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)

Time-Integrated Bioavailability Proxy for Actinides in a Contaminated Estuary

Actinides accumulate within aquatic biota in concentrations several orders of magnitude higher than in the seawater [the concentration factor (CF)], presenting an elevated radiological and biotoxicological risk to human consumers. CFs currently vary widely for the same radionuclide and species, which limits the accuracy of the modeled radiation dose to the public through seafood consumption. We propose that CFs will show less dispersion if calculated using a time-integrated measure of the labile (bioavailable) fraction instead of a specific spot sample of bulk water. Herein, we assess recently developed configurations of the diffusive gradients in thin films (DGT) sampling technique to provide a more accurate predictor for the bioaccumulation of uranium, plutonium, and americium within the biota of the Sellafield-impacted Esk Estuary (UK). We complement DGT data with the cross-flow ultrafiltration of bulk seawater to assess the DGT-labile fraction versus the bulk concentration. Sequential elution of Fucus vesiculosis reveals preferential internalization and strong intracellular binding of less particle-reactive uranium. We find significant variations between CF values in biota calculated using a spot sample versus using DGT, which suggest an underestimation of the CF by spot sampling in some cases. We therefore recommend a revision of CF values using time-integrated bioavailability proxies.

Adsorption of Plutonium onto Several Natural Sediments

ABSTRACT

The adsorption of plutonium (Pu) onto several natural sediments was studied using batch experiments to determine the different factors influencing Pu adsorption. These factors include the possible effect of different methods for adding Pu solution, the initial oxidation state of Pu, the particle size of the sediments, the pH, and the ionic strength of solution. The method used for adding Pu solution greatly influenced the species of Pu in sediment-groundwater systems and the adsorption behavior of Pu onto the sediments. Initial oxidation state of Pu, clay mineral content, particle size of sediment, pH, and ionic strength all appear to impact adsorption. The sediments preferred to adsorb Pu(IV) rather than Pu(V). The adsorption of Pu depended on the particle size and clay mineral content of the sediments; the higher the clay mineral content and the smaller the particle size, the more adsorption for Pu. The insoluble species of Pu formed in sediment-groundwater systems at pH 9.3 resulted in the decrease of adsorption capacity. The adsorption capacity increased with pH increasing at pH < 9. Ion exchange was the dominant mechanism for Pu adsorption onto the sediments at pH < 7.5. It appeared that the ionic strength had a stronger influence on adsorption capacity than pH.

Impact of soil organic matter on Pu migration in five Lithuanian surface soils

Pu distribution coefficient K_d variation was experimentally determined and examined in natural soil samples considering the type of soil, particle size, pH, the concentration of macroelements and organic matter content. This research was carried out with sand, silty sand, peat, clayey sand and clayey loam samples by applying ²³⁶Pu tracer in flow-through column tests. Due to relatively short contact time of 0.5-40 h the tests are considered as have not reached equilibrium state and represent the fast-moving contaminants retardation processes closer to field conditions. Every soil sample was fractionated into two particle size fractions: ≤0.25 mm and 0.25 ÷ 0.5 mm. Analysis revealed that K_d of Pu is higher for the smaller soil particle fraction (≤0.25 mm). The experimental study with 1.6, 4, 6 and 9 pH tracer solution revealed a tendency of elevated K_d when 4 pH and 6 pH solutions were applied, but obtained K_d values were not correlated with initial soil pH due to high buffering capacity of soils. This study shows a very significant influence (r = 0.98) of organic matter content on the Pu distribution coefficient. The K_d of Pu for the fine fraction of peat soil with high organic matter content (67%) reached maximum values of 6597 L/kg and 6200 L/kg when tracer solution was applied of pH = 4 and pH = 6, respectively. In comparison, the minimum K_d value of 3.9 L/kg was obtained for the coarse silty sand fraction with the lowest organic matter content of 1.3% at tracer pH = 1.6. A statistically reliable high correlations of r = 0.95 and 0.94 were also observed between K_d and specific soil elements Mg and Pb content in soils, respectively. The content of Fe in soils was significantly correlated (r = 0.67) with the K_d values of plutonium as well. However, the organic matter content in soils appeared to be the governing factor determining good correlations and causing the highest K_d of Pu values.

