Soil-plant transfer of plutonium and americium in contaminated regions of Belarus after the Chernobyl catastrophe

Experimental data are presented for the soil to plant transfer of plutonium and americium into the main species of grass vegetation of Belarusian grasslands contaminated as a result of the Chernobyl catastrophe of 1986. The content of radionuclides in pore soil solutions and the total reserve of biologically available forms of plutonium and americium in rooting layers of different soil varieties have been established. The distribution coefficients of (239,240)Pu and 241Am between the solid phase and pore waters of soils have been evaluated. The migration ability and biological availability of radionuclides in soils with different structures of the absorbing complex have been analyzed for various landscape conditions. The dependence of soil to plant transfer of plutonium and americium on the content and composition of organic matter, and other characteristics of the soil complex has been studied. On the basis of these data, predictions of the contamination levels of the main grass species of natural and agricultural ecosystems by 241Am are presented.     

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.     

An overview of BORIS: Bioavailability of Radionuclides in Soils

The ability to predict the consequences of an accidental release of radionuclides relies mainly on the level of understanding of the mechanisms involved in radionuclide interactions with different components of agricultural and natural ecosystems and their formalisation into predictive models. Numerous studies and databases on contaminated agricultural and natural areas have been obtained, but their use to enhance our prediction ability has been largely limited by their unresolved variability. Such variability seems to stem from incomplete knowledge about radionuclide interactions with the soil matrix, soil moisture, and biological elements in the soil and additional pollutants, which may be found in such soils. In the 5th European Framework Programme entitled Bioavailability of Radionuclides in Soils (BORIS), we investigated the role of the abiotic (soil components and soil structure) and biological elements (organic compounds, plants, mycorrhiza, and microbes) in radionuclide sorption/desorption in soils and radionuclide uptake/release by plants. Because of the importance of their radioisotopes, the bioavailability of three elements, caesium, strontium, and technetium has been followed. The role of one additional non-radioactive pollutant (copper) has been scrutinised in some cases. Role of microorganisms (e.g., K(d) for caesium and strontium in organic soils is much greater in the presence of microorganisms than in their absence), plant physiology (e.g., changes in plant physiology affect radionuclide uptake by plants), and the presence of mycorrhizal fungi (e.g., interferes with the uptake of radionuclides by plants) have been demonstrated. Knowledge acquired from these experiments has been incorporated into two mechanistic models CHEMFAST and BIORUR, specifically modelling radionuclide sorption/desorption from soil matrices and radionuclide uptake by/release from plants. These mechanistic models have been incorporated into an assessment model to enhance its prediction ability by introducing the concept of bioavailability factor for radionuclides.     

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.     

Radionuclide transfer from soil to fruit

The available literature on the transfer of radionuclides from soil to fruit has been reviewed with the aim of identifying the main variables and processes affecting the behaviour of radionuclides in fruit plants. Where available, data for transfer of radionuclides from soil to other components of fruit plant have also been collected, to help in understanding the processes of translocation and storage in perennial plants. Soil-to-fruit transfer factors were derived from agricultural ecosystems, both from temperate and subtropical or tropical zones. Aggregated transfer factors have also been collected from natural or semi-natural ecosystems. The data concern numerous fruits and various radionuclides. Soil-to-fruit transfer is nuclide specific. The variability for a given radionuclide is first of all ascribable to the different properties of soils. Fruit plant species are very heterogeneous, varying from woody trees and shrubs to herbaceous plants. In temperate areas the soil-to-fruit transfer is higher in woody trees for caesium and in shrubs for strontium. Significant differences between the values obtained in temperate and subtropical and tropical regions do not necessarily imply that they are ascribable to climate. Transfer factors for caesium are higher in subtropical and tropical fruits, while those for strontium, as well as for plutonium and americium, in the same fruits, are lower; these results can be interpreted taking into account different soil characteristics.     

Vertical migration of 60Co, 137Cs and 226Ra in agricultural soils as observed in lysimeters under crop rotation

