The influence of season and leaf age on concentrations of radiocaesium (137Cs), stable caesium (133Cs) and potassium in Agrostis capillaris

The transfer of radioactive caesium from soils to plants has been well researched. In contrast there is limited knowledge on natural stable 133Cs and its potential role as a predictor for radiocaesium behaviour. In a pot experiment with Agrostis capillaris close correlations were found between plant 137Cs and plant 133Cs concentrations (R2 90-96%). Season and leaf age had significant effects with concentrations increasing 10-30-fold between June and December. Simultaneously the plant concentrations of K, the nutrient analogue of Cs, decreased to around one third. In the soil the exchangeable fractions of K and 137Cs declined. No clear relationships were found between 137+133Cs in the plant and exchangeable K in the soil. However, at the end of the experiment the K content of the above-ground biomass was higher than the exchangeable pool in the soil, suggesting that depletion of soil K could be a key factor in the observed increase of plant 137+133Cs over time.

Arbuscular mycorrhizal fungi can decrease the uptake of uranium by subterranean clover grown at high levels of uranium in soil

Subterranean clover inoculated or not with the arbuscular mycorrhizal (AM) fungus Glomus intraradices was grown on soil containing six levels of 238U in the range 0-87 mg kg(-1). Increasing U concentration in soil enhanced the U concentration in roots and shoots of both mycorrhizal and nonmycorrhizal plants but had no significant effects on plant dry matter production or root AM colonization. Mycorrhizas increased the shoot dry matter and P concentration in roots and shoots, while in most cases, it decreased the Ca, Mg and K concentrations in plants. The AM fungus influenced U concentration in plants only in the treatment receiving 87 mg U kg(-1) soil. In this case, U concentration in shoots of nonmycorrhizal plants was 1.7 times that of shoots of mycorrhizal plants. These results suggested that mycorrhizal fungi can limit U accumulation by plants exposed to high levels of U in soil.

Radiocaesium and natural gamma emitters in mushrooms collected in Spain

Mushrooms can accumulate heavy metals in general, including radionuclides found in the nature. However, little attention has been paid to the radioactive content of mushrooms collected in Spain and the dose for the population due to their ingestion. To address this, we analysed the contents of 137Cs, 40K, 226Ra and 7Be present in different species of mushrooms, according to their genus and nutritional mechanism. We observed that mycorrhizal mushrooms accumulate 137Cs more than saprophytes, and vice versa for 7Be. 40K and 226Ra are accumulated to the same degree by the two groups of mushrooms. We estimated the dose due to ingestion of mushrooms in Spain to be 2 microSv/year, and the contribution of 40K and 226Ra to be generally greater than that of 137Cs. The contribution of 137Cs to the dose was calculated by taking into account the results of an experiment carried out under the controlled laboratory conditions, which showed that approximately 98% of 134Cs was associated with the readily digestible fraction of the mushrooms.

Seasonal changes of redox potential and microbial activity in two agricultural soils of tropical Australia: some implications for soil-to-plant transfer of radionuclides

Very little is known of the factors controlling soil-to-plant transfer of radionuclides in tropical environments. As part of an IAEA/FAO coordinated research project (CRP) designed to elucidate some of those factors, near-surface samples of two agricultural red-earth soils (Blain and Tippera) were collected from a study site in the Northern Territory. The climate is tropical monsoonal with crops being grown over the wet season from December to March/April. It is important to understand soil variables that may be related to this dramatic seasonality. In this investigation, soil redox state and microbial populations were assessed before and after the growing season with a view to generating hypotheses for future evaluation. The X-ray absorption near edge structure (XANES) technique was used to determine overall changes in the solid-state redox speciation of Fe and Mn in soils across the growing period. Fe speciation did not change but approximately 10% of the total Mn was oxidised from Mn(II) to Mn(III) and Mn(IV) in both soils between October 1999 and April 2000. An apparent disconnect between Fe and Mn was not unexpected given the >10 times higher concentration of Fe in the soils compared with Mn. These results have implications for the bioavailability of redox sensitive radionuclides such as Tc and Pu. Similarly, microbial population estimates were derived before and after the growing period. Total bacterial populations did not vary from 10(6) to 10(7) colonies per gram. Fungal populations increased over the growing season from 3-6 x 10(5) to 1-4 x 10(6) colonies per gram of soil. Fungi have the potential to decrease soil pH and hence increase the bioavailability of radionuclides such as Cs. In addition, fungi act to facilitate plant nutrition. This could lead to enhanced accumulation of nutrient analogues (e.g. Sr and Ra for Ca; Tc for Mn), but this effect may be masked by improved biomass production.

