Soil to plant transfer of 239 + 240Pu, 238Pu, 241Am, 137Cs and 90Sr from global fallout in flour and bran from wheat, rye, barley and oats, as obtained by field measurements

The Science of the Total Environment, 63 (1987) 111 124 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

SOIL

TO PLANT

TRANSFER

O F 239 + 240pu ' ~ S p u ' ~ l A m '

137Cs A N D

111

~Sr

FROM GLOBAL FALLOUT IN FLOUR AND BRAN FROM WHEAT, RYE, BARLEY AND OATS, AS OBTAINED BY FIELD MEASUREMENTS

K. BUNZL and W. KRACKE

Gesellschaft fiir Strahlen- und Umweltforschung Mtinchen, Institut fiir Strahlenschutz, D-8042 Neuherberg (F.R.G.) (Received July 14th, 1986; accepted August 8th, 1986)

ABSTRACT Crops of wheat, rye, barley and oats were grown on fields where the contamination of the soil with radionuclides resulted exclusively from global fallout debris. After machine harvesting and milling, the concentrations of 239÷24°pu,241Am,137Csand 9°Sr were determined separately in bran and flour, as well as in the soils. The concentrations of 239÷24°puin wheat, rye and barley flour were between 59 and 180 ttBq kg- 1, and in oat flour 1100#Bq kg 1. The range of concentrations of the other radionuclides for the four flours were: 241Am,5-70 ttBq kg i; i37Cs' 260-380mBq kg 1;90Sr' 310-1300mBq kg- 1. The corresponding concentrations of the radionuclides were always considerably higher in the bran than in the flour, with the exception of oats. The range of this factor depends on the cereal: for 2a9÷U0pu' 19--25; 241Am,10-38; 1~7Cs,4~; and for 9°Sr, 4-7. Similar differences between flour and bran were observed for stable elements, for example K, Ca and Fe. The soil-to-plant transfer factors of ~9+24°pu for wheat, rye and barley flour were between 0.00026 and 0.00078 (oats 0.017) and for bran between 0.0048 and 0.02 (oats 0.014). For 241Amthe corresponding values were somewhat lower. For the wheat, rye and barley flour the transfer factors of ~37Cswere between 0.013 and 0.018 (oats 0.069) and, for the bran, between 0.08 and 0.10 (oats 0.16). The corresponding values for 9°Sr were significantly higher: for flour between 0.08 and 0.14 (oats 1.2) and for bran between 0.62 and 0.93 (oats 1.5).

INTRODUCTION T h e p r e s e n c e o f l o n g - l i v e d r a d i o n u c l i d e s s u c h as 239+2t°pu a n d UlAm i n t h e e n v i r o n m e n t as a r e s u l t o f t h e g l o b a l f a l l o u t f r o m n u c l e a r w e a p o n t e s t i n g , o r from releases by the nuclear industry, has received considerable attention. T h e s e r a d i o n u c l i d e s c a n be t r a n s f e r r e d f r o m c o n t a m i n a t e d soil to p l a n t s a n d e n t e r food c h a i n s . A s a r e s u l t , t h e c o n t a m i n a t i o n of a g r i c u l t u r a l c r o p s b y r a d i o n u c l i d e s h a s b e e n i n v e s t i g a t e d i n n u m e r o u s g r e e n h o u s e a n d field e x p e r i m e n t s . F o r c e r e a l s , m o s t l y w h e a t a n d b a r l e y , t h e s o i l - t o - p l a n t t r a n s f e r of t r a n s u r a n i c e l e m e n t s h a s a l s o b e e n s t u d i e d ( S c h u l z et al., 1976; D a h l m a n a n d M c C l e o d , 1977; R o m n e y a n d W a l l a c e , 1977; S c h u l z , 1977; A d r i a n o et al., 1980a, b; M c L e o d et al., 1980, 1984; R o m n e y e t al., 1981, 1982; S c h u l t z a n d R u g g i e r i , 1981; C a w s e a n d B a k e r , 1985). I n g e n e r a l i n t h e s e i n v e s t i g a t i o n s t h e

