Concentrations and spatial distribution of plutonium in the terrestrial environment of the Marshall Islands

The Science of the Total Environment 229 Ž1999. 21]39

Concentrations and spatial distribution of plutonium in the terrestrial environment of the Marshall Islands q Steven L. SimonU , James C. Graham1, Andrew W. Borchert 1 Marshall Islands Nationwide Radiological Study, Majuro, Marshall Islands Received 2 January 1999; accepted 19 February 1999

Abstract Measurements of plutonium in the terrestrial environment of the Marshall Islands are one indicator of the degree of environmental contamination from nuclear weapons testing. Moreover, the spatial pattern of environmental plutonium concentrations is indicative of the pattern of total radionuclide deposition from all of the nuclear tests. Measurements of plutonium Ž 239q240 Pu. to discern the spatial pattern of deposition and the degree of contamination were made in soil samples collected from 1990 through 1993 at all 29 of the atolls which form the archipelago of the Marshall Islands. Measured concentrations ranged over nearly five orders of magnitude though the difference between the highest observed values and the estimated global fallout contribution was about four orders of magnitude. A strong gradient of increasing concentration of plutonium in soil was noted with increasing latitude between 9 and 11.5o N. The pattern of contamination and latitudinal increase was similar to that for 137Cs. Global fallout deposition of plutonium was estimated from two different data sets. Measurements of environmental plutonium in nearly 650 soil samples from the Marshall Islands are summarized and compared to estimates of the global fallout deposition. The ratio of 239q240 Pu in the soil to 137Cs in the soil over a 1000 km distance was also examined to determine if there was evidence for fractionation, i.e. differential deposition. It was found that the plutoniumrcesium ŽPurCs. ratio varied considerably with distance from the Bikini test site, decreasing with increasing distance. The ratio of PurCs was found to decrease about 133 times over the 1000 km distance. Q 1999 Elsevier Science B.V. All rights reserved. Keywords: Plutonium; Marshall Islands; Environmental contamination

q

Authors note: This report does not represent the opinion of the National Academy of Sciences or the National Research Council. Corresponding author. Present address: Board on Radiation Effects Research, National Academy of Sciences, Washington, DC 20418, USA. 1 Currently at Environmental Health Sciences, Radiation Control Office, Colorado State University, Ft. Collins, CO 80523-6021 USA.

U

0048-9697r99r$ - see front matter Q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 8 - 9 6 9 7 Ž 9 9 . 0 0 0 6 6 - 2

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S.L. Simon et al. r The Science of the Total En¨ ironment 229 (1999) 21]39

1. Introduction The worldwide distribution of plutonium in the environment has been of continuing interest to scientists over the decades since worldwide nuclear testing began. The initial interest arose due to the unique situation in which an element never before dispersed in the atmospheric environment was released in appreciable quantities over several years. The scientific community quickly realized that by tracking the long-lived radioactive by-products, considerable information could be acquired about global circulation patterns, processes of wet and dry deposition and the movement of contaminants within the terrestrial and aquatic ecosystems and food-chains. In addition, there were ongoing concerns about the effect of environmental plutonium on human health. As a result of regional nuclear weapons testing, the Marshall Islands were one of several sites worldwide that received plutonium contamination at a higher level than the average worldwide contamination level Ži.e. global background.. From 1946 through 1958 the US conducted nuclear weapons tests in the Marshall Islands at Bikini and Enewetak Atolls. In late-1989, the Republic of the Marshall Islands ŽRMI. commissioned a radiological monitoring program of the 29 atolls and four separate reef islands which compose the archipelago. The purpose of the Marshall Islands Nationwide Radiological Study ŽNWRS. was to document the geographic extent and levels of radioactive fallout contamination. The impetus for that survey was principally to bring to completion an independent radiological monitoring program as planned in the Compact of Free Association ŽUS Congress, 1986., an international agreement between the US and the Marshall Islands. The financial resources for conducting an independent monitoring program were provided for in that agreement. Prior to the beginning of the NWRS, 70% of the Marshall Islands had never been systematically monitored for residual radioactivity even though various types of monitoring had been conducted in the past. For example, immediately

following tests of the Ivy Ž1952. and Castle Ž1954. series, aerial surveys were conducted over the atolls with a scintillation-type instrument ŽEisenbud, 1953; Breslin and Cassidy, 1955.. After that, the largest monitoring program was an aerial gamma survey and a ground sampling program that covered approximately 30% of the nation ŽRobison et al., 1981; Tipton et al., 1981.. Those programs and all other monitoring efforts in recent years by the US Department of Energy concentrated on the northern atolls with a great deal of emphasis on the two test site atolls. The overall goals, methods and general study findings of the NWRS have recently been reported elsewhere ŽSimon and Graham, 1994, 1997; McEwan et al., 1997. though the emphasis in those reports was on environmental concentrations of 137 Cs, the principal radionuclide resulting in contemporary exposure ŽRobison, 1983; Robison et al., 1987, 1997; Simon and Graham, 1997.. The objective of this paper is to report on concentrations of environmental plutonium Ž 239q240 Pu. in the Marshall Islands, the spatial pattern of plutonium distribution and the geographic distribution of plutonium relative to 137 Cs. Only measurements from the terrestrial environment are reported here. 1.1. Background It is recognized that plutonium reached the atolls other than the test sites by atmospheric transport. Generally it is assumed that a portion of the total deposition at any location originated from nuclear tests by other countries at a variety of worldwide locations. Those sites contributing to global fallout in the Northern hemisphere included the Nevada Test Site ŽUSA., Novaya Zemlya ŽRussia., Semipalatinsk ŽKazakstan., Luc Bu Pu ŽLop Nor. ŽChina., Johnston Island, as well as the Marshall Islands. Thus, plutonium that is now in the environment of the atolls of the Marshall Islands could have arrived there by three paths: Ž1. as stratospheric Žglobal. fallout from test sites outside of the Marshall Islands; Ž2. as tropospheric Žlocal. fallout from tests at Bikini and Enewetak; and Ž3. as stratospheric Žglobal.

