Attitude of a group of Belgian stakeholders towards proposed agricultural countermeasures after a radioactive contamination: synthesis of the discussions within the Belgian EC-FARMING group.

In the case of radioactive contamination of the environment with an impact on the food chain, the remediation strategy will not only be based on scientific knowledge and technical experience, but will also be dictated by peculiarities of the country. These characteristics include the agro-industrial structure, the local and international economical contexts and the political configuration including the distribution of responsibilities and competencies. This paper identifies and illustrates the most relevant characteristics of the Belgian agricultural system and political environment; it also describes the past experience with food chain contamination, which is expected to influence the attitude of Belgian stakeholders, who would be involved in the setting up of countermeasure strategies for maintaining agricultural production and food safety. The picture drawn explains why several countermeasures aiming to reduce the contamination in food products, although scientifically sound and technically feasible, are hardly acceptable or even not acceptable at all, to the stakeholders.

Environmental monitoring and radioecology: a necessary synergy.

Environmental monitoring primarily aims through sampling or by the use of direct detection equipment to quantify the levels of radioactive substances and ionising radiation resulting from human activities and natural sources in the different compartments of the environment. Its objectives are very practical and include the quantification of the environmental sources of ionising radiation and the verification of compliance with regulatory requirements and permit limits for industrial, research and medical activities, as stated by their specific licence. Radioecology is a multidisciplinary science, which attempts to understand and to quantify the behaviour of radionuclides in the environment and the processes ruling their transport through natural and agricultural ecosystems to various receptors such as plants, animals and humans. A second facet of this science covers the assessment of the radiological dose to and effects on man and its environment from present, past or future, even hypothetical, nuclear activities. Despite their different immediate objectives, environmental monitoring and radioecology are complementary. Many examples illustrate the connections between these two approaches. For instance, transfer parameters generated by radioecological studies are necessary to estimate through models the radiological exposure of population, derive from the contamination level measured in a bio-indicator the quantity of radioactivity released from a nuclear installation, or identify potentially important pathways to be monitored. On the other hand, monitoring data will confirm important pathways suggested by radioecological modelling and provide site-specific data for the estimation of model parameters or actual data sets for the validation of transfer models.

Effects of low chronic doses of ionizing radiation on antioxidant enzymes and G6PDH activities in Stipa capillata (Poaceae).

Stipa capillata (Poaceae) seeds were harvested from a control area (displaying a gamma dose rate of 0.23 micro Sv h(-1)) (C plants) and from two contaminated areas (5.4 and 25 micro Sv h(-1)) on the Semipalatinsk nuclear test site (SNTS) in Kazakhstan. The plants were grown for 124 d in a greenhouse under controlled conditions and exposed to three different treatments: (0) control; (E) external gamma irradiation delivered by a sealed 137Cs source with a dose rate of 66 micro Sv h(-1); (E+I) E treatment combined with internal beta irradiation due to contamination by 134Cs and 85Sr via root uptake from the soil. The root uptake led to a contamination of 100 Bq g(-1) for 85Sr and 5 Bq g(-1) for 134Cs (of plant dry weight) as measured at harvest. The activity of SOD, APX, GR, POD, CAT, G6PDH, and MDHAR enzymes was measured in leaves. Under (0) treatment, all enzymes showed similar activities, except POD, which had higher activity in plants originating from contaminated areas. Treatment (E) induced an enhancement of POD, CAT, GR, SOD, and G6PDH activities in plants originating from contaminated areas. Only control plants showed any stimulation of APX activity. Treatment (E+I) had no significant effect on APX, GR, CAT, and POD activities, but MDHAR activity was significantly reduced while SOD and G6PDH activities were significantly increased. The increase occurred in plants from all origins for SOD, with a greater magnitude as a function of their origin, and it occurred only in plants from the more contaminated populations for G6PDH. This suggests that exposure to a low dose rate of ionizing radiation for almost a half century in the original environment of Stipa has led to natural selection of the most adapted genotypes characterized by an efficient induction of anti-oxidant enzyme activities, especially SOD and G6PDH, involved in plant protection against reactive oxygen species.

Use of the true absorption coefficient as a measure of bioavailability of radiocaesium in ruminants.