112

concentration of the radionuclide in the whole grain was determined; however, usually the milled grain, either the flour or the bran of a given cereal is processed into foodstuffs. Because the concentration of many elements in flour is considerably lower than in the bran, one can expect a similar behaviour for radionuclides. This has indeed been observed for ~°Sr and 137Csin wheat (Boeck, 1965), but a systematic study of several radionuclides and cereals does not seem to be available. Therefore, the purpose of the present study was to determine the concentrations of 23SPu, 23~24°Pu, 241Am, 137Csand 9°Sr separately in the bran and flour of wheat, barley, rye and oats, as well as in the soils in which they were grown. To assure the most realistic conditions possible, the crops were grown in fields where the soil was contaminated with the above radionuclides only from global fallout. The crops were machine harvested using a combine and milled in a flour mill. To minimize surface contamination of the grains by resuspended soil particles, the grains were washed and rinsed carefully before milling. The concentrations of the radionuclides in these washings were also determined. Due to the very low concentrations of Am and Pu in the cereals, a special procedure had to be developed for wet ashing 100 kg samples before the radiochemical separation of these elements. From the data obtained, soil-to-plant transfer factors were calculated for bran and flour separately. Under natural field conditions, the observed concentration of a radionuclide in a cereal will be the result of root uptake as well as of uptake resulting from surficial contamination of vegetative plant parts by resuspended particles during the period of growth. In addition, surficially attached radionuclides on the grain surface which are not removable by usual purification processes have to be considered. Surficially attached radionuclides will contribute to the contamination of commercially available cereal products, and, therefore, transfer factors obtained under natural field conditions will yield a realistic estimate of the radiation dose resulting from the consumption of cereals. Furthermore, the concentrations of the radionuclides observed in the bran and flour samples can be considered as current, background values, and may thus help to evaluate changes in the deposition of Pu and Am in the environment around nuclear energy facilities. MATERIALS AND METHODS

Sampling Winter wheat (Triticum aestivum), rye (Secale cereale), winter barley (Hordeum vulgare) and oats (Avena sativa) were grown in four fields located nort h of the small village of Untermfihlbach in Lower Bavaria, F.R.G. For the characterization of each soil, several samples were taken from the ploughed (Ap) horizon (0-25 cm) along the diagonals of each field and the most important parameters (pH, loss on ignition, cation exchange capacity, base saturation, total C, total N, total Ca, total K and Ca in an aqueous extract and

113 TABLE 1 Soil properties Crop soil Wheat

Rye

Barley

Oats

Soil texture

Sandy loam 4.6 4.3 10.1

Sandy loam Loamy sand 5.4 5.3 13.4

Silt loam

pH Loss on ignition (%) Cation exchange capacity (meq/100 g)a Base saturation (%) Total C (% of dry soil) Total N (% of dry soil) Total Ca (g kg 1) Ca in aqueous extract (g kg-1 soil) Total K (g kg 1) K in ammonium acetate extract (mg kg ' soil)

Sandy clay Sandy loam 5.0 5.1 11.7

39.8 1.12

43.8 1.44

58.1 1.66

33.1 1.0

0.12

0.14

0.17

0.06

4.8 0.034

4.8 0.044

7.6 0.068

1.9 0.029

12 2

14 4

15 5

4.6 3.7 9,4

12 1

BaC12 method, Mehlich (1953).

K in an ammonium acetate extract) determined. These values are given in Table 1. All grain samples were collected in J u n e / J u l y 1982 by machine harvesting using a combine, with the exception of rye, which was cut and threshed separately.

Purification To remove surficially attached dust and soil, 200 kg of each cereal grain were washed carefully. Four litres of grain in a sieve were immersed successively in three buckets containing 201 of distilled water while mixing by hand. After washing a total of 50 kg of grain the water in the buckets was replaced by fresh distilled water. While the water in the first bucket turned visibly dark during these washings, the water in the third bucket remained almost clear, which indicates that further washings would have relatively little effect in removing attached soil or dust. However, it does not imply that we removed all contaminants from the grain surface. The combined washings of each cereal (2401 per 200 kg) were acidified to pH 1 with nitric acid and stored for determination of radionuclides.

114 TABLE 2 Characterization of the cereals (the s t a n d a r d deviation of the values given is ca + 5% rel.)

Sample

Grade of flour (percentage of extraction)

Ash (%)

Dry weight (%)

K (gkg -~)

Ca (gkg -1)

Fe (gkg --1)

Wheat flour W h e a t bran

80

0.90

86 88

2.1 11

0.3 0.85

0.017 0.14

Rye flour Rye bran

82

0.90

88 87

2.3 8.5

0.27 0.88

0.105 0.19

Barley flour Barley bran

83

1.3

87 87

2.8 7.0

0.35 0.92

0.020 0.55

Oat flour Oat bran

36

1.8

90 88

2.7 4.0

0.51 0.72

0.081 0.084

Milling The washed grains were placed in plastic trays, dabbed with paper towels to remove interstitial water and air dried immediately at ~ 35°C to avoid germination. Milling was carried out by a commercially operated mill, weed seeds and other light-weight impurities were removed by passing a current of air through the samples. The mill was always cleaned carefully before the next cereal was processed. The resulting flours and brans were stored in plastic trays in thin layers to avoid spontaneous heating, Characteristic parameters of the flours and brans obtained (grade of flour, ash, dry weight, K, Ca and Fe) are given in Table 2.