S.L. Simon et al. r The Science of the Total En¨ ironment 229 (1999) 21]39

fallout from Bikini and Enewetak. Some of the plutonium from Marshall Islands nuclear tests circled the globe before being deposited in the Marshall Islands as part of worldwide global fallout. Such radioactivity is a complicating issue in determining the background level in the Marshall Islands above which the local contribution can be compared. For pragmatic as well as theoretical reasons, we use a definition of global fallout deposition to be the sum of radioactivity transported to the site of measurement via the stratosphere, regardless of whether it originated at or near the measurement site. This distinction is important so as to enable us to demonstrate a level of environmental radioactivity at each location in the Marshall Islands that is due to global fallout and above which any net deposition can be assumed to be a result of local tropospheric transport. Fortunately, the global fallout contribution can be estimated from deposition data reported at other locations in the mid-Pacific that were not within a reasonable distance or wind direction for tropospheric transport from the Marshall Islands tests. There is little doubt that the Marshall Islands tests contributed to the global fallout inventory in the Northern hemisphere. This can be concluded from examining the explosive yields of the tests conducted there. It has been reported that for tests greater than 1]2 MT explosive yield, 90]99% of the material is injected in the stratosphere ŽFerber, 1964; Perkins and Thomas, 1980; Holleman et al., 1987.. Eighteen of the 66 tests conducted in the Marshall Islands were larger than 1 MT. These 18 tests contributed 95% of the 1.08 times 10 5 kt TNT total explosive yield ŽDOE, 1994; Simon and Robison, 1997.. Using reported data on explosive yields from all countries ŽCarter and Moghissi, 1977., we estimated that the total yield worldwide from tests that were individually ) 1 MT in size was approximately 330 MT: 103 MT from US tests in the Marshall Islands, 203 MT from tests in the former USSR, 7 MT from tests by UK, 2 MT from tests by France and 15 MT from tests in China. For purposes of determining the relative contributions to global Žstratospheric . fallout in the

23

Northern hemisphere, the UK and French tests in the Southern hemisphere can be ruled out because of their small size and location. The relative contributions to Northern hemisphere global fallout were approximately 32, 63 and 5%, respectively, for the RMI, USSR and China tests. This is acknowledged to be a crude calculation and does not account for variations in weapon design, type and quality of fissile material, the ratios of fission yield to total yield, etc. Nonetheless, for discussion purposes, we estimate that approximately one-third of the global fallout inventory in the Northern hemisphere originated from US nuclear tests conducted in the Marshall Islands. Verification of the long-range transport of radioactive debris from the large tests in the Marshall Islands has been reported for decades. For example, the observations of Machta et al. Ž1956. following two of the largest tests, Ivy MIKE Ž1 November 1952, 10.4 MT explosive yield. and Castle BRAVO Ž1 March 1954, 15 MT yield., indicated that much of the material went into the stratosphere as a result of the high altitude of the nuclear cloud tops Ž30 000 and 35 000 m, respectively.. Radioactive material from MIKE was also dispersed in the troposphere, however, traveling eastward across North America, southern Europe and Asia and back to the Pacific in approximately 35 days. The analysis of Machta showed that fallout from BRAVO covered both Americas and extended into the south Atlantic. Compared to the short transit time for tropospheric fallout Žon the order of days., the global fallout removal half-time has been estimated to be between 2 and 4 years ŽKrey and Krajewski, 1970; Perkins and Thomas, 1980. with some estimates as high as 10 years ŽLeipunskii, 1970.. As indicated in the above discussion, background levels of 239q240 Pu in the Marshall Islands are a result of radioactive fallout dispersed globally by large nuclear weapons tested in various countries. The determination of the level of 239q240 Pu contamination resulting from global weapons testing provides a quantitative basis for apportioning the measured contamination at each atoll location into global and regional components. We examined two different data sets to

24

S.L. Simon et al. r The Science of the Total En¨ ironment 229 (1999) 21]39

estimate the global fallout contribution in the Marshall Islands. Data on levels of 239q240 Pu in soil at Northern equatorial locations from many studies were summarized by Holleman et al. Ž1987.. These data were one source of information used to estimate the plutonium contribution from global fallout. Of 234 values reported by Holleman from the Northern hemisphere, 27 were from samples acquired at latitudes between 6.5 and 13.3 o N. These locations had a mean latitude of 8.1o N and median of 7.3 o N, nearly the same as the Marshall Islands. All but one of the 27 sample values acquired at latitudes less than 13.5o N were reported in units of Bqrkg, the exception was reported in units of Bqrm2 . The exceptional value was converted to Bqrkg by assuming unit density Ža value we determined as typical for surface coral soils. and that the radioactivity was distributed only within the top 5 cm of soil. Summary statistics with and without the single value that required units conversion differed by less than 3%. The 27 data points were from locations of Paulau, Truk and Ponape Žall three in the Federated States of Micronesia., Guam ŽNorthern Marianna Islands., Majuro Atoll Žsouthern Marshall Islands. and the Panama Canal Zone. We computed summary statistics without the six Majuro data points or the single Canal Zone point: min.s 0.07 Bqrkg, max.s 3.4 Bqrkg, mean s 0.53 Bqrkg, median s 0.34 Bqrkg, S.D.s 0.73 Bqrkg, S.E.s 0.16 Bqrkg. The median value for the empirical Ž n s 20 total. and fitted distribution from the set of points without the Majuro or Canal Zone data coincide at approximately 0.34 Bqrkg. The distribution of values was determined to be close to log-normal by examining probability plots of the data on normal and logarithmic scales. The estimated population geometric mean ŽBqrkg., geometric standard deviation and geometric standard error were 0.33, 2.4, and 1.2, respectively. Sixty-eight percent of the observed values lay between 0.14 and 0.79 Bqrkg; that interval is consistent with the one sigma confidence interval on the median assuming log-normality. Ninety-five percent of values lay between 0.1 and 1.1 Bqrkg. Assuming

the Pu is contained in the top 5 cm of soil and a soil density of 1.0 grcm3 Ža value typical for coral soils, see Graham and Simon, 1996., an equivalent expression is 5]55 Bqrm2 . Another estimate of the global background level of 239q240 Pu was derived from the reported activity ratio of 137Cs to 239q240 Pu ŽCsrPu. from global fallout as investigated in the 1970s by the US Atomic Energy Commission’s Health and Safety Laboratory ŽHASL.. The CsrPu ratio was determined to be a constant value in the north temperate zone in 1979 with a value of 53 " 0.5 Ž1 S.D.. ŽBeck and Krey, 1983.. Decay correcting this value to 1993 Žthe mean date of our measurements. gave a CsrPu ratio of 38 " 0.4. Previously, the contribution of 137Cs by global fallout in the mid-Pacific was estimated to be between 400 and 800 Bqrm2 ŽSimon and Graham, 1996.. Using these various data, we estimated the level of plutonium from global fallout in the Marshall Islands: 137 239q240

Puglobal fallout ( s

Cs global fallout estimate w Cs r Pu x

400 800 to 38 38

Thus, this estimate for plutonium in the environment of the Marshall Islands from global fallout is between 10 and 21 Bqrm2 . Assuming unit density for the top 5 cm of soil and a uniform distribution within this layer, the contribution of plutonium from global fallout is estimated to be between 0.2 and 0.4 Bqrkg. The two estimates of plutonium deposition from global fallout in the mid-Pacific ŽNorthern hemisphere only. were consistent with one another.