The effect of sulfate-reducing bacteria on adsorption of 137Cs by soils from arid and tropical regions

Soils from different climatic regions of Australia were studied to determine their adsorption of (137)Cs, and the effect of microbial sulfate reduction on this adsorption. The soils consisted of a surface and regolith samples from the site of a proposed low and intermediate level radioactive waste repository in arid South Australia, and two red earth loam soils from an experimental plot in the tropical Northern Territory. The process of bacterial sulfate reduction substantially decreased the adsorption of (137)Cs to the arid and tropical soils, although extended incubation resulted in greater adsorption to the regolith sample. This could have implications for the mobility of radionuclides entering these soil ecosystems.

[Basic features of seasonal and multi-year dynamics of Cs-137 and Sr-90 in wood]

Basic features of seasonal and multiyear dynamics of accumulation of Chernobyl-derived 137Cs and 90Sr in wood are considered. Seasonal variation in the radionuclide concentration are shown to be more regular and predictable than the multiyear variation. Seasonal dynamics of 137Cs is opposite by trend to that of 90Sr. The multiyear dynamics of both 137Cs and 90Sr in the wood is variable and depends on chemical nature of individual radionuclide, type of landscape, kinetics of the radionuclide plant-available forms, and irreversible fixation of the radionuclides in the root-abundant soil layer.

Global fallout (137)Cs accumulation and vertical migration in selected soils from South Patagonia

The spatial distribution and vertical migration of global fallout (137)Cs were studied in soils from South Patagonia at the austral region of South America in semi-natural and natural environments located between 50-54 degrees S and 68-74 degrees W. The (137)Cs areal activity density varied from 222 to 858 Bq m(-2), and was found to be significantly positively correlated (p<0.001) with the mean annual precipitation rate. The fraction of the total activity density observed in steppe grass varied from <0.03% to 0.12% (median <0.07%) and is considerably lower than the results obtained at the South Shetland Islands (median 8%) and in other temperate environments in south-central Chile (median 0.2%). The median of the convection velocity v(s) of (137)Cs in the soil in such polar isotundra climate has been determined to be 0.056 cm y(-1). This value is higher than v(s) determined under polar climate (-0.012 cm y(-1)) and is near to the upper limit of v(s)-values determined in temperate environments from Chile (0.019 cm y(-1)). The median value of the diffusion coefficient D(s) (0.048 cm(2) y(-1)) is similar to D(s) observed in an Antarctic region (0.043 cm(2) y(-1)) and lower than D(s) in temperate regions of Chile (1.24 cm(2) y(-1)). About 35 years after the highest depositions, (137)Cs had penetrated to a depth of 6-14 cm in the Patagonian soils and can be expected to remain in the rooting zone of grass for many decades. Nevertheless, because of its low transfer to steppe grass observed at this region, the radioecological sensitivity of this ecosystem with respect to fallout radiocesium seems to be lower than in other polar regions.