In most studies quantifying the migration parameters - apparent migration velocity and apparent dispersion coefficient - of radionuclides in the soil by model calculations, these parameters are determined for undisturbed soils. For soils disturbed by ploughing, however, no such data are available in the literature. Therefore, in the present study, the migration parameters of (137)Cs, (60)Co and (226)Ra were estimated for ploughed soils by means of a convection-dispersion model. The depth distributions of the radionuclides were determined in four lysimeters (area: 1m(2), depth of soil monolith: 0.75m) filled with artificially contaminated soils of different types in July 1990. The lysimeters were cropped with agricultural plants. The soil in each lysimeter was ploughed manually once a year until 1996 (plough depth 20cm). In July 1999, soil samples were collected from three pits in each lysimeter. The depth distributions of all radionuclides proved to be very similar in each soil pit. The spatial variability of the depth distributions of a given radionuclide within the lysimeters was about the same as their variability between the four lysimeters. Evaluation of the migration parameters revealed that the convective transport of the radionuclides was always rather small or even zero, while the dispersive transport caused a "melting" process of the initially sharp activity edge at the lower border of the Ap horizon. These results are explained by the high evapotranspiration (80-90% of the total precipitation plus irrigation) and the small amounts of seepage water during the observation period of 9 years.     

Variability in background levels of surface soil radionuclides in the vicinity of the US DOE waste isolation pilot plant

Concentrations of radionuclides were measured in soils from a grid of locations surrounding the US Department of Energy Waste Isolation Pilot Plant (WIPP) in southeastern New Mexico and from a grid on a reference site approximately 20 km southeast of the WIPP site. Each of the two grids has 16 sampling locations (grid nodes) systematically distributed within an area of 16.580 ha. Sampling was conducted prior to the arrival of the first waste shipment at WIPP. Thus, the 137Cs and 23,240Pu in the soil are expected to have been deposited as global fallout, although the Gnome Site, 8.8 km southwest of the WIPP, is also a potential source of 239,240Pu and fission products. The reference grid has significantly higher concentrations of fallout and natural radionuclides than the WIPP grid. Up to 80% of the total variability in radionuclide concentrations across the two grids is attributable to differences between grid nodes. Differences between replicates within a location account for 44-50% of the variability in concentrations of the uranium isotopes, but only 11-17% of the variability in the concentrations of the other radionuclides. Samples having similar abundance of radionuclides were spatially aggregated across the terrain. The activity concentrations of the radionuclides were strongly correlated with the concentrations of Al and Pb, and with the percentages of sand, silt and clay in the soil. Normalizing radionuclide concentrations to the concentration of Al or percent fine particles can help adjust for differences in soil textures among samples and facilitate the detection of gradients or temporal changes in soil concentrations.     

Influence of soil texture on the distribution and availability of 238U, 230Th, and 226Ra in soils

The influence of soil texture on the distribution and availability of (238)U, (230)Th, and (226)Ra in soils was studied in soil samples collected at a rehabilitated uranium mine located in the Extremadura region in south-west Spain. The activity concentration (Bqkg(-1)) in the soils ranged from 60 to 750 for (238)U, from 60 to 260 for (230)Th, and from 70 to 330 for (226)Ra. The radionuclide distribution was determined in three soil fractions: coarse sand (0.5-2mm), medium-fine sand (0.067-0.5mm), and silt and clay (<0.067 mm). The relative mobility of the natural radionuclides in the different fractions was studied by comparison of the activity ratios between radionuclides belonging to the same radioactive series. The lability of these radionuclides in each fraction was also studied through selective extraction from the soils using a one-step sequential extraction scheme. Significant correlations were found for (238)U, (230)Th, and (226)Ra between the activity concentration per fraction and the total activity concentration in the bulk soil. Thus, from the determination of the activity concentration in the bulk soil, one could estimate the activity concentration in each fraction. Correlations were also found for (238)U and (226)Ra between the labile activity concentration in each fraction and the total activity concentration in bulk soil. Assuming that there is some particle-size fraction that predominates in the process of soil-to-plant transfer, the parameters obtained in this study should be used as correction factors for the transfer factors determined from the bulk soil in previous studies.     

Fractionation of natural radionuclides in soils from a uranium mineralized area in the south-west of Spain

A new version of a classical method was applied to study the distribution of natural radionuclides (238U, 230Th, and 226Ra) in the soil fractions obtained by a sequential extraction procedure. The potential significance of the fractions obtained with this method was tested on two very similar soils but with very different contents of the three radionuclides, collected in the proximity of a disused uranium mine located in the Extremadura region in the south-west of Spain. The results confirmed that, if only non-residual fractions are considered, the sequential method applied shows a characteristic speciation pattern of these natural radionuclides in this soil matrix, i.e., the distribution of each of the three radionuclides was very similar for the two soil samples.     

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.     

