Can we predict uranium bioavailability based on soil parameters? Part 2: soil solution uranium concentration is not a good bioavailability index

234U/238U disequilibrium and 235U/238U ratios measured using MC-ICP-MS in natural high background radiation area soils to understand the fate of uranium

The Chhatrapur-Gopalpur coastal area in Odisha, India is a well-known natural high background radiation (HBRA) area due to the abundance of monazite (a thorium bearing radioactive mineral) in beach sands and soils. Recent studies on Chhatrapur-Gopalpur HBRA groundwater have reported high concentrations of uranium and its decay products. Therefore, the soils of the Chhatrapur-Gopalpur HBRA are reasonably suspected as the sources of these high uranium concentrations in groundwater. In this report, first the uranium concentrations in soil samples were measured using inductively coupled plasma mass spectrometry (ICP-MS) and they were found to range from 0.61 ± 0.01 to 38.59 ± 0.16 mg kg^-１. Next, the ²³⁴U/²³⁸U and ²³⁵U/²³⁸U isotope ratios were measured to establish a baseline for the first time in Chhatrapur-Gopalpur HBRA soil. Multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) was used for measurement of these isotope ratios. The ²³⁵U/²³⁸U ratio was observed to be the normal terrestrial value. The ²³⁴U/²³⁸U activity ratio, was calculated to understand the secular equilibrium between ²³⁴U and ²³⁸U in soil and it varied from 0.959 to 1.070. To understand the dynamics of uranium in HBRA soil, physico-chemical characteristics of soil were correlated with uranium isotope ratios and this correlation of ²³⁴U/²³⁸U activity ratio indicated the leaching of ²³⁴U from Odisha HBRA soil.

Uranium transfer in grasses grown on mining waste and natural soil

The transfer of radionuclides from soil to the food chain often begins with uptake by plant root system. The roots of most angiosperms showed symbiosis with arbuscular mycorrhizal fungi (AMF) and to understand the transfer process of these toxic elements it is important to consider different physical, chemical and biological factors in soils. In the present study, three grass species (Poaceae), Zea mays, Chrysopogon zizanioides and Aristida setifolia were cultivated with and without organic fertilization in experimental blocks on natural soils, at Fazenda Vargem Formosa (VF) with low uranium (U) contents in the soil, and in the leached ore deposit at the Uranium Concentrate Unit Mine (URA) in Caetité (Uraniferous Province of Lagoa Real - Brazil). In the present study, the biomass production of plants, their rate of root colonization by AMF, the levels of U in soils, roots and leaves, as well as different physico-chemical parameters related to soil fertility were evaluated. The data analysis was performed using Artificial Neural Networks (ANNs), specifically Self-Organizing Maps (SOMs). The levels of available uranium in the soil ranges from 0.33 to 1.11 mg kg^-1 in VF and from 177.5 to 475.8 mg kg^-1 in URA. The results revealed high percentage of root AMF colonization, even in soils with high U contents. There was an inverse relationship between soil U content and its transfer to the plant organs, with U transfer rates being influenced by plant species and not by soil parameters. C. zizanioides had the lowest transfer factor to the shoot and the highest mass productivity under conditions of high U content in soil. The results indicate that C. zizanioides is an important species for use in the recovery of U mining areas.

Assessment of natural radionuclide transfer factors and partition coefficients in some Syrian soils

