Plutonium and 137Cs in surface water of the South Pacific Ocean

The present plutonium and 137Cs concentrations in South Pacific Ocean surface waters were determined. The water samples were collected in the South Pacific mid-latitude region (32.5 degrees S) during the BEAGLE expedition conducted in 2003-04 by JAMSTEC. 239,240Pu concentrations in surface seawater of the South Pacific were in the range of 0.5 to 4.1 mBq m(-3), whereas 137Cs concentrations ranged from 0.07 to 1.7 Bq m(-3). The observed 239,240Pu and 137Cs concentrations in the South Pacific were almost of the same level as those in the North Pacific subtropical gyre. The surface 239,240Pu in the South Pacific subtropical gyre showed larger spatial variations than 137Cs, as it may be affected by physical and biogeochemical processes. The 239,240Pu/137Cs activity ratios, which reflect biogeochemical processes in seawater, were generally smaller than that observed in global fallout, except for the most eastern station. The 239,240Pu/137Cs ratios in the South Pacific tend to be higher than that in the North Pacific. The relationships between anthropogenic radionuclides and oceanographic parameters such as salinity and nutrients were examined. The 137Cs concentrations in the western South Pacific (the Tasman Sea) and the eastern South Pacific were negatively correlated with the phosphate concentration, whereas there is no correlation between the 137Cs and nutrients concentrations in the South Pacific subtropical gyre. The mutual relationships between anthropogenic radionuclides and oceanographic parameters are important for better understanding of transport processes of anthropogenic radionuclides and their fate in the South Pacific.     

Vertical distributions of 239+240Pu activities and 240Pu/239Pu atom ratios in sediment cores: implications for the sources of Pu in the Japan Sea

The vertical distributions of 239+240Pu-specific activities and 240Pu/239Pu atom ratios in sediment cores of the Japan Sea were investigated. For comparison, the 239+240Pu-specific activities and 240Pu/239Pu atom ratios in the surface layer of bottom sediments in the Sea of Okhotsk were also determined. The 239+240Pu-specific activities in the surface layer sediments varied from 0.43 to 2.65 mBq/g in the Japan Sea. The lowest value was seen in the eastern Japan Basin, while the highest value was found in a sample collected from a coastal area off Hokkaido in the northern Japan Sea. It was found that the 240Pu/239Pu atom ratios in the surface sediments for the Japanese side of the Japan Sea ranged from 0.15 to 0.22. The 240Pu/239Pu atom ratios in the surface sediments collected from the Yamato Basin and Japan Basin were typically close to the global fallout value of 0.18, but higher 240Pu/239Pu atom ratios (0.20-0.22) were seen in samples from the northern Japan Sea. Vertical profiles of 240Pu/239Pu atom ratios revealed that these atom ratios were higher than the global fallout value in the deeper layers in the sediment cores from the northern Japan Sea on the Hokkaido coast and the Yamato Basin. In addition, the sediment cores from the northern Japan Sea near the Soya Strait and in the Ishikari Estuary had a constant vertical distribution of 240Pu/239Pu atom ratios with a value around 0.20. The results obtained in this work support the hypothesis that the prevailing ocean currents transported Pu derived from the Pacific Proving Grounds to the Japan Sea, and this contributed to the general elevated Pu inventory in the Japan Sea.     

Distribution of 137Cs, 238Pu and 239+240 Pu in sediments of the southeastern Brazilian shelf-SW Atlantic margin

In this work levels of 137Cs, 238Pu and 239+240 Pu as well as activity ratios of anthrophogenic radionuclides in sediment samples from the southeastern Brazilian shelf are presented. Instrumental gamma spectrometry was used to determine 137Cs and alpha spectrometry to determine 238Pu and 239+240 Pu after a radiochemical procedure. The levels ranged from 0.30 to 1.79 Bq kg(-1) for 137Cs, from 15 to 150 mBq kg(-1) for 238Pu and, from 18 to 117 mBq kg(-1) for 239+240 Pu. There was a bathymetric differentiation in the radionuclides distribution. 137Cs values were generally higher in the samples collected at water depths of less than 100 m. On the other hand, plutonium isotopes exhibit higher values at greater depths. The mean ratio of 239+240 Pu/137Cs obtained was 0.112+/-0.072 which is in agreement with the value reported for the Atlantic from atmospheric fallout of nuclear explosions in the past. The 238Pu/230+240 Pu ratios varied widely (from 0.339 to 2.088) and showed the influence of the SNAP-9A accident in the 238Pu levels for this area. The main goal of this work was to present unpublished anthropogenic radionuclide levels and activity ratios related to the contamination of the southeastern Brazilian shelf.     