Radiocaesium soil-to-plant transfer in tropical environments

In this work, soil-to-plant transfer factors of radiocaesium are predicted based on soil properties such as pH, organic matter content, exchangeable K+ and clay content valid for the tropical environments in Bangladesh, China and Japan, and using a previously published model. Due to insufficient data of soil properties in the selected regions, the average values of pH, organic matter content, exchangeable K+ and clay content were taken as the input model parameters within the ranges given for Asia. Nevertheless, a complete set of soil properties of Japanese soils was used to compare the measured and calculated TF values of radiocaesium for radish. The calculated TF values for radiocaesium are comparable with the measured values especially for leafy parts of a plant. However, calculated values for rice, an important crop in Asia are found to overestimate the measured values due to an overestimate of calculated CECs in soils in the selected regions. The empirical parameters used in the model need to be re-evaluated for the specific part of a plant and/or for a variety of different plants. Alternatively, a general conversion factor for each part of a plant and/or for a variety of different plants for a specific region is suggested for tropical environments.

Soil-water distribution coefficients and plant transfer factors for (134)Cs, (85)Sr and (65)Zn under field conditions in tropical Australia

Measurements of soil-to-plant transfer of (134)Cs, (85)Sr and (65)Zn from two tropical red earth soils ('Blain' and 'Tippera') to sorghum and mung crops have been undertaken in the north of Australia. The aim of the study was to identify factors that control bioaccumulation of these radionuclides in tropical regions, for which few previous data are available. Batch sorption experiments were conducted to determine the distribution coefficient (K(d)) of the selected radionuclides at pH values similar to natural pH values, which ranged from about 5.5 to 6.7. In addition, K(d) values were obtained at one pH unit above and below the soil-water equilibrium pH values to determine the effect of pH. The adsorption of Cs showed no pH dependence, but the K(d) values for the Tippera soils (2300-4100 ml/g) exceeded those for the Blain soils (800-1200 ml/g) at equilibrium pH. This was related to the greater clay content of the Tippera soil. Both Sr and Zn were more strongly adsorbed at higher pH values, but the K(d) values showed less dependence on the soil type. Strontium K(d)s were 30-60 ml/g whilst Zn ranged from 160 to 1630 ml/g for the two soils at equilibrium pH. With the possible exception of Sr, there was no evidence for downward movement of radionuclides through the soils during the course of the growing season. There was some evidence of surface movement of labelled soil particles. Soil-to-plant transfer factors varied slightly between the soils. The average results for sorghum were 0.1-0.3 g/g for Cs, 0.4-0.8 g/g for Sr and 18-26 g/g for Zn (dry weight) with the initial values relating to Blain and the following values to Tippera. Similar values were observed for the mung bean samples. The transfer factors for Cs and Sr were not substantially different from the typical values observed in temperate studies. However, Zn transfer factors for plants grown on both these tropical soils were greater than for soils in temperate climates (by more than an order of magnitude). This may be related to trace nutrient deficiency and/or the growth of fungal populations in these soils. The results indicate that transfer factors depend on climatic region together with soil type and chemistry and underline the value of specific bioaccumulation data for radionuclides in tropical soils.

Validation of ICRP metabolic models for the transuranics in a Japanese population

Distributions of (239,240)Pu and 241Am in the tissues of Japanese were determined and then compared to those estimated using recent ICRP metabolic models. Intakes by inhalation and ingestion were calculated and used as input to the ICRP-30 model or a combination of the ICRP-66 lung model and the ICRP-67 metabolic model. The (239,240)Pu distribution in the lung, liver, skeleton, kidney, and muscle using the combination ICRP-66 and 67 models agreed well with the measured data. However, the measured plutonium concentration in the spleen was higher than predicted and than found in the kidney or muscle and indicates that the spleen should be treated as a separate organ in the ICRP model. The fractional uptake via ingestion of (239,240)Pu was estimated to be 11% with 5 x 10(-4) as the f1 value. The combination of ICRP-66 and 67 models were adequate descriptors of the organ burdens of 241Am measured in Akita and Niigata district populations. The 241Am ingrowth from 241Pu taken into in the human body contributes 90% of the measured burden.