2. Methods The methodology reported here was used in various investigations of the NWRS ŽSimon et al., 1993; Graham and Simon, 1996; Simon and Graham, 1997.. These methods included techniques of environmental sampling, laboratory radiochemical and radiological sample analysis, statistical analysis and review of historical data.

S.L. Simon et al. r The Science of the Total En¨ ironment 229 (1999) 21]39

2.1. En¨ ironmental sampling Two different soil sampling protocols were used in the NWRS. Both protocols supported the primary monitoring technique of in situ measurement of gamma emitting radionuclides using high purity germanium ŽHPGe. detectors Žsee ICRU, 1994 for a review of in situ spectrometry.. Though the soil samples collected by the two methods were similar, the collection protocol differed for the purpose of supporting different study objectives. One protocol consisted of collecting surface soil samples at the locations of HPGe measurement sites. Depending upon location, the HPGe measurements sites were either part of a systematic grid or selected at random. In both cases, man-made buildings or concrete structures were avoided as were beaches or highly eroded areas. At each measurement site, three surface soil samples, each 15 times 15 times 5 cm deep, were obtained at random locations within 15 m of the HPGe detector. These type of samples were obtained only at northern atolls known or suspected of having high levels of radioactive contamination. The three subsamples from each measurement site were pooled to form a composite surface soil sample that was intended to be representative of the location of each in situ gamma measurement. These samples assisted in corroborating the estimates of the local inventory of gamma emitting nuclides that were calculated from the in situ spectrometry data. In addition, these samples were analyzed in the laboratory to characterize the level of environmental contamination with transuranic radionuclides. A second sampling method was the procurement of soil profiles sampled in 5-cm increments to a total depth of 30 cm. Over 200 soil profiles were acquired from locations over the entire nation during the survey of the Marshall Islands Žsee Graham and Simon, 1996.. Generally a ratio of one soil profile to each six in situ gamma measurements was maintained. The soil profiles were collected to characterize the vertical profile of 137 Cs activity, a parameter of considerable importance to estimating the areal inventory

25

ŽBeck et al., 1972; Helfer and Miller, 1988.. Only the top increment Ž0]5 cm. was analyzed for plutonium content; thus, only those data are reported here. Acquisition of all soil samples in the NWRS was by non-mechanized means except in a few instances when the NWRS was participating in intercomparison exercises with US Department of Energy laboratories Žlimited to Bikini and Rongelap Atoll.. In general, soil was carefully excavated by hand from the sides of a hand dug pit, then taken to the laboratory in Majuro for processing and analysis. Further details of the soil profile sampling methodology and the findings are presented in Graham and Simon Ž1996.. 2.2. Laboratory methods Soil samples were prepared in the laboratory as follows. Each soil sample was placed in a labeled aluminum container lined with absorbent paper for drying and gross weight measurement. Samples were dried at approximately 38 o C until there was less than 1% change in weight over the previous 24 h. After drying, the samples were weighed again and percent water calculated. Each sample was then sieved for at least 5 min using a mechanized shaker to separate particles ) 2 mm, 2.0]0.85 mm, 0.85]0.425 mm, 0.425]0.25 mm, 0.25]0.18 mm and - 0.18 mm. The sample weight for each particle size fraction was determined and the percent of total weight calculated. Particles ) 0.85 mm were mechanically crushed until less than 1% of the total sample weight contained particles greater than 0.85 mm. The crushed fraction was mixed with the original sample and a 500-, 1000- or 2000-ml aliquot was removed for gamma spectrometry analysis on an extended low-energy range HPGe detector. The mass of the aliquot was determined and used with the measured net count rate from the spectral peaks to determine the concentration of 241Am as well as 137 Cs. Activity in the sample was determined by comparison of the sample spectrum count-rates with the count-rates from a standard of identical geometry and density and containing known amounts of the radionuclides being assessed. Plutonium content was determined by alpha

1 3 1 1 4 3 4 2 1 2 1 4 3 4 9 3 3 3 3 4 2

Erikub Ebon Jabat Island Knox Jaluit Ailinglaplap Mili Lib Island Kili Island Namorik Jemo Island Aur Arno Maloelap Kwajalein Namu Ujae Lae Wotho Wotje Mejit Island

Prof Prof Prof Prof Prof Prof Prof Prof Prof Prof Prof Prof Prof Prof Prof Prof Prof Prof Prof Prof Prof

Type of soil samplesa

0.65 0.52 1.80 1.00 0.35 0.20 0.27 2.20 1.10 0.72 6.30 0.71 0.23 0.41 0.55 0.48 1.60 2.10 0.69 0.49 0.63

Number of samples per km2

0.17 0.25 0.34 0.37 0.35 0.50 0.35 0.48 0.72 0.73 0.76 0.58 0.56 0.52 0.54 0.71 0.67 0.97 0.74 0.77 1.11

] 0.25 ] ] 0.34 0.61 0.27 0.48 ] 0.73 0.53 0.51 0.47 0.45 0.64 0.49 1.07 0.68 0.64 1.11

Meanb ŽBqrkg.

Median ŽBqrkg.

4.70 2.30 2.60 2.20 ] 1.10 ] 2.10 2.90 4.30 4.10 2.20 3.10 1.90 3.80 6.10 5.70

] 1.70 ]

MaxrMin

] 26.50 ] ] 63.80 40.60 53.00 54.10 ] 7.00 ] 34.30 51.20 66.50 45.20 40.90 62.80 29.70 60.80 74.30 99.50

C.V. Ž%.

] 0.19]0.32 ] ] 0.13]0.60 0.26]0.61 0.24]0.63 0.30]0.67 ] 0.69]0.76 ] 0.40]0.85 0.30]0.86 0.21]0.92 0.24]1.0 0.47]1.0 0.37]1.2 0.65]1.2 0.32]1.2 0.25]1.5 0.33]1.9

Range ŽBqrkg.