Transfer factors of radium 226(²²⁶Ra), lead 210(²¹⁰Pb), polonium 210 (²¹⁰Po), uranium 238 (²³⁸U) and thorium 234(²³⁴Th) from five different agricultural soils in Syria to coriander, parsley and mint were investigated in a pot culture experiment. Geometric means of transfer factors (TF)were²²⁶Ra (0.13),²¹⁰Pb(0.03), ²¹⁰Po (0.02) and ²³⁸U (1.76) were within worldwide values, while TF values for ²³⁴Th (1.35) were higher than those recorded globally. The available transfer factor (ATF) values ranged between 0.03 and 1.45, 0.33 and 3.2, 0.10 and 3.36, 1.30 and 16.2 and 1.0 and 6.95 for²²⁶Ra,²¹⁰Pb,²¹⁰Po, ²³⁸U and ²³⁴Th, respectively. However, it is worth mentioning that the data from pot experiments may not represent field conditions. Liquid/solid partition coefficients (K_d)of ²²⁶Ra, U, ²¹⁰Pb and ²¹⁰Po for55 soils representing the dominant types of soils in Syria were also determined. Geometric means of K_d values ranged from 280 to1200, 750 to1600, 350to 4800 and 100-120 L kg^-1 for ²²⁶Ra, ²³⁸U, ²¹⁰Pb and ²¹⁰Po, respectively at pH = 4.0, and from 200 to 6700, 670 to 2400, 150 to 2100 and 100 to 160Lkg^-1at pH = 5.5, and from 370 to 790, 130 to 550, 60 to 330 and370 to 920Lkg^-1at pH = 7.0. The effects of soil mineral content, CEC, ECE, pH and soluble ions on the K_d values were investigated. In general, there were logarithmic relationships between the activity concentrations in soil and the K_d values (R² ranged from 0.59 to 1.00 at pH 4.0). There were no relationships between the K_d values and soil pH.

Bioaccumulation of potentially toxic elements from the soils surrounding a legacy uranium mine in Brazil

It is important to understand the environmental fate and potential risks posed by metals and metalloids around mines and in legacy mining areas. In order to assess the bioavailable concentrations of several potentially toxic elements (PTEs: As, Pb, Cd, Ni, Cu, Cr, Mn, Zn, Ba, U) and rare earth elements (REEs: La to Lu), a multi-method evaluation of their concentrations/fractionation/speciation in soils was related to their biouptake in corn, for a region surrounding a legacy U mine in Brazil. Chemical fractions of the PTE and REE in soils were determined using the BCR (Community Bureau of Reference) sequential extraction procedure; a single extraction with Ca(NO₃)₂ and the diffusion gradient in thin films (DGT) technique. All techniques were better correlated to the metals accumulated by the crops as compared to total metal concentrations. Ba, Cu, Mn and Zn were shown to have high mobility and high bioaccumulation factors in the corn. Concentrations of U, As, Cd, and Pb were above threshold concentrations and strongly correlated, suggesting that they had a similar anthropogenic source. Geospatial modeling agreed with results from principal component analysis, indicating multiple sources for the contamination. Results highlighted the need for multi-method approaches when evaluating the long-term risks posed by PTEs and REEs in agricultural soils.

Kinetics of uranium(VI) lability and solubility in aerobic soils

Uranium may pose a hazard to ecosystems and human health due to its chemotoxic and radiotoxic properties. The long half-life of many U isotopes and their ability to migrate raise concerns over disposal of radioactive wastes. This work examines the long-term U bioavailability in aerobic soils following direct deposition or transport to the surface and addresses two questions: (i) to what extent do soil properties control the kinetics of U speciation changes in soils and (ii) over what experimental timescales must U reaction kinetics be measured to reliably predict long-term of impact in the terrestrial environment? Soil microcosms spiked with soluble uranyl were incubated for 1.7 years. Changes in U^VI fractionation were periodically monitored by soil extractions and isotopic dilution techniques, shedding light on the binding strength of uranyl onto the solid phase. Uranyl sorption was rapid and strongly buffered by soil Fe oxides, but U^VI remained reversibly held and geochemically reactive. The pool of uranyl species able to replenish the soil solution through several equilibrium reactions is substantially larger than might be anticipated from typical chemical extractions and remarkably similar across different soils despite contrasting soil properties. Modelled kinetic parameters indicate that labile U^VI declines very slowly, suggesting that the processes and transformations transferring uranyl to an intractable sink progress at a slow rate regardless of soil characteristics. This is of relevance in the context of radioecological assessments, given that soil solution is the key reservoir for plant uptake.