Novel DGT Configurations for the Assessment of Bioavailable Plutonium, Americium, and Uranium in Marine and Freshwater Environments

Plutonium, americium, and uranium contribute to the radioactive contamination of the environment and are risk factors for elevated radiation exposure via ingestion through food or water. Due to the significant environmental inventory of these radioelements, a sampling method to accurately monitor their bioavailable concentrations in natural waters is necessary, especially since physicochemical factors can cause significant temporal fluctuations in their waterborne concentrations. To this end, we engineered novel diffusive gradients in thin-film (DGT) configurations using resin gels, which are selective for UO₂²⁺, Pu(IV + V), and Am(III) among an excess of extraneous cations. In this work, we also report an improved synthesis of our in-house ion-imprinted polymer resin, which we used to manufacture a resin gel to capture Am(III). The effective diffusion coefficients of Pu, Am, and U in agarose cross-linked polyacrylamide were determined in freshwater and seawater simulants and in natural seawater, to calibrate these configurations for environmental deployments.

Evaluation of Migration Radiological Equivalence for Dual Component Nuclear Waste in a Deep Geological Repository

ABSTRACT

The paper is concerned with the issue of achieving the radiological equivalence (the equivalence of radiation risks) of radioactive waste of nuclear reactors and corresponding mass of natural uranium, taking into account the different migration ability of radionuclides in geological formations and soil. This migration radiological equivalence is being investigated for the deep burial of radioactive waste in the case of the development of a two-component nuclear power system with the concurrent use of thermal neutron reactors and fast neutron reactors. Calculations were performed of radiation doses and radiation risks of cancer death arising from consumption of drinking water from a well above a disposal site. The radiation risk relating to a two-component nuclear power system is lower than that from natural uranium; i.e., after reaching the radiological equivalence (100 y of storage) over the timescale of 109 y, the principle of migration radiological equivalence is satisfied. It would take 106 y after radioactive waste disposal to reach the migration radiological equivalence if only thermal reactors were operated. As regards consumption of well drinking water, the radiation risk does not exceed 10-5 y-1 for a two-component nuclear power system, while being 10-3 y-1 (socially unacceptable level) for a power system using only thermal reactors. Radionuclides 241Am, 239Pu, and 240Pu in drinking water make the main contribution to the doses and radiation risks of people for 104 y after the disposal of radioactive waste.

Proposal of new distribution coefficients (Kd) of potentially toxic elements in soils for improving environmental risk assessment in the State of São Paulo, southeastern Brazil

Soil solid-solution distribution coefficients (K_d) are used in predictive environmental models to assess public health risks. This study was undertaken to determine K_d for potentially toxic elements (PTE) Cd, Co, Cr, Cu, Ni, Pb, and Zn in topsoil samples (0-20 cm) from 30 soils in the State of São Paulo, southeastern Brazil. Batch sorption experiments were carried out, and PTE concentrations in the equilibrium solution were determined by High Resolution Inductively Coupled Plasma Mass Spectrometry (HR-ICPMS). Sorption data was fitted to the Freundlich model. The K_d values were either obtained directly from the slope coefficients of C-type isotherms or derived from the slope of the straight line tangent to the non-linear L-type and H-type isotherms. Stepwise multiple regression models were used to estimate the K_d values through the combined effect of a number of soil attributes [pH_H2O, effective cation exchange capacity (ECEC) and contents of clay, organic carbon, and Fe (oxy)hydroxides]. The smallest variation in K_d values was recorded for Cu (105-4598 L kg^-1), Pb (121-7020 L kg^-1), Ni (6-998 L kg^-1), as variation across four orders of magnitude was observed for Cd (7-14,339 L kg^-1), Co (2-34,473 L kg^-1), and Cr (1-21,267 L kg^-1). The K_d values for Zn were between 5 and 123,849 L kg^-1. According to median values of K_d, PTE were sorbed in the following preferential order: Pb > Cu > Cd > Ni > Zn > Cr > Co. The K_d values were best predicted using metal-specific and highly significant (p < 0.001) linear regressions that included pH_H2O, ECEC, and clay contents. The K_d values reported in this study are a novel result that can help minimize erroneous estimates and improve both environmental and public health risk assessments under humid tropical edaphoclimatic conditions.

