Experimental studies of the transfer of neptunium from sea water to sediments and organisms (annelids and molluscs)

This paper deals with the physico-chemical behaviour of neptunium in sea water and with its capacity for transfer to coastal or deep-sea sediments and for uptake by two benthic species, Arenicola marina (L.) and Cerastoderma edule (L.). We used the ²³⁹Np isotope with mass concentrations in sea water of about 2·5 × 10⁻¹⁰−5·4×10⁻¹² g litre⁻¹, which are close to the concentrations observed in the environment for ²³⁷Np. The experimental results show that Np occurs in solution mainly as the neptunyl ion NpO₂⁺, but soluble anionic and neutral complexes and particulate forms were also detected. The distribution coefficients for the sediments varied from 350 to 6500. The carbonate phase plays a decisive role in neptunium transfer but other fixation processes are also possible. Concentration factors after 13 days were found to have values of 40 and 14 for the shells and soft parts of cockles, respectively, while polychaetes had a concentration factor of 2. Thus Np has a smaller probability of transfer to sediments and organisms than have plutonium and americium.     

Using sequential extraction techniques to assess the partitioning of plutonium and neptunium-237 from multiple sources in sediments from the Ob River (Siberia)

Sequential extraction techniques have been developed to assess partitioning of anthropogenic radionuclides (²⁴⁰Pu, ²³⁹Pu, and ²³⁷Np), originating from a variety of sources, as identified by using bulk sediment isotopic composition. Sediments were leached sequentially with a series of six chemical treatments designed to approximate different environmental processes that may occur or to selectively extract trace metals contained in different solid phases of the sediments (i.e., exchangeable, reducible, carbonate, organic, acid leachable, and refractory). Results indicate the majority of Pu and Np is similarly distributed within many of the extracted fractions, with the largest percentage (66–97%) of both elements being observed in sediments treated with buffered citrate dithionite (CDB), which targets easily reduced constituents such as Mn and Fe hydrous oxides. While these results do indicate an association of Pu and Np with redox sensitive elements, the environmental implications are unclear given that the CDB treatment is more extreme than naturally occurring conditions. Minor amounts of Np partition differently from Pu in sediments. The NH₄-acetate treatment, which is designed to liberate trace metals that are loosely adsorbed onto the surfaces of sedimentary materials such as hydrated iron oxides and humic substances, or present at exchangeable sites in clay minerals, mobilized ∼12% of the total Np while Pu levels were below detection. The H₂O₂ treatment, which is designed to liberate trace metals bound to organic matter, mobilized ∼8 and ∼1% of Np and Pu, respectively. These results indicate that a minor portion of the total Np may be affected by environmental conditions that have little or no effect on Pu. Between 7 and 24% of the Pu was observed in treatments designed to liberate Pu and Np that are tightly bound to lithogenic phases or refractory silicates. The ²⁴⁰Pu/²³⁹Pu observed in accessible and refractory fractions ranged between 0.11 and 0.18. With the exception of one sample, the ²⁴⁰Pu/²³⁹Pu isotope ratios measured in the refractory fractions are essentially identical having a mean ratio value of 0.123 ± 0.001 (1σ). The ²⁴⁰Pu/²³⁹Pu ratios provide isotopic evidence that suggests a portion of non-fallout contamination has a refractory nature. The presence of similarly low ²⁴⁰Pu/²³⁹Pu ratios in refractory fractions of sediments from the Ob and Irtysh Rivers suggests the existence of a source of refractory Pu which is consistent with refractory “hot particles” derived from surface tests at the Semipalitinsk test site.     