Temporal variation of Pu/Pu atom ratio and Pu inventory in water columns of the Japan Sea

The ^239+240Pu concentrations and ²⁴⁰Pu/²³⁹Pu atom ratios were determined by alpha spectrometry and double-focusing SF-ICP-MS for seawater samples obtained in 1984 and 1993 from the Yamato and Tsushima Basins of the Japan Sea in the western North Pacific margin. The total ^239+240Pu inventories in the whole water columns were approximately doubled during the period from 1984 to 1993 in the two basins. The increasing rates were estimated to be 5.1 Bq m⁻² yr⁻¹ in the Yamato Basin and 4.2 Bq m⁻² yr⁻¹ in the Tsushima Basin and they corresponded to ~ 0.02% of the annual ^239+240Pu inflow rate into the Japan Sea through the Tsushima Strait. The mean ²⁴⁰Pu/²³⁹Pu atom ratios were ~ 0.240 and significantly higher than the mean global fallout ratio of 0.18. Furthermore, there were no temporal or spatial variations of ²⁴⁰Pu/²³⁹Pu atom ratios during this period in the Japan Sea. The total ^239+240Pu inventories originating from the close-in fallout increased from 17.6 Bq m⁻² to 34.6 Bq m⁻² in the Yamato Basin and from 20.1 Bq m⁻² to 34.6 Bq m⁻² in the Tsushima Basin; however, the relative percentage of ~ 40% from the close-in fallout was unchanged during this period. A likely mechanism for the increasing Pu inventory would be the continuous inflow of the Tsushima Current from the western North Pacific, and the removal of Pu from surface waters by scavenging onto the settling particles, followed by regeneration of Pu from the settling particles during the downward transport.     

137Cs in the western South Pacific Ocean

The 137Cs activities were determined for seawater samples from the East Caroline, Coral Sea, New Hebrides, South Fiji and Tasman Sea (two stations) Basins of the western South Pacific Ocean by gamma spectrometry using a low background Ge detector. The 137Cs activities ranged from 1.4 to 2.3 Bq m(-3) over the depth interval 0-250 m and decreased exponentially from the subsurface to 1000 m depth. The distribution profiles of 137Cs activity at these six western South Pacific Ocean stations did not differ from each other significantly. There was a remarkable difference for the vertical profiles of 137Cs activity between the East Caroline Basin station in this study and the GEOSECS (Geochemical Ocean Sections Study) station at the same latitude in the Equatorial Pacific Ocean; the 137Cs inventory over the depth interval 100-1000 m increased from 400+/-30 Bq m(-2) to 560+/-30 Bq m(-2) during the period from 1973 to 1992. The total 137Cs inventories in the western South Pacific Ocean ranged from 850+/-70 Bq m(-2) in the Coral Sea Basin to 1270+/-90 Bq m(-2) in the South Fiji Basin. Higher 137Cs inventories were observed at middle latitude stations in the subtropical gyre than at low latitude stations. The 137Cs inventories were 1.9-4.5 times (2.9+/-0.7 on average) and 1.7-4.3 times (3.1+/-0.7 on average) higher than that of the expected deposition density of atmospheric global fallout at the same latitude and that of the estimated 137Cs deposition density in 10 degrees latitude by 10 degrees longitude grid data obtained by Aoyama et al. [Aoyama M, Hirose K, Igarashi Y. Re-construction and updating our understanding on the global weapons tests 137Cs fallout. J Environ Monit 2006;8:431-438], respectively. The possible processes for higher 137Cs inventories in the western South Pacific Ocean than that of the expected deposition density of atmospheric global fallout may be attributable to the inter-hemisphere dispersion of the atmospheric nuclear weapons testing 137Cs from the northern stratosphere to the southern one and its subsequent deposition, and water-bearing transport of 137Cs from the North Pacific Ocean to the western South Pacific.     