Decomposition of the samples Washings After addition of the tracers (~2Pu, 243Am) the combined washings of each cereal were reduced to 11 in a r ot a ry evaporator and filtered. The filtrate was evaporated to dryness, combined with the residue and ashed at 550°C (Harley, 1972) in a muffle furnace. After fuming with HF and HC104, the perchlorates were dissolved in dilute HNO3 and the solution made up to 11.

Soil Two hundred grams of air-dried soil, sieved to 2mm, was first ashed in a muffle oven at 550°C and, after addition of the tracers (242pu, 243Am), dissolved in HF and HC104 until a clear solution was obtained.

Flour, bran Because the concentrations of the transuranics in flour and bran are extremely small, 100 kg samples had to be analyzed to obtain reasonably small

115 analytical errors. For the wet disintegration of such large samples a special procedure was developed which employs a fermentation of the starch before wet ashing. For this purpose, 15 kg of flour (or bran) were mixed with 801 of water at 50°C using a 600 W stirrer and the tracers (242Pu, 243Am)and 5 ml of T60 (alpha-amylase Termamyl 60L, NOVO, Mainz) were added to dissolve the starch. After adjusting the pH to 5.8, the mixture was heated to 90°C for 30min and subsequently cooled to 5~60°C. For the saccharification of the starch, 20ml of SAN 200 L (Amyloglucosidase, NOVO, Mainz) was added, and the temperature maintained at 5~60°C for 30 min. After cooling to 30-35°C, the mixture was pumped into a 300 L fermentation tank, 100 g of yeast added, and a safety outlet attached. The fermentation process was allowed to proceed for 3-4 days at temperatures below 35°C, until no further CO2 generation was observable. In the case of bran the aqueous fraction was then separated with a suction filter, reduced by distillation in a rotary evaporator to a viscous liquid, combined again with the solid fraction (which is not present in the case of flour) and wet ashed with H202/HNO3. The remaining solution (~ 101) is somewhat opaque due to silicous residues. These were removed by centrifugation, dissolved in HF and HC104, taken up in dilute HNO3 and added to the liquid phase. It is advisable to determine the residual glucose content in the solution after the fermentation process. This value should not be much greater than 100-200 g per 15 kg of flour processed, otherwise spontaneous reactions in the subsequent wet ashing process might occur.

Determination of the radionuclides Caesium-137 was determined in all samples by direct gamma-spectrometry of aliquots of the solutions in a 1 1 Marinelli beaker, using a Ge(Li) detector and a multichannel analyzer. Strontium-90 in soil and washings was determined by low-level beta measurements of its daughter nuclide 90y, separated from 9°Sr by radiochemical techniques (Harley, 1972). In flour and bran it was determined according to Bunzl and Kracke (1981) in an aliquot of the solution. Plutonium-238, 239+24°pu and 241Am were separated from the solution by radiochemical procedures (Bunzl and Kracke, 1983, 1986) and determined by alpha spectrometry after electrodeposition on stainless steel discs. The overall chemical yield was, for the brans: Pu, 50-70%; Am, 30-40%, and for the soils: Pu, 50-80%; Am, 30-50%. The resolution of the alpha spectrum was 40-60 keV FWHM. In the case of flour (bran) the combined solution resulting from dissolving a total of 120 kg (30 kg) of material (wheat, rye, barley or oats) was used. Quality control was assured by determining 239+U°Pu and 241Am in the standard reference material " H u m a n Liver, NBS 4352" of the National Bureau of Standards. Calcium, K and Fe were determined in aliquots of the solutions by atomic absorption spectrometry with an error of 2-5% rel.

116 TABLE 3 Concentrations (mBq kg-1) of various radionuclides in the crop-soils (Ap-horizon) (the error given is one standard deviation of the counting statistics) Soil growing

