Radionuclides in the Environment - ACS Publications

24 Decrease of Bovine Cesium-137 Concentrations Following the Cessations of Atmospheric Nuclear Weapons Testing

Downloaded by UNIV OF CINCINNATI on May 30, 2016 | http://pubs.acs.org Publication Date: January 1, 1970 | doi: 10.1021/ba-1970-0093.ch024

CLIFTON BLINCOE and V. R. ΒΟΗΜΑΝ University of Nevada, Reno, Nev.

Cattle have been used since 1958 to study the fallout cesium -137 concentration in the biosphere. These studies have been made at three locations in Nevada and have included mea surement of both liver and skeletal muscle concentrations of Cs twice yearly. A comparison of yearling and mature cattle indicated no effect of these ages at slaughter on the Cs concentration. The half-period for decrease of bovine soft-tissue Cs in the absence of atmospheric nuclear weap ons testing averaged 0.9 year (0.5 ± 0.1 to 1.3 ± 0.5 years). There was no significant difference in the rate of decrease following the cessation of atmospheric nuclear weapons test ing in 1959 and in 1963. The same rate of decrease was observed at all three locations and in both muscle and liver. 137

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C e s i u m - 1 3 7 is the principle radionuclide of appreciable half-life and ^ high fission yield found in the edible tissues of animals. It also has gaseous precursors ( I and X e ) and can thus be expected in fallout from low temperature fission product releases to the atmosphere. Range beef cattle have been used as monitors of fallout radionuclides in the biosphere. They forage over large areas eating a variety of desert plants, thus integrating the biologically available fallout over a wide area. They are also large enough to provide the large samples necessary for determining quantitatively low concentrations of radionuclides. The ability of cattle to survive exclusively on desert range and their impor tance as human food also increases their desirability as monitors of biologically available radionuclides. Cattle have been maintained on desert range in three locations in Nevada since 1958 for studies of fallout accumulation. The data for C s , 137

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427 Freiling; Radionuclides in the Environment Advances in Chemistry; American Chemical Society: Washington, DC, 1970.

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E N V I R O N M E N T

I , and S r through 1962 have been published (4,5,6). These indicated that bovine thyroid I concentration was a good monitor of fresh fallout, that soft-tissue C s was a good monitor of biosphere contamination with long half-life radionuclides, and that ^ S r concentrations were largely cumulative. This report concerns the changes i n concentration of C s i n edible tissues of range cattle following the cessation of atmospheric nuclear weapons test series. 1 3 1

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1 3 1

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Methods Three herds of Hereford cattle have been maintained at three loca tions i n Nevada since 1958. One herd is on the Nevada Test Site of the U . S. Atomic Energy Commission ( N T S ) , another 80 km. east i n Delemar Valley ( D V ) , and the third 480 km. north of the D V herd at the Knoll Creek F i e l d Laboratory of the University of Nevada ( K C ) . These loca tions are indicated i n Figure 1. The locations, herds, and sampling pro cedures have been described i n detail (4, 5,6). Samples are taken twice yearly from all herds. Prior to 1963 a calf, a yearling, a two-year old, a three-year old, and a mature animal were slaughtered at each sampling period from each herd. Analysis of the data from these studies indicated that more useful data would be obtained by limiting the number of age groups used (5). Since 1963, three yearling and three mature animals have been taken from each herd at each sampling period. In all cases, the animals have lived their entire life i n the location indicated and have existed entirely on desert range except for winter supplementation with locally produced hay necessary at the K C location. Samples of liver and skeletal muscle from the round were analyzed for cesium-137 after wet digestion with nitric acid (3). The radiocesium was then separated by coprecipitation with cobalt cobalticyanide (2). C s was measured with a well-type scintillation counter (5.1 cm. diam eter and height N a l ( T l ) crystal), single-channel analyzer, and scaler by counting γ-ray energies of 0.662 ± 0.025 Mev. The standard deviation of analysis was found to be ± 0.032 p C i . C s / g r a m of tissue prior to 1963 and ± 0.050 p C i C s / g r a m in 1964-1965. Multichannel γ-ray spectrometry of the counting samples indicated that measurable quantities of C s or other fallout nuclides were not present, confirming that this procedure gives good decontamination from other fission products. Previous studies have failed to demonstrate the presence of C s i n cattle (5). Analysis of variance, linear regression, and other statistical procedures were performed using computer programs based on standard procedures 137

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1 3 5

(12,

U).