Pu data from soil samples from atolls of the Marshall Islands, ordered by maximum observed concentration ŽBqrkg.

No. of samples

239q240

Atollrisland

Table 1 Summary of

0.43]0.86 0.48]1.6 0.84]1.7 0.93]1.9 0.32]3.0 0.66]3.1 0.61]3.1 0.74]3.3 1.8]3.6 1.7]3.8 1.91]3.8 1.0]4.3 0.75]4.3 0.53]4.6 0.61]5.0 1.2]5.2 0.94]5.8 1.6]6.0 0.80]6.1 0.63]7.7 0.82]9.4

Žrange.

Ratio of observed Pu to global fallout estimates c 239q240

26 S.L. Simon et al. r The Science of the Total En¨ ironment 229 (1999) 21]39

19.09 649

Mean Total

Prof 2 prof, 1 css Css Prof 7 css, 4 prof Css Css 2 css, 1 prof Css Css Css Css Css

5.90

0.97

0.56 0.93 6.90 0.58 19.00 17.00 13.00 6.10 16.00 ; 36.00d 16.00 12.00 ; 32.00d

nra

nra

1.60 1.70 1.20 2.80 8.70 25.00 43.00 108.00 258.60 107.00 134.00 213.00 936.00e

nra

nra

1.50 1.30 1.70 3.30 10.30 23.50 55.80 169.00 258.00 141.00 364.00 515.00 1282.00e

65.50

4.30

2.40 54.70 7.50 7.70 3.70 4.60 43.40 5.10 216.40 995.20 366.30 117.80 24.10e

63.00

60.80

39.80 86.80 75.30 70.10 40.80 30.70 88.60 85.30 60.60 82.00 127.00 145.60 84.70e

nra

nra

0.85]2.0 0.042]2.3 0.61]4.6 1.0]7.7 5.8]21.3 9.1]41.4 5.9]254.0 65.1]334.0 2.4]519.0 0.69]683.0 6.98]2560.0 35.2]4140.0 186.2]4480.0e

b Mean c

css, composite surface soil Žpooled sample of three 15 times 15 times 5 cm deep.; prof, top 5-cm increment of 30-cm deep profile. value is equal to the single data value for atolls with one sample. Global fallout estimates used were 0.2]0.4 Bqrkg for 239q240 Pu Žsee text.. d Land area used for calculation is approximate. e Plutonium values estimated from gamma spectrometry measurements of 241Am and observed PurAm ratio; see Appendix A for details. nra, not applicable because of disproportionate sample numbers among atolls.

a

3.00

3 3 12 6 11 42 36 3 27 214 95 73 63

Median

Ailuk Taongi Ujelang Likiep Taka Utrik Ailinginae Bikar Rongerik Rongelap ŽS. Bikini Enewetak Rongelap ŽN.

Table 1 Ž Continued.

nra

nra

2.1]10.0 0.10]11.0 1.5]23.0 2.5]39.0 15.0]110.0 23.0]210.0 15.0]1300.0 160.0]1700.0 6.0]2600.0 1.7]3400.0 17.0]13 000.0 88.0]20 700.0 470.0]22 000.0e

S.L. Simon et al. r The Science of the Total En¨ ironment 229 (1999) 21]39 27

S.L. Simon et al. r The Science of the Total En¨ ironment 229 (1999) 21]39

28

spectrometry after chemical extraction from the soil. Extraction was based on three main procedures: complete soil digestion, extraction with anion exchange columns, followed by microprecipitation and plutonium mounting with neodymium fluoride. To maintain approximately equal counting times Žon the alpha spectrometer. necessary to attain a measurement precision of "10% at the 1 s confidence level, the soil mass was adjusted for each sample based on a prior gamma spectrometry measurement of the 241Am. The plutonium concentration was estimated from the 241Am measurement. The minimum detectable concentration of plutonium was estimated from the background count rate to be on the order of 0.04 Bqrkg for a 12-h counting period. 2.3. Statistical analysis Statistical techniques used here include stan-

dard summary statistics ŽTable 1., graphical analysis for the purpose of summarizing data ŽFigs. 1 and 2., linear and non-linear regression ŽFigs. 3 and 4., and probability plotting ŽFig. 5.. A procedure was also developed to estimate the 239q240 Pu concentration and uncertainty for a limited number of composite surface Ž0]5 cm. soil samples. The only samples for which estimation were necessary were from the northern islands of Rongelap Atoll. Rongelap Atoll was more heavily sampled than other atolls because of local and international interest Žsee Simon et al., 1997.. Of the 277 surface samples collected by the NWRS on Rongelap, 63 samples Ž23%. were from the northern islands of the atoll. However, because of resource limitations, those samples were not analyzed for plutonium content. The samples were analyzed for gamma emitters including 241 Am. Because of the usual strong correlation between 241Am and 239q240 Pu concentrations, we

Fig. 1. Measurements of 239q240 Pu ŽBqrkg. in surface Ž0]5 cm. soil from atolls of Marshall Islands. Atolls are ranked left to right according to maximum observed value. Median value is also shown and global background deposition is shown as a gray band.

S.L. Simon et al. r The Science of the Total En¨ ironment 229 (1999) 21]39

29

Fig. 2. Maximum observed value of 239q240 Pu in surface Ž0]5 cm. soil of atolls of the Marshall Islands ŽBqrkg. as a function of latitude ŽRongelap is divided into south and north portions; Majuro Atoll not shown..

were able to estimate the plutonium content in a manner similar to that used in other studies, e.g. ŽGilbert et al., 1975; Ibrahim et al., 1995.. Our estimation procedure is described in detail here Žsee Appendix A..