239+24°Pu

Wheat Rye Barley Oats

230 240 230 66

~SPu

241Am

'37Cs

90Sr

241Am

239+240pu _+ 3 +5 +5 +4

7.4 8.1 7.8 1.5

+ 0.5 _+ 0.5 + 0.7 _+ 0.4

60 71 65 19

+3 _+ 5 _+ 3 + 1

13800 17800 19500 5500

_+ 330 + 370 _+ 480 _+ 330

4100 3700 5500 1100

_+ 74 _+ 110 _+ 110 _+ 74

0.26 0.30 0.28 0.29

+ 0.01 + 0.02 _+ 0.01 + 0.02

RESULTS AND DISCUSSION

Soils The c o n c e n t r a t i o n s of 239÷24°Pu, 23Spu, 137Cs a n d ~ S r in the c r o p soils of w h e a t , rye a n d b a r l e y (Table 3) a r e t y p i c a l for soils in G e r m a n y c o n t a m i n a t e d only by global fallout ( B u n d e s m i n i s t e r i u m des I n n e r n , 1983; B a c h h u b e r et al., 1982, 1986). F o r 241Am in soils of G e r m a n y no r e p r e s e n t a t i v e d a t a is a v a i l a b l e for c o m p a r i s o n . V e r y s i m i l a r a c t i v i t y c o n c e n t r a t i o n s of this r a d i o n u c l i d e were, h o w e v e r , m e a s u r e d r e c e n t l y in soils of G r e a t B r i t i a n (Cawse a n d B a k e r , 1985). As e v i d e n t from T a b l e 3, the c o n c e n t r a t i o n s of all r a d i o n u c l i d e s i n v e s t i g a t e d are lower by a b o u t a f a c t o r of 4-5 in the soil w h e r e the oats were grown. It was l a t e r d i s c o v e r e d t h a t a forest h a d p r e v i o u s l y o c c u p i e d the site, a n d in p r e p a r i n g the g r o u n d for a g r i c u l t u r a l crops in 1973, the t r e e s w e r e cut a n d t h e top soil removed. T h e subsoil exhibited c o m p a r a t i v e l y low c o n c e n t r a t i o n s of radionuclides from global fallout. T h e r a t i o of23Spufi 39 ÷ 24°pu in all four soils w a s ~ 3%, and t h a t of 241Amf139+24°pu was 30% (Table 3).

Flour and bran The c o n c e n t r a t i o n s o b s e r v e d for 239+24°pu, ~38Pu, 241Am, 137Cs a n d 9°Sr are s h o w n in T a b l e 4. T h e c o n c e n t r a t i o n s of 239+24°pu in the flour of w h e a t r y e a n d b a r l e y w e r e b e t w e e n 59 and 1 8 0 # B q k g 1, while o a t flour exhibits a m u c h h i g h e r v a l u e (1100 #Bq kg-1). T h e c o r r e s p o n d i n g v a l u e s for 241Am also i n c r e a s e in the s e q u e n c e wheat, rye, barley, oats, f r o m ~<5.2 to 7 0 g B q k g 1. The conc e n t r a t i o n s of 137Cs in the flour w e r e r a t h e r s i m i l a r for the four types of flour, r a n g i n g b e t w e e n 260 a n d 380 m B q k g -1. C o n c e n t r a t i o n s of 9°Sr i n c r e a s e in the s e q u e n c e rye, wheat, barley, oats, from 310 to 1300mBq k g - l . The c o n c e n t r a tions of t h e s e r a d i o n u c l i d e s in the b r a n s of wheat, rye a n d b a r l e y w e r e a l w a y s c o n s i d e r a b l y l a r g e r t h a n those of the c o r r e s p o n d i n g flour. In the case of oats, the opposite is o b s e r v e d for P u a n d Am, but the differences b e t w e e n b r a n a n d flour for all r a d i o n u c l i d e s a r e m u c h smaller. W i t h the e x c e p t i o n of oats, the r a t i o of the c o n c e n t r a t i o n s of a r a d i o n u c l i d e in b r a n to t h a t in the flour depends

59 i 6 1100 ± 50

100 + 6 1900 _+ 70

180 ± 11 4500 ± 150

1100 ± 45 930 ± 50

Rye Flour Bran

Barley Flour Bran

Oats Flour Bran

239+24°pu (#Bqkg-')

Wheat Flour Bran

Sample

18 + 4

85 ± 9

57 + 4

24 ± 2

23Spu ( ~ B q k g 1)

70 ± 10 55 ± 10

31 + 4 310 _+ 26

11 + 2 160 _+ 16

~<5.2 200 ± 16

24'Am (pBqkg-~)

380 _+ 22 890 i 64

260 +_ 22 1590 ± 55

330 + 20 1600 + 70

260 + 15 1400 ± 90

137Cs ( m B q k g 1)

1300 + 50 1700 _+ 28

780 ± 30 3500 + 59

310 + 15 2300 ± 40

510 ± 22 3800 ± 66

9°Sr (mBqkg-1)

0.063 + 0.009 0.059 + 0.01

0.17 _+ 0.02 0.069 + 0.006

0.11 ± 0.02 0.084 + 0.009

-0.088 0.18 ± 0.02

241Am 239+24Opu

C o n c e n t r a t i o n s (means and s t a n d a r d deviation) of v a r i o u s r a d i o n u c l i d e s flour and b r a n

TABLE 4

14 _+ 0.9 11 i 0.8

8.9 + 0.7 8.2 + 0.5

6.6 + 0.6 10 i 0.7

140 _+ 11 220 ± 19

93 _+ 9 230 + 14

140 + 11 190 i 12

120 ± 9 130 ± 10

(mBq g - ' )

( m B q g 1)

3.4 + 0.4 7.7 + 0.5

137Cs/K

239m4Opu/Fe

2570 _+ 160 2400 i 100

2200 + 130 3700 +_ 200

1170 + 70 2600 + 130

1560 ± 100 4500 ± 220

(mBq g - ' )