Ecological and human health risk assessment of metals leached from end-of-life solar photovoltaics

Photovoltaic industry has shown tremendous growth among renewable energy sector. Though, this high installation rate will eventually result in generation of large volume of end-of-life photovoltaic waste with hazardous metals. In present study, reported leached metal contents from different photovoltaics in previous investigations were utilized for (i) potential fate and transport analysis to soil and groundwater and, (ii) estimating ecological and human health risks via dermal and ingestion pathways for child and adult sub-populations. The results indicate that the children are at highest risk, mainly due to lead (hazard quotient from 1.2 to 2.6). Metals, such as cadmium, lead, indium, molybdenum and tellurium pose maximum risks for child and adult sub-populations via soil-dermal pathway followed by soil-ingestion pathway. This is further proved by calculated high values of contamination factor and geo-accumulation index for cadmium (102.4), indium (238.9) and molybdenum (16.12). The estimated soil contamination is significant with respect to aluminium, silver, cadmium, iron, lead, however, groundwater contamination was insignificant. Exposure to polluted soils yields an aggregate hazard index (for non-cancer effects) > 1 for all four pathways, with soil dermal pathway as the major contributor. Lead poses significant cancer risk for all scenarios (average risk: 0.0098 to 0.047 (soil) and 2.1 × 10^-5 to 3.5 × 10^-5 (groundwater)), whereas acceptable non-cancer risk was observed for other metals from groundwater exposure. Further, variance contribution and spearman correlation coefficient analysis show that metal concentration, exposure frequency and ingestion rate are the main contributors towards overall uncertainty in risk estimates. More detailed assessment for environmentally-sensitive metals should be carried out by considering other field breakage scenarios also, although the assessment suggests low risk for majority of metals examined.

Deriving probabilistic soil distribution coefficients (Kd). Part 3: Reducing variability of americium Kd best estimates using soil properties and chemical and geological material analogues

The solid-liquid distribution coefficient (K_d) is a key input parameter in radioecological risk models. However, its large variability hampers its usefulness in modelling transport processes as well as its accuracy in representing soil-radionuclide interactions. To assist in the selection of K_d values and their cumulative distribution functions for study areas without site specific information, a critically reviewed dataset was developed, containing more than 5000 soil K_d entries for 83 elements and an additional 2000 entries of K_d data for 75 elements gathered from a selection of other geological materials. For the specific case of americium (Am), the dataset contained 109 entries for soils and 33 additional entries for sediment and subsoils. The analysis of the Am K_d soil dataset showed that values varied 4-orders of magnitude, and consequently the resulting Am K_d best estimate (geometric mean (GM): 4.6 × 10³ L kg^-1) lacked sufficient reliability. The objective of this study was to calculate cumulative distribution functions and statistically evaluate this dataset to determine if the Am K_d variability in soils could be reduced by considering various factors, including: 1) measurement methods, 2) key soil properties, 3) the use of chemical analogue data, and 4) the use of analogue data. Accounting for Am K_d experimental method (i.e., sorption vs. desorption; long-vs. short-term experiments) had little effect on reducing variability. However, accounting for key soil factors (i.e., organic matter content (OM), pH, soil texture) succeeded in reducing variability of Am K_d, especially when combining pH and OM. While previous data sets have used 20% OM content as a critical value to distinguish between mineral and organic soils, this study shows that this critical value should be reduced to 10% OM to minimize Am K_d variability. The inclusion in the dataset of Am K_d from other geological materials (e.g., gyttjas, tills, and subsoils) and K_d values from trivalent lanthanides (Ln (III)) and actinides (An (III)) (172 additional entries) did not statistically affect the Am K_d geometric means of the various pH and OM partial datasets. The larger composite dataset (> 310 entries), with both chemical analogues and geological material analogues to address data gaps, increased the statistical power for calculating K_d best estimates with lower variability, thereby enhancing their usefulness for radionuclide risk calculations.