3. Results and discussion 3.1. Data summary A total of 649 samples were analyzed for Pu in this study, 77 samples were the top increment Ž0]5 cm. of 30-cm deep profiles, 572 samples were composite surface soil Ž0]5 cm. samples acquired in the vicinity of HPGe measurements. Various summary statistics of the sampling program are provided in Table 1. The sampling density Žno. samplesrunit area. and the number of samples acquired and ana239q240

lyzed from each atoll were limited by financial resources and other practical constraints including the degree of local and national interest at each location. At two small uninhabited atolls, only a single sample was analyzed: Knox, an atoll which was at background level as determined by 137 Cs analysis ŽSimon and Graham, 1994, 1997. and Erikub that was approximately 2 times background. Because of the small size of these atolls, the number of samples when normalized to the land area of the atoll Ži.e. samples perrkm2 . was not far from the median sampling density at the other atolls ŽKnox s 1.0 samplerkm2 , Erikub s 0.7 samplerkm 2 ; median of all atolls s 1 samplerkm2 .. The median and mean number of samples analyzed per atoll was 3.0 and 19, respectively; the mean was highly skewed because of a few heavily sampled atolls Žsee Table 1.. The large number of samples from southern Rongelap Ž n s 214. was

30

S.L. Simon et al. r The Science of the Total En¨ ironment 229 (1999) 21]39

Fig. 3. Maximum observed value of 239q240 Pu in surface Ž0]5 cm. soil of atolls of the Marshall Islands ŽBqrm2 . as a function of the maximum observed value of 137 Cs ŽBqrm2 .. Locations not included in plot are Majuro Atoll Žno Pu data., Enewetak Atoll Žbecause of safety tests. and Taongi Žsee text.. Solid regression line was computed by linear least-squares.

made possible by a separately funded study ŽSimon, 1994; Simon et al., 1997.. The concentrations of 239q240 Pu in the analyzed soil samples covered a range of nearly five orders of magnitude, from a low value of 0.042 Bqrkg at Taongi to 4140 Bqrkg at Enewetak Ž4480 Bqrkg was estimated in one northern Rongelap Atoll sample.. The median measurement from each of the 34 atolls 2 reported in Table 1 covered a range of 5500 times from a low of 0.17 Bqrkg Ž n s 1. at Erikub Atoll to a high of 936 Bqrkg from northern Rongelap Atoll. The ratio of the maximum to the minimum

2 There are 29 atolls and five separate reef islands in the Republic of the Marshall Islands. We subdivided Rongelap Atoll into southern and northern sections for a total of 35 locations. Data is reported here for 34 of these 35 locations. No samples from Majuro Atoll were analyzed for plutonium content because of the high level of land disturbance.

observed data value ŽMaxrMin. from samples at each atoll covered the range from 1.7 ŽEbon, southern most atoll, n s 3. to 995 Žsouthern Rongelap, n s 214.. The median MaxrMin of all the atolls was 4.3; the mean was 65. The high mean value of the MaxrMin ratio was again skewed by Rongelap Atoll. The MaxrMin ratio is in indicator of the relative range of concentrations observed at each atoll. The locations with small MaxrMin values were generally those with the lowest concentrations and those at the greatest distances from the test sites Žsee Table 1.. This was demonstrated by the data for locations with maximum measurements equal or less than 2.0 Bqrkg Ž22 of 34 locations.. Only 17 of those 22 locations had more than a single measurement and can be examined for the MaxrMin ratio. Among the 17 atolls with more than a single measurement, the MaxrMin ratio varied from 1.1 to 6.1 with a median of 2.7.

S.L. Simon et al. r The Science of the Total En¨ ironment 229 (1999) 21]39

31

Fig. 4. Empirical data and regression model of PurCs ratio Žboth measured in Bqrkg. as a function of distance Žkm. from Bikini. Model fit from data of soil profiles. Median values Žblack dots. of surface Ž0]5 cm. sample data is for comparison only.

Hence, the range of observed values was not extraordinarily great at each of the atolls that were close to the global background level of contamination. The atolls that showed a wide range between the minimum and maximum observed value were the test site atolls ŽBikini, Enewetak. and those atolls closest to the test site atolls ŽRongerik, Ailinginae, and Rongelap.. The MaxrMin for those five atoll locations varied from 24 to 995. The range of observed values from each atoll can be inspected in Fig. 1 where all data collected are plotted. MaxrMin values which are indicative of the absolute spread of values were related to the coefficient of variation ŽC.V. Ž%. s 100 srx . which expresses the variation relative to the mean. MaxrMin values of approximately 2.0 or less

were generally indicative of C.V.s of 30% or less. MaxrMin values of 2.5 or less were indicative of C.V.s of 50% or less and MaxrMin values of 5.0]55.0 were generally indicative of C.V.s of approximately 85%. In one case Žsouthern Rongelap Atoll., the MaxrMin was 995 though the C.V. was only 82%. The median C.V. for all 28 atolls and four reef islands Žwithout weighting for atoll sample size. was 61% Žmean s 63%.. The collection and analysis of soil samples to only 5-cm depth limited the accuracy of determining the true total areal inventory ŽBqrm2 .. Wherever the plutonium penetrated deeper than 5 cm, the local inventory would have been underestimated. Generally there is good evidence for assuming that most of the plutonium activity from weapons fallout has remained within the top few centime-

32

S.L. Simon et al. r The Science of the Total En¨ ironment 229 (1999) 21]39

Fig. 5. Probability plot of 239q240 Pu: 241Am ratio Žboth measured in Bqrkg. in 481 surface soil samples from the northern Marshall Islands Žonly 20% of data points are shown.. Dotted line shows empirical fit to log-normal distribution with median equal to 2.14, geometric standard deviation s 1.025. Data were used to derive a conversion factor to estimate plutonium concentration from americium concentration Žsee Appendix A..

ters in undisturbed soils. For example, the migration of plutonium into the soil column at Nishiyama Ž3 km east of the hypocenter at Nagasaki. has been investigated and reported by Mahara and Kudo Ž1995.. This area has an annual precipitation rate of 2000 mm, not significantly different from the Marshall Islands which varies between 1000 and 3000 mm annually ŽNOAA, 1989a,b.. The mean 239q240 Pu vertical migration rate at Nishiyama was determined to be 1.25 mmryear. If that rate of migration is applied to the date of deposition from the largest test in the Marshall Islands ŽCastle BRAVO, 1 March 1954., the vertical migration would have been 5 cm over the years since deposition. The soil in Japan, however, is substantially different from that in the Marshall Islands, the former being of volcanic origin, the latter of coral origin. The lower binding ability of the coral soils is apparently responsible for greater vertical mi-

gration of plutonium in the Marshall Islands environment. For example, data on the concentration of 239q240 Pu in 214 vertical profiles was reported ŽRobison et al., 1982. from the US DOE sponsored Northern Marshall Islands Radiological Survey conducted in 1978. Those data showed that the average relative concentrations in the depth intervals 0]5, 5]10, 10]15, 15]25, and 25]40 cm were 1:0.38:0.18:0.074:0.029. Thus, the proportion of the areal inventory in the top 5 cm of the samples from the NWRS was on average 60% of the total. Because all of the areal inventory was probably not collected in our 5-cm deep samples, the true areal inventory of plutonium may be underestimated by as much as 40%. 3.2. Spatial distribution of plutonium The most obvious feature that we observed was a significant increase in the maximum sampled

