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BEHAVIOUR OF Cs AND Sr IN NATURAL ECOSYSTEMS AND THE POTENTIAL RADIATION EXPOSURE OF THEIR EXTENSIVE USE

Objective

The Chernobyl accident led to an enormous amount of measurements of 134Cs and 137Cs activities in, for example, wild berries and mushrooms. These data, which vary widely, are helpful for evaluating actual radiation exposure to man, but they are of no help in understanding the behaviour of radionuclides in forest ecosystems. The project Cycling of Radiocaesium and Strontium in Natural Ecosystems, investigates the fate of radiocaesium and strontium 90 in natural ecosystems in Belgium, Germany, Sweden and Italy, in order to improve the knowledge of the cycling mechanisms and the understanding of their short and long term behaviour.
The transfer of caesium-137 and strontium-90 from different soil horizons into mushrooms and plants has been investigated in different natural forest ecosystems. The influence of different parameters such as potassium and calcium on the transfer of the radionuclides has been analyzed. On the 3 selected coniferous ecosystems, activities between 5000 and 25000 Bq/m{2} have been measured for radiocaesium in the undisturbed soils in 1988. Between 85% and 95% of the activity was found in the organic O and L horizons. The strontium-90 activities range between 280 and 630 Bq/m{2}, implying that a large part of the total activity is due to fallout from nuclear weapons tests. Still 40% to 85% of the total activity was found in the upper O and L horizons. Laboratory experiments on the Kd value indicate that initially caesium is weakly bound in organic horizons. The Kd values in mineral horizons are similar to indicate that initially caesium is weakly bound in organic horizons. The Kd values in mineral horizons are similar to values found on farmland. The mobility of strontium is about the same in organic and mineral horizons.

Significantly higher caesium-137 activities have been measured in symbiotic species of mushrooms (50 to 20000 Bq/Kg freshweight) whereas saprophytes (less than 5 to 3000 Bq/Kg freshweight) and parasites (less than 5 to 1300 Bq/Kg freshweight) show generally lower values. The strontium-90 activities in mushrooms were below 1 Bq/Kg freshweight.

Autotrophic plants which are rooting mainly in the organic soil horizons have caesium-137 activities which are not significantly different from those in most of the mushrooms. The values ranged between 120 Bq/Kg freshweight for raspberries and 1340 Bq/Kg freshweight for blueberries. The behaviour of strontium-90 was similar to caesium-137 in green plants. The measured activities ranged between 7 Bq/Kg freshweight in pine needles and 37 Bq/Kg in blueberries.

Sampling has been undertaken at 2 forest sites, desiduous trees in Luxembourg (Mersch) and coniferous trees in Belgium (Florenville). The sampling techniques and methods were tested at both sites. There are also 5 secondary sampling sites with contrasting soils. At each site soil samples, tree parts (roots, stems and leaves) and herbaceous plants growing above sampled soils were collected in summer while in autumn fruit bodies of the main mycorrhizic and saprophytic species were collected. Similar sampling was done under a very contaminated pine stand, north of Gomel in Russia.

In total, 356 samples of plants, mushrooms and soil were taken from the sites. Half of them have been analyzed for their caesium-134, caesium-137 and potassium-40 content. Initial results highlight the importance of microsites under forest canopy in radionuclide transfer. These microsites influence the radionuclide availability and the rooting of plants (or the mycelium localization) in the different soil horizons. They also influence levels of radionuclide deposit on the soil, cocentrations in the different soil horizons and the density of herbaceous plants.

Work has been focused on diet composition and collection of muscle samples from moose killed during the hunting season. The caesium-137 activity concentration in muscle tissue in relation to deposition, diet, age and sex has been analyzed. Sampling of vegetation for biomass, nutritional and radionuclide analyses has been performed. Trapping of voles in different forest environments with different vegetation has been done, as well as sampling of muscle tissues from the red fox, a major predator on voles.