Modeling radium and radon transport through soil and vegetation

A one-dimensional flow and transport model was developed to describe the movement of two fluid phases, gas and water, within a porous medium and the transport of 226Ra and 222Rn within and between these two phases. Included in this model is the vegetative uptake of water and aqueous 226Ra and 222Rn that can be extracted from the soil via the transpiration stream. The mathematical model is formulated through a set of phase balance equations and a set of species balance equations. Mass exchange, sink terms and the dependence of physical properties upon phase composition couple the two sets of equations. Numerical solution of each set, with iteration between the sets, is carried out leading to a set-iterative compositional model. The Petrov-Galerkin finite element approach is used to allow for upstream weighting if required for a given simulation. Mass lumping improves solution convergence and stability behavior. The resulting numerical model was applied to four problems and was found to produce accurate, mass conservative solutions when compared to published experimental and numerical results and theoretical column experiments. Preliminary results suggest that the model can be used as an investigative tool to determine the feasibility of phytoremediating radium and radon-contaminated soil.

[Method of quantified complete estimation of soil properties for prediction of radionuclide accumulation by plants]

A method is founded on consideration of soil as three-phase system where soil solution reaction (pH), absorbing capacity (E) and content of organic matter (OM) are the main characteristics. The method of complete estimation of soil properties (CESP) for quantified analysis of radionuclide soil-to-plant TF dependence on soil properties is grounded and proposed. The area of an effective section (relative units)2 calculated as an area of the triangle with apexes lying in co-ordinates of normalised vectors pH, E and OM in three-dimensional space for complete estimation is used. It is shown that the dependence of 137Cs TF to various crops on CESP can be approximated by equation of power law.

Uptake and release of cesium-137 by five plant species as influenced by soil amendments in field experiments

Phytoextraction field experiments were conducted on soil contaminated with 0.39 to 8.7 Bq/g of 137Cs to determine the capacity of five plant species to accumulate 137Cs and the effects of three soil treatments on uptake. The plants tested were redroot pigweed (Amaranthus retroflexus L. var. aureus); a mixture of redroot pigweed and spreading pigweed (A. graecizans L.); purple amaranth (A. cruteus L.) x Powell's amaranth (A. powellii S. Watson), referred to here as the amaranth hybrid; Indian mustard [Brassica juncea (L.) Czern.]; and cabbage (Brassica oleracea L. var. capitata). For control plants, the concentration ratios (CR) of 137Cs were greatest for redroot pigweed and the amaranth hybrid, with average CR values of 1.0 +/- 0.24 and 0.95 +/- 0.14, respectively. The lowest value was for Indian mustard at 0.36 +/- 0.10. The soil treatments included (i) application of NH4NO3 solution to the soil after plants had matured, (ii) addition of composted manure to increase organic matter content of the soil, (iii) combination of the manure and ammonium solution treatments, and (iv) controls. The ammonium solution gave little overall increase in accumulation of 137Cs. The use of composted manure also had little influence, but the combination of the composted manure with application of ammonium solutions had a distinctly negative effect on plant uptake of 137Cs. On average the fraction of 137Cs taken up from the soil was reduced by 57.4 +/- 1.2% compared with controls. This was the result of release of competing ions, primarily Ca, from the manure and was observed across all five plant species tested. The application of ammonium solution took place in the last two weeks before harvest. The reduction of plant 137Cs content, by addition of the ammonium solution, as it interacted with the manure, indicates that substantial quantities 137Cs can be released from the shoots of plants as a result of sudden changes in soil solution chemistry.

Reduction of crop contamination by soil resuspension within the 30-km zone of the Chernobyl nuclear power plant

A field experiment was conducted within the 30-km zone of the Chernobyl Nuclear Power Plant to analyze whether the application of mulching reduced resuspension of 137Cs contaminated soil in oat (Avena sativa) crops. In 1993, we applied a mulch treatment at a dose of 200 g m(-2), and soil resuspension was measured by estimating soil loadings onto plant surfaces from Ti concentrations in plants. In 1994, two mulch doses were applied, 200 and 50 g m(-2), and we estimated the contribution of soil resuspension by using artificial resuspension collection devices (ARC). In the 1993 experiment between 4.6 and 34.4% of the plant's total 137Cs contamination was attributed to external soil contamination. The mean amount of soil-derived 137Cs attached to vegetation was 124.7 Bq kg(-1)(plant) in control plots and 53.7 Bq kg(-1)(plant) in mulched plots. In the 1994 experiment, covering the soil with a mulch layer decreased the radiocesium content in ARC by about 70%. Results obtained in these experiments suggest that soil resuspension was a significant mechanism for plant contamination and that mulching was effective in reducing that contamination.