Development and application of an in situ x-ray fluorescence spectrometer for underwater sediment analysis

An energy-dispersive X-ray fluorescence spectrometer has been developed that provides in situ analyses for thirty elements in the 5 to 50 ppm range (dry weight) in sediments at water depths to 100 m. The analyzer consists of a radionuclide excitation source, ¹⁰⁹Cd, and a solid cryogen-cooled Si(Li) detector with a 0.2-mm beryllium window. The spectrometer has been designed to analyze elements in sediment surfaces as well as a function of sediment depth. The spectrometer has been utilized on a minisubmersible and from a surface vessel. This instrument was applied to several environmental studies. The in situ spectrometer is compared with a laboratory X-ray fluorescence spectrometer.     

Experimental studies on the bioaccumulation of plutonium from sea water and a deep-sea sediment by clams and polychaetes

The bioavailability of ²³⁷Pu in the (III + IV) and (V + VI) oxidation states from sea water and a NE Atlantic deep-sea sediment has been studied for clams (Venerupis decussata) and polychaete worms (Hermione hystrix). After 22 days' exposure in sea water transfer factors (TF) had not reached equilibrium, and were 74 ± 5 and 61 ± 1 for clams and 370 ± 10 and 275 ± 11 for polychaetes, for Pu (III + IV) and Pu (V + VI) respectively. Depuration rates after sea water exposure followed a single exponential form ($\text{T}{}_{\mathrm{<mtext>b</mtext><mtext>1</mtext><mtext>2</mtext>}}\text{= 50}\text{days}$) for clams, and for polychaetes at least a two component from with $\text{T}{}_{\mathrm{<mtext>b</mtext><mtext>1</mtext><mtext>2</mtext>}}\text{= 1·3}\text{days}$ and 54 days. TF values for ²³⁷Pu bioaccumulation from sediments were very low, 6 × 10^?3 and 5 × 10^?2 for clams and polychaetes, respectively, after 20 days' exposure.Plotonium appears to be about 50% more bioavailable to the same species than americium as measured by laboratory experiments using a similar deep-sea sediment. Depuration experiments with clams after 40 days' exposure to labelled sediment indicated loss followed a single exponential form with a $\text{T}{}_{\mathrm{<mtext>b</mtext><mtext>1</mtext><mtext>2</mtext>}}\text{= 24}\text{days}$. Significant differences in the behaviour of Pu introduced into the uptake and loss experiments in different oxidation states were not observed in general.     

Plant uptake of neptunium-237 and technetium-99 under field conditions

Plant uptake of ⁹⁹Tc freshly added to soils was compared with uptake of ⁹⁹Tc ‘aged’ in soil for more than a decade. A combination of alkaline soil and freshly added ⁹⁹Tc resulted in elevated uptake into radish foliage (plant/soil concentration ratios ranged from 37 to 46). Neptunium-237 was freshly added to all soils. Neptunium uptake via plant roots into foliage was strongly affected by soil pH. Neptunium uptake was greatest from acidic soils. The observed plant/soil concentration ratios for ²³⁷Np under field conditions were approximately 10⁻² from acidic soils (pH 5·6–5·7) and were comparable to field concentration ratios for ²³⁹Pu, that is 10⁻³, from a basic soil (pH 7·5).     

Particle and solution phase depth distributions of transuranics and 55Fe in the North Pacific

In situ large volume filtration and chemisorption techniques were used to collect samples from the North Pacific for radiochemical analyses of fallout transuranics and ⁵⁵Fe in filterable and filtered phases. The data cover several locations for surface collections and a detailed depth profile north of Hawaii at 30°N. The observed partition of these nuclides between suspended particulate and filtered phases is directly linked to the rates at which they are moved downward through the water column in association with sinking particles. Particulate phases in open ocean surface waters contain higher Pu (12–35%) than subsurface particulates (which average about 6%). ²⁴¹Am was found to exhibit much stronger particle association—typically close to 50% in both surface and subsurface waters—in accord with its known greater particle reactivity. In the high Pu deep water layer, particle associated Pu dropped to close to 1% of total Pu concentration. Together with a correlated increase in the proportion of oxidized Pu in this layer close to the sediment-water interface, this is taken as clear evidence of remobilization Pu from particles at, or near to, the interface. ⁵⁵Fe distributions on filtered particulates indicate a much deeper depth distribution relative to the transuranics. This may reflect both a higher particle association reactivity in respect to scavenging and a longer exposure history to scavenging (especially relative to ²⁴¹Am—produced by in situ decay).     

