New best estimates for radionuclide solid–liquid distribution coefficients in soils. Part 2. Naturally occurring radionuclides

Predicting the transfer of radionuclides in the environment for normal release, accidental, disposal or remediation scenarios in order to assess exposure requires the availability of an important number of generic parameter values. One of the key parameters in environmental assessment is the solid liquid distribution coefficient, K_d, which is used to predict radionuclide–soil interaction and subsequent radionuclide transport in the soil column. This article presents a review of K_d values for uranium, radium, lead, polonium and thorium based on an extensive literature survey, including recent publications. The K_d estimates were presented per soil groups defined by their texture and organic matter content (Sand, Loam, Clay and Organic), although the texture class seemed not to significantly affect K_d. Where relevant, other K_d classification systems are proposed and correlations with soil parameters are highlighted. The K_d values obtained in this compilation are compared with earlier review data.     

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.     

Distributions of radionuclide sorption coefficients (Kd) in sub-surface sediments and the implications for transport calculations

The effect of the spatial variability of K_d on calculations of contaminant travel time in the vadose zone was determined. Depth discrete measurements of K_d were made for a suite of radionuclides (¹⁰⁹Cd, ⁵⁷Co, ⁶⁰Co, ⁸⁵Sr, ¹³⁷Cs, and ⁸⁸Y) utilizing a sediment core from the E-Area at the Savannah River Site. The K_d’s were ordered as ⁸⁵Sr²⁺ < ¹³⁷Cs⁺ < ¹⁰⁹Cd²⁺ < ⁵⁷Co²⁺ = ⁶⁰Co²⁺ << ⁸⁸Y³⁺ and the values generally fell below or near the lowest quartile of values reported in the literature. Correlations were generally weak between soil properties and K_d values. Most importantly, all of the K_d distributions could be reasonably approximated as log-normal. Deterministic and stochastic calculations of contaminant travel time to the water table were made. The deterministic calculations were based on each of three conceptual models of the vadose zone: complete stratification (17 strata, each with a different K_d), two strata (two sections of the vadose zone, each characterized by a single, average K_d), and unstratified (a single zone with an average K_d). Stochastic calculations were based on log-normal fits to the K_d data. The two strata model generally yielded travel times 2× greater than those in the completely stratified model. The unstratified model yielded travel times that were between 3 and 5 times greater than the completely stratified model. The stochastic mean travel times were comparable to those of the two strata model.     

Comparison of distribution coefficients for americium and curium: Effects of pH and naturally occurring colloids

Distribution coefficients (K_D) were measured by a batch technique for sorption of americium-243 and curium-244 to Lake Michigan sediment from four different natural surface waters. The dependences of K_D values on pH and on the concentrations of naturally occurring colloidal organic carbon (COC) were studied. The comparison of K_D values showed that, while changes in pH and in COC concentrations cause significant changes in sorption for both americium and curium, neither variable causes a significant difference in sorption between these two elements.     

Laboratory studies of the diffusive transport of Cs and Co through potential waste repository soils

Tests using reconstituted samples have been performed to assess the diffusive transport of ¹³⁷Cs and ⁶⁰Co through natural regolith materials from a region in South Australia being considered for a radioactive waste repository. A double diffusion cell apparatus made of polycarbonate resin was developed to estimate the effective diffusion (D_e) and sorption coefficients (K_d) that allowed large withdrawals from the source and collector cells and has enabled tests with low concentrations of radioactivity. An alternative to porous stainless steel filter plates has also been used to reduce uncertainty in test interpretation. Analysis of the transient data used a staged method of the Laplace transform to take into consideration the volume of the samples withdrawn from the apparatus during testing. At test completion samples were cut into slices and analysed for radionuclide concentration. Data obtained from the sliced samples confirmed that both numerical and experimental data produced acceptable mass balance. The D_e values obtained in this study were of the order of 10⁻⁶ cm² s⁻¹ for both species, higher than previously published data. The K_d values from the diffusion and batch sorption tests were in reasonable agreement for ¹³⁷Cs, but an order of magnitude different for ⁶⁰Co. The sorption of the latter radionuclide was strongly pH dependent, and this dependency during diffusion tests would benefit from further investigation.     