9°Sr/Ca

118 TABLE 5 Ratio of radionuclide c o n c e n t r a t i o n in b r a n to t h a t in flour (the e r r o r given is one s t a n d a r d deviation) Sample

239+240pu

24~Am

137Cs

90Sr

Wheat Rye Barley Oats

19 19 25 0.85

~<38 15 _+ 3 10 + 2 0.71 + 0.2

5.4 4.5 6.1 2.3

7.5 7.4 4.5 1.3

_+ 2 + l + 2 _+ 0.06

+ 0.9 + 1 _+ 1 _+ 0.2

_+ 0.8 _+ 0.8 + 0.9 + 0.3

strongly on the radionuclide considered (see Table 5). The enrichment factors of bran over flour are highest for 239+2t°Pu (factor of ~ 20) and 241Am (factor of 10-38), while 137Cs and 9°Sr are enriched in the brans by a factor of only 4 to 7. One reason for the exceptional behaviour of oats is the rather low extraction percentage (36%) of oat flour compared with the other flours (see Table 2). In general, oats are not used for the production of flour. If, as in the present case, there is a need to produce an oat flour with a comparably low ash content as obtained for the other flours, this can be achieved only at the cost of the flour yield. Nevertheless, the oat flour obtained in this way was considerably darker in colour than the other flours and exhibited the highest ash content (see Table 2).

Ratios of radionuclides in flours, brans, soils and deposition The ratios 239+2t°Pu/137Cs, 239+24°Pu/~Sr and 241Am/239+~°Pu are given in Table 6. The Pu/Cs ratio is much lower in the bran and flour than that found in the soil and deposition. If we assume that these radionuclides were deposited on the surface of the vegetation directly by fallout or by resuspended soil material, this observation can be explained in two ways: (i) In the surficially attached material Pu is more soluble than Cs and is removed from the surface more effectively by weathering (e.g. leaching by rain) during the vegetation period. (ii) Pu and Cs are taken up by the leaf or grain, Cs being preferred to Pu. Because fallout plutonium is known to be a rather insoluble oxide (Bondietti and Sweeton, 1977) compared with Cs, the second possibility seems to be much more likely. Therefore, direct contamination of the brans without subsequent uptake of the radionuclide was obviously not the reason for the observed activity of the samples. This seems to be true for Sr as well, because also in this case the ratio Pu/Sr in flour and brans is much lower than that of the soil or the deposition. The preferential uptake of Cs and Sr compared with Pu was always significantly higher for wheat than for barley and, with the exception of oats, lower for brans than for the flours. The ratios observed for Am/Pu in the soil and in the bran and flour indicate a small preferential uptake of Pu compared with Am; this uptake is higher for the brans than for the flour of rye and barley. For wheat, the opposite behaviour is observed, and for oats no

119 TABLE 6 Radionuclide ratios in flours, brans and soil (all ratios were multiplied by 104) Sample

239+ 240pu/137Cs

239+240pu/~Sr

Wheat flour Wheat bran

2.3 + 0.3 7.8 +_ 0.6

1.1 +_ 0.1 2.9 _+ 0.1

~<880 1800 +_ 200

Rye flour Rye bran

3.0 _+ 0.3 11 + 0.7

3.2 + 0.3 8.3 + 0.3

1100 + 200 840 _+ 90

Barley flour Barley bran

6.9 + 0.7 28 _+ 1

2.3 +_ 0.2 12 + 0.2

1700 ± 200 690 ± 60

Oat flour Oat bran

28 _+ 2 10 +_ 0.9

8.5 +_ 0.5 5.5 _+ 0.3

630 + 90 590 ± 100

SoiF

130 ± 21

555 ± 95

2829 + 170

Deposition in 1982b

330

1000

241AIII/239+ 24Opu

-

aAverage of the four soils. bH6tzl et al. (1983). difference b e t w e e n b r a n and flour is found. The u p t a k e of the r a d i o n u c l i d e s by the r o o t s could also explain the a b o v e observations, but with the d a t a available, this c o n c l u s i o n c a n n o t be drawn. The h i g h c o n c e n t r a t i o n s of the r a d i o n u c l i d e s in b r a n s also suggest t h a t the low c o n c e n t r a t i o n s f o u n d in the flour are possibly the result of b r a n residues, and t h a t the a c t u a l v a l u e s for pure flour w o u l d be even lower. If the a c t i v i t y in the flour is solely due to b r a n c o n t a m i n a t i o n , t h e n the r a t i o of the c o n c e n t r a tion of a r a d i o n u c l i d e in the flour to t h a t in the b r a n s h o u l d be c o n s t a n t for a given cereal, n a m e l y the c o n c e n t r a t i o n of b r a n in the flour. I n s p e c t i o n of Table 5 (where the r e c i p r o c a l of this r a t i o is listed), shows, however, t h a t this is n o t the case for wheat, rye and barley. Thus, for these cereals, b r a n impurities in the flour are n o t a m a j o r s o u r c e of c o n t a m i n a t i o n . F o r oats, on the o t h e r hand, these r a t i o s are r a t h e r similar (Table 5), and it c a n n o t be excluded t h a t b r a n residues c o n t r i b u t e to the observed presence of the r a d i o n u c l i d e s in the flour. This is p r o b a b l y due to the difficulties in o b t a i n i n g an o a t flour with a low ash content, as m e n t i o n e d before.