239 + 240Pu and 137Cs concentrations in fish, cephalopods, crustaceans, shellfish, and algae collected around the Japanese coast in the early 1990s

Marine organisms, i.e. fish, cephalopods, crustaceans, shellfish, and algae, were collected in the early 1990s along the Sea of Japan coast and the Japanese Pacific coast and analyzed for their 239 + 240Pu and 137Cs concentrations. The 239 + 240Pu concentrations in muscle of fish were below 0.4 mBq/kg wet wt. and the lowest among the analyzed marine organisms. Most 137Cs concentrations in muscle of fish ranged from 100 to 300 mBq/kg wet wt. Higher concentrations of 239 + 240Pu, ranging from 1.6 to 5.7 mBq/kg wet wt., were observed in viscera of cephalopods than in their muscle. The 239 + 240Pu concentrations in whole soft tissues of bivalves varied approximately one order of magnitude from 0.8 to 6.1 mBq/kg wet wt., while 137Cs concentrations had little variation, being approximately 60 mBq/kg wet wt. The 239 + 240Pu concentrations in algae had a wide variation, ranging from 1.7 to 42.3 mBq/kg wet wt., and were higher than those of the other marine organisms. No statistically significant difference in mean concentrations of 239 + 240Pu was detected among the whole soft tissues of bivalves, viscera of cephalopods and crustaceans, and whole bodies of cephalopods and crustaceans within the 95% confidence limit. The mean concentrations of 137Cs became higher in the order, cephalopods and crustaceans and bivalves, algae, viscera of fish, muscles of fish. The mean concentrations of 239 + 240Pu were comparable for algae collected along the Japan Sea coast and the Pacific coast. Furthermore, the difference in mean concentrations of 137Cs in algae between the Japan Sea coast and the Pacific coast was not statistically significant within the 95% confidence limit. These results can be considered to indicate no definite influence from radioactive dumping into the Japan Sea by the former USSR and Russia with respect to radioactive pollution of marine organisms collected along the Japanese coast.     

Arctic Ocean sea ice drift origin derived from artificial radionuclides

Since the 1950s, nuclear weapon testing and releases from the nuclear industry have introduced anthropogenic radionuclides into the sea, and in many instances their ultimate fate are the bottom sediments. The Arctic Ocean is one of the most polluted in this respect, because, in addition to global fallout, it is impacted by regional fallout from nuclear weapon testing, and indirectly by releases from nuclear reprocessing facilities and nuclear accidents. Sea-ice formed in the shallow continental shelves incorporate sediments with variable concentrations of anthropogenic radionuclides that are transported through the Arctic Ocean and are finally released in the melting areas. In this work, we present the results of anthropogenic radionuclide analyses of sea-ice sediments (SIS) collected on five cruises from different Arctic regions and combine them with a database including prior measurements of these radionuclides in SIS. The distribution of ¹³⁷Cs and ^239,240Pu activities and the ²⁴⁰Pu/²³⁹Pu atom ratio in SIS showed geographical differences, in agreement with the two main sea ice drift patterns derived from the mean field of sea-ice motion, the Transpolar Drift and Beaufort Gyre, with the Fram Strait as the main ablation area. A direct comparison of data measured in SIS samples against those reported for the potential source regions permits identification of the regions from which sea ice incorporates sediments. The ²⁴⁰Pu/²³⁹Pu atom ratio in SIS may be used to discern the origin of sea ice from the Kara-Laptev Sea and the Alaskan shelf. However, if the ²⁴⁰Pu/²³⁹Pu atom ratio is similar to global fallout, it does not provide a unique diagnostic indicator of the source area, and in such cases, the source of SIS can be constrained with a combination of the ¹³⁷Cs and ^239,240Pu activities. Therefore, these anthropogenic radionuclides can be used in many instances to determine the geographical source area of sea-ice.     