The major factors important for variation in radiocaesium activity in moose tissue, using a multivariate linear model, are deposition at collection site and year of collection. Age showed a negative correlation coefficient suggesting higher levels in calves. Other results are in preparation.

A study is being done on the distribution of radiocaesium in all plants present in a natural forest in order to analyze the main factors responsible for radiocaesium transfer within the ecosystem. Samples of vascular plants, lichens, bryophytes and mushrooms have been taken. The average depth of the root systems of all the vascular plants has been estimated to study the relations between total contamination of the plants and their root absorption from different soil layers. A first statistical elaboration, of approximately 40% of the measurements, showed a very high degree of correlation between radiological data and the ecology of the species.

Higher plants, mushrooms, lichens and bryophytes were considered in a study to find a suitable bioaccumulator for a rapid production of deposition maps in mountain areas. Bryophytes appeared to be the most suitable and a test study was carried out on 9 species. The absorbing capacity for each species has been measured in the laboratory with samples collected from different inclinations and positions in relation to the trees. The relationship between contamination and thickness of the moss carpets was also studied. A sampling strategy for deposition mapping in the Carnic Alps has been adopted following these results.

The caesium budget for a terrestrial boreal forest ecosystem has been analyzed. Physical decay governs the change in the content of caesium-137 and, to a greater degree, caesium-134. A significant output (5 to 10%) occurs by water leaving the catchment area. This constituts a considerable source with regard to input to the aquatic recipients, particularly fresh water fish.

The redistribution of a primary deposit of radioactive caesium between the boreal forest compartments participating in the caesium turnover seems to be fast. Similar proportions of the old (nuclear weapons) and new (Chernobyl) caesium-137 content within the pine tree, perennial plants and the upper organic horizons in the soil was obtained in less than 5 years after the Chernobyl accident. This is not the case in those fractions of the system with a known slow turnover (eg lichens, mosses and the inner core of large trees) which constitute secondary sources of considerable importance in the long term behaviour of caesium-137.

The ratio of concentration of caesium-137 in moose meat to cummulative ground deposition per unit area is not significantly different over the 5 years, probably due to the slow release of caesium-137 from the secondary sources.

The potential contribution to the collective dose to the Swedish population from forest products, such as moose and berries, is at the same level as meat, milk and milk products from the agricultural area, assuming that caesium will behave similarly in the future as in the past 2 decades. There are several factors, however, such as environmental pollution which may affect the behaviour of caesium.

The last link in the ecological transport chain of caesium in the boreal forest to man is under study including methods for the assessment of body content in different critical groups and in the general population. The concentration of caesium in moose meat is a significant contribution to the amount of caesium in humans. Samples have been obtained from moose killed in traffic accidents and during the hunting season to build up a picture of the local activity concentration variation.

Alternative methods to whole body measurements, for assessing the internal contamination of gamma emitting radionuclides in humans, have been investigated.
The first method is the analysis of human tissues from antropsy or surgical procedures. A representative sample of the population can be selected and adjusted with respect to age and sex. Measurements were carried out on 3 individuals to check the procedure and calculations involvd and a fairly good agreement was obtained.
The second method is the measurement of activity concentration in excreta. If the biological half life is known, the body burden, under equilibrium ingestion and excretion, can be determined by this method. It was tested on a group of Laplanders whose whole body content is measured regularly. There is a linear relationship between daily urinary excretion and total body activity measured simultaneously. If equilibrium is assumed, the slope of the fitted line is proportional to the average half life of caesium in the studied population. The same slope was obtained on the 2 occasions that both measurements were made.

Parameters of relevance are under investigation in order to model the dynamic and retention of caesium and strontium in a forest environment. The study involves the interaction of nuclides in different soil systems and the accumulation and redistribution of caesium and strontium in trees and understory vegetation. Sampling has been done in a mixed stand of spruce and pine exposed to fallout from nuclear bomb tests. As well as the study on the dynamic of caesium and strontium in the environment, the relationship between the caesium and potassium content in different compartments in the trees has been studied by potassium-40 measurements.