137Cs soil-to-plant transfer for individual species in a semi-natural grassland. Influence of potassium soil content

In the present study we assessed the radiocaesium uptake by plants in order to piece together information on factors affecting the uptake processes, particularly K supply and plant species differences. Vegetation uptake from soil contaminated by the Chernobyl accident was compared at two semi-natural grasslands. The Cs/K discrimination factor (DF), which is often used to evaluate a plant's efficiency in absorbing nutrients from soil, was estimated. The obtained DF values (0.01 to 0.8) vary with K soil concentrations and plant species, indicating that the (40)K is more efficiently absorbed than (137)Cs. The soil-to-plant relationship was evaluated by means of the transfer factor (TF). The (137)Cs TF(sp) values obtained from separated plant species varied within the range of 0.016 to 0.400 (site 1) and 0.017 to 0.171 (site 2). When mixed grass samples were considered a large variation was observed, mainly for site 1. The (137)Cs TF(mix) ranges were: 0.018 to 0.250 for site 1 and 0.017 to 0.167 for site 2. These values fall within the range of TFs commonly reported (0.0001-1). Our present data suggest that these pastures are apt for forage use. Different plant species presented different individual behavior regarding their (137)Cs TF(sp) when the (40)K soil activity concentration was taken in account. For most of the species analyzed, we observed a gradual decrease in the individual (137)Cs TF(sp) when the (40)K soil activity concentration was increased, with the exception of Taraxacum officinale at one of the sampling sites.

Uptake rates of thorium progeny in a semiarid environment

The release rates and transformation processes that influence the mobility, biological uptake, and transfer of radionuclides are essential to the assessment of the health effects in the food chain and ecosystem. This study examined concentrations of 222Th in both soil and vegetation at a closed military training site, Kirtland Air Force Base (KAFB), New Mexico. Brazilian sludge was intentionally introduced into the topsoil in the early 1960s to simulate nuclear weapon accidents. Soil (60) and vegetation (120) samples were collected from 1996 to 2000 and analyzed for radionuclides and progeny. High-resolution gamma-ray spectroscopy was used to determine radionuclide activities. The results indicate that the thorium progeny were the predominant contaminant in soil and vegetation. Concentration ratios (CRs) were calculated based on actinium levels.

Recommendations for radioecological screening levels for terrestrial plants

This paper presents recommended radionuclide concentrations in soil that may be associated with a dose of 10 mGy d(-1) (1.0 rad d(-1)) to sensitive tissues of higher plants. A dose rate of 10 mGy d(-1) is used in this investigation as a basis for radioecological screening because it corresponds to a dose rate recommended by the IAEA specifically for terrestrial plants as a dose that is not anticipated to have an adverse effect on terrestrial ecosystems. In deriving these radioecological screening levels for plants, specific consideration is given to the internal and external exposure of the root hairs of higher plants to both penetrating and non-penetrating radiation in soil. None of the radioecological screening levels for plants evaluated in this paper are more restrictive than the default soil screening levels recommended by the NCRP for the protection of humans.