Determination of distribution coefficient of uranium from physical and chemical properties of soil

Uranium is a long lived radioactive element which is naturally present in minute concentrations in igneous, sedimentary and metamorphic rocks. These rocks when subjected to weathering results in the formation of soil which also has traces of uranium. Distribution coefficient (K_d) is a crucial parameter in environmental assessment which is used to predict the interaction and transport of uranium in groundwater. The objective of the study is to estimate the K_d of uranium in soils and to develop a relation between this and the soil parameters. Seven rock samples and twenty three soil samples were collected during this study. The K_d of rock samples of different grain sizes where determined and the soil samples were analysed for electrical conductivity, pH, grain size, bulk density, particle density, porosity, calcium carbonate, cation exchange capacity and K_d. The K_d of the soil increases with increase in soil pH up to 6, after which it gradually decreases. Multiple regression analysis was performed to quantify the effect of various soil parameters on soil K_d and equations were statistically significant. Thus, soil K_d in a region could be predicted using limited soil properties with such statistically significant equations.

The transfer of fallout plutonium from paddy soil to rice: A field study in Japan

Reported transfer factor (TF) values of Pu from paddy soil to rice are rather scarce, despite the radiotoxicity of Pu and the irreplaceable role of rice in Asian peoples' diets. Here, we conducted a field study to investigate the transfer of global fallout Pu from paddy soil to rice grain (hulled rice) in Japan. The ²⁴⁰Pu/²³⁹Pu atomic ratios in two rice grain samples out of 16 samples were determined and the ratios corresponded well with the global fallout value. The soil-to-rice TF_Pu in 12 Japanese prefectures ranged from 4.5 × 10^-6 to 1.2 × 10^-4 with a geometric mean of 3.3 × 10^-5. The TFs of rice obtained in this study were compatible to the TFs for the broad heading "cereals" compiled in the IAEA Technical Report Series No. 472. Weak correlations were found between the TF and the investigated soil characteristics such as soil pH and loss on ignition. Regarding the TFs for cerium (Ce) and thorium (Th) which are commonly considered as Pu analogues, we observed no significant correlations between the log(TF_Pu) and log(TF_Ce) or log(TF_Pu) and log(TF_Th). On the other hand, interestingly, a significantly positive correlation (r = 0.795, p < 0.001) was observed between log(TF_Pu) and log(TF_U). In view of the observed similarity of TF values for U and Pu from soil to rice, we thought that using the easy-to-measure TF_U to estimate TF_Pu from soil to rice might be suggested although the mechanism was unclear.

Multiple environmental factors influence 238U, 232Th and 226Ra bioaccumulation in arbuscular mycorrhizal-associated plants

Ecological consequences of low-dose radioactivity from natural sources or radioactive waste are important to understand but knowledge gaps still remain. In particular, the soil transfer and bioaccumulation of radionuclides into plant roots is poorly studied. Furthermore, better knowledge of arbuscular mycorrhizal (AM) fungi association may help understand the complexities of radionuclide bioaccumulation within the rhizosphere. Plant bioaccumulation of uranium, thorium and radium was demonstrated at two field sites, where plant tissue concentrations reached up to 46.93 μg g^{-1 238}U, 0.67 μg g^{-1 232}Th and 18.27 kBq kg^{-1 226}Ra. High root retention of uranium was consistent in all plant species studied. In contrast, most plants showed greater bioaccumulation of thorium and radium into above-ground tissues. The influence of specific soil parameters on root radionuclide bioaccumulation was examined. Total organic carbon significantly explained the variation in root uranium concentration, while other soil factors including copper concentration, magnesium concentration and pH significantly correlated with root concentrations of uranium, radium and thorium, respectively. All four orders of Glomeromycota were associated with root samples from both sites and all plant species studied showed varying association with AM fungi, ranging from zero to >60% root colonisation by fungal arbuscules. Previous laboratory studies using single plant-fungal species association had found a positive role of AM fungi in root uranium transfer, but no significant correlation between the amount of fungal infection and root uranium content in the field samples was found here. However, there was a significant negative correlation between AM fungal infection and radium accumulation. This study is the first to examine the role of AM fungi in radionuclide soil-plant transfer at a community level within the natural environment. We conclude that biotic factors alongside various abiotic factors influence the soil-plant transfer of radionuclides and future mechanistic studies are needed to explain these interactions in more detail.