Plutonium isotopes in seas around the Korean Peninsula

239+240Pu concentrations and 240Pu/239Pu atom ratios in coastal seas adjacent to the Korean Peninsula were determined, during the period 1999 to 2002, to assess the current distribution and to identify sources of Pu isotopes. 239+240Pu concentrations in surface waters ranged from 3.1 to 22.3 mBq m(-3) with higher concentrations in winter than in summer. 239+240Pu concentrations in seawaters around the Korean Peninsula are greater than that in the western North Pacific. 240Pu/239Pu atom ratios ranged from 0.18 to 0.33 with an average value of 0.25+/-0.03, which is significantly higher than the global fallout average. The 240Pu/239Pu atom ratios of the 2000 m deep entire water column in the south western part of the East Sea (Sea of Japan) was comparable to that observed in waters near Bikini Atoll. The higher 240Pu/239Pu atom ratios than that of global fallout may be explained by the hypothesis that the earlier input signal of low 240Pu/239Pu atom ratio (0.18-0.19) of global fallout plutonium in seas adjacent to the Korean Peninsula is being gradually diluted by the high 240Pu/239Pu atom ratio plutonium transported from the tropical Pacific Proving Grounds via prevailing ocean current.     

Measurement of 240Pu/239Pu isotopic ratios in soils from the Marshall Islands using ICP-MS.

Nuclear weapons tests conducted by the United States in the Marshall Islands produced significant quantities of regional or tropospheric fallout contamination. Here we report on some preliminary inductively coupled plasma-mass spectrometry (ICP-MS) measurements of plutonium isolated from seven composite soil samples collected from Bikini, Enewetak and Rongelap Atolls in the northern Marshall Islands. These data show that 240Pu/239Pu isotopic signatures in surface soils from the Marshall Island vary significantly and could potentially be used to help quantify the range and extent of fallout deposition (and associated impacts) from specific weapons tests. 137Cs and 60Co were also determined on the same set of soil samples for comparative purposes.     

Temporal record of Pu isotopes in inter-tidal sediments from the northeastern Irish Sea

A depth profile of ²³⁹Pu and ²⁴⁰Pu specific activities and isotope ratios was determined in an inter-tidal sediment core from the Esk Estuary in the northeastern Irish Sea. The study site has been impacted with plutonium through routine radionuclide discharges from the Sellafield nuclear reprocessing plant in Cumbria, NW England. A pronounced sub-surface maximum of ~ 10 kBq kg^-1 was observed for ^239 + 240Pu, corresponding to the peak in Pu discharge from Sellafield in 1973, with a decreasing trend with depth down to ~ 0.04 kBq kg^-1 in the deeper layers. The depth profile of ^239 + 240Pu specific activities together with results from gamma-ray spectrometry for ¹³⁷Cs and ²⁴¹Am was compared with reported releases from the Sellafield plant in order to estimate a reliable sediment chronology. The upper layers (1992 onwards) showed higher ^239 + 240Pu specific activities than would be expected from the direct input of annual Sellafield discharges, indicating that the main input of Pu is from the time-integrated contaminated mud patch of the northeastern Irish Sea. The ²⁴⁰Pu/²³⁹Pu atom ratios ranged from ~ 0.03 in the deepest layers to > 0.20 in the sub-surface layers with an activity-weighted average of 0.181. The decreasing ²⁴⁰Pu/²³⁹Pu atom ratio with depth reflects the changing nature of operations at the Sellafield plant from weapons-grade Pu production to reprocessing spent nuclear fuel with higher burn-up times in the late 1950s. In addition, recent annual ²⁴⁰Pu/²³⁹Pu atom ratios in winkles collected during 2003-2008 from three stations along the Cumbrian coastline showed no significant spatial or temporal differences with an overall average of 0.204, which supports the hypothesis of diluted Pu input from the contaminated mud patch.     

