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Plutonium in tree rings from France and Japan
Appl. Radiat. lsot. Vol. 46, No. 11, pp. 1271-1278, 1995
Pergamon
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Plutonium in Tree Rings from France and Japan J.-P. G A R R E C l, T. S U Z U K I , 2 Y. M A H A R A , 3 D. C. S A N T R Y , 4 S. M I Y A H A R A , 5 M . S U G A H A R A , 6 J. Z H E N G 7 a n d A. K U D O 7 JLaboratoire Pollution Atmospherique, INRA, 54280 Champenoux, France, :Department of Environmental Engineering, Niigata University, Niigata, Japan, ~Abiko Research Laboratory, CRIEP1, Chiba, Japan, 4Institute for National Measurements Standards, NRCC, Ottawa, Canada, 5Department of Marine Chemistry, Nagasaki University, Nagasaki, Japan, 6Department of Environmental Engineering, Osaka Sangyo University, Daito-shi, Japan and qnstitute for Environmental Research and Technology, NRCC, Ottawa, Canada Plutonium, along with other radionuclide concentrations, was measured in evergreen tree rings from two different locations. This was used as an information source for the past two centuries. Tree rings are a product of annual layers and thus chronological information is clearly visible. Three trees were harvested in 1988-1990: a French white fir (137 years old) and a spruce tree (177 years old) from the France-Germany border near Nancy, France and a sugi (78 years old) from Nagasaki, Japan. The uniform branchless part of the trunks from the harvested trees were immediately separated into a set of tree ring samples each of which contained 3-20 years of growth. The separated samples were mechanically powdered, dried at 105°C to obtain the dry weight, ashed at 350°C to measure 4°K, ~34Csand mCs and ashed again at 600°C to determine 239+240pu"The highest 239÷:40pu concentration of 30.0 mBq/kg of dry wood was obtained from the tree rings from Nagasaki, located at the centre of the local fallout of the Pu A-bomb detonated in 1945. This concentration peak was, however, observed in tree rings of 1965-67. The concentration was only 2.9 mBq/kg for the tree rings of 1944-46. The contribution of the local fallout on the surface soils from the A-bomb was 181 mBq/cm: at the harvested area of the tree, while the contribution of global fallout by many weapons testing was 5.9 mBq/cm: (or 3.3% total fallout in the region). The reason for the over 20 year time lag of 239+:40pu uptake by the tree rings is unknown because many factors influence the routes of Pu into the tree rings. Also the chemical form of Pu in surface soils may have been changed by the surrounding environment.The highest concentration in the tree rings from France was 9.4 mBq/kg which is about 31% of Nagasaki :39+:4Opuconcentration. The harvested area did not have any recorded Pu sources other than global fallout. An interesting result was that the distribution of ~Cs and~37Cs concentrations in the French white fir was different from Nagasaki. Data suggested that these new radionuclide inputs were from the Cbernobyl accident. The mobility (or diffusion coefficient) of cesium is 2-8 cmS/yr in the portion of heart-wood tree rings 0955-1870). Although tree rings can record chronological inputs of various trace elements, some elements cannot be used. These exceptions would be elements that: (1) are mobile within tree rings; (2) have littlte understood entry routes to the tree rings (via roots, leaves or barks); and (3) have unknown biogeochemical behaviour in the surrounding environment. Further investigation is warranted to use tree rings as a tool to record past environmental history.
Introduction One of reliable chronological information source is the tree rings which show not only the seasons of every year but the annual layers for many millenniums. Other sources are available, such as ice cores, lake or ocean sediment cores, volcanic ash layers in soils and sedimentary rocks and others. Each method, including tree rings, has various limitations. The ice cores accurately preserve past environmental conditions with a high time resolution. They are, however, available in limited locations which are usually far from human activities. Sediment cores are generally superior for long time spans (millions of years), but have a poor time resolution. Furthermore, some elements and materials are not preserved in their
original forms and/or concentrations within the sediment layers due to the dissolution (escape) into the liquid phase. Volcanic ash layers create a field of a specific historic time of eruptions. However, eruptions occur at random and the precise chronology can not be defined between events. Tree rings have further advantages because trees grow within close proximity to human activities, thus, the samples contain the records of human activity and the close locations make the samples inexpensive relative to the ice cores and the deep ocean sediment cores. The disadvantages of using tree rings are that:
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(1) the mobility of all recorded elements or materials is not fully known;
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J.-P. Garrec et al.
(2) the records are filtered by the biological activities; and (3) the degree of bioavailability for elements and/or materials, especially man-made element plutonium, in the surrounding environment is poorly understood.
Nagasaki sugi investigation with additional data from the French tree rings to attempt to clarify: (1) whether plutonium is recorded in tree rings similarly to ~4C; (2) if not, how it is recorded; and (3) what factors influence the concentrations of Pu in the tree rings.
The mobility of elements within tree rings is in itself a very interesting subject. It was recently found that An additional location, a preserved natural forest mCs and 4°K are mobile in red spruce and sugi (Kohno in the France-German border near Nancy, France, et al., 1988; Kudo et al., 1993), while 14C, 9°Sr was selected for background data, where no known (Bondietti et al., 1990), B (Peterson and Anderson, Pu input exists other than the global fallout, while 1990), Cd, Zn and Pb (Suzuki, 1975) are immobile in the Nagasaki site had an instantaneous Pu input various species of tree rings. On the other hand, mCs in 1945 immediately after the detonation of the is immobile in non-living materials such as soils Pu A-bomb. Unfortunately, the harvested site in and sediments, while 9°Sr is mobile (Mahara and France had a considerable input of the Chernobyl Miyahara, 1984; Mahara et al., 1988). In the Nagasaki accident in 1986 and this made the data more difficult 239+24°Pu study of sediment cores collected from a to interpret. Some data reported in this article have freshwater reservoir, it was noted that 10% of the Pu been published elsewhere (Kudo et al., 1993; Mahara was mobile, while 137Cswas immobile (Mahara et al., et al., 1995). 1988; Kudo et al., 1991a). At Nagasaki, the first Pu analysis in sugi tree rings revealed a few Pu concentration peaks in the tree rings, which indicated Materials and Methods that Pu is not mobile (Kudo et al., 1993). At the same time, the highest peak concentration was in 1965-67 Three species of tree rings, spruce (Picea alies), tree rings, which did not represent any conceivable Pu French white fir (Abies alba), and sugi (Cryptomeria input in the region. In addition to the mobility, the japonica D. Don) were harvested from two locations, routes of Pu into the tree rings and the biochemical in France in 1989 (for fir) and 1990 (for spruce) and reactions during the transportation into tree rings are in Japan in 1988. The French harvested site (called inadequately understood at present. Furthermore, the Lanotone High Vosges with an altitude of 430 m) was behaviour and biogeochemical reaction of Pu in the a well preserved mixed natural forest on hilly ground, surrounding environments are unknown. Hence, as shown in Fig. 1. Nagasaki sugi results of Pu were not conclusive for the A 78 year old sugi tree was harvested at the site complex system of the living and non-living shown in Fig. 2. The site was 2.8 km east of the environment. explosion hypocentre, where the local fallouts of The objective of this article is to summarize 239 + 24Opuand 137Cswere highest (Kudo et al., 1991a,b).