Influence of root exudation of white lupine (Lupinus albus L.) on uranium phytoavailability in a naturally uranium-rich soil

Mechanisms of uranium (U) transfer from soil to plants remain poorly understood. The kinetics of supply of U to the soil solution from solid phases could be a key point to understand its phytoavailability and implications for environmental risk assessment. Root activity, particularly the continuous release of organic acids in the rhizosphere, could have an effect on this supply. We tested the impact of citrate exudation by roots of Lupinus albus, either P-sufficient (P+) or P-deficient (P-), on the phytoavailability of U from a naturally contaminated soil (total content of 413 mg U kg^-1) using a rhizotest design. Combined effects of P (P-/P+ used to modulate plant physiology) and citrate (model exudate) on the solubilization of U contained in the soils were tested in closed reactors (batch). The batch experiment showed the existence of a low U available pool (0.4% total U) and high accessibility (k_d' around 20 L kg^-1) which was not significantly affected by P treatment or citrate concentrations. Analysis of U, Fe, Ca, P and citrate concentrations in the batches suggested a complex combination of mechanisms and factors including desorption, resorption, precipitation, co-sorption. On rhizotest, L. albus plants extracted 0.5-0.75% of the total U and between 25 and 40% of the estimated available U present in the rhizotest in 5 days. Uranium accumulation at the whole plant level (20 mg U kg^-1_d.w., shoot to root ratio around 10^-3) seemed to be dependent neither on the plant P nutrition status nor citrate exudation level, possibly in relation with the equivalent accessibility of U whatever the growth conditions. Yet differential translocation to shoots seemed to be positively correlated to citrate exudation.

A study on possible use of Urtica dioica (common nettle) plants as uranium (234U, 238U) contamination bioindicator near phosphogypsum stockpile

The aim of this study was to determine uranium concentrations in common nettle (Urtica dioica) plants and corresponding soils samples which were collected from the area of phosphogypsum stockpile in Wiślinka (northern Poland). The uranium concentrations in roots depended on its concentrations in soils. Calculated BCF and TF values showed that soils characteristics and air deposition affect uranium absorption and that different uranium species have different affinities to U. dioica plants. The values of ²³⁴U/²³⁸U activity ratio indicate natural origin of these radioisotopes in analyzed plants. Uranium concentration in plants roots is negatively weakly correlated with distance from phosphogypsum stockpile.

Comparison of two sequential extraction procedures for uranium fractionation in contaminated soils

Two sequential extraction procedures were carried out on six soils with different chemical properties and contamination history to estimate the partitioning of uranium (U) between different soil fractions. The first standard method (method of Schultz) was specifically developed for actinides, while the second one (method of Rauret) was initially created for heavy metals. Reproducibility of both methods was compared by means of the coefficient of variation (CV). A soil-to-plant transfer experiment was also carried out with ryegrass to verify if one of the extracted fractions efficiently predicted plant uptake. In artificially contaminated soils, most of the U was retrieved from the exchangeable and the carbonates fractions. In soils with high natural levels of U or contaminated by industrial activity, most of the U was found in the less available fractions. Different U concentrations were found in the fractions which were supposed to be comparable in the two methods. Extracted fractions following Schultz differentiated more strongly between the tested soils but no relationships with soil parameters could be established. As expected, the highest U transfer factors (TF) were observed for ryegrass grown on artificially contaminated soils and the lowest on soils with high natural concentrations or industrial contamination, in agreement with the extraction procedures. No good relation was found between the soil-to-shoot TF and the extracted U concentrations. On the other hand, the U concentration in the roots, the U concentration in the shoots and the soil-to-root TF are well correlated to the U concentration determined in the first extracted fractions (so called exchangeable fractions) from the method of Schultz. We conclude that the extraction method according to Schultz should be preferably used for U, and that the exchangeable fraction can be proposed as a potential indicator to evaluate plant uptake in soils.