Results and

Discussion

This study encompasses two periods without extensive atmospheric contamination by nuclear weapons testing. A n informal voluntary testing moratorium was observed from Oct. 31,1958 until Sept. 1, 1961. A limited

Freiling; Radionuclides in the Environment Advances in Chemistry; American Chemical Society: Washington, DC, 1970.


24.

B L I N C O E

A N D

Figure

B O H M A N

1.

Bovine

Locations

429

Cesium-137

of the test herds of cattle

range

nuclear test ban treaty was signed on Aug. 5, 1963 and continues i n effect to the present time. Only occasional, low fission yield, atmospheric nuclear detonations by France and China have occurred during these periods before 1967. Otherwise all detonations ceased during the informal testing moratorium, but underground testing continued during the limited test ban treaty. These tests have occasionally released limited quantities of radioactivity to the atmosphere. The data collected following these cessations of atmospheric nuclear weapons tests were first studied to ascertain the experimental variables influencing the C s concentration i n bovine soft tissues and then to ascertain the rates of decrease of C s i n these tissues in the absence of atmospheric nuclear weapons testing. 137

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Analysis of variance was made on the 1960 muscle data and the 1961 liver data as representative of concentrations during the informal



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testing moratorium. Time was not a factor in the analysis of variance, and spring and fall data for a given year were considered replicates of each other since only one animal per age group was slaughtered at any loca tion at any one time (5). Analysis of variance was also made on the data collected from 1963 to and including the spring 1966 sampling as repre sentative of tissue concentrations during the first few years of the limited nuclear test ban treaty period. The major factors studied were the tissue sampled (muscle or liver), age of the animals (yearling or mature), location i n the state ( K C , D V or N T S ) , and time. A l l factors and inter actions not containing age as a variable were highly significant ( P > 0.01). Location i n the state significantly affected soft-tissue concentra tions of C s . This was expected since the study was designed originally to ascertain the effect of distance from the Nevada Test Site on fallout in cattle. The observation that liver and muscle had significantly different concentrations of radiocesium was in agreement with a previously re ported study (5) indicating a highly significant correlation of liver and muscle fallout C s concentration with the liver increasing i n C s rela tively slower than muscle. Since age was not a factor i n the concentrations of C s i n soft tissues, data from yearling and mature cattle were com bined i n all subsequent statistical studies. The samples collected after cessation of atmospheric nuclear weap ons tests in 1958 and 1963 were examined to ascertain the relative rate of decrease of soft-tissue cesium-137 (Table I ) . In view of the analysis of variance studies, each tissue and location were considered separately, 137

1 3 7

1 3 7

1 3 7

Table I. Half-Period for Decrease of Bovine Cesium-137 Concentrations Following the Cessation of Atomospheric Nuclear Weapons Tests 1959"·

Tissue Location KC DV NTS

10 10 9

Liver

KC DV NTS

8 6 6

β 6 0 d β ;

—

a

No. of Half-Penod, Samples years

Muscle

Average

1963

6

Sampling No. of Periods Half-Period, Samples Included" years

0.84 ± 0.14* 0.54 ± 0.10 2.29 ± 2.67

36 30 42

F63-S66 S64-S66 S63-S66

0.96 ± 0.11 1.33 ± 0.39 0.78 ± 0.06

0.88 0.59 0.83 0.74

36 30 42

F63-S66 S64-S66 S63-S66

1.01 0.11 1.34 ± 0.52 1.00 ± 0.12 1.07 ± 0.09'

± ± ± ±

0.17 0.04 0.27 0.07'·'

Year of termination of atmospheric nuclear weapon tests. S59-S61 data used. S = spring sampling; F == fall sampling. Standard error of estimate. The muscle-NTS value excluded from the average. Standard error of the mean.