S.L. Simon et al. r The Science of the Total En¨ ironment 229 (1999) 21]39

soil concentration of plutonium with increasing latitude between 9 and 11.5o N ŽFig. 2.. Also seen was evidence of a relatively symmetrical decrease at latitudes greater than 11.5o N though only two sampling locations were available north of the location where the peak occurred Žsee Fig. 2.. This pattern could be interpreted to represent the cross-section of a single Gaussian type plume or a multiple set of plumes that moved along W to E parallels. However, it is known that the wind directions at different altitudes varied considerably at the times of the 18 large nuclear tests ŽDNA, 1979.. Thus, the observed deposition pattern is likely a product of complex dispersion patterns which moved the nuclear clouds 3 in a variety of directions. The soil contamination data alone is not sufficient information to explain the phenomena which resulted in the observed spatial pattern. However, the spatial pattern itself is informative about the relative degree of contamination of the atolls. The maximum observed concentration ŽBqrkg in top 5 cm. at each atoll is plotted in Fig. 2 as a function of latitude. We believe that these data points of maximum concentration can be interpreted to be closest to the original value of deposition at each atoll. This is most likely a good assumption for atolls that lie at distances of a 100 km or more from the test sites though it is less certain for the test sites or nearby atolls. The rationale of that hypothesis is associated with the lack of land surface variations. All islands in the Marshall Islands are coral and are virtually flat with highly porous soil; the result is that precipitation is quickly absorbed into the soil. Standing water following storms is very rare; furthermore, there is little evidence of erosion from runoff which might lead to collection of radioactivity in localized areas. The weathering process in general decreases the radioactivity inventory in the upper soil horizons over whole islands and does not result in significant localized

3

Nuclear cloud refers to the condensed water vapor, dust and radioactive debris resulting from the detonation of a nuclear weapon on or near to the earth’s surface w38x.

33

variations. Thus, the observed high values were probably not a result of physical or bio-accumulation but rather a remnant of the original deposition. It should be again noted that the Pu areal inventory data at some locations may be underestimated by ; 40% as described in Section 3.1, however, this would apply to all locations equally. One caveat should be noted for the data presented in Fig. 2. For those atolls where only a single sample was analyzed Žsee Table 1., the data value reported is likely closer to the average concentration for the atoll rather than to the maximum. The data from each of the five locations with only a single sample are probably underestimates of the maximum value, however, those five locations are also at great distance from the test sites and are at low concentrations. The variation between mean and maximum at these locations is probably not more than 3 times. 3.3. Pu measurement data relati¨ e to estimated global fallout deposition Irrespective of our hypothesis that the maximum observed value best represents the original deposition, the strong gradient in maximum areal inventory with change in latitude was clearly observed. Atolls at latitudes greater than 9 o N show evidence of having received local fallout deposition. This conclusion is in agreement with analysis of soil samples for 137 Cs ŽSimon and Graham, 1994, 1997.. The ratio of observed values of 239q240 Pu relative to the estimate of global fallout was expressed as a range Žsee Table 1.. This range accounted for the twofold variation in best estimates of global fallout deposition. The ratio of observed 239q240 Pu to the estimated global fallout deposition was less than 4 times for 11 of 34 atollrisland locations Žsee Table 1.. Ten atollrisland locations were less than 10 times higher than global fallout. Twelve atolls were more than 10 times global fallout deposition. Enewetak Atoll had locations with the highest contamination, up to 20 000 times global background. The high ratio at Enewetak was not representative of the entire atoll, however. The

34

S.L. Simon et al. r The Science of the Total En¨ ironment 229 (1999) 21]39

median ratio Žobserved plutoniumrglobal fallout estimate. at Enewetak Atoll was approximately 670. 3.4. The spatial distribution of plutonium relati¨ e to cesium The spatial pattern of environmental plutonium in the RMI was found to be similar to the spatial pattern of 137 Cs deposition ŽSimon and Graham, 1994, 1997.. Though there is no obvious reason to expect a different deposition pattern for the two radionuclides, these data provide confirmation of the similarity of their spatial distribution. Two pieces of evidence support this finding. First, the increase in soil concentration of plutonium with increasing latitude between 9 and 11.5o N ŽFig. 2. was similarly observed for 137 Cs Žsee Fig. 2 of Simon and Graham, 1997.. Second, we examined the relationship between the empirical data of plutonium and cesium in soil at each atoll. In Fig. 3, we plotted the maximum observed value of 239q240 Pu at each atoll as a function of the maximum observed value of 137 Cs. In this figure, the Pu and Cs data are not from the same samples, rather they represent the maximum observations at each atoll regardless of the sample identity. A strong linear relationship Ž R 2 s 0.985. between the two variables was determined. The power ‘b’ of the fitted equation ŽPu s a times Cs b . is close to unity Ž bs 1.093. indicating a near perfect proportional relationship between the two concentrations over five orders of magnitude. In Fig. 3, data are not included for Majuro, Enewetak and Bikar Atolls, though the reasons for exclusion differed for each. No Pu measurements were made at Majuro Atoll because the level of soil disturbance there has been high. Majuro is the capital city and the most heavily populated of all the atolls. Enewetak was the site of at least one safety test 4 which resulted in excessively high local concentrations of plu-

4 A safety test is the purposeful destruction of fissionable material with high-explosives and is usually not accompanied by significant nuclear fission. High environmental levels of plutonium contamination may result within a localized area.

tonium. The data from Bikar Atoll Ž n s 3. were excluded because the mean value of the PurCs ratio Žs 15.2. was more than 100 S.D. Ž s s 0.128. away from the mean PurCs ratio for the other atolls combined Žs 0.135.. The soil from Bikar was extremely high in organic matter due to the high content of avian excrement Ž‘guano’.. It is our hypothesis that cesium was originally deposited at Bikar in approximately the same ratio to plutonium as at other atolls but has since migrated downward more rapidly, thus elevating the PurCs ratio in surface soil. The relationship of plutonium to cesium concentrations in the soil was also examined as a function of distance from the Bikini Atoll test site. A significant body of literature exists describing variations of the radionuclide composition of nuclear clouds as a function of transit time or downwind distance Žsee for example, Lai and Freiling, 1970; Stevenson, 1970; Glasstone and Dolan, 1977.. Differential depletion of the various radionuclides from the nuclear cloud is described as fractionation ŽGlasstone and Dolan, 1977.: Any of several processes, apart from radioactive decay, which results in change in the composition of the radioactive debris. As a result of fractionation, the delayed fallout generally contains relatively more strontium-90 and cesium-137, which have gaseous precursors, than does the early fallout from a surface burst.