Biogeochemical behaviour and bioremediation of uranium in waters of abandoned mines

The discharges of uranium and associated radionuclides as well as heavy metals and metalloids from waste and tailing dumps in abandoned uranium mining and processing sites pose contamination risks to surface and groundwater. Although many more are being planned for nuclear energy purposes, most of the abandoned uranium mines are a legacy of uranium production that fuelled arms race during the cold war of the last century. Since the end of cold war, there have been efforts to rehabilitate the mining sites, initially, using classical remediation techniques based on high chemical and civil engineering. Recently, bioremediation technology has been sought as alternatives to the classical approach due to reasons, which include: (a) high demand of sites requiring remediation; (b) the economic implication of running and maintaining the facilities due to high energy and work force demand; and (c) the pattern and characteristics of contaminant discharges in most of the former uranium mining and processing sites prevents the use of classical methods. This review discusses risks of uranium contamination from abandoned uranium mines from the biogeochemical point of view and the potential and limitation of uranium bioremediation technique as alternative to classical approach in abandoned uranium mining and processing sites.

A review of the behaviour of U-238 series radionuclides in soils and plants

The U-238 series of radionuclides is of relevance in a variety of environmental contexts ranging from the remediation of former uranium mining and milling facilities to the deep geological disposal of solid radioactive wastes. Herein, we review what is known concerning the behaviour of radionuclides from the U-238 decay chain in soils and plants. This review is intended to provide a single comprehensive source of information to anyone involved in undertaking environmental impact assessment studies relating to this decay chain. Conclusions are drawn relating to values and ranges of distribution coefficients appropriate to uranium, thorium, radium, lead and polonium in different soil types and under various environmental conditions. Similarly, conclusions are drawn relating to plant:soil concentration ratios for these elements for different plant and soil types, and consideration is given to the distribution of these elements within plants following both root uptake and foliar application.

Adsorption and desorption of uranium (VI) in aerated zone soil

In this paper, the adsorption and desorption behavior of uranium (VI) in aerated zone soil (from Southwest China) was systematically investigated using a static experimental method in order to provide useful information for safety assessment of the disposal of (ultra-)low uraniferous radioactive waste, as well as a potential remediation method for uranium-contaminated soils. The adsorption behavior of uranium (VI) was firstly studied by batch experiments as functions of contact time, pH, liquid/solid ratio, temperature, colloids, minerals and coexistent ions. The results indicated that the adsorption of uranium (VI) by natural soil was efficient at an initial concentration of 10 mg/L uranium (VI) nitrate solution with 100 mg natural soil at room temperature when pH is about 7.0. The adsorption was strongly influenced by the solution pH, contact time, initial concentration and colloids. The adsorption equilibrium for uranium (VI) in soil was obtained within 24 h and the process could be described by the Langmuir adsorption equation. For uranium (VI) desorption, EDTA, citric acid and HNO(3) were evaluated under different conditions of temperature, concentration and proportion of liquid to solid. The adsorbed uranium (VI) on natural soil could be easily extracted by all these agents, especially by HNO(3), implying that the uranium-contaminated soils can be remedied by these reagents.

A review of multiple stressor studies that include ionising radiation

Studies were reviewed that investigated the combined effects of ionising radiation and other stressors on non-human biota. The aim was to determine the state of research in this area of science, and determine if a review of the literature might permit a gross generalization as to whether the combined effects of multi-stressors and radiation are fundamentally additive, synergistic or antagonistic. A multiple stressor database was established for different organism groups. Information was collected on species, stressors applied and effects evaluated. Studies were mostly laboratory based and investigated two-component mixtures. Interactions declared positive occurred in 58% of the studies, while 26% found negative interactions. Interactions were dependent on dose/concentration, on organism's life stage and exposure time and differed among endpoints. Except for one study, none of the studies predicted combined effects following Concentration Addition or Independent Action, and hence, no justified conclusions can be made about synergism or antagonism.

Element interactions and soil properties affecting the soil-to-plant transfer of six elements relevant to radioactive waste in boreal forest

Cobalt (Co), lead (Pb), molybdenum (Mo), nickel (Ni), uranium (U), and zinc (Zn) are among the elements that have radioactive isotopes in radioactive waste. Soil-to-plant transfer is a key process for possible adverse effects if these radionuclides are accidentally released into the environment. The present study aimed at investigating factors affecting such transfer in boreal forest. The plant species studied were blueberry (Vaccinium myrtillus), May lily (Maianthemum bifolium), narrow buckler fern (Dryopteris carthusiana), rowan (Sorbus aucuparia) and Norway spruce (Picea abies). Regression analyses were carried out to investigate the effects of the chemical composition and physical properties of soil on the soil-to-leaf/needle concentration ratios of Co, Mo, Ni, Pb, U and Zn. Soil potassium (K), magnesium (Mg), manganese (Mn), phosphorus (P) and sulphur (S) concentrations were the most important factors affecting the soil-to-plant transfer of the elements studied. Soil clay and organic matter contents were found to significantly affect plant uptake of Mo, Pb and U. Knowledge of the effects of these factors is helpful for interpretation of the predictions of radioecological models describing soil-to-plant transfer and for improving such models.