Washings I f the a m o u n t of acid-insoluble residues f o u n d in the w a s h i n g s of the four cereals are compared, the l a r g e s t q u a n t i t i e s are found in the w a s h i n g s of b a r l e y and oats, a n d c o n s i d e r a b l y less ( ~ 33%) in w h e a t a n d rye. The d e t e r m i n a t i o n of the ash c o n t e n t of these residues revealed t h a t t h e y consisted n o t only of r e s u s p e n d e d soil, but to a c o n s i d e r a b l e e x t e n t also of o r g a n i c m a t e r i a l (plant v e g e t a t i v e portions) as a r e s u l t of the c o m b i n i n g process d u r i n g h a r v e s t i n g .

120 The highest ash contents were observed for the residues in the washing of barley (60%), which indicates a high percentage of resuspended soil, and the lowest for rye (3.5%), which shows that in this case the residues consisted essentially of organic material. The ash content of the residues in the washings of wheat (22%) and oats (10%) was intermediate. This also explains the observation that the ratios of the radionuclides observed in the washings (Pu/ Cs = (30-50) × 104; Pu/Sr = (50-100) × 104; Am/Pu = (500-2000) × 104) fall between the corresponding ratios found in the soils and the bran samples (see Table 6). One can argue that washing the grains is not a realistic procedure, because grains are not cleaned in this way for the commercial production of flours or brans. However, if we assume that all the activity of a radionuclide removed by washing would otherwise increase the activity concentration of the bran correspondingly, we can calculate the concentrations of the radionuclides of unwashed brans from the volume of washings used and the measured radionuclide concentrations in the washings. The results show that the radionuclide concentrations of the brans (as given in Table 4) would be larger for 239+~4°Pu by 37 - 104%, for 241Amby 23-345%, for '37Cs by 14-61% and for 9°Sr by 2-18%. By far the largest percentage increase was always observed for barley bran, and the smallest for rye bran. In normal processing of grain, mill cleaning procedures would remove some of the impurities which we removed by washing; therefore, the values given above are upper limits.

Soil-to-plant transfer factors From the observed activity concentrations of the radionuclides in the plant parts and in the corresponding soils, the soil-to-plant transfer factors (TF) can be calculated according to: Bq per kg air dry plant material TF = Bq per kg air dry soil of the 0-25 cm layer

(1)

If one prefers to calculate the TF with respect to oven-dried plant material, this can be done with the dry weights given for the flours and brans in Table 2. On average, the resulting TF-values are about 10% higher. The TFs calculated according to Eqn (1) are given for 239+24°Pu, 241Am, 137Cs and 9°Sr in Table 7. As can be seen, the TFs for the brans are considerably higher than those of the flours, with the exception of oats, where the TFs are rather similar. It is also evident that the TFs for the flours and brans increase in the sequence: Am < Pu

< < Cs < < Sr

For a given radionuclide the differences in the TFs for the four cereals are comparatively small. Because the soils on which they were grown were not exactly identical (see Table 1), it is uncertain to which extent these differences are plant specific or the result of different soil properties.