• Berlin
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P is (~)---Sompling Site
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Fig. 1. Sampling site of French white fir and spruce in France.
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Plutonium in tree rings from France and Japan
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The surface soil concentration of 230+24°pu w a s 64.5 mBq/g (dry wt) and that of '37Cs was 87.4 mBq/g. The site has been preserved as a water catchment forest for a drinking water reservoir which was built in 1904. The forest was artificially planted, thus the same size (age) trees were growing 3-6 m apart (each tree occupying about 10 m2). The tree chosen was about 20 m tall and the diameter of its trunk was 55 cm at 1.5 m above the ground (Kudo et al., 1993). The width of the tree rings ranged from a maximum 6.8 mm in 1941 to a minimum of 0.4 mm in 1959. Because the site was protected by a hill nearby, the tree was shielded from any direct effect from the A-bomb blast. A portion of branchless trunk (about 1.5 m above ground), stripped of bark, was separated into a set of samples representing 3-20 years growth. The first samples contained the annual layers up to 1985 and the rest of the samples were collected separating various annual layers. The collected samples were mechanically powdered and dried at 105°C for 24 h to obtain a dry weight, which ranged from approx. 500-2000 g. After ashing at 350°C, the samples were measured for their 4°K, mCs and '37Cs concentrations using a low background Ge(Li) y-ray spectrometer for a long counting time. The radioactivity was decay-corrected to 9 August 1990 (45 years after the A-bomb explosion) for sugi and 26 April 1986 (Chernobyl accident) for trees in France. For Pu analysis, samples were further ashed at 600°C for several days in order to oxidize all organic materials. The content of ash at 600°C in the samples was lower in the outer sapwood layers compared to inner heartwood layers. The concentration of 2394-240pu was measured with an ct particle spectrometer following a chemical
separation using 236pu a s an internal standard. Each ash sample was dissolved into HNO3, purified by an ion exchanger and electroplated onto a stainless steel disk. The chemical procedures were similar to those used in the United States (Harley, 1972), Japan (Japanese Science and Technology Agency, 1979) and Germany (Sch/ittelkopf, 1981). The procedure was routinely calibrated using IAEA standard soil samples (SD-N-I/1 and Soil-6) and the ~t spectrometer was calibrated with 24'Am sources standardized by 4n c~-7 anti-coincidence counting. The detection limit was 0.45 mBq/kg dry wood (90% confidence with a maximum variation of +25%). No appreciable amount of 23Spu and 2a2pu w a s measured in the tree rings.
Results and Discussion The concentrations of 239+ 24Opu in the tree rings from France and Japan are shown in Fig. 3 as a function of time (tree growth). The highest concentration recorded was 30.0 mBq/kg for the Nagasaki sugi, while the highest were 2.7 mBq/kg for the French white fir and 9.4 mBq/kg for the spruce, where no Pu input other than the global fallout was recorded. The Nagasaki harvest site was contaminated in 1945 by the local fallout of 239+ 240pu ' 181 mBq/cm 2, from the A-bomb which was the source of 97% of the combined Pu deposits of the local and the global fallout. The reason for a relatively high Pu content for the spruce compared to the Nagasaki sugi (or 31% of Nagasaki content) was not clear. If the source of Pu for the French harvested site was only from the global fallout, its highest concentration should be about 1 mBq/kg instead 9.4 mBq/kg (or 3% of the Nagasaki sugi).
1274
J.-P. Garrec
hypothesis of a 22 year time lag for the sugi can be applicable to the tree rings in France and if the Pu source was only global fallout and the greatest deposition was in 1963, the highest concentration in the tree rings in France should be in 1985. The data shown in Fig. 3 is inconclusive to prove this hypothesis. Another interesting point in Pu concentrations in France and Japan was that there was 239+ 240pu in the tree rings prior to 1945. The release of the man-made Pu into the environment occurred only after 1945, while the French white fir contained 3.6 mBq/kg in 1930-39 and 0.86 mBq/kg in 1926--28 for the sugi. The explanation for the Pu detection prior to 1945 may be explained by a fact that the xylem usually transports trace elements into 25 year older tree rings. The year 1945 of the Nagasaki A-bomb minus 25 years was the year 1920. The considerable Pu content (3.6 mBq/kg)
Three possible reasons for the high Pu content were:
(1) unknown sources of Pu, excluding Chernobyl; (2) higher bioavailability of Pu deposited in Nancy area than in Nagasaki; and (3) 9 and 3 times higher Pu uptake by the spruce and the French white fir, respectively, than that of the sugi. The growing time of the tree rings which had the highest concentration of 239+24°pu was interesting because they were in 1980-84 for the French white fir, 1985-89 for the spruce and 1965-67 for the sugi. At these particular periods, there were no Pu inputs available in both France and Japan. One probable explanation for Nagasaki sugi was that it took 22 years for Pu to be bioavailable for the tree rings. If this 40
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Fig. 3.23~÷2,Opuconcentrations in tree rings from France and Japan.
Plutonium in tree rings from France and Japan in 1930-39 might be caused by the xylem transportation into 25 year older tree rings. Another hypothesis to explain Pu content in tree rings prior to 1945 may be that there are two types o f 239+ 2'l°Pu in the tree rings, one of which is mobile by the xylem transportation and the other is not. Further research is warranted to prove these hypothesis using a larger sample size (at least 2000 g in dry wt) with a few annual layers. Surprisingly, Pu from the 1945 nuclear explosion played a minor role in the Pu concentration profile for the sugi in Japan, since the Pu content was only 2.9 mBq/kg in the 194~ 46 sample compared to 30.0 mBq/kg in the 1965-67 sample. The Pu bomb exploded at a mere 2.8 km from this sugi tree and the site of the tree harvested was the centre of the local fallout or 'black rain'. Why did the sample of 1944-46 contain so little Pu in the tree rings? Some conceivable answers are as follows, (a) The only Pu directly entering to the tree rings of 1944-46 was via the leaves, not the roots or the bark. The deposition of Pu on the surface soils must remain at the top few millimeters (Mahara and Miyahara, 1984), and thus the roots might not be able to absorb Pu immediately from the surface soils. (b) Although Pu deposition by the local fallout was considerable at this site (Kudo et al., 1991b), the local fallout might contain relatively large Pu particles compared to those of the global fallout, which were < 1/~m in diameter. The Pu in the larger sized particles of the local fallout had less chance to be absorbed through the leaves of the sugi tree, or was less bioavailable. The diameter of sugi stomata ranged from 10 to 15/2m. (c) The deposition period of the local fallout was < 24 h while that of the global fallout was many years. An additional piece of information was that this site was struck by two severe typhoons with heavy rainfall and gusting winds within 2 months after the nuclear detonation. These storms could have washed the local fallout off the leaves.