Distribution of fallout radionuclides (7Be, 137Cs, 210Pb and 239,240Pu) in soils of Taiwan

Depth profiles and cumulative deposition of four fallout radionuclides (7Be, 137Cs, 210Pb and 239,240Pu) were determined in presumably undisturbed soils in Taiwan. Inventories of these radionuclides in different areas correlate significantly with each other (except 7Be) and with mean annual rainfall, providing a necessary condition for the development of soil erosion studies in Taiwan. However, the data show very large spatial variability between and within landscape units, reflecting the steep topographic and meteorological gradients in the island. Thus, the application of fallout radionuclides to study soil conservation in Taiwan is expected to be a demanding task; it will call for dense sampling even at undisturbed reference sites.     

The presence of some artificial and natural radionuclides in a Eucalyptus forest in the south of Spain

Long-lived artificial radionuclides (137Cs, 90Sr) were studied in a Eucalyptus plantation located in the south-west of Spain. Radionuclide concentrations were determined in different types of samples corresponding to specific forest components (soil, trees, herbs and litter). Depth profile distributions were obtained in two selected core soils. Two layers were separately measured in three other cores. The concentration factor, defined as the ratio between the mean activity concentration in a component and the mean activity concentration in the soil, was calculated for each component. The biomass of different components was estimated in order to evaluate the total density concentration (Bq/ha) of the artificial radionuclides (137Cs, 90Sr) in the Eucalyptus plantation. The transfer of the radionuclides between the different forest components can be inferred from the results. Additionally, other naturally occurring radionuclides (40K, 226Ra, 228Ra, 228Ac) were determined for comparison. Transport of radionuclides from forest to a nearby pulp mill is also discussed.     

New best estimates for radionuclide solid–liquid distribution coefficients in soils. Part 3: miscellany of radionuclides (Cd,Co, Ni,Zn, I,Se, Sb,Pu, Am,and others)

New best estimates for the solid–liquid distribution coefficient (K_d) for a set of radionuclides are proposed, based on a selective data search and subsequent calculation of geometric means. The K_d best estimates are calculated for soils grouped according to the texture and organic matter content. For a limited number of radionuclides this is extended to consider soil cofactors affecting soil–radionuclide interaction, such as pH, organic matter content, and radionuclide chemical speciation. Correlations between main soil properties and radionuclide K_d are examined to complete the information derived from the best estimates with a rough prediction of K_d based on soil parameters. Although there are still gaps for many radionuclides, new data from recent studies improve the calculation of K_d best estimates for a number of radionuclides, such as selenium, antimony, and iodine.     

Evaluation of DNA damage in the root cells of Allium cepa seeds growing in soil of high background radiation areas of Ramsar-Iran

Plants are unique in their ability to serve as in situ monitors for environmental genotoxins. We have used the alkaline comet assay for detecting induced DNA damage in Allium cepa to estimate the impact of high levels of natural radiation in the soils of inhabited zones of Ramsar. The average specific activity of natural radionuclides measured in the soil samples for (226)Ra was 12,766Bqkg(-1) whereas in the control soils was in the range of 34-60Bqkg(-1). A positive strong significant correlation of the DNA damage in nuclei of the root cells of A. cepa seeds germinated in the soil of high background radiation areas with (226)Ra specific activity of the soil samples was observed. The results showed high genotoxicity of radioactively contaminated soils. Also the linear increase in the DNA damage indicates that activation of repair enzymes is not triggered by exposure to radiation in HBRA.     

Establishment of control site baseline data for erosion studies using radionuclides: a case study in East Slovenia

The aim of the present study was to establish a reference site and its soil characteristics for use of fallout radionuclides in erosion studies in Slovenia. Prior to this study, no reference site and baseline data existed for Slovenia for this purpose. In the agricultural area of Gori?ko in East Slovenia, an undisturbed forest situated in Šalamenci (46°44’N, 16°7’E), was selected to establish the inventory value of fallout ¹³⁷Cs and to establish a baseline level of multi-elemental fingerprint (major, minor, trace elements including heavy metals) and naturally occurring radionuclides in soils. A total of 20 soil profiles were collected at four 10 cm depth increments for evaluation of baseline level of ¹³⁷Cs inventory. An exponential distribution for ¹³⁷Cs was found and the baseline level inventory was established at 7300 ± 2500 Bq m⁻² with a coefficient of variation of 34%. Of this mean present-day inventory, approximately 45% is due to the Chernobyl contribution.The physical degradation of soils through erosion is linked with biochemical degradation. This study introduces an approach to establishment of the naturally occurring radionuclide and elemental fingerprints baseline levels at a reference site which can provide comparative data to those from neighbouring agricultural fields for assessment of soil redistribution magnitude using fallout radionuclides. In addition, this information will be used to determine the impact of soil erosion processes and agricultural practices on soil quality and redistribution within agricultural landscapes in Slovenia.     