Distribution of artificial radionuclides in deep sediments of the Mediterranean Sea

Artificial radionuclides enter the Mediterranean Sea mainly through atmospheric deposition following nuclear weapons tests and the Chernobyl accident, but also through the river discharge of nuclear facility effluents. Previous studies of artificial radionuclides impact of the Mediterranean Sea have focussed on shallow, coastal sediments. However, deep sea sediments have the potential to store and accumulate pollutants, including artificial radionuclides. Deep sea marine sediment cores were collected from Mediterranean Sea abyssal plains (depth > 2000 m) and analysed for ^239,240Pu and ¹³⁷Cs to elucidate the concentrations, inventories and sources of these radionuclides in the deepest areas of the Mediterranean. The activity — depth profiles of ²¹⁰Pb, together with ¹⁴C dating, indicate that sediment mixing redistributes the artificial radionuclides within the first 2.5 cm of the sedimentary column. The excess ²¹⁰Pb inventory was used to normalize ^239,240Pu and ¹³⁷Cs inventories for variable sediment fluxes. The ^239,240Pu/²¹⁰Pb_xs ratio was uniform across the entire sea, with a mean value of 1.24 × 10^− 3, indicating homogeneous fallout of ^239,240Pu. The ¹³⁷Cs/²¹⁰Pb_xs ratio showed differences between the eastern (0.049) and western basins (0.030), clearly significant impact of deep sea sediments from the Chernobyl accident. The inventory ratios of ^239,240Pu/¹³⁷Cs were 0.041 and 0.025 in the western and eastern basins respectively, greater than the fallout ratio, 0.021, showing more efficient scavenging of ^239,240Pu in the water column and major sedimentation of ¹³⁷Cs in the eastern basin. Although areas with water depths of > 2000 m constitute around 40% of the entire Mediterranean basin, the sediments in these regions only contained 2.7% of the ^239,240Pu and 0.95% of the ¹³⁷Cs deposited across the Sea in 2000. These data show that the accumulation of artificial radionuclides in deep Mediterranean environments is much lower than predicted by other studies from the analysis of continental shelf sediments.     

Large particle flux of 239+240Pu on the continental margin of the East China Sea

Settling particles were collected from three locations in the East China Sea continental margin and analyzed for 239+240Pu. Two types of sediment traps were used, cylindrical traps and conical time-series traps. Surface sediment samples collected from five locations were also analyzed for 239+240Pu. Data from cylindrical traps showed there was a clear tendency for total mass fluxes to increase with depth at all three stations, and there was an especially large increase near the bottom. 239+240Pu concentrations in settling particles increased with depth from 1.76 mBq/g at 97-m depth to 3.0 mBq/g at 120-m depth and ranged from approximately 3 to 4 mBq/g at depths greater than 120 m. 239+240Pu concentrations collected in the near-bottom traps were approximately two times higher than those in the underlying surface sediments. Like total mass fluxes there was a clear tendency for 239+240Pu fluxes to increase with depth at every station, and the highest 239+240Pu fluxes were observed near the bottom. 239+240Pu concentrations in the time-series traps had little variation throughout the sampling period, though the total mass fluxes showed a large variation. A high variability of 239+240Pu fluxes occurred in very short period of time (1/2 day). The large fluxes of 239+240Pu might be attributed to episodic lateral transport of particles that flow down the continental slope with the nepheloid layer which was considered to be significant for 239+240Pu transport on the continental slope in the East China Sea.     

238Pu and 239+240Pu inventory and distribution through the lower Rhone valley terrestrial environment (Southern France)