1275
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Time (yr) Fig. 4. Radioactive cesium distribution in French white fir tree rings [after Mahara et al. (1995)].
Bq/kg) for their outermost tree ring samples (1986-89 and 1986-88, respectively) was 316 fold, despite the fact that the sugi was also harvested after the Chernobyl accident. The magnitude of this difference indicates that there must have been massive recent inputs of'34Cs and ~37Csinto the French white fir, while there was no visible input for the sugi. The Nagasaki sugi was 7800 km east of the Chernobyl compared to 1600 km west for the French white fir. This fact further confirms that the radionuclides released from the U.S.S.R. accident were not evenly distributed throughout the northern hemisphere. Surprisingly, there was a shorter half-life cesium The French white fir samples provided a unique isotope, '34Cs, in the French white fir tree rings. As opportunity to observe the movement of cesium within mentioned earlier, both '34Cs and ~37Cs isotopes tree rings after the Chernobyl accident. Fortunately showed parallel concentration profiles throughout the 137 years and the concentration of each isotope the contents of 7 emitting radionuclides were measured immediately after the harvest and thus the concen- followed a straight line on a semi-log scale, decreasing trations of two cesium radionuclides, ~34Cs (half-life from the 1986-89 sample to the 1870--90 sample, 2.062 yr) and ~37Cs (30.1 yr) were accurately deter- Fig. 4. Different cesium isotopes should have the same mined. The observed distribution of ~3*Cs and '37Cs chemical behaviour in the tree rings. These facts within the tree rings parallels each other during the confirm an instantaneous input of the two cesium history of the tree (from 1871 to 1989). The isotopes into the tree rings. The concentration ratio of ~34Cs/~37Cswas a constant concentration decreased from the outer rings to the inner ones. This decreasing concentration was not value of 34%, though there were some lower ratios in both old tree rings of the 1891-1910 and the 1871-1890 observed in the sugi tree rings harvested from Japan, ~samples. The higher ~37Cs concentration, relative to Fig. 4. The concentration (25.3 Bq/kg dry wood) of ~37Csin '34Cs, was considered to be due to existing '37Cs the 1986-89 sample was 23.2 times higher than the background, resulting from prior nuclear fallout, that highest concentration (1.09 Bq/kg) of the 78 year old had migrated through the tree rings before the sugi. In fact, the difference between concentrations for U.S.S.R. accident. The ratio of'34Cs#37Cs ranged from 14 to 58% for the surface soils, sediments and grasses the French white fir (25.3 Bq/kg) and the sugi (0.080
1276
J.-P. Garrec et al.
in Europe, North America and Asia (Aoyama et al., 1986; Devell et al., 1986; Thomas and Martin, 1986; Pringle and Vermeer, 1986; Ballestra et al., 1987; Denschlag et al., 1987; Papastefanou et al., 1988; Tobler et al., 1988; Smith and Ellis, 1990; Wieland et al., 1993). Two radionuclides from the accident, ~34Cs and L37Cs,were moving within the French white fir from the outer rings toward the inner rings. The movement was not hindered by the transition area between the sapwood and the heartwood, though there were some disturbances in the mobility of the nuclide. Apparently, one of the driving forces of this radionuclide mobility is the concentration difference (molecular diffusion) between the outer tree rings (9.02 Bq/kg for 134Cs and 25.3 Bq/kg for mCs) and the inner rings (0.158 Bq/kg for ~34Cs and 1.68 Bq/kg for mCs). In order to verify that this hypothesis of a simple molecular diffusion is the major driving force for ~34Cs and '37Cs, a mathematical model was proposed assuming two radionuclides entered in a cylinder, tree rings, as follows: Ot -
r
rO-~r
- 2C
where C = ~34Csor mCs concentration in the various tree ring samples (Bq/kg dry wood); Co = ~3"Csor mCs concentration in 1989-86 tree ring sample (Bq/kg dry wood); r = radius (era) of the French whiter fir (r = 27 for the 1989 tree rings, and r = 0 in 1852); D = diffusion coefficient of ~3"Cs or mCs (cm2/yr); 2 = radionuclide decay constant; and t = time in years (on 26 April 1986 t = 0 and November 1989 t = 3.5). Solving above equation using the given conditions, Initial conditions: C = 0, 0 _< r ~ a, t = 0. Boundary conditions: C = Co e x p ( - 2t)r <_ a, t > 0
activity is strong and newly created xylem moves in a complicated pattern during the seasons of the year. Therefore, the movement of the radionuclides can not be explained by the simple molecular diffusion model, though data in Fig. 5 indicate that the molecular diffusion is still one of the dominant driving force for the isotopes. Further study is warranted to clarify these points by harvesting the same species of trees with a similar age from the sites in France and Japan. In this way, the mechanism of cesium movement demonstrated in this article may be further confirmed. A similarity in the concentration profiles between mCs and 4°K was well demonstrated, for example, in the Nagasaki tree rings, though 4°K showed about 150-200 times higher radioactivity than that of mCs, Fig. 6. In other words, both K and Cs elements are very mobile within tree rings. In the case of Nagasaki sugi, the profile showed two distinctive patterns: (a) a rapid increase zone (sapwood) of 17 years between 1971 and 1988; and (b) a nearly constant level (of heartwood) during 1917-I 970. The transition between sapwood and heartwood for this tree was considered to be in the 1971-73 sample whose ash (600°C) content was 0.99% (Cutter and Guyette, 1993: Kudo et al., 1995). Conclusions
Based on the analysis of the tree rings from France and Japan, the following conclusions were obtained concerning plutonium and other radionuclides: (I) The plutonium can be analyzed within tree rings which have a chronological record, especially in the Nagasaki sugi tree rings. The highest concentration of Pu was 30.0 mBq/kg for the sample of 1965-67. There is a small Pu peak for 1944--46 sample.
(where a = radius of tree ring, 27 cm).