Limitations of existing methods to describe the bioavailability of dietary radionuclides to ruminants (the transfer coefficient and apparent absorption coefficient) have led to the alternative suggestion of using the true absorption coefficient (A(t)). Various approaches to estimating A(t) for radiocaesium, involving the intravenous administration of a second isotope, are presented and discussed with reference to results from studies in which a range of radiocaesium sources were examined in sheep. Although estimates of A(t) differed between the sources, they were reasonably consistent between measurement techniques. Those methods which involved the estimation of endogenous faecal excretion of radiocaesium could be used with previously contaminated animals and did not require continuous administrations of radiocaesium isotopes, but gave unreliable results for sources of low bioavailability. Methods based on estimating the turnover rate of dietary radiocaesium through blood plasma were sufficiently sensitive to measure A(t) for the range of sources studied. However, they require previously uncontaminated animals and continuous administration of both isotopes for approximately 7 days. Bioavailability is more effectively measured as A(t) than as the transfer or apparent absorption coefficients since A(t) does not incorporate factors relating to the metabolism of radiocaesium in the tissues of the animal. The results of these studies show that differences in transfer coefficients between sheep and cattle and between sheep of differing ages are not due to variation in absorption across the gut. The potential for applying these approaches to other radioactive elements is discussed.

Development and testing of a revised dynamic model of radiocaesium transfer to sheep tissues.

The model of radiocaesium transfer to sheep presented by Galer et al. provides reliable predictions only for sheep of a similar body weight to those used in the development of the model (approximately 30 kg). To extend the applicability of the model, it was necessary to re-parameterise it in terms of activity concentrations in tissues rather than total activities within them (although for gut compartments the use of activity has been retained). The rate coefficients for the new model have been estimated by fitting the model to the data used by Galer et al. which was derived from a single "calibration" experiment. The new model was found to account for 94% of the observed variation in the data (n = 42), a result similar to that obtained by Galer et al. The model has also been tested against data not used in its development but obtained from four separate experiments undertaken by three different laboratories. Good agreement between the predictions of the new model and observations was found for most circumstances and for several breeds of sheep with different body weights. It is concluded that the new model provides a useful dynamic description of radiocaesium transfers to the tissues of sheep of different breeds and under different contamination scenarios.

The use of an in vitro technique to predict the absorption of dietary radiocaesium by sheep.

The validity of an in vitro extraction technique to predict the availability for absorption of radiocaesium in the sheep gut has been assessed. The technique (a 2-h extraction with caesium chloride) was found to be valid for sources with a low availability for transfer across the gut, but inappropriate for ionic radiocaesium or radiocaesium incorporated internally within herbage. For such radiocaesium sources, which have a high in vitro availability, no correlation was found between in vitro extraction and true absorption measurements. A true absorption value of 0.80 is recommended for these sources, although values for individual sheep in the range 0.60 to 1.00 should be expected.

Radiocaesium metabolism in pregnant ewes and their progeny.

Pregnant ewes have been chronically contaminated during pregnancy and lactation with Cs-134 administered as CsCl in the food. Plateau concentrations in excreta and blood were reached after about 20 d of contamination. At delivery, the contamination levels in lamb's tissue were lower than those in the corresponding organs from an ewe; they increased rapidly as the lambs received milk from their contaminated dam, up to higher levels than in ewes. The transfer coefficient were estimated in various tissues from an ewe slaughtered at delivery (muscles 0.57 d/kg, kidneys 0.50 d/kg, liver 0.33 d/kg, blood 0.02 d/kg) and milk (0.09 d/l). During the decontamination phase, the radioactivity was excreted from the ewe's body according to a two exponential pattern with half-times of about 1.3 and 20 d. The retention in muscles, kidneys and liver from the ewes was characterised by two compartments with longer half-times of 2.3 to 2.9 d and 123 to 174 d respectively. The contamination levels in organs from lambs separated from their dam 3 days after birth and fed uncontaminated artificial milk decreased with a global half-time of about 10 d.

Assessment of dose to man from releases of 99Tc in fresh water systems.