Accumulation of Am by suspended matter, diatoms and aquatic weeds of the Yenisei River

In this work we experimentally estimated the capacities of the key components of the Yenisei River (Russia): particulate suspended matter (seston), diatom microalgae, and submerged macrophytes for accumulating ²⁴¹Am from water. In our experiments large particles of seston (>8 μm), comparable in size with diatoms, took up most of americium from water. The accumulation of americium by isolated diatom algae (Asterionella formosa and Diatoma vulgare) was lower than by total seston. The concentration factors (CFs) of ²⁴¹Am for seston of the Yenisei River in our experiments were (2.8-6.9)·10⁵; for diatoms - (1.5-4.2)·10⁴. The CFs for aquatic plant Elodea canadensis were within the same order of magnitude as those for diatoms. Activity concentration and CFs of ²⁴¹Am were nearly the same in experiments under dark and light conditions. This is indicative of an energy independent mechanism of americium uptake from the water by diatoms and submerged macrophytes.     

Uptake of curium (244Cm) by five benthic marine species (Arenicola marina,Cerastoderma edule,Corophium volutator,Nereis diversicolor and Scrobicularia plana): Comparison with americium and plutonium

Curium (²⁴⁴Cm) uptake from contaminated sea water was studied in five benthic marine species: two bivalve molluscs (Scrobicularia plana and Cerastoderma edule), two polychaete annelids (Arenicola marina and Nereis diversicolor) and one amphipod crustacean (Corophium volutator). The concentrations in the whole organisms relative to the concentration in the sea water (concentration factors) were: 700 for the amphipods (after 11 d of accumulation), 140 for the cockles (after 28 d), 80 for the scrobicularia (after 23 d) and ∼30 for the two annelids (after >20 d). All species except S. plana accumulated americium and curium similarly; S. plana accumulated similar amounts of curium and plutonium.     

Uptake and loss of 134Cs and 60Co by the Baltic bivalve Macoma baltica in a laboratory microcosmos

Accumulation and clearance of ¹³⁴Cs and ⁶⁰Co by the Baltic bivalve Macoma baltica were experimentally investigated in a laboratory microcosmos. The nuclides were added to the water and the activities in bivalve flesh, shell, feces and sediment were determined at regular intervals. The uptake was quite rapid, 40% (Cs) and 55% (Co) of the final steady state values being attained after 24 h. The subsequent releases were also rapid, 50% (Cs) and 40% (Co), of the accumulated activity being lost within 6 days. The experiments demonstrated that the major intake route following short-term releases of activity will be from the water column and that the close relationship between activity in water and organism can thus be used for predictive purposes without the complication of radionuclide uptake from contaminated sediments. However, for longer periods, the subsequent intake of sediments will generate a significant exposure pathway for this deposit-feeding bivalve.     

The accumulation of plutonium by the edible winkle (Littorina littorea L.)

Laboratory experiments were carried out to measure the accumulation of ²³⁷Pu by the edible winkle from labelled sea water, food (seaweed), and silt. The equilibrium whole body concentration factor for ²³⁷Pu accumulation from sea water at 10°C was estimated as 34 with a biological half-time of 9 d. After 50 d accumulation, 83% of the measured whole body activity was present on the shell. In the flesh, the major sites for accumulation were the head/foot complex and the digestive gland. Depuration was found to be biphasic with components having half-times of 10 and 193 d respectively. Pu loss, following uptake from labelled Fucus spiralis, was biphasic with half-times of 1 and 69 d for the two components. Assimilation efficiency was tentatively estimated as 7%. Pu accumulation from labelled silt was low with an estimated transfer factor of 0.004; Pu loss was biphasic, with half-times similar to those for the seaweed experiment. Comparison of the laboratory-derived results with those for winkles from the environment showed that the food pathway was the main route for Pu accumulation, with the ingestion of silt playing a critical role in determining the final flesh Pu burden.     