=
a
n=l
~,Jl(a~t,)
1.0 7-
exp(-2t)
Figure 5 shows the result of calculation using a proposed diffusion model, using diffusion coefficients of D = 2 cm2/yr, D = 5 cm2/yr and D = 8 cm2/yr. The non-dimensional concentration of l~Cs and mCs are used and shown in Fig. 5. The French white fir data settles within the diffusion coefficient of 5-8 cm2/yr in the heartwood tree rings which seems to be before 1940s. Of course, there is no variation between two radionuclides and the results prove that the proposed hypothesis (diffusion coefficient is the major driving force) is explaining the natural phenomenon occurring in the heartwood region of the French white fir.
The proposed diffusion model did not precisely describe the movement of the isotopes for the sapwood region (1940s-1980s). In this region, the biological
• 137Cs °
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Fig. 5. Diffusion coefficientof'~Cs and mCs within tree rings of French white fir.
Plutonium in tree rings from France and Japan 106 8 6 4 O
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(2) There seems to be two types of plutonium, mobile and immobile within the tree rings. The immobile portion of plutonium might be used as a chronological plutonium record for the surrounding environment, though there are still considerable unknown factors involved. Further investigation is warrented in this subject, especially mobile and immobile plutonium which is produced in the environment. (3) The French white fir provided a unique opportunity to observe the movement of cesium (using both ~34Cs and t37Cs) within the tree tings. The major driving force was identified as the molecular diffusion by comparing data with data calculated using a mathematical model. Acknowledgements--The authors thank Dr D. L. Singleton, Ms S. Bohme and Ms K. M. Schliewen of the National Research Council of Canada, Canada and Mr P. Gelhaye at the Centre de Recherches Forestieres, INRA de Nancy, France for their contribution. This investigation was partially supported by the Monbusyo International Scientific Research Program of Japan and the Toyota Foundation.
References Aoyama M., Hirose K., Suzuki Y., Inoue H. and Sugimura Y. (1986) High level radioactive nuclides in Japan in May. Nature 321, 819-820. Ballestra S. B., Holm E., Walton A. and Whitehead N. E. (1987) Fallout deposition at Monaco following the Chernobyl accident. J. Environ. Radioact. 5, 391. Bondietti E. A., Momoshima N., Shortle W. C. and Smith K. T. (1990) A historical perspective on divalent cation trends in red spruce stemwood and the hypothetical
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relationship to acidic deposition. Can. J. Forest Res. 20, 1850. Cutter B. E. and Guyette R. P. (1993) Anatomical, chemical and ecological factors affecting tree species choice in dendrochemistry studies. J. Environ. Quality 22, 611. Denschlag H. O., Diel A., Glasel K. H., Heimann R., Karfrell N., Knitz U., Menke H., Trautmann N. and Weber M. (1987) Fallout in the Mainz area from the Chernobyl reactor accident. Radiochim. Acta 41, 163. Devell L., Tovedal H., Bergstrom U., Appelgren A., Chyssler J. and Anderson L. (1986) Initial observations of fallout from the reactor accident at Chernobyl. Nature 321, 192. Harley J, H. (1972) Health and Safety Laboratory Procedures Manual. Office of Scientific and Technical Information, HASL-300. U.S. Energy Research and Development Administration, Oak Ridge, TN, U.S.A. Japanese Science and Technology Agency (1979) Analytical procedure for plutonium (in Japanese). Radiation Management Series 12. Japan Chemical Analysis Centre, Chiba-shi, Japan. Kohno M., Koizumi Y., Okumura K. and Mito I. (1988) Distribution of environmental cesium-137 in tree rings. J. Environ. Radioact. 8, 15. Kudo A., Mahara Y., Kauri T. and Santry D. C. (1991 a) Fate of plutonium released from the Nagasaki A-bomb, Japan. Water Sci. Technol. 23, 291. Kudo A., Mahara Y., Santry D. C., Miyahara S. and Garrec J.-P. (1991b) Geographical distribution of fractionated local fallout from the Nagasaki A-bomb. J. Environ. Radioact. 14, 305. Kudo A., Suzuki T., Santry D. C., Mahara Y., Miyahara S. and Garrec J.-P. (1993) Effectiveness of tree rings for recording Pu history at Nagasaki, Japan. J. Environ. Radioact. 21, 55. Kudo A., Koerner R. M., Fisher D. A., Bourgeois J., Santry D. C., Mahara Y. and Sugahara M. (1995) Plutonium from Nagasaki A-bomb as a possible tracer for global transport, using existing initial conditions and ice cores. NATO Advanced Research Workshop: Ice Core Studies of Global Biogeochemical Cycles, Annecy, France, 1993 (submitted). Mahara Y. and Miyahara S. (1984) Residual plutonium migration in soil of Nagasaki. J. Geophys. Res. 89, 7931. Mahara Y., Kudo A., Kauri T., Santry D. C. and Miyahara S. (1988) Mobile Pu in reservoir sediments of Nagasaki, Japan. Hlth Phys. 54, 107. Mahara Y., Garrec J.-P., Santry D. C., Suzuki T., Miyahara. and Kudo A. (1995) Distribution and movement of ~J4Cs and mCs within French white fir tree rings, using the fallout from the Chernobyl accident (Submitted). Papastefanou C., Manolopoulou and Charalambous S. (1988) Radiation measurements and radiological aspects of fallout from the Chernobyl accident. J. Em~iron. Radioact. 7, 49. Peterson D. L. and Anderson D. R. (1990) Content of chemical elements in tree rings of Lodgepole Pine and White Bark Pine from a Subalpine Sierra Nevada Forest. United States Department of Agriculture Forest Service Research Paper, PSW-200. Pacific Southwest Research Station, Forest Service, Berkeley, CA, U.S.A. Pringle D. M. and Vermeer W. J. (1986) Gamma-ray spectrum of Chernobyl fallout. Nature 321,596. Schiittelkopf H. (1981) Development of an Analytical Method for Plutonium in Fantogram Region and its Procedure for Environmental Samples (in German). Kernforschungszentrum Karlsruhe, Germany. Smith J. N. and Ellis K. M. (1990) Time dependent transport of Chernobyl radioactivity between atmospheric and lichen phases in eastern Canada. J. Environ. Radioact. 11, 151-168.