The objective of this paper is to evaluate the dose to man from releases of 99Tc in a fresh water system and to identify the biospheric transfer parameters to which the total dose is the most sensitive. Only internal exposure is taken into account, as the external irradiation leads to a negligible dose contribution. Two release modes were considered: continuous (routine) releases and accidental releases. The concentrations in the biospheric compartments subsequent to routine releases were calculated according to International Atomic Energy Agency procedures. For the accidental releases, a more dynamic approach was adopted, especially for the milk and meat compartments. A routine-release scenario typical for the Mol site has been applied, and the biospheric compartment leading to the highest dose contribution was shown to be the irrigated grain. The biospheric transfer parameters to which the first-year doses were the most sensitive consisted mainly of the mass interception factor for grain and the milk transfer factor. The doses in following years were very dependent on the value of the root zone removal rate. The accidental-release scenario resulted in committed dose equivalent that are strongly influenced by the time of year at which the release occurs.

Technetium absorption and turnover in monogastric and polygastric animals.

Technetium (Tc) released into the environment can reach animals in various chemical forms: as pertechnetate (TcO-4) in drinking water or deposited on the surface of vegetables and forage plants, or as Tc bioincorporated into plants and associated with various plant constituents. In addition to being influenced by chemical speciation in the diet, absorption, metabolism, and retention of Tc in animals are modified by the treatment that the alimentary bolus undergoes during its passage through the gastrointestinal tract. This behavior differs markedly between polygastric and monogastric animals. We have, therefore, studied the fate of 99mTc given in the diet either as TcO-4 or bioincorporated into maize in rats (as an example of a monogastric animal) and in sheep (as an example of a polygastric animal). Urine and feces were collected and assayed for Tc activity by gamma spectrometry. Animals were sacrificed at different times after contamination, and the Tc content of tissues was determined. The pattern of absorption, excretion and, to a certain degree, of organ distribution and retention depended on animal species and species of Tc administered. Excretion was by feces and urine, and several metabolic components could be discerned. A component of very short half-time in urine suggests that newly absorbed Tc is more readily excreted than that already bound by tissues. The highest tissue concentrations were found in the thyroid. Retention of Tc was, however, most pronounced in bone and skin. Hair contains considerable amounts of Tc and may serve as a bioindicator of Tc contamination.

Long-term availability of Tc deposited on soil after accidental releases.

Considerable uncertainties are associated with the transfer of Tc from soil to plant. Mathematical models built to simulate that transfer generally use a single value derived from short-term experiments. Recent observations have, however, given a clear indication that the soil-to-plant transfer varies with time, depending on the biogeochemical cycling of this element. The long-term behavior of Tc in soils accidentally contaminated has been studied for 4 y in lysimeters exposed to natural climatic conditions. An important fraction of the deposit (70%) is rapidly removed from the 20-cm plow layer with a half-time of 54 +/- 2 d; the remaining fraction tends to persist in the soil for long periods of time (half-time = 30 +/- 16 y). Immediately after the deposit, the plant-to-soil concentration ratios are very high (about 400); they decrease to six 3 y after the contamination. The shape of the evolution of the contamination measured in grass after an accidental deposit is best-fitted to a sum of two exponential functions; the environmental half-times estimated for the two components are about 30 d and 2 to 3 y, respectively.

Bioavailability of Tc incorporated in plant material.

The effective removal rate of radiopollutants from contaminated soil usually has been estimated by considering only the radioactive decay constant. This approximation, however, becomes a matter of concern when assessments are performed for long release periods, for long-lived radionuclides and for radionuclides exhibiting relatively high values of plant-to-soil concentration ratios. Previous results obtained for 99Tc showed that phenomena other than radioactive decay may be of importance in soil depletion and that uptake by plants, as well as mechanisms affecting the availability, must be taken into account. In agricultural practice, ignorance of these mechanisms may lead to inaccurate predictions of dose levels. Harvest losses have already been theoretically discussed, and removal constants of soil radioactivity from harvest and leaching have been estimated. Validation of the soil-plant model for Tc including these depletion processes is thus necessary, and minilysimeter experiments were conducted. Contaminated plant material was reincorporated into the soil and was allowed to undergo humification; plants were cultivated on these soils and their Tc uptake was studied. Results showed that an important part of the recycled, bioincorporated Tc is immediately and highly available to plants. The results are discussed within the framework of the plant-material degradation.