Deriving probabilistic soil distribution coefficients (Kd). Part 1: General approach to decreasing and describing variability and example using uranium Kd values

A general approach is presented to derive probabilistic radionuclide distribution coefficients (K_d) in soils from a K_d dataset. The main aim was to derive informed estimates with a low inherent uncertainty by restricting the K_d value data to subsets based on key soil factors and the experimental approach used to calculate the K_d value (e.g., sorption and desorption tests). As an example, the general approach was applied to uranium (U) K_d values that are part of a critically reviewed dataset containing more than 5000 soil K_d entries for 83 elements and an additional 2000 entries of K_d data for 75 elements gathered from a selection of other, non-soil, geological materials. The overall soil U K_d dataset included 196 values spanning a range of four orders of magnitude (1-67,000 L kg^-1), with additional 50 entries for other geological materials. Whereas the effect of the experimental approach could be disregarded, major factors in decreasing U K_d variability were pH and organic matter content (OM). Limitation in the number of entries made it difficult to use texture information (sand, silt, clay) to further decrease U K_d variability. The integrated combination of pH + OM permitted some soil groups to have U K_d confidence intervals as narrow as two orders of magnitude. Specifically for U K_d, data in the Mineral (< 20% OM) and Organic (≥ 20% OM) partial datasets were significantly different. Analogue data from geological materials other than soils, such as subsoil, till and gyttja (a lacustrine mud having elevated organic matter (OM) contents), were also statistically evaluated to determine whether they could be used to fill U K_d data gaps. It was shown that U K_d from subsoils and tills, but not gyttjas, could be used to enhance soil U K_d datasets. Selection of probabilistic K_d values for risk modelling can be made more reliably and with less uncertainty by using appropriate geochemical data representative of the study site to narrow the wide range of potential K_d values.

Kinetics of 99Tc speciation in aerobic soils

Technetium-99 is a significant and long-lived component of spent nuclear fuel relevant to long-term assessments of radioactive waste disposal. Whilst ⁹⁹Tc behaviour in poorly aerated environments is well known, the long-term bioavailability in aerobic soils following direct deposition or transport to the surface is less well understood. This work addresses two questions: (i) to what extent do soil properties control ⁹⁹Tc kinetics in aerobic soils and (ii) over what experimental timescales must ⁹⁹Tc kinetics be measured to make reliable long-term predictions of impact in the terrestrial environment? Soil microcosms spiked with ⁹⁹TcO₄^- were incubated for 2.5 years and ⁹⁹Tc transformations were periodically monitored by a sequential extraction, which enabled quantification of the reaction kinetics. Reduction in soluble ⁹⁹Tc was slow and followed a double exponential kinetic model including a fast component enhanced by low pH, a slow component controlled by pH and organic matter, and a persistently soluble ⁹⁹Tc fraction. Complexation with soil humus was key to the progressive immobilisation of ⁹⁹Tc. Evidence for slow transfer to an unidentified 'sink' was found, with estimated decadal timeframes. Our data suggest that short-term experiments may not reliably predict long-term ⁹⁹Tc solubility in soils with low to moderate organic matter contents.

Using chemical fractionation and speciation to describe uptake of technetium, iodine and selenium by Agrostis capillaris and Lolium perenne

To understand the dynamic mechanisms governing soil-to-plant transfer of selenium (Se), technetium-99 (⁹⁹Tc) and iodine (I), a pot experiment was undertaken using 30 contrasting soils after spiking with ⁷⁷Se, ⁹⁹Tc and ¹²⁹I, and incubating for 2.5 years. Two grass species (Agrostis capillaris and Lolium perenne) were grown under controlled conditions for 4 months with 3 cuts at approximately monthly intervals. Native (soil-derived) ⁷⁸Se and¹²⁷I, as well as spiked ⁷⁷Se, ⁹⁹Tc and ¹²⁹I, were assayed in soil and plants by ICP-MS. The grasses exhibited similar behaviour with respect to uptake of all three elements. The greatest uptake observed was for ⁹⁹Tc, followed by ⁷⁷Se, with least uptake of ¹²⁹I, reflecting the transformations and interactions with soil of the three isotopes. Unlike soil-derived Se and I, the available pools of ⁷⁷Se, ⁹⁹Tc and ¹²⁹I were substantially depleted by plant uptake across the three cuts with lower concentrations observed in plant tissues in each subsequent cut. Comparison between total plant offtake and various soil species suggested that ⁷⁷SeO₄^2-, ⁹⁹TcO₄^- and ¹²⁹IO₃^-, in soluble and adsorbed fractions were the most likely plant-available species. A greater ratio of ¹²⁷I/¹²⁹I in the soil solid phase compared to the solution phase confirmed incomplete mixing of spiked ¹²⁹I with native ¹²⁷I in the soil, despite the extended incubation period, leading to poor buffering of the spiked available pools. Compared to traditional expressions of soil-plant transfer factor (TF_total), a transfer factor (TF_available) expressed using volumetric concentrations of speciated 'available' fractions of each element showed little variation with soil properties.