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Suzuki T. (1975) Historical concentrations of Cd, Zn and Pb in tree rings harvested at a Smelter, Japan (in Japanese). J. Japanese Forestry Soc. 57, 45-52. Thomas A. J. and Martin J. M. (1986) First assessment of Chernobyl radioactive plume over Paris. Nature 321, 817-819. Tobler L., Bajo S. and Wyttenback A. (1988) Deposition of
134,137Cs from Chernobyl Fallout on Norway Spruce and forest soil and its incorporation into spruce twigs. J. Environ. Radioact. 6, 225-245. Wieland E., Santchi P. H., Hohener P. and Strum M. (1993) Scavenging of Chernobyl mCs and naturaFt°Pb in Lake Senpach, Switzerland. Geochim. Cosmochim. Acta 57, 2959-2979~
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Plutonium in Tree Rings from France and Japan J.-P. G A R R E C l, T. S U Z U K I , 2 Y. M A H A R A , 3 D. C. S A N T R Y , 4 S. M I Y A H A R A , 5 M . S U G A H A R A , 6 J. Z H E N G 7 a n d A. K U D O 7 JLaboratoire Pollution Atmospherique, INRA, 54280 Champenoux, France, :Department of Environmental Engineering, Niigata University, Niigata, Japan, ~Abiko Research Laboratory, CRIEP1, Chiba, Japan, 4Institute for National Measurements Standards, NRCC, Ottawa, Canada, 5Department of Marine Chemistry, Nagasaki University, Nagasaki, Japan, 6Department of Environmental Engineering, Osaka Sangyo University, Daito-shi, Japan and qnstitute for Environmental Research and Technology, NRCC, Ottawa, Canada Plutonium, along with other radionuclide concentrations, was measured in evergreen tree rings from two different locations. This was used as an information source for the past two centuries. Tree rings are a product of annual layers and thus chronological information is clearly visible. Three trees were harvested in 1988-1990: a French white fir (137 years old) and a spruce tree (177 years old) from the France-Germany border near Nancy, France and a sugi (78 years old) from Nagasaki, Japan. The uniform branchless part of the trunks from the harvested trees were immediately separated into a set of tree ring samples each of which contained 3-20 years of growth. The separated samples were mechanically powdered, dried at 105°C to obtain the dry weight, ashed at 350°C to measure 4°K, ~34Csand mCs and ashed again at 600°C to determine 239+240pu"The highest 239÷:40pu concentration of 30.0 mBq/kg of dry wood was obtained from the tree rings from Nagasaki, located at the centre of the local fallout of the Pu A-bomb detonated in 1945. This concentration peak was, however, observed in tree rings of 1965-67. The concentration was only 2.9 mBq/kg for the tree rings of 1944-46. The contribution of the local fallout on the surface soils from the A-bomb was 181 mBq/cm: at the harvested area of the tree, while the contribution of global fallout by many weapons testing was 5.9 mBq/cm: (or 3.3% total fallout in the region). The reason for the over 20 year time lag of 239+:40pu uptake by the tree rings is unknown because many factors influence the routes of Pu into the tree rings. Also the chemical form of Pu in surface soils may have been changed by the surrounding environment.The highest concentration in the tree rings from France was 9.4 mBq/kg which is about 31% of Nagasaki :39+:4Opuconcentration. The harvested area did not have any recorded Pu sources other than global fallout. An interesting result was that the distribution of ~Cs and~37Cs concentrations in the French white fir was different from Nagasaki. Data suggested that these new radionuclide inputs were from the Cbernobyl accident. The mobility (or diffusion coefficient) of cesium is 2-8 cmS/yr in the portion of heart-wood tree rings 0955-1870). Although tree rings can record chronological inputs of various trace elements, some elements cannot be used. These exceptions would be elements that: (1) are mobile within tree rings; (2) have littlte understood entry routes to the tree rings (via roots, leaves or barks); and (3) have unknown biogeochemical behaviour in the surrounding environment. Further investigation is warranted to use tree rings as a tool to record past environmental history.
Introduction One of reliable chronological information source is the tree rings which show not only the seasons of every year but the annual layers for many millenniums. Other sources are available, such as ice cores, lake or ocean sediment cores, volcanic ash layers in soils and sedimentary rocks and others. Each method, including tree rings, has various limitations. The ice cores accurately preserve past environmental conditions with a high time resolution. They are, however, available in limited locations which are usually far from human activities. Sediment cores are generally superior for long time spans (millions of years), but have a poor time resolution. Furthermore, some elements and materials are not preserved in their
original forms and/or concentrations within the sediment layers due to the dissolution (escape) into the liquid phase. Volcanic ash layers create a field of a specific historic time of eruptions. However, eruptions occur at random and the precise chronology can not be defined between events. Tree rings have further advantages because trees grow within close proximity to human activities, thus, the samples contain the records of human activity and the close locations make the samples inexpensive relative to the ice cores and the deep ocean sediment cores. The disadvantages of using tree rings are that:
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(1) the mobility of all recorded elements or materials is not fully known;
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J.-P. Garrec et al.
(2) the records are filtered by the biological activities; and (3) the degree of bioavailability for elements and/or materials, especially man-made element plutonium, in the surrounding environment is poorly understood.
Nagasaki sugi investigation with additional data from the French tree rings to attempt to clarify: (1) whether plutonium is recorded in tree rings similarly to ~4C; (2) if not, how it is recorded; and (3) what factors influence the concentrations of Pu in the tree rings.
The mobility of elements within tree rings is in itself a very interesting subject. It was recently found that An additional location, a preserved natural forest mCs and 4°K are mobile in red spruce and sugi (Kohno in the France-German border near Nancy, France, et al., 1988; Kudo et al., 1993), while 14C, 9°Sr was selected for background data, where no known (Bondietti et al., 1990), B (Peterson and Anderson, Pu input exists other than the global fallout, while 1990), Cd, Zn and Pb (Suzuki, 1975) are immobile in the Nagasaki site had an instantaneous Pu input various species of tree rings. On the other hand, mCs in 1945 immediately after the detonation of the is immobile in non-living materials such as soils Pu A-bomb. Unfortunately, the harvested site in and sediments, while 9°Sr is mobile (Mahara and France had a considerable input of the Chernobyl Miyahara, 1984; Mahara et al., 1988). In the Nagasaki accident in 1986 and this made the data more difficult 239+24°Pu study of sediment cores collected from a to interpret. Some data reported in this article have freshwater reservoir, it was noted that 10% of the Pu been published elsewhere (Kudo et al., 1993; Mahara was mobile, while 137Cswas immobile (Mahara et al., et al., 1995). 1988; Kudo et al., 1991a). At Nagasaki, the first Pu analysis in sugi tree rings revealed a few Pu concentration peaks in the tree rings, which indicated Materials and Methods that Pu is not mobile (Kudo et al., 1993). At the same time, the highest peak concentration was in 1965-67 Three species of tree rings, spruce (Picea alies), tree rings, which did not represent any conceivable Pu French white fir (Abies alba), and sugi (Cryptomeria input in the region. In addition to the mobility, the japonica D. Don) were harvested from two locations, routes of Pu into the tree rings and the biochemical in France in 1989 (for fir) and 1990 (for spruce) and reactions during the transportation into tree rings are in Japan in 1988. The French harvested site (called inadequately understood at present. Furthermore, the Lanotone High Vosges with an altitude of 430 m) was behaviour and biogeochemical reaction of Pu in the a well preserved mixed natural forest on hilly ground, surrounding environments are unknown. Hence, as shown in Fig. 1. Nagasaki sugi results of Pu were not conclusive for the A 78 year old sugi tree was harvested at the site complex system of the living and non-living shown in Fig. 2. The site was 2.8 km east of the environment. explosion hypocentre, where the local fallouts of The objective of this article is to summarize 239 + 24Opuand 137Cswere highest (Kudo et al., 1991a,b).