238Pu, 239,240Pu, 241Am, 90Sr and 137Cs in soils around nuclear research centre Rez, near Prague

Forty-four soil samples were taken around the nuclear research centre Rez, near Prague. The mean activity concentrations of 238Pu, 239,240Pu, 241Am, 90Sr and 137Cs in uncultivated soil were 0.010, 0.26, 0.12, 2.7 and 23 Bq.kg(-1), respectively. Contents of radionuclides in cultivated soil were lower and in forest soil higher than in uncultivated soil. The mean activity ratios of 238Pu/239,240Pu, 241Am/239,240Pu, 90Sr/239,240Pu and 239,240Pu/137Cs in uncultivated soil were 0.041, 0.47, 10.9 and 0.013, respectively. The mean activity ratios in cultivated and forest soils were close to the values given above. It follows from the results that the source of 239,240Pu, 90Sr and 137Cs in the studied area is deposition from atmospheric nuclear tests, in the case of 137Cs also deposition from Chernobyl accident. The contribution of the research centre effluents was not proved for these radionuclides. Increased activity ratio of 241Am/239,240Pu indicates the presence of 241Am in the soils studied emanating from sources other than nuclear tests. Uniform distribution of the 241Am/239,240Pu activity ratio around the nuclear research centre and the absence of an area with evidently higher activity ratio, including at sites lying in the main wind direction, suggest that the additional activity of 241Am does not originate from the nuclear research centre. The additional source might be the deposition following the Chernobyl accident.     

Interpolation of solid/liquid partition coefficients,K(d), for iodine in soils

The measurement of K(d) is difficult for most radionuclides: a different value is expected for every different soil. This study explored a modification of the constituent-K(d) approach used to estimate K(d) in geological materials. Here we selected five soils of very different compositions, four were field soils and one was an artificial potting soil. The soils were blended together in ratios of 1:1, 1:3 and 1:1:2 for all possible (60) 2- and 3-soil combinations. The K(d) was measured for each soil and each of the combined soils using additions of stable iodine. Our hypothesis was that the weighted average of the K(d)s of the original, unblended soils, weighted by the blending ratios, would be a reasonable estimate of the measured K(d)s of the blended soils. The ratios of expected/measured K(d) values did not deviate significantly from unity (a geometric mean of 0.91) for the four field soils. This result suggests that K(d) in the combined field soils could be estimated by the weighted average K(d) for the constituent soils. The resulting variation is consistent with other estimation methods. The practical implication of this finding is that, with K(d) data for a few benchmark soils in a region, one could estimate K(d) for other soils. The potting soil did not conform, and there are several possible explanations for this.     

Migration ability of radionuclides in soil-vegetation cover of Belarus after Chernobyl accident

This article illustrates the experimental experience achieved in the research of the self-restoration of radioactive-contaminated natural ecosystems. The main directions of studies were: the content and geochemical stability of "hot" particles in radioactive fallout from Chernobyl accident; the physicochemical forms (water-soluble, exchangeable, mobile and fixed) of Cs-137, Sr-90, Pu-239, 240 and Am-241 in the wide varieties of soils; the biological accessibility of radionuclides and their contents in soil pore solutions; and the dynamics and migration parameters of radionuclides vertical redistribution in different landscape conditions.     

Vertical distribution, migration rates, and model comparison of actinium in a semi-arid environment

Vertical soil characterization and migration of radionuclides were investigated at four radioactively contaminated sites on Kirtland Air Force Base (KAFB), New Mexico to determine the vertical downward migration of radionuclides in a semi-arid environment. The surface soils (0-15 cm) were intentionally contaminated with Brazilian sludge (containing (232)Thorium and other radionuclides) approximately 40 years ago, in order to simulate the conditions resulting from a nuclear weapons accident. Site grading consisted of manually raking or machine disking the sludge. The majority of the radioactivity was found in the top 15 cm of soil, with retention ranging from 69 to 88%. Two models, a compartment diffusion model and leach rate model, were evaluated to determine their capabilities and limitations in predicting radionuclide behavior. The migration rates of actinium were calculated with the diffusion compartment and the leach rate models for all sites, and ranged from 0.009 to 0.1 cm/yr increasing with depth. The migration rates calculated with the leach rate models were similar to those using the diffusion compartment model and did not increase with depth (0.045-0.076, 0.0 cm/yr). The research found that the physical and chemical properties governing transport processes of water and solutes in soil provide a valid radionuclide transport model. The evaluation also showed that the physical model has fewer limitations and may be more applicable to this environment.