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Radiation doses and pathways to man from semi-natural ecosystems

Objective


In order to determine which pathways are of most importance and how the various radionuclides are partitioned within a typical semi-natural ecosystem, all participating laboratories co-operated in a joint study on the inventory and distribution of caesium-137 and strontium-90 in a semi-natural woodland ecosystem in Sweden. A similar study was also carried out in a Danish pine forest by the Risoe National Laboratory.

A study to explain the transfer of radionuclides within semi-natural ecosystems was undertaken at NERC. As part of this work a new caesium-137 sequential extraction technique for soils with relatively low radiocaesium contamination has been developed. This procedure is considered to be more representative of field conditions than existing techniques, most of which work with dry soils. The method was subsequently applied to peat samples taken from a range of heather dominated ecosystems grown on organic soils in Scotland, England and Ireland. The results show that, in general, the caesium-137 activity concentrations in heather plants are more highly correlated with water extractable caesium-137 than with the total caesium-137 content of the soil. The ammonium acetate extractable caesium-137 is also likely to be a better indicator of caesium-137 availability for heather root uptake than the use of traditional transfer factors for organic soils.

Laboratory experiments to investigate the Cs/K and Sr/Ca relationships have been undertaken by the University of Thessaloniki using a range of plant species typical of semi-natural ecosystems. In the experiments the effects of nutrient status, physico-chemical characteristics of the soil and the root distribution of plant species on Cs and Sr uptake were evaluated.

An investigation of the factors affecting Sr uptake from soil to plants showed that C.E.C. and available Ca were the parameters of most importance. The uptake of stable Cs and Sr by plants with the different forms of the elements in the soil was also investigated by means of a pot experiment followed by sequential extraction techniques.

Methods of characterising and quantifying the available radiocaesium, other than the traditional transfer factors, have been investigated by the RPII. A study of site variability shows that there is less variability in the caesium-137 content of Calluna vulgaris Juncus squarrous than of the underlying soils. The use of plant-to-plant ratios instead of the traditional ratio systems also yields better results as these ratios exhibit less variation.

A series of studies of the transfer of radiocaesium from the forest ecosystem to man have been carried out at the University of Uppsala. Experiments investigating the transfer of radiocaesium from soil to various forest plant species show that species related differences are more important factors in determining this transfer than soil related parameters. Particular attention was given to the fungal species as these are known to be significant accumulators of caesium-137.

In Ireland, a study of the accumulation and retention of caesium-137 in peatland vegetation was carried out between 1989 and 1993 by the RPII. Most species showed a seasonal pattern in caesium-137 concentration with higher concentrations in summer.

In the Faroe Islands, Risoe has been measuring the levels of caesium-137 in milk since 1962. Effective half-life calculations for caesium-137 derived from weapons tests yield a value of 3 years, while the value for caesium-137 derived from the Chernobyl accident is 2 years. This difference in value for caesium-137 from different sources has been observed for other ecosystem compartments, but it is expected that, over time, any difference will be eroded as the more recent radiocaesium deposited following the Chernobyl accident becomes fixed to the soil minerals. The estimated effective ecological half-life for strontium-90 in Faroese milk is 4 years. This value may also change (particularly in areas close to the Chernobyl region) if strontium-90, initially deposited as insoluble particles, dissolves and becomes available for plant uptake.

As a final step in this project, Risoe National Laboratory co-ordinated an assessment of committed average doses to individuals in Denmark, the U.K., Ireland, Greece and Sweden arising from Chernobyl contamination.
CONSUMPTION OF FOODSTUFFS PRODUCED FROM SEMI-NATURAL ECOSYSTEMS CONTAMINATED WITH ARTIFICIAL RADIOACTIVITY CAN CONTRIBUTE A SIGNIFICANT RADIATION DOSE OVER SEVERAL DECADES TO THE GENERAL POPULATION AND TO CRITICAL GROUPS IN PARTICULAR. MANY FACTORS CONTROLLING THE UPTAKE OF Cs-137 ARE NOT YET FULLY UNDERSTOOD AND VERY FEW DATA ARE AVAILABLE FOR Sr-90. THIS LIMITS THE DEGREE TO WHICH RADIATION DOSES FROM SEMI-NATURAL ECOSYSTEMS CAN BE EVALUATED AND ALLOWS LIMITED PREDICTIVE CAPABILITY FOLLOWING DEPOSITION OF RADIONUCLIDES.

THE FOLLOWING SPECIFIC TOPICS WILL BE ADDRESSED AS PART OF THE RESEARCH CONTRACT.
- THE IMPORTANCE OF FUNGI IN DETERMINING THE RADIONUCLIDES BURDEN OF GRAZING ANIMALS WILL BE STUDIED. SEASONAL, SITE AND YEAR-TO-YEAR VARIABILITY WILL BE ASSESSED AND THE EFFECTS OF CLIMATE ON PRODUCTION OF FRUITBODIES EVALUATED. THIS ALLOW A BETTER ESTIMATE OF RADIONUCLIDE TRANSFER TO MAN. - LABORATORY STUDIES USING PLANT SPECIES TYPICAL OF SEMI-NATURAL ECOSYSTEMS WILL INVESTIGATE Cs/K AND Sr/Ca RELATIONSHIPS. THE EXPERIMENTS WILL INCLUDE THE EFFECTS OF NUTRIENT STATUS, PHYSICO-CHEMICAL CHARACTERISTICS OF THE SOIL AND ROOTING DISTRIBUTION.
- THE USE OF TRANSFER FACTORS, DERIVED FOR AGRICULTURAL ECOSYSTEMS HAVE PROVEN TO BE LESS APPROPRIATE FOR SEMI-NATURAL ECOSYSTEMS. FACTORS SUCH AS Cs-EXTRACTABILITY, SEASONAL VARIABILITY IN UPTAKE, DISTRIBUTION WITHIN SOIL PROFILES AND ECOLOGICAL HALF-LIVES WILL ALL BE STUDIED WITH A VIEW TO REFINING CURRENT DEFINITIONS TO PROVIDE MORE RELIABLE PREDICTIVE CAPABILITY. - A STUDY OF STABLE STRONTIUM LEVELS AT A DISUSED MINE IN AN UPLAND AREA OF SCOTLAND WILL PROVIDE DATA ON POSSIBLE INDICATOR SPECIES FOR Sr-90 UPTAKE. THE USE OF GALLUNA VULGARIS AS A BIOINDICATOR FOR Cs-137 WILL BE FURTHER EXAMINED.

AN IMPORTANT PART OF THE PROGRAM WILL BE ECOSYSTEM COMPARISON AND EVALUATION OF THE EXTENT TO WHICH RESULTS OBTAINED IN ONE COUNTRY CAN BE APPLIED IN ANOTHER. A SERIES OF INTERCOMPARISON EXCERCICES FOR DETERMINATION OF STABLE AND ARTIFICIAL RADIONUCLIDES AND OF SAMPLING TECHNIQUES WILL BE PERFORMED.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

RADIOLOGICAL PROTECTION INSTITUTE OF IRELAND
Address
Clonskeagh Square 3
14 Dublin
Ireland

Participants (4)

Aristotle University of Thessaloniki
Greece
Address
University Campus
54006 Thessaloniki
Natural Environment Research Council
United Kingdom
Address
Windermere Road
LA11 6JU Grange-over-sands - Cumbria
Risø National Laboratory
Denmark
Address

4000 Roskilde
The Swedish University of Agricultural Sciences
Sweden
Address

750 07 Uppsala