The assumption of linearity in soil and plant concentration ratios: An experimental evaluation

We have evaluated one of the main assumptions in the use of concentration ratios to describe the transfer of elements in the environment. The ratios examined in detail were the ‘concentration ratio’ (CR) of leaf to soil and the ‘partition coefficient’ of (K_d) of solid- to liquid-phase concentrations in soil. The ‘translocation ratio’ of berry to leaf was also computed when possible. Use of these ratios implies a linear relationship between the concentrations. Soil was experimentally contaminated to evaluate this linearity over more than a 1000-fold range in concentration. A secondary objective was to determine CR and K_d values in a long-term (2 y) outdoor study using a peat soil and a slow-growing native plant species, specifically blueberries (Vaccinium angustifolium). The elements I, Se, CS, Pb and U wer chosen as environmentally important elements expected to provide a broad range in CR and K_d values. the reuslts indicated that relationships of leaf and leachate concentrations were not consistently linearly related to the total soil concentrations for each of the elements. The modelling difficulties implied by these concentration dependencies can be partially offset by including in the models the strong negative correlation between CR and K_d. The error introduced by using a mean value of the ratios for Se or U resulted in up to a ten-fold increase in variability for CR and a three-fold increase for K_d.     

Two-dimensional numerical modeling of Sr transport in an unsaturated Chinese loess under artificial sprinkling

⁹⁰Sr is a fission byproduct of uranium and plutonium, and it presents a major health problem in the environment. A field test on the transport of various radionuclides including ⁹⁰Sr in an unsaturated Chinese loess was conducted under artificial rain conditions from July 1997 to August 2000. The vertical concentration distribution of ⁹⁰Sr displayed an unusual profile of double concentration peaks, which were separated by a thin (0.7 cm) source layer. In order to interpret the double-peak concentration profile, the transport of ³H and ⁹⁰Sr in the unsaturated Chinese loess under artificial sprinkling conditions was simulated using WATERM, a numerical code for simulating flow field, and NESOR, also a numerical code but for simulating nuclide migration. The models were able to adequately simulate the double-peak concentration profile. The observation suggested that the fine arenaceous quartz layer, though 0.7 cm thick, formed a capillary barrier together with the local loess, which prevented water from penetrating. A significant discrepancy was observed between the model-fitted distribution coefficient (K_d) of ⁹⁰Sr and that determined from independent laboratory experiments, which can be attributed to a number of factors such as the capillary barrier effect, solution-to-solid ratio and soil water content. Therefore, when the model is used for predictive purposes where K_d is used as an input parameter, K_d must be determined under well controlled conditions by taking into account these factors as well as the heterogeneity in the field.     

Probabilistic distribution coefficients (Kds) in freshwater for radioisotopes of Ag,Am, Ba,Be, Ce,Co, Cs,I, Mn,Pu, Ra,Ru, Sb,Sr and Th – implications for uncertainty analysis of models simulating the transport of radionuclides in rivers

The objective of this study was to provide operational probability density functions (PDFs) for distribution coefficients (K_ds) in freshwater, representing the partition of radionuclides between the particulate and the dissolved phases respectively. Accordingly, the K_d variability should be considered in uncertainty analysis of transport and risk assessment models. The construction of PDFs for 8 elements (Ag, Am, Co, Cs, I, Mn, Pu and Sr) was established according to the procedure already tested in Durrieu et al. [2006. A weighted bootstrap method for the determination of probability density functions of freshwater distribution coefficients (K_ds) of Co, Cs, Sr and I radioisotopes. Chemosphere 65 (8), 1308–1320]: (i) construction of a comprehensive database where K_ds values obtained under various environments and parametric conditions were collected; (ii) scoring procedure to account for the ‘quality’ of each datapoint (according to several criteria such as the presentation of data (e.g. raw data vs mean with or without replicates), contact time, pH, solid-to-liquid ratio, expert judgement) in the construction of the PDF; (iii) weighted bootstrapping procedure to build the PDFs, in order to give more importance to the most relevant datapoints. Two types of PDFs were constructed: (i) non-conditional, usable when no knowledge about the site of concern is available; (ii) conditional PDFs corresponding to a limited range of parameters such as pH or contact time; conditional PDFs can thus be used when some parametric information is known on the site under study. For 7 other radionuclides (Ba, Be, Ce, Ra, Ru, Sb and Th), a simplified procedure was adopted because of the scarcity of data: only non-conditional PDFs were built, without incorporating a scoring procedure.     