Studies of Np and Pu in the marine environment of Swedish-Danish waters and the North Atlantic Ocean

The long-lived anthropogenic radionuclides (237)Np, (239)Pu and (240)Pu were determined in marine environmental samples (seaweed and seawater) collected from Swedish-Danish waters and the North Atlantic Ocean at various locations on different occasions during the period 1991-2001. The measurements were performed with sector field Inductively Coupled Plasma Mass Spectrometry (ICP-MS) and conventional alpha spectrometry. The (237)Np activity concentrations in Fucus vesiculosus and surface seawater from the Swedish west coast and Danish waters ranged from 0.16+/-0.02 to 1.02+/-0.09 mBq kg(-1) (dry weight) and 0.65+/-0.02 to 1.69+/-0.02 mBq m(-3), respectively, depending on the location and sampling year. Most of the (237)Np in these waters is believed to originate from the Sellafield nuclear reprocessing plant, with some contribution from global fallout. The (240)Pu/(239)Pu atomic ratios in F. vesiculosus samples are reported in this study with an overall average of 0.17+/-0.03. The (237)Np and (239)Pu activity concentrations observed in surface seawater collected in North Atlantic waters ranged from 0.16+/-0.01 to 0.62+/-0.08 mBq m(-3) and from 0.64+/-0.05 to 4.27+/-0.08 mBq m(-3), respectively, and the (237)Np/(239)Pu atomic ratios were a good indicator of conservative behaviour of Np in marine waters.     

Revision and meta-analysis of selected biosphere parameter values for chlorine, iodine, neptunium, radium, radon and uranium

There is a continual supply of new experimental data that are relevant to the assessment of the potential impacts of nuclear fuel waste disposal. In the biosphere, the traditional assessment models are data intensive, and values are needed for several thousand parameters. This is augmented further when measures of central tendency, statistical dispersion, correlations and truncations are required for each parameter to allow probabilistic risk assessment. Recent reviews proposed values for 10-15 key element-specific parameters relevant to (36)Cl, (129)I, (222)Rn, (226)Ra, (237)Np and (238)U, and some highlights from this data update are summarized here. Several parameters for Np are revised downward by more than 10-fold, as is the fish/water concentration ratio for U. Soil solid/liquid partition coefficients, Kd, are revised downward by 10-770-fold for Ra. Specific parameters are discussed in detail, including degassing of I from soil; sorption of Cl in soil; categorization of plant/soil concentration ratios for U, Ra and Np; Rn transfer from soil to indoor air; Rn degassing from surface water; and the Ca dependence of Ra transfers.     

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.     

Radium and radium-daughter nuclides in carbonates: a brief overview of strategies for determining chronologies

Radium isotopes have been used extensively to trace the movement of groundwater as well as oceanic water masses, but these radionuclides (and their daughters) are also useful chronometers for the determination of the time scales of other Earth and environmental processes. The purpose of this overview is to present the application of Ra and Ra daughters in the dating of carbonates. We show that the choice of dating method (decay of excess radionuclide or ingrowth of daughter) depends strongly on the parent/daughter activity ratios in the water in which the carbonate was precipitated. Thus freshly precipitated carbonates uniformly show excesses of ²²⁶Ra relative to its parent ²³⁰Th, and ²²⁶Ra decay can provide ages of carbonates over Holocene time scales. In contrast, carbonates are precipitated in waters of greatly varying ²¹⁰Pb/²²⁶Ra. Corals, deep-sea hydrothermal vent clams and the shelled cephalopod Nautilus live in waters with significant dissolved ²¹⁰Pb and all show excesses of ²¹⁰Pb in their carbonate. Bivalve molluscs from nearshore and coastal waters, and carbonates deposited from groundwater environments (e.g. travertines) in which ²¹⁰Pb is efficiently scavenged from solution, show deficiencies of ²¹⁰Pb relative to ²²⁶Ra. In contrast, fish otoliths strongly discriminate against ²¹⁰Pb regardless of the environment in which the fish lives. Deficiencies of ²²⁸Th relative to ²²⁸Ra are common in all carbonates. Useful time ranges for the ²¹⁰Pb/²²⁶Ra and ²²⁸Th/²²⁸Ra chronometers are ∼100 y and ∼10 y, respectively.     