121 TABLE 7 Soil-to-plant transfer factors for flours and brans Sample

230+240pu

24,Am

,37Cs

Wheat flour Wheat bran

0.00026 + 0.00003 0.0048 ± 0.0002

~<0.000086 0.0033 ± 0.0003

0.019 + 0.001 0.10 ± 0.007

0.12 + 0.006 0.93 + 0.02

Rye flour Rye bran

0.00042 ± 0.00003 0.0079 _+ 0.0003

0.00015 ± 0.00003 0.0023 ± 0.0003

0.018 + 0.001 0.09 ± 0.004

0.084 ± 0.005 0.62 + 0.02

Barley flour Barley bran

0.00078± 0.00005 0.020 ± 0.0008

0.00047± 0.00006 0.0048 ± 0.0005

0.013 _+ 0.001 0.082 ± 0.003

0.14 + 0.006 0.64 + 0.02

0.017 ± 0.001 0.014 + 0.001

0.0037 ± 0.0006 0.0029 ± 0.0005

0.069 ± 0.006 0.16 +_ 0.02

1.2 ± 0.09 1.5 + 0.1

Oat flour Oat bran

OOSr

The values of the TFs given in Table 7 m a y be c o m p a r e d with c o r r e s p o n d i n g values in the literature. However, as m e n t i o n e d earlier, u s u a l l y only values for the whole g r a i n are available. F o r w h e a t g r a i n g r o w n in a greenhouse, TF v a l u e s for P u r a n g e b e t w e e n 1.1 x 10 7 and 3.8 x 10 -6 and for Am b e t w e e n 5.3 x 10 ~ and 3.0 x 10 5 (Schultz et al., 1976). R o m n e y et al. (1981) observed T F values of b e t w e e n l.1 x 10 7 a n d 9 . 7 x 10 7 for P u and b e t w e e n 6.0 z 10 7 and 7.2 z 10 ~ for Am for the r o o t u p t a k e of w h e a t g r a i n c u l t i v a t e d in 200-liter c o n t a i n e r s of seven artificially c o n t a m i n a t e d soils. Similar v a l u e s were also observed for b a r l e y grain, as reviewed by A d r i a n o et al. (1980a) and Schultz (1977). F o r cereals in general, Frissel (1985) gives t r a n s f e r factors, depending on the soil type: Am, 10 6 _ 10 5; Pu, 3 x 10 .5 - 6 x 105; Cs, 1.1 z 10-2 - 1.6 x 10 2 a n d S r , 9 x 10 .2 - 1.9 x 10 1. C o n s i d e r a b l y h i g h e r T F v a l u e s for w h e a t g r a i n were f o u n d (Pu, TF = 10-3) if this cereal is g r o w n u n d e r field c o n d i t i o n s and w h e n a surficial c o n t a m i n a tion by direct aerial deposition of P u - b e a r i n g particles (e.g. from a nuclear-fuel chemical s e p a r a t i o n facility) or r e s u s p e n s i o n from the soil was present (Adriano et al., 1980b; M c L e o d et al., 1980). Recently, t r a n s f e r factors were d e t e r m i n e d for some field-grown crops g r o w n at different l o c a t i o n s in G r e a t B r i t a i n in soils c o n t a i n i n g the r a d i o n u c l i d e s only from the global fallout, as in o u r i n v e s t i g a t i o n (Cawse and Baker, 1985). F o r b a r l e y grains, T F values of Am b e t w e e n 0.0323 and 0.77 and for P u b e t w e e n 0.011 and 0.42 were reported. F o r w h e a t g r a i n the TF of Am was ~<0.03. The t r a n s f e r factors w h i c h we observed for P u and Am in the flours and b r a n s of the four cereals (Table 7), are significantly h i g h e r t h a n those r e p o r t e d for g r e e n h o u s e experiments (see above), w h e r e r o o t u p t a k e is most i m p o r t a n t . As m e n t i o n e d previously, this m a y be due to a d d i t i o n a l u p t a k e of r e s u s p e n d e d P u or Am by the v e g e t a t i v e p l a n t parts. On the o t h e r hand, the g r e e n h o u s e e x p e r i m e n t s were performed u s i n g soils with a m u c h h i g h e r P u and Am conc e n t r a t i o n ( ~ 1 - 1 0 k B q g ' ) t h a n in the present i n v e s t i g a t i o n (0.2mBqg-1). Therefore, the h i g h e r T F values w h i c h we observed m a y be the result of the

122 extremely low Pu and Am concentrations in the soil. Wildung and Garland (1974) also observed a marked increase in the TF of Pu for shoots and roots of barley when the soil Pu concentration was decreased. Compared with the transfer factors of Am in barley grains determined by Cawse and Baker (1985), (see above) our values are at least one order of magnitude lower (barley bran, 0.0048; barley flour, 0.00047). For Pu we find TF = 0.02 for barley bran, which is within the range given by these authors for barley grain. These differences may be due to different soil properties or other factors: e.g., resuspension effects or cleaning procedures of the grains. The transfer factors for Cs and Sr which we observed for wheat, rye and barley flours are similar to those given for cereals in general by Frissel (1985) (see above); whereas the TF values which we found for oat flour and bran as well as for the brans of wheat, rye and barley are higher.