Transfer of U, Al and Mn in the water-soil-plant (Solanum tuberosum L.) system near a former uranium mining area (Cunha Baixa, Portugal) and implications to human health

Knowledge about metals in crops, grown in contaminated soils around mine sites, is limited and concerns about exposure to hazardous elements through the consumption of contaminated foodstuff, are high. In this study a field experiment was carried out in two agricultural soils located near a former uranium mine area (Cunha Baixa, Portugal). The purpose of the study was to assess the effect of irrigation water quality on soil-potato (Solanum tuberosum L.) crop system and to evaluate if the consumption of the crop represents health risk to the local villagers. The soils were divided in two plots: one irrigated with contaminated water (U: 1.03-1.04mg/L; Al: 7.5-8.00mg/L; Mn: 4.52mg/L) and the other with uncontaminated water (U: 14-10μg/L; Al: 17-23μg/L; Mn: 2.4-5.7μg/L). After irrigation and potato growth, only soil characteristics, as salinity and total U and Mn concentrations were significantly different from those measured at the beginning of the experiment. Within the potato plants, elements were mostly translocated and concentrated in the aerial part: stems and leaves (U: 73-87%; Al: 85-96%; Mn: 85-94%), which minimize the risk of contamination of the edible tissue. In potato tubers, the highest average concentrations (121-590μg U/kg; 25-64mg Al/kg; 12-13mg Mn/kg dry weight) were registered at soil plots irrigated with contaminated water. Uranium and Al were mostly concentrated in the potato peel (88-96 and 76-85%, respectively), and Mn (67-78%) in the pulp, which reinforces the importance of removing peel to minimize human exposure. The risk analysis calculated for non-cancer health effects (hazard quotient), related only to the exposure through the consumption of this basic foodstuff, revealed safety for Cunha Baixa village residents (adults and children) even when potato crop was grown on U enriched soils and irrigated with contaminated water.

Influence of plant activity and phosphates on thorium bioavailability in soils from Baotou area, Inner Mongolia

Harm of thorium to living organisms is governed by its bioavailability. Thorium bioavailability in the soil-plant system of Baotou rare earth industrial area was studied using pot experiments of wheat and single extraction methods. The effects of wheat growth stage and phosphate on thorium bioavailability were also investigated. Based on extractabilities of various extraction methods (CaCl(2), NH(4)NO(3), EDTA, HOAc) and correlation analysis of thorium uptake by wheat plant and extractable thorium, a mixture of 0.02M EDTA+0.5M NH(4)OAc (pH 4.6) was found suitable for evaluation of thorium bioavailability in Baotou soil, which could be predicted quantitatively by multiple regression models. Because of differences of wheat root activities, thorium bioavailability in rhizosphere soil was higher than in bulk soil at tillering stage, but the reverse occurred at jointing stage. Phosphate addition induced the mineralization of soluble thorium by forming stable thorium phosphate compounds, and reduced thorium bioavailability in soil.

Uptake of uranium and thorium by native and cultivated plants

Large part of available literature on biogeochemistry of uranium and thorium refers to the studies performed either in highly contaminated areas or in nutrient solutions that have been artificially 'spiked' with radionuclides. Effects of background levels of natural radioactivity on soil-grown plants have not been studied to the same extent. In this paper, we summarised results of greenhouse and field experiments performed by the author from 2000 to 2006. We examined some of the factors affecting transfer of U and Th from soil to plants, differences in uptake of these radionuclides by different plants, relationships between U and Th in soil and in plants, and temporal variations of U and Th in different plant species. Concentrations of radionuclides (critical point for experimental studies on biogeochemistry of U and Th--rare trace elements in non-contaminated regions) and essential plant nutrients and trace elements were determined by instrumental neutron activation analysis.