In many past publications, fractionation referred only to fission products. Because the dispersed plutonium is largely unfissioned nuclear fuel ŽGlasstone and Dolan, 1977., plutonium was generally not considered in analyses of fractionation. However, plutonium and cesium are the two main elements still remaining from the nuclear tests that are of public and scientific interest. Thus, we have examined evidence for fractionation between these two elements. Our observations indicate that plutonium was depleted from the nuclear cloud earlier or at shorter distances as compared to cesium. This conclusion emerged from analyzing the change in the plutoniumrcesium ratio ŽPurCs. over a range

S.L. Simon et al. r The Science of the Total En¨ ironment 229 (1999) 21]39

of distances from less than 50 km from the BRAVO test site to nearly 1000 km distance. Prior to determining the PurCs ratios in this study, Simon et al. Ž1997. compared the 137Cs inventory measured to 30-cm depth as part of the NWRS with reported inventory measurements in the northern Marshall Islands for samples to 60cm depth ŽRobison et al., 1994.. From that comparison, the 30-cm samples were estimated to account for 67% of the total deposition of 137 Cs, not significantly different than the situation for plutonium Žsee Section 3.1 above.. Thus, the ratio of PurCs reported here Žwith sampling depths of 5 and 30 cm, respectively. is approximately the same as a ratio determined from samples taken to depths necessary to collect the complete inventory Ž40 cm for Pu and 60 cm for Cs., i.e.: w 239q240 Pu x 0y 5cm r w 137 Cs x 0y30 cm ( w 239q240 Pu x 0y 40 cm r w 137 Cs x 0y60 cm Plutonium from the top 5 cm of soil and 137 Cs from the top 30-cm depth of soil were used for this analysis as described above. Our procedure for determining PurCs ratios consisted of: Ž1. determining the net plutonium concentration above global background Žmeasured concentration minus 0.34 Bqrkg.; Ž2. converting the plutonium concentration units ŽBqrkg. to areal inventory ŽBqrm2 . by multiplying by 50 5 ; Ž3. determining the net cesium areal inventory above global background Žmeasured areal inventory minus 600 Bqrm2 .; and Ž4. calculating the ratio of the net plutonium to the net cesium. Because many samples from the southern atolls were near to background levels or barely detectable, the net plutonium or cesium values were often near to zero or negative. We necessarily removed those data from the distance analysis because of the logarithmic transformation used. Using the method described above, there were 67 non-zero data points from which to determine a distance relationship for the PurCs ratio. Using

5 Fifty kilograms per meter squared in the depth increment 0]5 cm assuming unit density.

35

those data, we fit the PurCs ratio data with distance using unweighted non-linear least squares Žsee Fig. 4.: Pu r Cs s 1.85= exp Ž y0.00483= km. w R2 s 0.18x . From this fitted equation, a decrease of 38% in the PurCs ratio is predicted per 100-km distance from the Bikini test site. The equation also predicts that the PurCs ratio should equal 0.026 at a distance of 883 km. A PurCs ratio of 0.026 is equivalent to the reported ratio of CsrPu Ž38.4 in 1993. in the northern hemisphere as derived from Beck and Krey Ž1983.. That the above fitted relationship is weak does not come unexpected. An important factor contributing to a poor fit is that approximately 40% of the Pu and Cs likely permeated below the depth of sampling. Because the degree of permeation of each radionuclide would have varied for each sample Ždepending on local variations in soil quality, disturbance, etc.., considerable fluctuations in the PurCs ratio were found, thus, obscuring to some degree the true fractionation effect. Additional PurCs data were available from the composite surface soil samples taken at gamma spectrometry measurement sites. These data were not used in the estimation of the regression line because the data are from a few locations that were heavily sampled. The PurCs ratio was computed for these data similarly to the samples from the soil profiles but they were used only for comparison to the estimated regression line. The median values of the composite surface soil samples and their 95% confidence intervals are shown in Fig. 4. The 95% C.I. for each of those locations is a non-parametric estimate ŽGeigy, 1982; Gilbert, 1987.. The median values from these additional sampling locations are generally consistent with the estimated regression line. Our interpretation of these findings is that fallout particles bearing mainly plutonium were removed from the nuclear cloud sooner, i.e. at earlier times andror shorter distances than fallout particles bearing mainly 137 Cs. Thus, these data provide evidence of differential deposition

36

S.L. Simon et al. r The Science of the Total En¨ ironment 229 (1999) 21]39

Žfractionation effect. between cesium and plutonium at distances less than 1000 km from the test site. The reasonable hypotheses to explain the differential deposition Žfractionation effect. are several. First, the temperature at which the various elements condense from the superheated nuclear cloud vary considerably. For example, the boiling points of plutonium oxide and cesium are 3200 o C q and 669 o C, respectively. Below these temperatures, the elements would be expected to condense, thus, plutonium which is a refractory element would condense at an earlier time after the detonation than would the cesium which is more volatile ŽGlasstone and Dolan, 1977.. Furthermore, there are other metallic elements in the nuclear cloud due to the vaporization of the weapons casing and associated equipment including steel towers which were sometimes used to support the weapons. Metals used in steel such as cobalt, iron, manganese, molybdenum, tungsten and zirconium all have relatively high boiling points Ž2870 o , 2750 o , 1962 o , 4612 o , 5600 o , 4377 o C, respectively; Weast, 1990.. These elements would be expected to condense at temperatures well above that of cesium, thus providing condensation nuclei for the plutonium or plutonium oxide. At sufficiently low temperatures for the cesium to condense, the cloud would be partially depleted of the heavier particles which would have been removed by gravitational settling ŽGlasstone and Dolan, 1977.. In addition, plutonium, in particular the oxide form, is highly insoluble as compared to cesium. The reasons for the fractionation effect between plutonium and cesium are thus likely related to differences in their condensation temperatures as well as to differences in their chemical affinity for condensation nuclei. Both processes result in the likelihood of plutonium to form particles susceptible to more rapid gravitational settling. Our findings agree with the qualitative statements of Glasstone and Dolan, Ž1977, Sections 9.06]9.10..