Iodine budget in forest soils: Influence of environmental conditions and soil physicochemical properties

Due to its longevity, radioisotope ¹²⁹I is a health concern following potential releases in the environment which raises questions about residence and exposure times relevant for risk assessments. We determined ¹²⁷I concentrations (as a surrogate for ¹²⁹I) in a series of French forest soils (i.e. litters, humus and mineral soils) under different vegetation and climate conditions in order to identify the major processes affecting its accumulation and persistence in the soil column. The input fluxes linked to rainfall, throughfall and litterfall were also characterized. Main results obtained showed that: (i) rainfall iodine concentrations probably influenced those of litterfall through absorption by leaves/needles returning to the ground; (ii) throughfall was the major iodine input to soils (mean = 83%), compared to litterfall (mean = 17%); (iii) humus represented a temporary storage of iodine from atmospheric and biomass deposits; (iv) iodine concentrations in soils depended on both the iodine inputs and the soil's ability to retain iodine due to its organic matter, total iron and aluminium concentrations; (v) these soil properties were the main factors influencing the accumulation of iodine in the soil column, resulting in residence times of 419-1756 years; and (vi) the leaching of iodine-containing organic matter dissolved in soil solution may be an important source of labile organic iodine for groundwater and streams.

Investigation of 3H, 99Tc, and 90Sr transport in fractured rock and the effects of fracture-filling/coating material at LILW disposal facility

Batch adsorption, batch diffusion, and flow-through column experiments were conducted using groundwater and fractured rock collected in unsaturated zone to increase our understanding of sorption and transport behavior of radionuclides. Increasing K_d values were observed in the sequence ⁹⁰Sr, ⁹⁹Tc, and ³H regardless of the geological media tested. For all sorbing radionuclides, K_d values for the fracture-filling/coating material were observed to be higher than those for without fracture-filling/coating material regardless of the groundwater. These higher K_d values are the result of zeolite mineral in filling/coating material of fractured rock. The batch diffusion and flow-through column experiments were also conducted using the same fractured rock sample, and the results of diffusion and column experiments showed similar trend of radionuclide sorption and transport to sorption experiment. In this study, sorption K_d of radionuclide was determined and used to increase our understanding of radionuclide retardation through fracture-filling/coating materials.