New best estimates for radionuclide solid–liquid distribution coefficients in soils,Part 1: radiostrontium and radiocaesium

Best estimates for the solid-liquid distribution coefficients (K_d) of radiostrontium and radiocaesium for various soil types, were derived from geometric means (GM) calculated from grouping soils by texture and organic matter content, and also using soil cofactors governing soil–radionuclide interaction. The K_d (Sr) GM for Sand, Loam, Clay and Organic groups were similar, although the value for the Sand group was significantly lower. The Sr cofactor approach, based on the ratios of cation exchange capacity (CEC) to Ca and Mg concentrations in the soil solution, leads to K_d (Sr) GM with a lower variability, from which best estimates could be proposed. The K_d (Cs) GM for Sand and Organic groups differed, although similar values were obtained for Loam and Clay groups. Grouping the K_d (Cs) according to the Radiocaesium Interception Potential (RIP) and the RIP divided by the K concentration in the soil solution also allows to suggest K_d (Cs) best estimates with a lower variability.     

Some physical and chemical features of the variability of kd distribution coefficients for radionuclides

k_d distribution coefficients are widely used as a first approach to the understanding and determination of the eventual fate of metals and radionuclides released into the aquatic environment. For some radionuclides, k_d varies within a range of more than two orders of magnitude. Based on some physical and chemical aspects of the interactions between the dissolved and the particulate phases, some mathematical equations have been developed to explain some features of the observed k_d variability quantitatively. Thus, we have studied the decrease in particle size of k_d in the case of man-made radionuclides interacting with natural particles, and its increase in the case of naturally occurring radionuclides. For the latter, the dependence of k_d on the instantaneous water state has also been investigated. The study also shows the competition effects, particularly the dependence of k_d on pH and salinity. Finally, the effects produced by the size distribution of a given population of particles or the mixture of different populations, are also included. The present work provides for each situation, experimental support from literature data and/or modelling exercises.     

Tidal variations in dissolved and particulate phase radionuclide activities in the Esk estuary,England, and their distribution coefficients and particulate activity fractions

Measurements have been made of the radionuclide activities, particularly for plutonium isotopes, of the particulate and dissolved fractions of the tidal waters of the Esk Estuary. A wide range of K_d values indicates the lack of equilibrium conditions in this dynamically changing environment. The particulate activity contributes a major part of the activity of the suspension, with reworked estuarine sediment providing an important component of this. The estuarine circulation system results in the dispersion of particulate radionuclides into the low salinity upper reaches of the estuary, where plutonium isotopes are apparently mobilised, enriching the dissolved fraction activity.     

Variability of the sorption of Cs, Zn, Sr, Co, Cd, Ce, Ru, Tc and I at trace concentrations by a forest soil along a transect

The variability of the sorption properties (characterized by the distribution coefficient K_d) of a forest soil for Cs, Zn, Sr, Co, Cd, Ce, Ru, Tc, and I were determined along a transect of 150 m in regular intervals of 3 m. Radioactive tracers were used to ensure trace concentrations of these elements. For comparison, for each soil sample the loss on ignition, as a measure of the soil organic matter content, and the pH were also determined. On average, the K_d values increase in the sequence Tc < I < Ru ≈ Co ≈ Zn ≈ Sr < Cd < Ce < Cs; the spatial variability in the sequence I < Zn ≈ Sr ≈ Co ≈ Cd ≈ Ru ≈ Tc < Cs, and I ≈ Ce < Tc. Correlation analysis revealed that, when moving along the transect, the K_d values for some elements changed in a similar, and for others in a dissimilar pattern. In a few cases a correlation between the pH or the loss on ignition and the K_d values was also present. The spatial structure of the data was examined by semivariograms. For the elements Co, Cd, Zn, and Sr, the K_d values showed an almost periodic behaviour along the transect, which is probably due to periodic changes of some soil properties in a forest with regularly spaced trees.     

Proposal for new best estimates for the soil solid–liquid distribution coefficient and soil-to-plant transfer of nickel

The objective of this study was to compile data, based on an extensive literature survey, for the soil solid–liquid distribution coefficient (K_d) and soil-to-plant transfer factor (TF) for nickel. The K_d best estimates were calculated for soils grouped according to texture and organic matter content (sand, loam, clay and organic) and soil cofactors affecting soil–nickel interaction, such as pH, organic matter, and clay content. Variability in K_d was better explained by pH than by soil texture.     