Transfer of 241Am and 237Pu from euphausiid moults to a carbonate-rich marine sediment

Concentrations of ²⁴¹Am and ²³⁷Pu adsorbed onto moulted exoskeletons from the euphausiid Megancytiphanes norvegica decreased exponentially with 50% retention times of 3–7 d when moults were incubated in filtered seawater with small amounts of a carbonate-rich sediment. Over 95% of sediment weight was present as < 43 μm silt particles and 91 ± 4% of radioactivity lost from moults was recovered in this size fraction. Adsorption of both actinides (atoms μm⁻²) was greatest in the medium-fine sand fraction which had the highest carbonate content. These particles constituted <0·4% of total sediment dry weight but their reactivity (atoms μm⁻² surface: atoms μm⁻³ in solution) was 10³–10⁴ times higher than similar quotients for <43 μm particles. The enrichment shows that non-homogeneous distributions may arise between particle types when actinides such as ²⁴¹Am and ²³⁷Pu sorbed to surfaces of organic particles are transferred to sediments.     

Accumulation of 90Sr in the bone tissue of northern mole voles in the head portion of the East Ural Radioactive Trace

The accumulation of ⁹⁰Sr in the bone tissue of northern mole voles (Ellobius talpinus Pallas, 1770) living under chronic exposure to ⁹⁰Sr in the center of the Eastern Ural Radioactive Trace (EURT) with a ⁹⁰Sr contamination density of 37 MBq/m² or 1000 Ci/km² has been evaluated. Northern mole voles live under-ground and are characterized by family organization of populations and an extremely weak tendency toward dispersal. While there are no sex- or age-related differences in ⁹⁰Sr accumulation, sevenfold interindividual differences have been found. Hereditary (familial) determination of the variability of ⁹⁰Sr accumulation has been demonstrated. The probable causes of the difference in the familial component from the results of laboratory experiments on inbred mice are considered. The age-related inversion of ⁹⁰Sr accumulation previously found in other mammalian species in the EURT area has been confirmed. A possible mechanism of its formation is suggested.     

A survey of environmental radioactivity measurements in the United States

A survey of selected radiochemical laboratories in the United States was conducted to estimate the relative numbers and types of analyses performed nationwide for the measurement of environmental radio-activity and to identify the regulatory requirements which affect the selection of analytical procedures and media. Information was sought from each laboratory regarding its parent facility, whether the laboratory performs its own analyses or contracts them out, and the technical and regulatory requirements for which the laboratory performed analyses.Analyses of water, air-particulate, and soil and sediment samples contribute the greatest workload. Many fewer vegetation, milk, diet, gas, or bioassay samples are processed. The most common measurements, especially for water, are for gross-α and gross-β activity. More than 50 different specific-radionuclide analyses are being performed with varying degree of regularity by the 161 laboratories surveyed. γ-spectrometry is the most frequently used method for specific-radionuclide analyses; the use of Ge (Li) detectors for γ-spectrometric measurements now exceeds that of NaI (Tl) detectors. Other radionuclides for which specific analyses are frequently performed by a large percentage (25–75%) of the laboratories include ⁹⁰Sr, ¹³¹I, ²²⁶Ra, ^238,239Pu, ^234,235,238U, and tritium in water (HTO).     