• Berlin
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Fig. 1. Sampling site of French white fir and spruce in France.
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Plutonium in tree rings from France and Japan
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The surface soil concentration of 230+24°pu w a s 64.5 mBq/g (dry wt) and that of '37Cs was 87.4 mBq/g. The site has been preserved as a water catchment forest for a drinking water reservoir which was built in 1904. The forest was artificially planted, thus the same size (age) trees were growing 3-6 m apart (each tree occupying about 10 m2). The tree chosen was about 20 m tall and the diameter of its trunk was 55 cm at 1.5 m above the ground (Kudo et al., 1993). The width of the tree rings ranged from a maximum 6.8 mm in 1941 to a minimum of 0.4 mm in 1959. Because the site was protected by a hill nearby, the tree was shielded from any direct effect from the A-bomb blast. A portion of branchless trunk (about 1.5 m above ground), stripped of bark, was separated into a set of samples representing 3-20 years growth. The first samples contained the annual layers up to 1985 and the rest of the samples were collected separating various annual layers. The collected samples were mechanically powdered and dried at 105°C for 24 h to obtain a dry weight, which ranged from approx. 500-2000 g. After ashing at 350°C, the samples were measured for their 4°K, mCs and '37Cs concentrations using a low background Ge(Li) y-ray spectrometer for a long counting time. The radioactivity was decay-corrected to 9 August 1990 (45 years after the A-bomb explosion) for sugi and 26 April 1986 (Chernobyl accident) for trees in France. For Pu analysis, samples were further ashed at 600°C for several days in order to oxidize all organic materials. The content of ash at 600°C in the samples was lower in the outer sapwood layers compared to inner heartwood layers. The concentration of 2394-240pu was measured with an ct particle spectrometer following a chemical
separation using 236pu a s an internal standard. Each ash sample was dissolved into HNO3, purified by an ion exchanger and electroplated onto a stainless steel disk. The chemical procedures were similar to those used in the United States (Harley, 1972), Japan (Japanese Science and Technology Agency, 1979) and Germany (Sch/ittelkopf, 1981). The procedure was routinely calibrated using IAEA standard soil samples (SD-N-I/1 and Soil-6) and the ~t spectrometer was calibrated with 24'Am sources standardized by 4n c~-7 anti-coincidence counting. The detection limit was 0.45 mBq/kg dry wood (90% confidence with a maximum variation of +25%). No appreciable amount of 23Spu and 2a2pu w a s measured in the tree rings.
Results and Discussion The concentrations of 239+ 24Opu in the tree rings from France and Japan are shown in Fig. 3 as a function of time (tree growth). The highest concentration recorded was 30.0 mBq/kg for the Nagasaki sugi, while the highest were 2.7 mBq/kg for the French white fir and 9.4 mBq/kg for the spruce, where no Pu input other than the global fallout was recorded. The Nagasaki harvest site was contaminated in 1945 by the local fallout of 239+ 240pu ' 181 mBq/cm 2, from the A-bomb which was the source of 97% of the combined Pu deposits of the local and the global fallout. The reason for a relatively high Pu content for the spruce compared to the Nagasaki sugi (or 31% of Nagasaki content) was not clear. If the source of Pu for the French harvested site was only from the global fallout, its highest concentration should be about 1 mBq/kg instead 9.4 mBq/kg (or 3% of the Nagasaki sugi).
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J.-P. Garrec
hypothesis of a 22 year time lag for the sugi can be applicable to the tree rings in France and if the Pu source was only global fallout and the greatest deposition was in 1963, the highest concentration in the tree rings in France should be in 1985. The data shown in Fig. 3 is inconclusive to prove this hypothesis. Another interesting point in Pu concentrations in France and Japan was that there was 239+ 240pu in the tree rings prior to 1945. The release of the man-made Pu into the environment occurred only after 1945, while the French white fir contained 3.6 mBq/kg in 1930-39 and 0.86 mBq/kg in 1926--28 for the sugi. The explanation for the Pu detection prior to 1945 may be explained by a fact that the xylem usually transports trace elements into 25 year older tree rings. The year 1945 of the Nagasaki A-bomb minus 25 years was the year 1920. The considerable Pu content (3.6 mBq/kg)
Three possible reasons for the high Pu content were:
(1) unknown sources of Pu, excluding Chernobyl; (2) higher bioavailability of Pu deposited in Nancy area than in Nagasaki; and (3) 9 and 3 times higher Pu uptake by the spruce and the French white fir, respectively, than that of the sugi. The growing time of the tree rings which had the highest concentration of 239+24°pu was interesting because they were in 1980-84 for the French white fir, 1985-89 for the spruce and 1965-67 for the sugi. At these particular periods, there were no Pu inputs available in both France and Japan. One probable explanation for Nagasaki sugi was that it took 22 years for Pu to be bioavailable for the tree rings. If this 40
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Fig. 3.23~÷2,Opuconcentrations in tree rings from France and Japan.
Plutonium in tree rings from France and Japan in 1930-39 might be caused by the xylem transportation into 25 year older tree rings. Another hypothesis to explain Pu content in tree rings prior to 1945 may be that there are two types o f 239+ 2'l°Pu in the tree rings, one of which is mobile by the xylem transportation and the other is not. Further research is warranted to prove these hypothesis using a larger sample size (at least 2000 g in dry wt) with a few annual layers. Surprisingly, Pu from the 1945 nuclear explosion played a minor role in the Pu concentration profile for the sugi in Japan, since the Pu content was only 2.9 mBq/kg in the 194~ 46 sample compared to 30.0 mBq/kg in the 1965-67 sample. The Pu bomb exploded at a mere 2.8 km from this sugi tree and the site of the tree harvested was the centre of the local fallout or 'black rain'. Why did the sample of 1944-46 contain so little Pu in the tree rings? Some conceivable answers are as follows, (a) The only Pu directly entering to the tree rings of 1944-46 was via the leaves, not the roots or the bark. The deposition of Pu on the surface soils must remain at the top few millimeters (Mahara and Miyahara, 1984), and thus the roots might not be able to absorb Pu immediately from the surface soils. (b) Although Pu deposition by the local fallout was considerable at this site (Kudo et al., 1991b), the local fallout might contain relatively large Pu particles compared to those of the global fallout, which were < 1/~m in diameter. The Pu in the larger sized particles of the local fallout had less chance to be absorbed through the leaves of the sugi tree, or was less bioavailable. The diameter of sugi stomata ranged from 10 to 15/2m. (c) The deposition period of the local fallout was < 24 h while that of the global fallout was many years. An additional piece of information was that this site was struck by two severe typhoons with heavy rainfall and gusting winds within 2 months after the nuclear detonation. These storms could have washed the local fallout off the leaves.