Distribution and mobilization of U, Th and 226Ra in the plant-soil compartments of a mineralized uranium area in south-west Spain

The activity concentrations of natural uranium isotopes (238U and 234U), thorium isotopes (232Th, 230Th and 225Th) and 226Ra were studied in soil and vegetation samples from a disused uranium mine located in the Extremadura region in the south-west of Spain. The results allowed us to characterize radiologically the area close to the installation and one affected zone was clearly manifest as being dependent on the direction of the surface water flow from the mine. The activity concentration mean values (Bq/kg) in this zone were: 10,924, 10,900, 10,075 and 5,289 for 238U, 234U, 230Th and 226Ra, respectively, in soil samples and 1,050, 1,060, 768 and 1,141 for the same radionuclides in plant samples. In an unaffected zone, the activity concentration mean values (Bq/kg) were: 184, 190, 234 and 7251 for 235U, 234U, 230Th and 226Ra, respectively, in soil samples and 28. 29, 31 and 80 in plant samples. The activity concentrations obtained for 232Th and 228Th showed that the influence of the mine was only important for the uranium series radionuclides. The relative radionuclide mobilities were determined from the activity ratios. Correlations between radionuclide activity concentrations and stable element concentrations in the soil samples helped to understand the possible distribution paths for the natural radionuclides.     

Biosorption of uranium by chemically modified Rhodotorula glutinis

The present paper reports the biosorption of uranium onto chemically modified yeast cells, Rhodotorula glutinis, in order to study the role played by various functional groups in the cell wall. Esterification of the carboxyl groups and methylation of the amino groups present in the cells were carried out by methanol and formaldehyde treatment, respectively. The uranium sorption capacity increased 31% for the methanol-treated biomass and 11% for the formaldehyde-treated biomass at an initial uranium concentration of 140 mg/L. The enhancement of uranium sorption capacity was investigated by Fourier transform infrared (FTIR) spectroscopy analysis, with amino and carboxyl groups were determined to be the important functional groups involved in uranium binding. The biosorption isotherms of uranium onto the raw and chemically modified biomass were also investigated with varying uranium concentrations. Langmuir and Freundlich models were well able to explain the sorption equilibrium data with satisfactory correlation coefficients higher than 0.9.     

Impact of water quality parameters on the sorption of U(VI) onto hematite

In this study, the sorption of U(VI) from aqueous solution on hematite was studied as a function of various water quality parameters such as contact time, pH, ionic strength, soil humic acid (HA) or fulvic acid (FA), solid content and temperature by using a batch technique. The results demonstrated that the sorption of U(VI) was strongly dependent on ionic strength at pH < 6.0, and outer-sphere surface complexation may be the main sorption mechanism. The sorption was independent of ionic strength at pH > 6.0 and the sorption was mainly dominated by inner-sphere surface complexation. The presence of HA/FA increases U(VI) sorption at low pH, whereas decreases U(VI) sorption at high pH. The thermodynamic parameters (ΔH⁰, ΔS⁰, and ΔG⁰) were calculated from the temperature dependent sorption isotherms, and the results suggested that U(VI) sorption was a spontaneous and endothermic process. The results might be important for the application of hematite in U(VI) pollution management.     

Fractionation of natural radionuclides in soils from the vicinity of a former uranium mine ?irovski vrh, Slovenia

As a result of former uranium mining and milling activities at ?irovski vrh, Slovenia, 0.6 million tons of uranium mill tailings (UMT) were deposited onto a nearby waste pile Boršt. Resulting enhanced levels of natural radionuclides in UMT could pose threat for the surrounding environment. Therefore, sequential extraction protocol was performed to assess mobility and bioavailability of ²³⁸U, ²³⁴U, ²³⁰Th and ²²⁶Ra in soils from the waste pile and its surrounding. The radionuclides associated with exchangeable, organic, carbonate, Fe/Mn oxides and residual fraction, respectively, were determined. Results showed that the highest activity concentrations for the studied radionuclides were on the bottom of the waste pile. In non-contaminated locations, about 80% of all radionuclides were in the residual fraction. Considering activity concentrations in the UMT, ²³⁸U and ²³⁴U are the most mobile. Mobility of ²²⁶Ra is suppressed by high sulphate concentrations and is similar to mobility of ²³⁰Th.     