Because of atmospheric global fallout due to thermonuclear tests performed between 1945 and 1980 and to the American SNAP 9A satellite explosion in 1964, plutonium's long-lived alpha emitter isotopes ((238)Pu, (239)Pu and (240)Pu) can be found everywhere in the environment. In the soils of the lower Rhone valley, over a surface area of approximately 11,000 km(2), existing results allow estimation of the total inventory resulting from this origin of 551 GBq as regards these radionuclides. The Marcoule Nuclear Reprocessing Plant (NRP) has been releasing (238)Pu and (239+240)Pu into the environment for over 40 years in the region and is as such a second likely source for Pu input. This article gives a global review of plutonium distribution in this particular region. It is shown that releases from Marcoule, although accounting for less than 2% of the total Pu inputs into the terrestrial environment, are responsible for localised and measurable Pu enrichments close to the facility (+2.8 GBq), and in the Rhone delta where sediments from the river settled during the past floods (+0.1 GBq). Irrigating with water from the Rhone River also allowed the transfer of approximately 3.8 GBq of (238)Pu and (239+240)Pu to the soils for the whole 1960-1998 period. These inputs, distributed over wide irrigated surface areas, do not induce significant increases of soil Pu-activity levels. In a second section, this study confirms that most of the plutonium existing in the terrestrial environment is accumulated in the soil. Less than 0.1% of the activities exist currently in the plant compartments, and the flows exported by agricultural productions are very low, although we are interested here in a French region where agriculture prevails.     

Reconstructing historical radionuclide concentrations along the east coast of Ireland using a compartmental model

A mathematical model is presented that simulates the annually averaged transport of radionuclides, originating from the BNFL reprocessing plant at Sellafield, throughout the Irish Sea. The model, CUMBRIA77, represents the processes of radionuclide transport and dispersion in the marine environment and allows predictions of radionuclide concentration in various environmental media, including biota, to be made throughout the whole of the Irish Sea. In this paper we describe the use of the model to reconstruct the historical activity concentrations of 137Cs and 239+240Pu in a variety of environmental media in the western Irish Sea and along the Irish east coast back to 1950. This reconstruction exercise is of interest because only limited measurements of 137Cs and 239+240Pu activity are available prior to the 1980s. The predictions were compared to the available measured data to validate their accuracy. The results of the reconstruction indicate that activity concentrations of 137Cs in the western Irish Sea follow a similar, though slightly delayed and smoothed, profile to the discharges from the Sellafield site, with concentrations at the time of peak discharge (the mid-1970s) being around an order of magnitude higher than those measured in the 1980s and 1990s. By contrast, the concentrations of 239+240Pu at the time of peak discharges were similar to those presently measured. These differences reflect the distinct marine chemistries of the two nuclides, in particular the higher propensity of plutonium to bind to sediments leading to extended transport times. Despite these differences in behaviour the doses to Irish seafood consumers from 137Cs remain significantly higher than those from 239+240Pu.     

Characterization of Pu concentration and its isotopic composition in a reference fallout material

Because there is no reference material for fallout plutonium isotope monitoring, preparation of such a material is necessary for quality control of fallout radionuclides analysis for atmospheric environmental studies. In this work, we report the characterization of Pu activity and its isotopic composition in a reference fallout material prepared by the Meteorological Research Institute (MRI), Japan. This material was prepared from samples collected at 14 stations throughout Japan in 1963-1979, with reference values of ¹³⁷Cs, ⁹⁰Sr and ^239 + 240Pu activities. We analyzed the activities of ^239 + 240Pu and ²⁴¹Pu, and the atom ratios of ²⁴⁰Pu/²³⁹Pu and ²⁴¹Pu/²³⁹Pu using an isotope dilution sector-field inductively coupled plasma mass spectrometry (SF-ICP-MS). The ^239 + 240Pu activities in this fallout material using acid leaching and total digestion were 6.56 ± 0.20 mBq/g and 6.79 ± 0.16 mBq/g, respectively. Atom ratios of ²⁴⁰Pu/²³⁹Pu were 0.1915 ± 0.0030 and 0.1922 ± 0.0044, respectively. Both ²⁴⁰Pu/²³⁹Pu and ²⁴¹Pu/²³⁹Pu atom ratios were slightly higher than those of global fallout, which could be attributed to the deposition of fallout radionuclides resulting from the Chinese nuclear weapons tests conducted in the 1970s. The dominant host phases of ^239 + 240Pu were found to be organic matter-sulfides (70%) with a relative high ²⁴⁰Pu/²³⁹Pu atom ratio, and Fe-Mn oxides (19%) using a sequential extraction method.     