Simplified method for detecting tritium contamination in plants and soil

Cost-effective methods are needed to identify the presence and distribution of tritium near radioactive waste disposal and other contaminated sites. The objectives of this study were to (i) develop a simplified sample preparation method for determining tritium contamination in plants and (ii) determine if plant data could be used as an indicator of soil contamination. The method entailed collection and solar distillation of plant water from foliage, followed by filtration and adsorption of scintillation-interfering constituents on a graphite-based solid phase extraction (SPE) column. The method was evaluated using samples of creosote bush [Larrea tridentata (Sessé & Moc. ex DC.) Coville], an evergreen shrub, near a radioactive disposal area in the Mojave Desert. Laboratory tests showed that a 2-g SPE column was necessary and sufficient for accurate determination of known tritium concentrations in plant water. Comparisons of tritium concentrations in plant water determined with the solar distillation-SPE method and the standard (and more laborious) toluene-extraction method showed no significant difference between methods. Tritium concentrations in plant water and in water vapor of root-zone soil also showed no significant difference between methods. Thus, the solar distillation-SPE method provides a simple and cost-effective way to identify plant and soil contamination. The method is of sufficient accuracy to facilitate collection of plume-scale data and optimize placement of more sophisticated (and costly) monitoring equipment at contaminated sites. Although work to date has focused on one desert plant, the approach may be transferable to other species and environments after site-specific experiments.

Effect of soil amendments on radiocesium transfer to alfalfa

Greenhouse experiments were carried out to evaluate the effect of different soil-based countermeasures on radiocesium transfer to Medicago saliva (alfalfa) grown on artificially contaminated loamy-clayey soil. Various rates of potassium, ammonium, and Prussian Blue supplements were applied, and the uptake of radiocesium by control and treated alfalfa plants was monitored during four growth periods. Transfer factors ranging between 0.06 and 0.02 were determined for control plants. Application of potassium at rates higher than 0.1 meq per 100 g soil was found to suppress radiocesium uptake, the effect being more pronounced at increasing fertilization rates. On the contrary, soil treatment with ammonium enhanced the bio-accumulation of radiocesium, indicating that Cs+ ions, previously unavailable to plant roots, were released from soil particles. Prussian Blue supplements had practically no effect on soil-to-alfalfa transfer of the radionuclide.

Effect of industrial pollution on the distribution of 137Cs in soil and the soil-to-plant transfer in a pine forest in SW Finland

The objective of this study was to evaluate the effect of industrial pollution on the distribution of radiocaesium in soil and on its transfer from soil to plants. The study was started in September 2000 in four Scots pine stands located at distances of 0.5, 2, 4 and 8 km along a transect running SE from the Cu-Ni smelter at Harjavalta in SW Finland. Annual emissions from the smelter in 1990 were 80 t of Cu, 31 t of Ni and 9000 t of SO(2), and in 1999 these were 5.9, 0.8 and 3400 t, respectively. At each site, soil profiles were sampled with a corer, and samples were separated into litter (L), organic soil layer (O) and mineral soil layers (B, E). Mushrooms, lichens (Cladina spp. and Cetraria islandica), lingonberry (Vaccinium vitis-idaea) and crowberry (Empetrum nigrum) plants were collected at each site, except at a distance of 0.5 km, where only mushrooms were available. In the organic soil layer, 137Cs activity decreased from 8000 Bq/m(2) at a distance of 8 km from the smelter to 1500 Bq/m(2) at a distance of 0.5 km; in litter, 137Cs concentration increased from 6300 Bq/m(2) at 8 km to 14000 Bq/m(2) at 0.5 km. 137Cs activity concentration decreased significantly in plants, mushrooms and lichens as the pollution load increased. In lichens, 137Cs activity decreased from 910 Bq/kg at 8 km to 170 Bq/kg at 2 km, while in lingonberry it decreased from 1470 to 20 Bq/kg and in crowberry from 310 to 20 Bq/kg. Aggregated transfer factors for 137Cs decreased in a similar way in lingonberry from 7.6x10(-2) m(2)/kg at 8 km to 7.7x10(-4) m(2)/kg at 2 km and in crowberry from 1.6x10(-2) to 7.9x10(-4) m(2)/kg.