Diffusive gradient in thin FILMS (DGT) compared with soil solution and labile uranium fraction for predicting uranium bioavailability to ryegrass

The usefulness of uranium concentration in soil solution or recovered by selective extraction as unequivocal bioavailability indices for uranium uptake by plants is still unclear. The aim of the present study was to test if the uranium concentration measured by the diffusive gradient in thin films (DGT) technique is a relevant substitute for plant uranium availability in comparison to uranium concentration in the soil solution or uranium recovered by ammonium acetate. Ryegrass (Lolium perenne L. var. Melvina) is grown in greenhouse on a range of uranium spiked soils. The DGT-recovered uranium concentration (C(DGT)) was correlated with uranium concentration in the soil solution or with uranium recovered by ammonium acetate extraction. Plant uptake was better predicted by the summed soil solution concentrations of UO(2)(2+), uranyl carbonate complexes and UO(2)PO(4)(-). The DGT technique did not provide significant advantages over conventional methods to predict uranium uptake by plants.

ICRP Publication 114. Environmental protection: transfer parameters for reference animals and plants

In Publication 103 (ICRP, 2007), the Commission included a section on the protection of the environment, and indicated that it would be further developing its approach to this difficult subject by way of a set of Reference Animals and Plants (RAPs) as the basis for relating exposure to dose, and dose to radiation effects, for different types of animals and plants. Subsequently, a set of 12 RAPs has been described in some detail (ICRP, 2008), particularly with regard to estimation of the doses received by them, at a whole-body level, in relation to internal and external radionuclide concentrations; and what is known about the effects of radiation on such types of animals and plants. A set of dose conversion factors for all of the RAPs has been derived, and the resultant dose rates can be compared with evaluations of the effects of dose rates using derived consideration reference levels (DCRLs). Each DCRL constitutes a band of dose rates for each RAP within which there is likely to be some chance of the occurrence of deleterious effects. Site-specific data on Representative Organisms (i.e. organisms of specific interest for an assessment) can then be compared with such values and used as a basis for decision making. It is intended that the Commission's approach to protection of the environment be applied to all exposure situations. In some situations, the relevant radionuclide concentrations can be measured directly, but this is not always possible or feasible. In such cases, modelling techniques are used to estimate the radionuclide concentrations. This report is an initial step in addressing the needs of such modelling techniques. After briefly reviewing the basic factors relating to the accumulation of radionuclides by different types of biota, in different habitats, and at different stages in the life cycle, this report focuses on the approaches used to model the transfer of radionuclides through the environment. It concludes that equilibrium concentration ratios (CRs) are most commonly used to model such transfers, and that they currently offer the most comprehensive data coverage. The report also reviews the methods used to derive CRs, and describes a means of summarising statistical information from empirical data sets. Emphasis has been placed on using data from field studies, although some data from laboratory experiments have been included for some RAPs. There are, inevitably, many data gaps for each RAP, and other data have been used to help fill these gaps. CRs specific to each RAP were extracted from a larger database, structured in terms of generic wildlife groups. In cases where data were lacking, values from taxonomically-related organisms were used to derive suitable surrogate values. The full set of rules which have been applied for filling gaps in RAP-specific CRs is described. Statistical summaries of the data sets are provided, and CR values for 39 elements and 12 RAP combinations are given. The data coverage, reliance on derived values, and applicability of the CR approach for each of the RAPs is discussed. Finally, some consideration is given to approaches where RAPs and their life stages could be measured for the elements of interest under more rigorously controlled conditions to help fill the current data gaps.

Radium and uranium levels in vegetables grown using different farming management systems

Vegetables grown with phosphate fertilizer (conventional management), with bovine manure fertilization (organic management) and in a mineral nutrient solution (hydroponic) were analyzed and the concentrations of (238)U, (226)Ra and (228)Ra in lettuce, carrots, and beans were compared. Lettuce from hydroponic farming system showed the lowest concentration of radionuclides 0.51 for (226)Ra, 0.55 for (228)Ra and 0.24 for (238)U (Bq kg(-1) dry). Vegetables from organically and conventionally grown farming systems showed no differences in the concentration of radium and uranium. Relationships between uranium content in plants and exchangeable Ca and Mg in soil were found, whereas Ra in vegetables was inversely correlated to the cation exchange capacity of soil, leading to the assumption that by supplying carbonate and cations to soil, liming may cause an increase of U and a decrease of radium uptake by plants. The soil to plant transfer varied from 10(-4) to 10(-2) for (238)U and from 10(-2) to 10(-1) for (228)Ra.