±

d

BLiNCOE

A N D

BOHMAN

Bovine Cesium-137

431


24.

Figure 2. Decrease of muscle cesium-137 concentrations in range cattle following the cessation of atmospheric nuclear weapons testing. Dashed line indicates the regression equation calculated only on 1959 and 1960 data. but data from yearling and mature cattle were pooled. The relative rate of decline is expressed as half-periods. Figures 2 and 3 give data for muscle and liver C s concentrations at the N T S location. These data 137


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are representative of those collected and indicate the regression equations used to calculate the half-period. It should be pointed out that these half-periods have little relation to the biological half-life of C s . The


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Figure 3.

Decrease of liver cesium-137 concentrations in range cattle following the cessation of atmospheric nuclear weapons testing


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A N D

BOHMAN

433

Bovine Cesium-137

biological half-life i n cattle is reported to be 15-30 days ( J J , 13), which is more than an order of magnitude less than the observed half-period for loss of C s following the cessation of atmospheric nuclear weapons testing. The relatively short biological half-life and the long half-period observed i n these studies indicate that range cattle are always in a steady state with their intake of fallout cesium-137. The levels of soft tissue C s i n range cattle, thus, depend almost exclusively on their intake of cesium-137. Since pulmonary absorption of C s would be a very minor source of this isotope, except i n specialized circumstances, we feel the soft-tissue C s concentrations found reflect the oral intake of this isotope and, hence, the plant contamination, both internal and external, of the range species grazed. Foliar contamination is the most significant route of entry of C s into the food chain (JO) and can be affected by many factors. It would be changed by rain removal of surface contamination, wind removal of surface contamination, death and removal of plants and parts of plants, changing rate of atmospheric fallout deposition, etc. A l l half-periods observed were similar. The average half-period for decrease of muscle and liver fallout C s i n Nevada range cattle following the cessation of atmospheric nuclear weapons testing i n 1959 was 0.7 year, and that following the cessation of atmospheric nuclear weapons testing in 1963 was 1.1 years. This difference was not significant. Thus fallout C s in the biosphere behaved the same, following the cessation of atmospheric nuclear weapons testing in 1959 and i n 1963. This would indicate that the limited releases of fission products, which have occurred during the limited test ban treaty, have not significantly influenced the biosphere concentration of cesium-137. The half-period for decrease of muscle cesium-137 concentration in N T S animals after the cessation of atmospheric nuclear weapons testing in 1959 appears anomalous (Table I and Figure 2). In addition to being considerably greater (2.3 years) than the other half-periods found, its standard deviation was larger than the value. If one considers only the initial rate of decrease of radiocesium in these muscle samples, one finds a half-period of 0.9 year which is quite consistent with other data. E x amination of these data, in comparison with data from liver (Figures 2 and 3), indicates that only muscle decreased its rate of decline in 1960. It should also be noted that the values during 1960 were only about twice the standard error of analysis. Thus, analytical error alone is not an improbable cause of these anomalous values. The three test herds were exposed to different mixtures of local and world-wide fallout. W h e n atmospheric nuclear weapons testing ceases, animals in the N T S herd reach a greater maximum in soft-tissue C s concentration faster than animals in the other two herds (5). This is because the D V and K C herds respond primarily to world-wide fallout which 137