1275
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Bq/kg) for their outermost tree ring samples (1986-89 and 1986-88, respectively) was 316 fold, despite the fact that the sugi was also harvested after the Chernobyl accident. The magnitude of this difference indicates that there must have been massive recent inputs of'34Cs and ~37Csinto the French white fir, while there was no visible input for the sugi. The Nagasaki sugi was 7800 km east of the Chernobyl compared to 1600 km west for the French white fir. This fact further confirms that the radionuclides released from the U.S.S.R. accident were not evenly distributed throughout the northern hemisphere. Surprisingly, there was a shorter half-life cesium The French white fir samples provided a unique isotope, '34Cs, in the French white fir tree rings. As opportunity to observe the movement of cesium within mentioned earlier, both '34Cs and ~37Cs isotopes tree rings after the Chernobyl accident. Fortunately showed parallel concentration profiles throughout the 137 years and the concentration of each isotope the contents of 7 emitting radionuclides were measured immediately after the harvest and thus the concen- followed a straight line on a semi-log scale, decreasing trations of two cesium radionuclides, ~34Cs (half-life from the 1986-89 sample to the 1870--90 sample, 2.062 yr) and ~37Cs (30.1 yr) were accurately deter- Fig. 4. Different cesium isotopes should have the same mined. The observed distribution of ~3*Cs and '37Cs chemical behaviour in the tree rings. These facts within the tree rings parallels each other during the confirm an instantaneous input of the two cesium history of the tree (from 1871 to 1989). The isotopes into the tree rings. The concentration ratio of ~34Cs/~37Cswas a constant concentration decreased from the outer rings to the inner ones. This decreasing concentration was not value of 34%, though there were some lower ratios in both old tree rings of the 1891-1910 and the 1871-1890 observed in the sugi tree rings harvested from Japan, ~samples. The higher ~37Cs concentration, relative to Fig. 4. The concentration (25.3 Bq/kg dry wood) of ~37Csin '34Cs, was considered to be due to existing '37Cs the 1986-89 sample was 23.2 times higher than the background, resulting from prior nuclear fallout, that highest concentration (1.09 Bq/kg) of the 78 year old had migrated through the tree rings before the sugi. In fact, the difference between concentrations for U.S.S.R. accident. The ratio of'34Cs#37Cs ranged from 14 to 58% for the surface soils, sediments and grasses the French white fir (25.3 Bq/kg) and the sugi (0.080
1276
J.-P. Garrec et al.
in Europe, North America and Asia (Aoyama et al., 1986; Devell et al., 1986; Thomas and Martin, 1986; Pringle and Vermeer, 1986; Ballestra et al., 1987; Denschlag et al., 1987; Papastefanou et al., 1988; Tobler et al., 1988; Smith and Ellis, 1990; Wieland et al., 1993). Two radionuclides from the accident, ~34Cs and L37Cs,were moving within the French white fir from the outer rings toward the inner rings. The movement was not hindered by the transition area between the sapwood and the heartwood, though there were some disturbances in the mobility of the nuclide. Apparently, one of the driving forces of this radionuclide mobility is the concentration difference (molecular diffusion) between the outer tree rings (9.02 Bq/kg for 134Cs and 25.3 Bq/kg for mCs) and the inner rings (0.158 Bq/kg for ~34Cs and 1.68 Bq/kg for mCs). In order to verify that this hypothesis of a simple molecular diffusion is the major driving force for ~34Cs and '37Cs, a mathematical model was proposed assuming two radionuclides entered in a cylinder, tree rings, as follows: Ot -
r
rO-~r
- 2C
where C = ~34Csor mCs concentration in the various tree ring samples (Bq/kg dry wood); Co = ~3"Csor mCs concentration in 1989-86 tree ring sample (Bq/kg dry wood); r = radius (era) of the French whiter fir (r = 27 for the 1989 tree rings, and r = 0 in 1852); D = diffusion coefficient of ~3"Cs or mCs (cm2/yr); 2 = radionuclide decay constant; and t = time in years (on 26 April 1986 t = 0 and November 1989 t = 3.5). Solving above equation using the given conditions, Initial conditions: C = 0, 0 _< r ~ a, t = 0. Boundary conditions: C = Co e x p ( - 2t)r <_ a, t > 0
activity is strong and newly created xylem moves in a complicated pattern during the seasons of the year. Therefore, the movement of the radionuclides can not be explained by the simple molecular diffusion model, though data in Fig. 5 indicate that the molecular diffusion is still one of the dominant driving force for the isotopes. Further study is warranted to clarify these points by harvesting the same species of trees with a similar age from the sites in France and Japan. In this way, the mechanism of cesium movement demonstrated in this article may be further confirmed. A similarity in the concentration profiles between mCs and 4°K was well demonstrated, for example, in the Nagasaki tree rings, though 4°K showed about 150-200 times higher radioactivity than that of mCs, Fig. 6. In other words, both K and Cs elements are very mobile within tree rings. In the case of Nagasaki sugi, the profile showed two distinctive patterns: (a) a rapid increase zone (sapwood) of 17 years between 1971 and 1988; and (b) a nearly constant level (of heartwood) during 1917-I 970. The transition between sapwood and heartwood for this tree was considered to be in the 1971-73 sample whose ash (600°C) content was 0.99% (Cutter and Guyette, 1993: Kudo et al., 1995). Conclusions
Based on the analysis of the tree rings from France and Japan, the following conclusions were obtained concerning plutonium and other radionuclides: (I) The plutonium can be analyzed within tree rings which have a chronological record, especially in the Nagasaki sugi tree rings. The highest concentration of Pu was 30.0 mBq/kg for the sample of 1965-67. There is a small Pu peak for 1944--46 sample.
(where a = radius of tree ring, 27 cm).
=
a
n=l
~,Jl(a~t,)
1.0 7-
exp(-2t)
Figure 5 shows the result of calculation using a proposed diffusion model, using diffusion coefficients of D = 2 cm2/yr, D = 5 cm2/yr and D = 8 cm2/yr. The non-dimensional concentration of l~Cs and mCs are used and shown in Fig. 5. The French white fir data settles within the diffusion coefficient of 5-8 cm2/yr in the heartwood tree rings which seems to be before 1940s. Of course, there is no variation between two radionuclides and the results prove that the proposed hypothesis (diffusion coefficient is the major driving force) is explaining the natural phenomenon occurring in the heartwood region of the French white fir.