14C and 210Pb in NE atlantic sediments: Evidence of biological reworking in the context of radioactive waste disposal

Scavenging by the seabed and by sedimentary particles in the deep ocean may have a significant effect on the removal of artificial radionuclides released to the water column as a result of radioactive waste disposal operations. Biological activity in the upper layers of the sediment column will enhance the rate of removal for those particle-reactive radionuclides with a short half-life relative to the turnover time of the upper mixed layer. For longer-lived radionuclides the rate of sediment accumulation will determine the ultimate rate of removal.The rate of sediment accumulation and extent of biological mixing of deep-sea sediment from three areas of the NE Atlantic Ocean have been investigated using ¹⁴C and ²¹⁰Pb data. Treatment of the radiocarbon with a simple box model provided estimates of sedimentation rate (ω), mixed layer depth (L), mixed layer age (T_ML) and the age of material arriving at the surface (T₀), which were broadly similar to previously published values from other ocean basins. Box cores from the Iberia Abyssal Plain, Madeira Abyssal Plain and from the NEA low-level radioactive waste dumpsite yielded sedimentation rates in the range 0.8 to 2.2 cm ky^?1 over the upper 16–25 cm. Continuous particle mixing appears to be taking place to a depth of 4 to 6 cm below the present sediment-water interface. Closely spaced vertical sampling of core 161-2 for ²¹⁰Pb allowed a biodiffusion coefficient (D_B) to be calculated (4 × 10^?9cm²s^?1), treating bioturbations as a diffusive term and sedimentations as an advective term in a simple mathematical model. In general, values of D_B in deep ocean sediments fall within the range 1–10 × 10^?9cm²s^?1, two orders of magnitude lower than nearshore values. From a review of published data it is concluded that biological mixing takes place extensively in the deep ocean; it appears to be fairly constant on a basin-wide scale and amenable to incorporation in mathematical models of radionuclide behaviour in the water column.     

Distribution of long-lived radionuclides of the 238U series in the sediments of a small river in a uranium mineralized region of Spain

A study is presented on the distribution and mobilization of the natural U isotopes (238U and 234U), 230Th, and 226Ra in the sediments of a small river crossing an uranium mineralized zone where a disused uranium mine is located. Due to the preferential directions for surface run-off waters and to the mine's situation, one sampling point along the river bed was identified as a point of accumulation of radionuclides. The average values of the activity concentrations (Bq/kg) in this sediment sample were 5,025, 5,055, 5,915 and 1,694 for 238U, 234U, 230Th and 226Ra, respectively, while the respective average values of the activity concentrations (Bq/kg) for the sediment sample considered to give the background level were 125, 124, 131 and 370. Isotopic ratios between the descendants of 238U served to clarify some paths of distribution, involving the soils nearest to the sampling points and the location of these points with respect to the disused mine. The differences in behaviour found between the uranium, thorium and radium isotopes were associated to the mobility of these radionuclides in the fluvial system studied. Correlations between the radionuclide activity concentration ratios and stable element concentrations in the sediment samples were also investigated.     

Comparison of laboratory uranium sorption data with 'in situ distribution coefficients' at the Koongarra uranium deposit, Northern Australia

Distribution coefficients derived from laboratory sorption experiments are commonly used to model the migration of long-lived radionuclides in the environment. However, it has been suggested that field measurements in natural systems ('in situ distribution coefficients') may provide a more accurate indication of 'true' partitioning coefficients than laboratory experiments. In this paper, the relationship between field and laboratory sorption data for uranium is evaluated, using data from the Koongarra uranium deposit in Northern Australia. An extensive suite of laboratory sorption measurements and in situ partitioning data for U has been obtained at this site. A valid comparison can only be made when the calculation of field partitioning is based on U in 'accessible' phases (rather than total U in the solid) and U species in true solution (i.e. excluding particles). In this study, accessible U was estimated using a chemical extraction and the results were verified using an isotope exchange technique. A satisfactory correspondence between field and laboratory partitioning data was obtained when the pH values and partial pressures of CO2 in laboratory sorption experiments were similar to those found in the field. Under these conditions, the measured laboratory sorption ratios (Rd) and in-field partitioning values (Pacc) for U at Koongarra were in the range between approximately 1 x 10(3) and 2 x 10(4) ml/g. However, the distribution of U in solid and groundwater phases at Koongarra is extremely heterogeneous. This variability must be taken into account when modelling radionuclide migration at this site.