Sorption and speciation of selenium in boreal forest soil

Sorption and speciation of selenium in the initial chemical forms of selenite and selenate were investigated in batch experiments on humus and mineral soil samples taken from a 4-m deep boreal forest soil excavator pit on Olkiluoto Island, on the Baltic Sea coast in southwestern Finland. The HPLC-ICP-MS technique was used to monitor any possible transformations in the selenium liquid phase speciation and to determine the concentrations of selenite and selenate in the samples for calculation of the mass distribution coefficient, K_d, for both species. Both SeO₃^2- and SeO₄^2- proved to be resistant forms in the prevailing soil conditions and no changes in selenium liquid phase speciation were seen in the sorption experiments in spite of variations in the initial selenium species, incubation time or conditions, pH, temperature or microbial activity. Selenite sorption on the mineral soil increased with time in aerobic conditions whilst the opposite trend was seen for the anaerobic soil samples. Selenite retention correlated with the contents of organic matter and weakly crystalline oxides of aluminum and iron, solution pH and the specific surface area. Selenate exhibited poorer sorption on soil than selenite and on average the K_d values were 27-times lower. Mineral soil was more efficient in retaining selenite and selenate than humus, implicating the possible importance of weakly crystalline aluminum and iron oxides for the retention of oxyanions in Olkiluoto soil. Sterilization of the soil samples decreased the retention of selenite, thus implying some involvement of soil microbes in the sorption processes or a change in sample composition, but it produced no effect for selenate. There was no sorption of selenite by quartz, potassium feldspar, hornblende or muscovite. Biotite showed the best retentive properties for selenite in the model soil solution at about pH 8, followed by hematite, plagioclase and chlorite. The K_d values for these minerals were 18, 14, 8 and 7 L/kg, respectively. It is proposed that selenite sorption is affected by the structural Fe(II) in biotite, which is capable of inducing the reduction of SeO₃^2- to Se(0). Selenite probably forms a surface complex with Fe(III) atoms on the surface of hematite, thus explaining its retention on this mineral. None of the minerals retained selenate to any extent.

Field study of time-dependent selenium partitioning in soils using isotopically enriched stable selenite tracer

A better understanding of selenium fate in soils at both short and long time scales is mandatory to consolidate risk assessment models relevant for managing both contamination and soil fertilization issues. The purpose of this study was thus to investigate Se retention processes and their kinetics by monitoring time-dependent distribution/speciation changes of both ambient and freshly added Se, in the form of stable enriched selenite-77, over a 2-years field experiment. This study clearly illustrates the complex reactivity of selenium in soil considering three methodologically defined fractions (i.e. soluble, exchangeable, organic). Time-dependent redistribution of Se-77 within solid-phases having different reactivity could be described as a combination of chemical and diffusion controlled processes leading to its stronger retention. Experimental data and their kinetic modeling evidenced that transfer towards less labile bearing phases are controlled by slow processes limiting the overall sorption of Se in soils. These results were used to estimate time needed for (77)Se to reach the distribution of naturally present selenium which may extend up to several decades. Ambient Se speciation accounted for 60% to 100% of unidentified species as function of soil type whereas (77)Se(IV) remained the more abundant species after 2-years field experiment. Modeling Se in the long-term without taking account these slow sorption kinetics would thus result in underestimation of Se retention. When using models based on Kd distribution coefficient, they should be at least reliant on ambient Se which is supposed to be at equilibrium.

Distribution and speciation of ambient selenium in contrasted soils, from mineral to organic rich

Selenium adsorption onto oxy-hydroxides mainly controls its mobility in volcanic soils, red earths and soils poor in organic matter (OM) while the influence of OM was emphasized in podzol and peat soils. This work aims at deciphering how those solid phases influence ambient Se mobility and speciation under less contrasted conditions in 26 soils spanning extensive ranges of OM (1-32%), Fe/Al oxy-hydroxides (0.3-6.1%) contents and pH (4.0-8.3). The soil collection included agriculture, meadow and forest soils to assess the influence of OM quality as well. Trace concentrations of six ambient Se species (Se(IV), Se(VI) and 4 organo-Se compounds) were analyzed by HPLC-ICP-MS in three extractants (ultrapure water, phosphate and sodium hydroxide) targeting Se associated to different soil phases. The Kd values determined from ultrapure water extraction were higher than those reported in commonly used short-term experiments after Se-spiking. Correlations of ambient Se content and distribution with soil parameters explained this difference by an involvement of slow processes in Se retention in soils. The 26 Kd values determined here for a wide variety of soils thus represent a relevant database for long-term prediction of Se mobility. For soils containing less than 20% OM, ambient Se solubility is primarily controlled by its adsorption onto crystalline oxy-hydroxides. However, OM plays an important role in Se mobility by forming organo-mineral associations that may protect adsorbed Se from leaching and/or create anoxic zones (aggregates) where Se is immobilized after its reduction. Although for the first time, inorganic Se(IV), Se(VI) and organo-Se compounds were simultaneously investigated in a large soil collection, high Se proportions remain unidentified in each soil extract, most probably due to Se incorporation and/or binding to colloidal-sized OM. Variations of environmental factors regulating the extent of OM-mineral associations/aggregation may thus lead to changes in Se mobility and bio-availability.