Effect of humic acid on the bioavailability of radionuclides to rice plants

We investigated the effect of humic acid and solution pH on the uptake of the radionuclides, (83)Rb, (137)Cs, (54)Mn, (65)Zn, (88)Y, (102)Rh, and (75)Se in rice plants by the multitracer technique. The addition of humic acid to a culture medium containing SiO(2) increased the uptake of Mn and Zn at pH 4.3, whereas their uptake was decreased at pH 5.3. Humic acid depressed the uptake of Y at both pHs. The uptake of Se, which does not interact with humic acid, was not affected by its presence. These results suggest that uptake of the radionuclides by the rice plant is regulated by the affinity of radioactive nuclides for humic acid, as well as by the soil solution's pH.

The effective and environmental half-life of 137Cs at Coral Islands at the former US nuclear test site

The United States (US) conducted nuclear weapons testing from 1946 to 1958 at Bikini and Enewetak Atolls in the northern Marshall Islands. Based on previous detailed dose assessments for Bikini, Enewetak, Rongelap, and Utirik Atolls over a period of 28 years, cesium-137 (137Cs) at Bikini Atoll contributes about 85-89% of the total estimated dose through the terrestrial food chain as a result of uptake of 137Cs by food crops. The estimated integral 30, 50, and 70-year doses were based on the radiological decay of 137Cs (30-year half-life) and other radionuclides. However, there is a continuing inventory of 137Cs and 90Sr in the fresh water portion of the groundwater at all contaminated atolls even though the turnover rate of the fresh groundwater is about 5 years. This is evidence that a portion of the soluble fraction of 137Cs and 90Sr inventory in the soil is lost by transport to groundwater when rainfall is heavy enough to cause recharge of the lens, resulting in loss of 137Cs from the soil column and root zone of the plants. This loss is in addition to that caused by radioactive decay. The effective rate of loss was determined by two methods: (1) indirectly, from time-dependent studies of the 137Cs concentration in leaves of Pisonia grandis, Guettarda specosia, Tournefortia argentea (also called Messerschmidia), Scaevola taccada, and fruit from Pandanus and coconut trees (Cocos nucifera L.), and (2) more directly, by evaluating the 137Cs/90Sr ratios at Bikini Atoll. The mean (and its lower and upper 95% confidence limits) for effective half-life and for environmental-loss half-life (ELH) based on all the trees studied on Rongelap, Bikini, and Enewetak Atolls are 8.5 years (8.0 years, 9.8 years), and 12 years (11 years, 15 years), respectively. The ELH based on the 137Cs/90Sr ratios in soil in 1987 relative to the 137Cs/90Sr ratios at the time of deposition in 1954 is less than 17 years. The magnitude of the decrease below 17 years depends on the ELH for 90Sr that is currently unknown, but some loss of 90Sr does occur along with 137Cs. If the 15-year upper 95% confidence limit on ELH (corresponding to an effective half-life of 9.8 years) is incorporated into dose calculations projected over periods of 30, 50, or 70 years, then corresponding integral doses are 58, 46 and 41%, respectively, of those previously calculated based solely on radiological decay of 137Cs.

Availability of 99Tc in undisturbed soil cores

Models for safety assessment of radioactive waste repositories need accurate values of the soil-to-plant transfer of radionuclides. In oxidizing environments, (99)Tc is expected to occur as pertechnetate ((99)TcO(4)(-)). Due to its high mobility, leaching of this element in the field might be important, potentially affecting the reliability of estimated transfer parameters of (99)Tc as measured in closed experimental systems such as hydroponics or pot experiments. The aim of this experiment was to measure the leaching of (99)Tc in undisturbed irrigated soil cores under cultivation as well as plant uptake and to study the possible competition between the two transfer pathways. Undisturbed soil cores (50 x 50 cm) were sampled from a Rendzic Leptosol (R), a colluvial Fluvic Cambisol (F) and a Dystric Cambisol (D) using PVC tubes (three cores sampled per soil type). Each core was equipped with a leachate collector at the bottom, allowing the monitoring of (99)Tc leaching through the cores. Cores were placed in a greenhouse and maize (Zea mays L., cv. DEA, Pioneer) was sown. After 135 d, maize was harvested and radioactivity determined in both plant and water samples. Results showed that during the growing period, leaching of (99)Tc was limited, due to the high evapotranspiration rate of maize. After harvest, leaching of (99)Tc went on because of the absence of evapotranspiration. Effective uptake (EU) of (99)Tc in leaves and grains was calculated. EU reached 70% of the input in the leaves and was not significantly different among soils. These results confirmed those obtained from pot experiments, even though leaching was allowed to occur in close-to-reality hydraulical conditions. As a consequence, it was concluded that pot experiments are an adequate surrogate for more complex "close-to-reality" experimental systems for measuring transfer factors.