The uptake of plutonium-238, 239, 240, americium-241, strontium-90 and caesium-137 into potatoes

Transfer factors have been measured for ²³⁹Pu + ²⁴⁰Pu, ²⁴¹Am, ⁹⁰Sr and ¹³⁷Cs into potatoes grown in a blend of soil which included silt polluted with radioactive waste discharged into the Irish Sea. The experiment has been carried out over four seasons and attempts to assess the radiological consequences to the consumer which would arise if potatoes were to be grown in land heavily contaminated with the silt. During the course of the experiment plutonium and americium became less available for uptake, ⁹⁰Sr became slightly more available and ¹³⁷Cs remained nearly constant in its availability. The values of the transfer factors are among the lowest reported for these radionuclides into potatoes.     

Rapid dating of recent sediments in Loch Ness: inductively coupled plasma mass spectrometric measurements of global fallout plutonium

The (239+240)Pu activity profile is determined for a sediment core collected from 170-m depth at Loch Ness, Scotland. These measurements use magnetic sector inductively coupled plasma mass spectrometry for rapid determination of Pu activities and (240)Pu/(239)Pu atom ratios. A (239+240)Pu detection limit of 0.1 Bq/kg is obtained for 2 g of acid-leached sediment; (242)Pu is used as a spike isotope. The Pu activity profile exhibits a maximum of 42.7+/-0.3 Bq/kg (239+240)Pu in the 9-10-cm depth interval. The position of this maximum coincides with peaks in the (241)Am and (137)Cs activity profiles. These peak activities are ascribed to the 1963/1964 peak fallout from atmospheric testing of nuclear weapons. The (240)Pu/(239)Pu atom ratios are in the range 0.15-0.20, in agreement with the expected range of 0.166-0.194 for Northern Hemisphere fallout, and do not suggest the presence of other contributing sources. This study demonstrates that ICPMS has considerable potential for rapid determination of the chronology of post-1950 sediments, and also for validating (210)Pb dates where chronologies over longer time-scales are needed.     

Radionuclide behaviour and transport in a coniferous woodland ecosystem: vegetation, invertebrates and wood mice, Apodemus sylvaticus

Activity concentrations of radionuclides (134Cs, 137Cs, 238Pu, 239 + 240Pu and 241Am) were measured in vegetation, invertebrates and wood mice, Apodemus sylvaticus, collected in Lady Wood, a coniferous woodland in the vicinity of the British Nuclear Fuels reprocessing plant at Sellafield, Cumbria, UK. Vegetation was of low diversity and biomass with activity concentrations ranging from 1 to 5 Bq kg-1 (134Cs), 0.3-0.5 Bq kg-1 (238Pu), 0.8-8 Bq kg-1 (239 + 240Pu), and 0.6-16 Bq kg-1 (241Am), dry wt. Caesium-137 activity concentrations were high compared to the reference site in Cheshire, varying between 65 and 280 Bq kg-1. Marked inter-specific and temporal differences in radionuclide activity concentrations were recorded for invertebrate populations. Caesium-137, 238Pu, 239 + 240Pu and 241Am activity concentrations in detritivorous invertebrates were consistently higher than in all other invertebrate groups reflecting contamination of the leaf litter. The activity concentrations in detritivores increased during the autumn and winter, reflecting changes in diet as food sources varied throughout the year. Activity concentrations in invertebrates caught in Lady Wood were generally an order of magnitude higher than for the reference site. Activity concentrations in wood mice varied between 7 and 150 Bq kg-1 (137Cs), 0.1-0.3 Bq kg-1 (238Pu), 0.1-0.6 Bq kg-1 (239 + 240Pu) and 0.2-0.4 Bq kg-1 (241Am). There were clear differences in the activity concentration of 137Cs (P < 0.01), 239 + 240Pu (P < 0.05) and 241Am (P < 0.05) in animals caught in Lady Wood compared to the reference site. However, the activity concentrations for 238Pu were similar at both sites, reflecting a low gastrointestinal transfer. Seasonal variation in activity concentrations was observed for 137Cs, 238Pu and 241Am. This variation is attributed to changes in the age structure of the population and diet throughout the year.     