Gaining insights into reactive fluid-fractured rock systems using the temporal moments of a tracer breakthrough curve

In this paper, we show that the tracer breakthrough curves (BTCs), when the tracer chemically interacts with the solid matrix of a fractured rock, are considerably different than when it does not. Of particular interest, is the presence of a long pseudo steady state zone in the BTCs, where the tracer concentration is more or less constant over a long period of time. However, such a zone of constant concentration is not visible when either the tracer does not interact with the solid, or does so at an extremely fast rate. We show that these characteristics of the BTCs could be correlated to the parameters of the system. We develop expressions for the mean residence time and its variance for a chemically active and inactive tracer. We show that chemical interaction between the tracer and the solid increases the mean residence time and the increase depends on the distribution coefficient. We also show that the variance of residence time for a chemically active tracer is much larger than that for an inactive tracer, and it depends on both the distribution coefficient and the rate of chemical reaction. We verify these calculations against synthetic tracer BTCs, where the temporal moments are calculated by numerically integrating the tracer evolution curves. Even though we developed the mathematical expressions assuming an idealized fracture-matrix system, we believe that the mathematical expressions developed in this paper can be useful in gaining insights into reactive transport in a real fractured rock system.

Assessment of individual radionuclide distributions from the Fukushima nuclear accident covering central-east Japan

A tremendous amount of radioactivity was discharged because of the damage to cooling systems of nuclear reactors in the Fukushima No. 1 nuclear power plant in March 2011. Fukushima and its adjacent prefectures were contaminated with fission products from the accident. Here, we show a geographical distribution of radioactive iodine, tellurium, and cesium in the surface soils of central-east Japan as determined by gamma-ray spectrometry. Especially in Fukushima prefecture, contaminated area spreads around Iitate and Naka-Dori for all the radionuclides we measured. Distributions of the radionuclides were affected by the physical state of each nuclide as well as geographical features. Considering meteorological conditions, it is concluded that the radioactive material transported on March 15 was the major contributor to contamination in Fukushima prefecture, whereas the radioactive material transported on March 21 was the major source in Ibaraki, Tochigi, Saitama, and Chiba prefectures and in Tokyo.

Iodide sorption and partitioning in solid, liquid and gas phases in soil samples collected from Japanese paddy fields

Sorption kinetics of iodide (I(-)), which is one of the major inorganic chemical forms of iodine in soil environments, were studied under four sets of experimental conditions characterised by temperature or biological activity. We compared partitioning ratios in solid, liquid and gas phases in soils as well as soil-soil solution distribution coefficients (K(d)s) at two different temperatures 4 and 23 °C, for 63 paddy soil samples collected throughout Japan. Interestingly, (125)I emission from soil was observed; the partitioning ratios in gas phase ranged from 0 to 27 % at 4 °C and from 0 to 42 % at 23 °C. In addition, the authors found that K(d) values at 23 °C had good correlation with pH though there was no correlation between K(d) values at 4 °C and pH because of the difference in biological activity.

Selenium dynamics in boreal streams: the role of wetlands and changing groundwater tables

The concentrations of selenium in 10 catchments of a stream network in northern Sweden were monitored over two years, yielding almost 350 observations of selenium concentrations in streamwater. The export of selenium was found to be systematically greater from forests than from mires. Accounting for atmospheric deposition, which was monitored over four years, there was a net accumulation of selenium in mires, while the export from forest soils was approximately equal to the atmospheric deposition. In forest dominated catchments the concentrations of selenium oscillated rapidly back and forth from high to low levels during spring floods. High selenium concentrations coincided with rising groundwater tables in the riparian forest soils, while low selenium concentrations were associated with receding groundwater. Thermodynamic modeling indicated that precipitation of elemental selenium would occur under reducing conditions in the riparian soils. Since changes in the redox conditions are likely to occur near the transition from the unsaturated to the saturated zone, it is hypothesized that the transport of selenium from forest soils to streams is controlled by redox reactions in riparian soils.

