of caesium in forest soil columns (undisturbed profile), following a surface contamination, has been investigated. This study has suggested the risk of misleading interpretations of activity measurements if expressed on a weight basis. Caesium speciation suggests that mineral components, combined with the organic fraction in upper organic horizons, could play a role in the high caesium retention in the surface layer of the soil.
A comparative synthesis of the distribution in the different compartments of a Belgian (Vielsalm) and a Soviet (Chernobyl) forest ecosystem has been produced. Sequential extractions of forest soils from Chernobyl have shown their weak capacity to desorb caesium. Transfer from soil to plant seems related to root system development.
The in situ determination of aerosol deposition mechanisms is still in progress. Rain water and aerosols are regularly collected above and under coniferous canopies and the results obtained are under examination and interpretation.
Characterized radionuclides or surrogates in the form of aerosols and droplets will be applied to tree canopies in the field and laboratory studies. Aerosols containing radionuclides in the physicochemical form arising in an accident will be simulated. These and other types of aerosol will be applied to the foliage of various species under controlled aerodynamic conditions in a wind tunnel and/or in the field. Wet deposition studies will be performed using a rain simulator. A complementary study will be carried out in the field measuring interception by spruce of natural aerosols and precipitation.
Loss of radionuclides will be measured in both the field and laboratory, and weathering half lives, resuspension, foliar absorption, translocation and leaching quantified. Comparative investigations will be carried out between Mediterranean and North European woodlands, with emphasis on the dynamics of radionuclide pathways in different ecosystem compartments, (eg litter and soils). Data from all the studies will be used to validate and refine mathematical models for radionuclide transfer after wet or dry deposition. Interlaboratory comparisons will aim at a high level of quality assurance.
The research programme is divided between 4 groups in the following way :
Imperial College (ICST) are undertaking controlled wet and dry deposition studies using a rain simulator and wind tunnel followed by measurements of field loss processes, including resuspension.
CEA, Fontenay are studying the same processes but under field conditions and also undertaking mathematical modelling of the transfer processes involved in a forest ecosystem.
UCL, Louvain are using aerosols generated in a simulated reactor accident system to study the physicochemical effects on deposition together with the effect of leaf surface characteristics on interception and retention. Field studies of natural aerosols deposited in North European and Mediterranean forest canopies enable geographical effects to be studied.
University of Barcelona are studying pathways of radionuclides in Mediterranean forest ecosystems both in the field and laboratory and these results will be compared with those of UCL, Louvain.
Fields of science (EuroSciVoc)
CORDIS classifies projects with EuroSciVoc, a multilingual taxonomy of fields of science, through a semi-automatic process based on NLP techniques. See: The European Science Vocabulary.
CORDIS classifies projects with EuroSciVoc, a multilingual taxonomy of fields of science, through a semi-automatic process based on NLP techniques. See: The European Science Vocabulary.
- natural sciences chemical sciences inorganic chemistry alkali metals
- natural sciences chemical sciences inorganic chemistry transition metals
- natural sciences chemical sciences inorganic chemistry alkaline earth metals
- natural sciences chemical sciences nuclear chemistry radiation chemistry
- natural sciences mathematics applied mathematics mathematical model
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Coordinator
LONDON
United Kingdom
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