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continues to rise for some time after testing stops, whereas the N T S herd also responds to local fallout deposited simultaneously with testing. By the time the K C and D V herds reach their maximum C s concentrations, this higher radiocesium concentration i n the N T S herd has decreased to the same level as the other two herds. After atmospheric nuclear weapons testing stops, the factors decreasing the radiocesium concentration of the animal diets would operate at the same relative rate for both worldwide and local fallout, and the continued deposition of world-wide fallout would be at the same absolute rate i n a l l three locations. One would thus expect the C s concentration to decrease somewhat faster at the N T S location because of the local nature of more of the fallout. This tendency is apparently masked by the continual deposition and removal of world-wide fallout. Our data (Table I) indicate the relative rate of decrease of C s i n the biosphere is essentially the same at a l l three locations. One would expect the initial rate of decline to be affected most by local fallout, but the dispersion of the data is too great to calculate a reliable initial rate of decrease of the N T S herd compared with the other two herds. Also, the N T S herd moves from place to place within the Nevada Test Site, thus ingesting various mixtures of fresh and worldwide fallout. This would also tend to obscure the presumably more rapid decrease i n local than world-wide fallout C s . 137


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The half-periods observed here are about the same as the half-period in humans calculated from published data (1, 8,9). Data for the radiocesium concentration in milk is subject to appreciable variation within a given year, owing primarily to the varying feed sources of dairy cattle during a year. Using the yearly data published by Campbell et al. (7) and Anderson et al. (1) we calculated a half-period of 1 to 1.7 years— again somewhat longer than i n Nevada range cattle. The slower decline of fallout C s i n cow's milk probably reflects the consumption of stored feed. W h i l e range cattle are continually exposed to biosphere cesium-137, dairy cattle consume a large portion of their radiocesium from stored feeds which represent biosphere contamination at least six months before consumption. This decreases the rate of dechne of fallout C s i n milk. Measurement of milk cesium-137 secreted by range beef cattle is not practical because of the limited lactation period and milk production of beef cattle and because of the added handling of the cattle that would be required. 137

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Acknowledgments The authors acknowledge the assistance of G . R. Farmer, D . V . M . and E . L . Fountain, D . V . M . with the early sampling periods and of the Southwestern Radiological Health Laboratory, U . S. Public Health Service with the later sampling periods.


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Literature

AND

BOHMAN

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Cited

(1) Anderson, Ε. C , Ward, G. M . , Holland, J. Ζ., Langham, W. H . , U. S. A i . Energy Comm. Repf, TID-7632,1962. (2) Blincoe, C , J. Agr. Food Chem. 9, 127 (1961). (3) Blincoe, C , Anal. Chem. 34, 715 ( 1962). (4) Blincoe, C , Bohman, V. R., Fountain, E. L . , /. Agr. Food Chem. 12, 414 (1964). (5) Ibid., 13,157 (1965). (6) Bohman, V. R., Blincoe, C , Wade, M . Α., Lesperance, A. L . , /. Agr. Food Chem. 14, 413 (1966). (7) Campbell, J. F., Murthy, G. K., Lewis, K. H . , Straub, G. P., "Radioactive Fallout, Soils, Plants, Foods, Man," E . B. Fowler, E d . , Elsevier, New York 1965 (8) Eisenbud, M.', Federation Proc. 22,1410 (1963). (9) McNeill, K. G.^Harrison, J. E., Nature 216,1340 ( 1967). (10) Russell, J . S., "Atomic Energy Waste: Its Nature, Use and Disposai," Interscience, New York, 1961. (11) Sansom, B. F., J. Agri. Sci. 66, 389 (1966). (12) Snedecor, G. W., "Statistical Methods Applied to Experiments in Agri culture and Biology," Iowa State University Press, Ames, Iowa, 1956. (13) Wasserman, R. H . , Lengemann, F. W., Thompson, J. C , Jr., Comar, C. L . , "Radioactive Fallout, Soils, Plants, Foods, Man," Ε. B. Fowler, E d . , Elsevier, New York, 1965. (14) Youden, W. J . , "Statistical Methods for Chemists," p. 17, Wiley, New York, 1951. R E C E I V E D June 4, 1968. This project was supported in part by the U . S. Atomic Energy Commission, Contracts AT(04-3) 34 and AT(04-3) 509. Nevada Agri cultural Experiment Station Projects 190 and 394. Journal Series 84.


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