Influence of humic acid on the Am(III) sorption onto kaolinite

The sorption of americium(III), (Am(III)), onto kaolinite was studied in batch experiments in the absence and presence of humic acid (HA) ([Am(III)]0=1×10−6ߙM, [HA]0=0 or 10ߙmg/L, I=0.01ߙM NaClO4, pH=3–10, p CO2=10−3.5ߙatm, solid-to-liquid ratio (S/L)=1 or 4ߙg/L). The results show that the Am(III) sorption onto kaolinite is influenced by S/L, the presence of HA and the pH value. In the absence of HA, Am(III) exhibits a very strong and almost pH independent sorption onto kaolinite at the S/L ratio of 4ߙg/L. In the presence of HA, there are small differences in the Am(III) sorption compared to the HA free system. At pH values 5, HA very slightly enhances the sorption of Am(III). Conversely, at pH values ≥5.5, the presence of HA decreases the sorption of Am(III) due to the formation of dissolved Am(III) humate complexes. The decrease of S/L from 4 to 1ߙg/L has a significant effect on the Am(III) sorption onto kaolinite. A sorption edge occurs at pH 6 and the influence of carbonate on the Am(III) sorption at higher pH values becomes evident. Furthermore, the influence of HA on the Am(III) sorption onto kaolinite is more pronounced. The Am(III) sorption results are compared to literature data and to those of U(VI) sorption onto kaolinite obtained under the same experimental conditions.

Long-term and long-range migration of radioactive fallout in a Karst system

Mountainous areas are often covered by little evolved soils from which deposited radionuclides can potentially leak into the vadose zone. In the Swiss Jura mountains, we observed unusual isotopic ratios of nuclear weapon test (NWTs) fallout with an apparent loss of NWTs plutonium relative to ¹³⁷Cs of Chernobyl origin in thinner soils. Here, we studied the karstic watershed of a vauclusian spring to determine the residence times of plutonium, ²⁴¹Am, and ⁹⁰Sr deposited by global fallout and their respective mobility in carbonaceous soils. The results show that ⁹⁰Sr is washed most efficiently from the watershed with a residence time of several hundred years. The estimated plutonium residence time is more than 10 times higher (in the range of 5000-10,000 years), and the ²⁴¹Am residence time is double that of plutonium. The spring water ²⁴¹Am/²³⁹+²⁴⁰Pu isotopic ratio is lower (0.12 - 0.28) than found in watershed soils (0.382 ± 0.077). Similar differences are found in aquatic mosses (²⁴¹Am/²³⁹+²⁴⁰Pu isotopic ratio 0.05-0.12), which are permanently submerged in spring waters. In contrast to plutonium, ⁹⁰Sr is leached from these mosses with 0.5M HCl, demonstrating that strontium is probably associated with calcium carbonate precipitations on the mosses. The higher plutonium to americium isotopic ratio found in the samples of spring water and mosses at the outlet of the karst shows that plutonium mobility is enhanced.

Laboratory studies of the diffusive transport of 137Cs and 60Co through potential waste repository soils

Tests using reconstituted samples have been performed to assess the diffusive transport of (137)Cs and (60)Co 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) cm(2) 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 (137)Cs, but an order of magnitude different for (60)Co. The sorption of the latter radionuclide was strongly pH dependent, and this dependency during diffusion tests would benefit from further investigation.

Laboratory experiments to characterize radiochloride diffusion in unsaturated soils

Diffusion transport of (36)Cl was examined in seven soils under unsaturated conditions in tubes packed with two portions of each soil having different (36)Cl activity concentrations. Apparent diffusion coefficients (D(a)) derived from diffusion profiles varied within a narrow range (from 3x10(-10) to 7x10(-10) m(2) s(-1)) confirming the minor effect of soil properties on the diffusion of a non-reactive radionuclide like (36)Cl. Instead, packing conditions had a major effect. Solid-liquid distribution coefficients (K(d)) derived from D(a) (0.02-0.2 L kg(-1)) were systematically lower than those obtained from batch experiments (0.6-1.0 L kg(-1)), but with a similar variation pattern among soils. The low values of K(d) (Cl) confirmed an almost negligible radiochloride-soil interaction.