Feasibility of using U to reconstruct close-in fallout deposition from the Hiroshima atomic bomb

The first results on the feasibility of using ²³⁶U to reconstruct the level and spatial distribution of close-in fallout deposition from the Hiroshima A-bomb are reported, coupled with the use of global fallout ¹³⁷Cs and ^239 + 240Pu. The results for global fallout ²³⁶U in soil samples (0-30 cm) from Ishikawa prefecture showed that the deposition density of ²³⁶U from the global fallout can be accurately evaluated using AMS. All deposited ²³⁶U, ¹³⁷Cs and ^239 + 240Pu appeared to have been recovered using 30-cm cores. It was also noted from the depth profiles for ²³⁶U/^239 + 240Pu and ²³⁶U/¹³⁷Cs ratios that the downward behavior of ²³⁶U in the soil was apparently similar to that of ^239 + 240Pu, while the ¹³⁷Cs was liable to be retained in upper layers compared with ²³⁶U and ^239 + 240Pu. The accumulated levels were 1.78 × 10¹³ atoms m^− 2 for ²³⁶U, 4340 Bq m^− 2 for ¹³⁷Cs and 141 Bq m^− 2 for ^239 + 240Pu. The ratios of ²³⁶U/¹³⁷Cs and ²³⁶U/^239 + 240Pu were (4.10 ± 0.12) × 10⁹ and (1.26 ± 0.04) × 10¹¹ atoms Bq^− 1, respectively. Results of ²³⁶U, ¹³⁷Cs and ^239 + 240Pu measurements for the seven soil cores (0-30 cm) from Hiroshima were discussed on the basis of ratios of ²³⁶U/¹³⁷Cs and ²³⁶U/^239 + 240Pu by comparing with those from the background area in Ishikawa, indicating that the global fallout dominates the current level of ²³⁶U accumulation in soil in the Black-rain area around Hiroshima after the Hiroshima bomb, and the contribution of the close-in fallout ²³⁶U produced by the Hiroshima A-bomb seems difficult to observe.     

Model testing of radioactive contamination by Sr, Cs and Pu of water and bottom sediments in the Techa River (Southern Urals, Russia)

This paper presents results of testing models for the radioactive contamination of river water and bottom sediments by ⁹⁰Sr, ¹³⁷Cs and ^239,240Pu. The scenario for the model testing was based on data from the Techa River (Southern Urals, Russia), which was contaminated as a result of discharges of liquid radioactive waste into the river. The endpoints of the scenario were model predictions of the activity concentrations of ⁹⁰Sr, ¹³⁷Cs and ^239,240Pu in water and bottom sediments along the Techa River in 1996. Calculations for the Techa scenario were performed by six participant teams from France (model CASTEAUR), Italy (model MARTE), Russia (models TRANSFER-2, CASSANDRA, GIDRO-W) and Ukraine (model RIVTOX), all using different models. As a whole, the radionuclide predictions for ⁹⁰Sr in water for all considered models, ¹³⁷Cs for MARTE and TRANSFER-2, and ^239,240Pu for TRANSFER-2 and CASSANDRA can be considered sufficiently reliable, whereas the prediction for sediments should be considered cautiously. At the same time the CASTEAUR and RIVTOX models estimate the activity concentrations of ¹³⁷Cs and ^239,240Pu in water more reliably than in bottom sediments. The models MARTE (^239,240Pu) and CASSANDRA (¹³⁷Cs) evaluated the activity concentrations of radionuclides in sediments with about the same agreement with observations as for water. For ⁹⁰Sr and ¹³⁷Cs the agreement between empirical data and model predictions was good, but not for all the observations of ^239,240Pu in the river water-bottom sediment system. The modelling of ^239,240Pu distribution proved difficult because, in contrast to ¹³⁷Cs and ⁹⁰Sr, most of models have not been previously tested or validated for plutonium.