Depleted uranium mobility across a weapons testing site: isotopic investigation of porewater, earthworms, and soils

The mobility and bioavailability of depleted uranium (DU) in soils at a UK Ministry of Defence (UK MoD) weapons testing range were investigated. Soil and vegetation were collected near a test-firing position and at eight points along a transect line extending approximately 200 m down-slope, perpendicular to the firing line, toward a small stream. Earthworms and porewaters were subsequently separated from the soils and both total filtered porewater (<0.2 microm) and discrete size fractions (0.2 microm-100 kDa, 100-30 kDa, 30-3 kDa, and <3 kDa)obtainedvia centrifugal ultrafiltration were examined. Uranium concentrations were determined by inductively coupled plasma optical emission spectrometry (ICP-OES) for soils and ICP-mass spectrometry (MS) for earthworms and porewaters, while 235U:238U atom ratios were determined by multicollector (MC)-ICP-MS. Comparison of the porewater and earthworm isotopic values with those of the soil solids indicated that DU released into the environment during weapons test-firing operations was more labile and more bioavailable than naturally occurring U in the soils at the testing range. Importantly, DU was shown to be present in soil porewater even at a distance of approximately 185 m from the test-firing position and, along the extent of the transect was apparently associated with organic colloids.

Method of diffusive gradients in thin films (DGT) compared with other soil testing methods to predict uranium phytoavailability

The measurement of diffusive gradients in thin films (DGT) has been proposed as a surrogate for metal uptake by plants. A small-scale experiment was performed to test the predictive capacity of the DGT method with respect to uranium availability and uptake by ryegrass. Correlation analyses were performed to compare the results obtained with the DGT device with more conventional bioavailability indices - concentration of uranium in pore water or in selective extracts. Six soils with different uranium contamination history and with distinct soil characteristics were used for the availability tests and the uptake experiment. The four uranium bioavailability indices screened were highly correlated, indicating that at least partially comparable uranium pools were assessed. The uranium concentration in the pore water was a better predictor for uranium uptake by ryegrass than amounts of uranium recovered following extraction with 0.11 M CH3COOH or 0.4 M MgCl2, the fractions considered exchangeable according to, respectively, the BCR or NIST standardized sequential extraction methods. The DGT measured concentration, C(DGT), was also highly correlated with plant uptake but the significance level was sensitive to the value of the diffusion coefficient (pH depend or not) used to calculate C(DGT). From the results obtained it could not be concluded that the DGT method would have an additional value in assessing uranium bioavailability.

Plant-induced changes in soil chemistry do not explain differences in uranium transfer

A greenhouse experiment was set up with maize, ryegrass, Indian mustard, wheat and pea to evaluate to what extent differences in uranium (U) transfer factors can be explained by root-mediated changes in selected soil properties. The experiment involved an acid and an alkaline soil contaminated with (238)U. U soil-to-shoot transfer factors (TFs) ranged between 0.0005 and 0.021 on the acid soil and between 0.007 and 0.179 on the alkaline soil. Indian mustard showed the highest U uptake in shoots and maize the lowest. The root TFs, only available for the acid soil, ranged from 0.58 for maize and Indian mustard to 1.38 for ryegrass. The difference in U uptake between the two soils and the five plants was only partially explained by the different initial U concentrations in soil solution or differences in soil properties in the two soils. However, we obtained a significant relation for differences in shoot TFs observed between the two soils when relating shoot TFs with concentration of UO(2)(2+) and uranyl carbonate complexes in soil solution (R(2)=0.88). The physiological mechanisms by which root-to-shoot U transfer is inhibited or promoted seemed at least as important as the plant-induced changes in soil characteristics in determining soil-to-shoot TFs.