4. Conclusions and summary Data from the laboratory analysis of soil sam-

ples for 239q240 Pu content have been used to corroborate the 137 Cs spatial pattern of contamination of the Marshall Islands. Similar results were found for both radionuclides in terms of ranking the atolls by median or maximum concentration ŽSimon and Graham, 1997.. A significant and steep increase in the maximum observed plutonium concentration at each atoll was noted with increasing latitude between 9 and 11.5o N. At latitudes greater than 11.5o N, the plutonium concentration decreased. This phenomenon is believed to be a result of wide dispersion and dilution of the nuclear clouds with increasing distance. Numerous atolls in the southern Marshall Islands Žthose - 9 o N latitude. as well as two atolls in the northern part of the nation ŽBikar at 12.2 o N and Taongi at 14.5o N. have lower environmental plutonium concentrations than those atolls near to 11.5o N, the approximate latitude of the Bikini and Enewetak test site. Environmental concentrations of plutonium varied considerably over the archipelago of the Marshall Islands from background levels Žapproximately 0.2]0.4 Bqrkg in the top 5 cm. up to 20 000 times the global deposition level. The samples above 1000 Bqrkg were few and were limited to locations at only three atolls ŽBikini, Enewetak and Rongelap.. Those three atolls had the largest sampling programs, but the higher observed concentrations are not believed to be a consequence of more intensive sampling. The PurCs ratio was found to vary with distance, decreasing by approximately 38% per 100 km distance from the Bikini test site. This conclusion was reached from our analysis of surface soil samples collected from over the entire nation. The PurCs ratio reached a value consistent with Northern hemispheric locations at approximately 880 km from the Bikini test site. Because of the weakness of the statistical fit of the PurCs data, the estimated rate of change of the ratio with increasing distance should be considered with some caution. Fractionation from any single nuclear event is dependent on many specific variables related to weapon design and circumstances of the detonation. The data used in this analysis

S.L. Simon et al. r The Science of the Total En¨ ironment 229 (1999) 21]39

are the cumulative result of numerous fallout events, differing weather conditions and other variables. We believe that the observation of a fractionation effect is a more important finding than the numerical fit of the data. Several mechanisms are believed to be responsible for the fractionation effect between plutonium and cesium with increasing distance. In particular, the cooling of the nuclear cloud as it travels produces particles of differing radionuclide composition because of the different temperature at which each radionuclide condenses from the superheated state. Furthermore, because of differences in the affinity of each radionuclide to attach to condensation nuclei in the atmosphere, particles of differing size and radionuclide composition are formed as the cloud travels. Differences in gravitational settling for the various sized particles result in changes in the radionuclide composition of the cloud during transit. Acknowledgements The measurements reported here were conducted by the Marshall Islands Nationwide Radiological Study and were supported by the Government of the Republic of the Marshall Islands. Sample collection was assisted by A. Barron and S. Duffy. Laboratory analysis of plutonium was assisted by S. Como, R. Thomas and T. Schmidt. Assistance in developing methods for plutonium analysis in coral soil was provided by S.A. Ibrahim of Colorado State University. Assistance in developing protocols for sampling, the analysis of 137 Cs, as well as review of all the collected data was provided by the Scientific Advisory Panel to the Nationwide Radiological Study. H.L. Beck of the Department of Energy’s Environmental Measurements Laboratory provided a number of useful suggestions to improve this manuscript.

37

tonium concentration by a laboratory method described in Section 2.2. Some samples, however, were only analyzed by gamma ray spectrometry for gamma emitters including 137 Cs and 241Am. The objective of this section is to present details of how estimates of plutonium concentration were made for this latter group of samples. The concentration of 241Am in composite surface soil samples Žthree samples 15 times 15 times 5 cm deep each. was determined by analysis of gamma spectra as measured in the laboratory with extended range high-purity germanium ŽHPGe. detectors. For standardization, a laboratory-mixed gamma standard traceable to the National Institutes of Standards and Technology ŽNIST. was used. Both 241Am and 239q240 Pu were measured in a total of 481 samples. The radionuclide concentration data were used to determine an empirical ratio between 239q240 Pu ŽBqrkg. and 241Am ŽBqrkg.. The median ratio was used as a conversion factor from the 241Am concentration measured in other samples. Ratios of the two radionuclides were determined and summary statistics of the population of ratios were computed: mean s 2.72, S.D.s 1.75, S.E.s 0.080 Ž n s 481., median s 2.14, skewness s 2.38. Although there were some moderately extreme values of the PurCs ratio, 90% of the ratio data were within the range of 1.0]6.0 Žsee Fig. 5.. Normal and log-normal probability plots of the calculated ratios were examined to determine the statistical distribution of the data. The PurAm ratio data were closer to a log-normal than normal distribution as was evident from the near straight line of the ratio data on a log-probability scale ŽFig. 5.. Therefore the median value of 2.14 was used as a robust estimate of the conversion factor to determine the concentration of 239q240 Pu from the measured concentration of 241 Am: w 239q240 Pu x estimate s 2.14= w 241 Amx measured

Appendix A: Estimation procedures for plutonium concentration in soil Soil samples from most of the northern islands of the Marshall Islands were analyzed for plu-

Readers should note that 241Am is a decay product of 241 Pu Ž t 1r2 ( 15 years.. Though 84% of the 241 Pu had decayed to americium at the time of the americium measurements Ž1994., the ratio of

S.L. Simon et al. r The Science of the Total En¨ ironment 229 (1999) 21]39

38 241

Am: 241 Pu we determined was specific to that point in time. The uncertainty of the predicted plutonium value was evaluated by error propagation. The geometric standard error of the PurAm ratios was determined as follows:

Geometric S.E.s exp

S.D. Ž ln x i .

'n

s 1.025 where S.D. Žln x i . s the standard deviation of the natural logarithm of the ratios; and n s the number of samples Ž481.. The error of the estimated 239q240 Pu concentrations was calculated as follows: 239q240

Pu Ž Bq r kg. " 1 s s Ž 2.14" G.S.E.. = Ž 241 Am" sAm .

239q240

Pu Ž Bq r kg. " 1 s

s w 2.14=241Amx "

(

G.S.E.2 q

2 sAm% 100

where, s the median ratio between 239q240 Pu and 241Am; GSE s the geometric standard error of ratios of 239q240 Pu to 241Am; 241 Am s the concentration of 241Am ŽBqrkg. measured in the sample; sAm% s the percent error in the measured 241 Am ŽBqrkg. concentration.

2.14

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