Bioavailability of sediment-associated and low-molecular-mass species of radionuclides/trace metals to the mussel Mytilus edulis

Sediments can act as a sink for contaminants in effluents from industrial and nuclear installations or when released from dumped waste. However, contaminated sediments may also act as a potential source of radionuclides and trace metals to the water phase due to remobilisation of metals as dissolved species and resuspension of particles. The marine mussel Mytilus edulis is a filter-feeding organism that via the gills is subjected to contaminants in dissolved form and from contaminants associated to suspended particles via the digestive system. In this paper the bioavailability of sediment-associated and seawater diluted Cs, Co, Cd and Zn radioactive tracers to the filtering bivalve M. edulis has been examined. The mussels were exposed to tracers diluted in ultrafiltered (<10 kDa) seawater (Low Molecular Mass form) or to tracers associated with sediment particles from the Stepovogo Fjord at Novaya Zemlya in short-term uptake experiments, followed by 1-month depuration experiments in flow-through tanks. A toxicokinetic model was fitted to the uptake and depuration data, and the obtained parameters were used to simulate the significance of the two uptake pathways at different suspended sediment loads and sediment-seawater distribution coefficients. The results of the model simulations, assuming steady state conditions, suggest that resuspended particles from contaminated sediments can be a highly significant pathway for mussels in the order ¹⁰⁹Cd ≌ ⁶⁵Zn < ¹³⁴Cs < ⁶⁰Co. The significance increases with higher suspended sediment load and with higher K_d. Furthermore, the experimental depuration data suggest that Cs is retained longer and Co, Cd and Zn shorter by the mussels when associated with ingested sediments, than if the metals are taken up from the low molecular mass (LMM) phase.     

Sources of anthropogenic radionuclides in the environment: a review

Studies of radionuclides in the environment have entered a new era with the renaissance of nuclear energy and associated fuel reprocessing, geological disposal of high-level nuclear wastes, and concerns about national security with respect to nuclear non-proliferation. This work presents an overview on sources of anthropogenic radionuclides in the environment, as well as a brief discussion of salient geochemical behavior of important radionuclides. We first discuss the following major anthropogenic sources and current developments that have lead, or could potentially contribute, to the radionuclide contamination of the environment: (1) nuclear weapons program; (2) nuclear weapons testing; (3) nuclear power plants; (4) uranium mining and milling; (5) commercial fuel reprocessing; (6) geological repository of high-level nuclear wastes that include radionuclides might be released in the future, and (7) nuclear accidents. Then, we briefly summarize the inventory of radionuclides ⁹⁹Tc and ¹²⁹I, as well as geochemical behavior for radionuclides ⁹⁹Tc, ¹²⁹I, and ²³⁷Np, because of their complex geochemical behavior, long half-lives, and presumably high mobility in the environment; biogeochemical cycling and environment risk assessment must take into account speciation of these redox-sensitive radionuclides.     

Speciation patterns of actinides in natural waters: a laboratory investigation

The results of laboratory investigations of actinide speciation in ground waters of varying compositions are summarized, re-evaluated and further interpreted, with the inclusion of additional data. Actinide speciation (solubility, oxidation state distributions) in these water samples exhibits great variability at 10⁻¹⁰ to 10⁻⁸ M total actinide concentrations. Americium solubility appears to be controlled by the formation of radiocolloids. neptunium is predictably oxidized and soluble in these water samples, with only a few exceptions. Plutonium exhibits widely varying oxidation state distributions—and hence solubility— despite the presence of 6 mg liter⁻¹ dissolved oxygen in all experiments. Plutonium solubility is enhanced by carbonate and fluoride but is diminished by sulfate in reducing waters. In general, these results are compatible with the limited information available for actinide speciation from previous in-situ investigations but they emphasize the paucity of accurate thermodynamic data to describe these systems.