Spatial variability of fallout-90Sr in soil and vegetation of an alpine pasture

According to the soil-to-plant transfer concept generally used in dose assessment modeling, the plant uptake of a radionuclide should depend linearly on its concentration in the soil. In order to validate this concept for (90)Sr in a semi-natural ecosystem, plant and soil samples were taken at 100 plots of a 100 x 100 m(2) area within an alpine pasture near Berchtesgaden, Germany. At three plots, the vertical distribution of (90)Sr in the soil was determined in addition. A statistically significant correlation between the soil and plant concentration of (90)Sr was not detectable (Spearman correlation coefficient R=-0.116, p>0.05) within the range of the Sr-concentration covered (15-548 Bq kg(-1) dry soil and 17-253 Bq kg(-1) dry plant material). Thus, the prerequisite of the soil-to-plant transfer concept was not fulfilled for (90)Sr at this site. Organic carbon and total nitrogen were also determined in the soil samples. Both elements were highly correlated (R=0.912, p<0.001), their ratio being C/N=10.9+/-0.7. While C was positively correlated with the (90)Sr concentrations in the soil (R=0.342, p<0.001), negative correlations were observed for the plant concentrations (R=-0.286, p<0.01) and the concentration ratios (R=-0.444, p<0.001) of (90)Sr. These results are compared with those recently obtained for (137)Cs by Bunzl et al. (J Environ Radioactiv 48 (2000) 145).

Sustainability of radiologically contaminated territories

Decisions on appropriate countermeasures in the case of a contamination of large areas due to fallout from a weapon's detonations or a serious reactor or reprocessing plant accident depend on fallout level and the time period required for the exposure levels to fall below a given intervention limit. This is particularly relevant for countermeasures with high costs and substantial consequences to the population involved, such as relocation or soil perturbation. Natural processes which result in a decrease in activity concentrations in foodstuffs and external exposure with time, are least detrimental to the soil among all countermeasures proposed for rehabilitation of contaminated areas. The impact of these natural self-healing effects (soil sustainability) on the temporal activity reduction in foodstuffs, as well as their effect on the time period required for exposure levels to decrease to sufficiently low levels to allow unrestricted re-utilization and re-settlement of the land, are discussed. Depending on the type of foodstuff, in Central Europe this temporal decrease follows an effective half life of about 0.25-1.3 y from first to second year after fallout, followed by a decrease in milk with an effective half-life of 1-2 y in the next 7 y and about 5 y thereafter, while in cereals and vegetables in Austria a decrease by 2.3 y and in fruit by 1.3 y over a period of more than a decade is observed. In contrast, the decrease in foodstuffs derived from semi-natural environs is much slower by an effective half-life of roughly 8 y. The time-span for adequate recovery of a contaminated territory, therefore, depends on the fallout level and the radionuclides involved and the contribution of natural and semi-natural derived foodstuffs to the diet. It is shown that for 137Cs and a typical Central European diet the time span amounts to about 1 y for a deposition of 300 kBq m(-2) and to about 10 y for a deposition of 10,000 kBq m(-2). Thus it is demonstrated that natural restoration effects may contribute significantly to the environmentally safe and sustainable resettlement of an area substantially contaminated with fission products.