The interaction of caesium with soil constituents is under study in 8 soil systems with different stand and soil characteristics. The soils investigated have been classified as cambisols, podsols and histosols below stands of pine, spruce, birch, beach, alnus and oak. Sampling has been conducted in order to study the partial equilibrium of caesium from the fallout in the different soil systems and the interaction and distribution with time of caesium deposited in 1986. The areas have been characterized in respect of soil classification, clay and organic matter content, pH and cation exchange capacity.

The dynamic of caesium in a forest environment during the period immediately after the fallout from the Chernobyl accident is under study in a heavily contaminated Scots pine stand. Sampling of soil and vegetation has been performed immediately after the accident and 4 years later. Measurements of the samples are in progress.
Seven laboratories from Belgium, Italy, Sweden and Germany are involved in this research project. The investigations take place in different forests sites in their countries. For joint investigations, boreal forests in Sweden and beech forests in the Italian Alps have been chosen.

The experimental strategy is focusing on the fate of radiocaesium and strontium 90 after their deposition in forest ecosystems. One major point is the uptake of caesium and strontium from soil to plants. Until now it is not completely understood why the uptake rates of caesium in natural ecosystems are significantly higher than on agriculturally used areas. Natural ecosystems are characterized by undisturbed soils with organic and mineral horizons. In a first step, the distribution of radiocaesium and strontium 90 within the different layers are measured. To improve the knowledge of the plant availability, soil parameters like nutrient concentration, exchange capacity, kd values, etc. are determined. Additionally, plant parameters such as mycelium and rooting depths, potassium and calcium distribution within the plants are analyzed. The importance of various parameters on the transfer of caesium and strontium will be analyzed by statistical methods.

The plant and herbivore interactions are analyzed by a special programme on moose. Moose meat contributes to about 5% to 10% of the average meat consumption in Sweden and is therefore of radiological interest. To obtain the radionuclide intake rate, the migration of single moose is observed during the year as well as their seasonal consumption habits. Later they are shot and caesium 134 and 137 activities in meat are measured. The correlation of radiocaesium and potassium intake rates will be tested.

For long term considerations, the loss of radionuclides from the ecosystems by migration and runoff is analyzed. Further studies are concerned with the distribution of caesium and strontium within the ecosystems, the antagonisms of caesium and potassium as well as strontium and calcium, etc.

To test how the caesium 134 and 137 activity in forest ecosystems is reflected in human populations, whole body activities are measured in groups which preferably consume mushrooms, wild berries and moose meat. Their results will be compared with measurements of persons whose consumption rates of these products are low.

The natural ecosystems will be modelled at least on three levels : in a first step, different equations for each transfer will be tested for their reliabilities. In a second step the distribution and cycling of caesium and strontium within the whole system under short term and long term aspects will be described. On the third level, the potential radiation exposure of man by the extensive use of natural ecosystems will be calculated.

Coordinator

BUNDESAMT FUER STRAHLENSCHUTZ
Address
1,Ingolstädter Landstraße 1
85764 Oberschleissheim
Germany

Participants (6)

National Defence Research Establishment
Sweden
Address
20,Cementvägen
901 82 Umeä
Statens Strälskyddsinstitut
Sweden
Address

104 01 Stockholm
THE SWEDISH UNIVERSITY OF AGRICULTURAL SCIENCES
Sweden
Address
10,Arrheniusplan 2C
750 07 Uppsala
UNIVERSITE DE LIEGE*ULG
Belgium
Address
Place Du 20 Aout 7
4000 Liege
UNIVERSITÀ DEGLI STUDI DI TRIESTE
Italy
Address
Via Giorgieri 10
34100 Trieste
University of Umeä
Sweden
Address

901 87 Umeä