The proposed diffusion model did not precisely describe the movement of the isotopes for the sapwood region (1940s-1980s). In this region, the biological
• 137Cs °
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Fig. 5. Diffusion coefficientof'~Cs and mCs within tree rings of French white fir.
Plutonium in tree rings from France and Japan 106 8 6 4 O
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(2) There seems to be two types of plutonium, mobile and immobile within the tree rings. The immobile portion of plutonium might be used as a chronological plutonium record for the surrounding environment, though there are still considerable unknown factors involved. Further investigation is warrented in this subject, especially mobile and immobile plutonium which is produced in the environment. (3) The French white fir provided a unique opportunity to observe the movement of cesium (using both ~34Cs and t37Cs) within the tree tings. The major driving force was identified as the molecular diffusion by comparing data with data calculated using a mathematical model. Acknowledgements--The authors thank Dr D. L. Singleton, Ms S. Bohme and Ms K. M. Schliewen of the National Research Council of Canada, Canada and Mr P. Gelhaye at the Centre de Recherches Forestieres, INRA de Nancy, France for their contribution. This investigation was partially supported by the Monbusyo International Scientific Research Program of Japan and the Toyota Foundation.
References Aoyama M., Hirose K., Suzuki Y., Inoue H. and Sugimura Y. (1986) High level radioactive nuclides in Japan in May. Nature 321, 819-820. Ballestra S. B., Holm E., Walton A. and Whitehead N. E. (1987) Fallout deposition at Monaco following the Chernobyl accident. J. Environ. Radioact. 5, 391. Bondietti E. A., Momoshima N., Shortle W. C. and Smith K. T. (1990) A historical perspective on divalent cation trends in red spruce stemwood and the hypothetical
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relationship to acidic deposition. Can. J. Forest Res. 20, 1850. Cutter B. E. and Guyette R. P. (1993) Anatomical, chemical and ecological factors affecting tree species choice in dendrochemistry studies. J. Environ. Quality 22, 611. Denschlag H. O., Diel A., Glasel K. H., Heimann R., Karfrell N., Knitz U., Menke H., Trautmann N. and Weber M. (1987) Fallout in the Mainz area from the Chernobyl reactor accident. Radiochim. Acta 41, 163. Devell L., Tovedal H., Bergstrom U., Appelgren A., Chyssler J. and Anderson L. (1986) Initial observations of fallout from the reactor accident at Chernobyl. Nature 321, 192. Harley J, H. (1972) Health and Safety Laboratory Procedures Manual. Office of Scientific and Technical Information, HASL-300. U.S. Energy Research and Development Administration, Oak Ridge, TN, U.S.A. Japanese Science and Technology Agency (1979) Analytical procedure for plutonium (in Japanese). Radiation Management Series 12. Japan Chemical Analysis Centre, Chiba-shi, Japan. Kohno M., Koizumi Y., Okumura K. and Mito I. (1988) Distribution of environmental cesium-137 in tree rings. J. Environ. Radioact. 8, 15. Kudo A., Mahara Y., Kauri T. and Santry D. C. (1991 a) Fate of plutonium released from the Nagasaki A-bomb, Japan. Water Sci. Technol. 23, 291. Kudo A., Mahara Y., Santry D. C., Miyahara S. and Garrec J.-P. (1991b) Geographical distribution of fractionated local fallout from the Nagasaki A-bomb. J. Environ. Radioact. 14, 305. Kudo A., Suzuki T., Santry D. C., Mahara Y., Miyahara S. and Garrec J.-P. (1993) Effectiveness of tree rings for recording Pu history at Nagasaki, Japan. J. Environ. Radioact. 21, 55. Kudo A., Koerner R. M., Fisher D. A., Bourgeois J., Santry D. C., Mahara Y. and Sugahara M. (1995) Plutonium from Nagasaki A-bomb as a possible tracer for global transport, using existing initial conditions and ice cores. NATO Advanced Research Workshop: Ice Core Studies of Global Biogeochemical Cycles, Annecy, France, 1993 (submitted). Mahara Y. and Miyahara S. (1984) Residual plutonium migration in soil of Nagasaki. J. Geophys. Res. 89, 7931. Mahara Y., Kudo A., Kauri T., Santry D. C. and Miyahara S. (1988) Mobile Pu in reservoir sediments of Nagasaki, Japan. Hlth Phys. 54, 107. Mahara Y., Garrec J.-P., Santry D. C., Suzuki T., Miyahara. and Kudo A. (1995) Distribution and movement of ~J4Cs and mCs within French white fir tree rings, using the fallout from the Chernobyl accident (Submitted). Papastefanou C., Manolopoulou and Charalambous S. (1988) Radiation measurements and radiological aspects of fallout from the Chernobyl accident. J. Em~iron. Radioact. 7, 49. Peterson D. L. and Anderson D. R. (1990) Content of chemical elements in tree rings of Lodgepole Pine and White Bark Pine from a Subalpine Sierra Nevada Forest. United States Department of Agriculture Forest Service Research Paper, PSW-200. Pacific Southwest Research Station, Forest Service, Berkeley, CA, U.S.A. Pringle D. M. and Vermeer W. J. (1986) Gamma-ray spectrum of Chernobyl fallout. Nature 321,596. Schiittelkopf H. (1981) Development of an Analytical Method for Plutonium in Fantogram Region and its Procedure for Environmental Samples (in German). Kernforschungszentrum Karlsruhe, Germany. Smith J. N. and Ellis K. M. (1990) Time dependent transport of Chernobyl radioactivity between atmospheric and lichen phases in eastern Canada. J. Environ. Radioact. 11, 151-168.
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Suzuki T. (1975) Historical concentrations of Cd, Zn and Pb in tree rings harvested at a Smelter, Japan (in Japanese). J. Japanese Forestry Soc. 57, 45-52. Thomas A. J. and Martin J. M. (1986) First assessment of Chernobyl radioactive plume over Paris. Nature 321, 817-819. Tobler L., Bajo S. and Wyttenback A. (1988) Deposition of
134,137Cs from Chernobyl Fallout on Norway Spruce and forest soil and its incorporation into spruce twigs. J. Environ. Radioact. 6, 225-245. Wieland E., Santchi P. H., Hohener P. and Strum M. (1993) Scavenging of Chernobyl mCs and naturaFt°Pb in Lake Senpach, Switzerland. Geochim. Cosmochim. Acta 57, 2959-2979~