Comparative distribution of radionuclides and nutrient elements in the cereals It is well known that some radionuclides exhibit chemical and metabolic behaviour which is similar to that of corresponding essential nutritional elements (Davis, 1963). For example, 9°Sr is considered to be an analogue to Ca, and 13VCsas an analogue to K. Recently, studies by Wirth (1985) have indicated that one should also include the concentrations of essential elements in transfer studies. For this purpose, the discrimination factor has been defined as the transfer coefficient soil-plant of the radionuclide divided by the transfer coefficient of the corresponding essential element. However, to obtain meaningful results there is a need to use only those concentrations of both the radionuclide and the essential element in the soil which are actually available for the root uptake. At present, a method which allows us to unambiguously determine these plant-available concentrations, especially for Pu and Am, is not available, nor are the analogues to these transuranics known. Moreover, as mentioned before, in the present case it is not clear whether root uptake is the prevailing mechanism for the uptake of the radionuclides. For these reasons, discrimination factors were not calculated. Analogues, however, also seem to be present within the plant. If one compares the concentrations of the radionuclides in the flours and brans of the four cereals with the corresponding values of the essential elements K, Ca and Fe in these samples (Table 2 and 4), one sees immediately that, with the exception of oats, the brans always exhibit much higher concentrations. A correlation analysis, employing Spearman correlation coefficients, shows a close positive correlation between the concentration of 137Cs and K (significance level 99%), 9°Sr and Ca (99%), and 239+24°Puand Fe (99%). For this reason we calculated, for all vegetation samples~ the ratios 137Cs/K, 9°Sr/Ca and 239÷~°Pu/Fe (Table 6). As expected, these ratios vary much less than the individual values. However, since brans exhibit higher concentrations of all nutrient elements and radionuclides than the corresponding flours, even combinations where analogues are unlikely, e.g. 9°Sr/K, show significant correlations in our sample material.

123 T h e r e f o r e , to d e t e c t a n a l o g u e s , i t w o u l d be m o r e a p p r o p r i a t e to t e s t t h e signific a n c e of t h e c o r r e l a t i o n b e t w e e n a r a d i o n u c l i d e a n d t h e p r o p o s e d a n a l o g u e (e.g. 9°Sr a n d Ca) s e p a r a t e l y for t h e f l o u r a n d t h e b r a n s a m p l e s . H o w e v e r , i n t h e p r e s e n t case, t h e n u m b e r of s a m p l e s ( f o u r e a c h ) is t o o s m a l l for s u c h a test. ACKNOWLEDGMENTS T h e a u t h o r s w o u l d l i k e to t h a n k M r s V. T s c h S p p a n d M r M. H a i m e r l for v a l u a b l e t e c h n i c a l a s s i s t a n c e , F. D i e t l for t h e d e t e r m i n a t i o n of t h e s t a b l e e l e m e n t s i n t h e s a m p l e s , F. P f i r r m a n n for t h e d e t e r m i n a t i o n o f t h e soil t e x t u r e s , G. P r S h l a n d S. H e l f f e r i c h for s t i m u l a t i n g d i s c u s s i o n s a n d ~ ' P e r o x i d - C h e m i e H S l l r i e g e l s k r e u t h " for s u p p l y i n g t h e h y d r o g e n p e r o x i d e . REFERENCES Adriano, D.C., A. Wallace and E.M. Romney, 1980a. Uptake of transuranic nuclides from soil by plants grown under controlled environmental conditions. In: W.C. Hanson (Ed.), Transuranic Elements in the Environment. U.S. Dept. of Energy, DOE/TIC-22800, pp. 336-360. Adriano, D.C., J.C. Corey and R.C. Dahlman, 1980b. Plutonium contents of field crops in the Southeastern United States, In: W.C. Hanson (Ed.), Transuranic Elements in the Environment. U.S. Dept. of Energy, DOE/TIC-22800, pp. 381-402. Bachhuber, H., K. Bunzl, W. Schimmack and I. Gans, 1982. The migration of 137Cs and 9°Sr in multilayered soils: Results from batch, column, and fallout investigations. Nucl. Technol., 59: 291-301. Bachhuber, H., K. Bunzl and W. Schimmack, 1986. Spatial variability of fallout-~37Csin the soil of a cultivated field. Environ. Monit. Asess., in press. Boeck, K., 1965. Die Radioaktivit/itsiiberwachung yon Lebensmittelimporten, insbesondere von Getreide. In: Uberwachung der Radioaktivitiit von Lebensmitteln. Gersbach und Sohn, Miinchen, pp. 183-191. Bondietti, E.A. and F.H. Sweeton, 1977. Transuranic speciation in the environment. In: M.G. White and P.B. Dunaway (Eds), Transuranics in the Environment. Energy Research & Development Administration, Las Vegas, pp. 449472. Bundesministerium des Innern, 1983. Umweltradioaktivit/it und Strahlenbelastung. Bundesminsiterium des Innern, Bonn. Bunzl, K. and W. Kracke, 1981. 239+24°pu, 137Cs, 9°Sr and 4°K in different types of honey. Health Phys., 41: 554-558. Bunzl, K. and W. Kracke, 1983. Fallout 3zg/24°puand 238pu in human tissues from the Federal Republic of Germany. Health Phys., 44: 441-451. Bunzl, K. and W. Kracke, 1986. Simultaneous determination of plutonium and americium, in biological and environmental samples, J. Radioanal. Nucl. Chem., in press. Cawse, P.A. and S.J. Baker, 1985. Soil to plant transfer factors for 24~Amand additional data for ~37Csand 239~240pu' determined by field measurements. International Union of Radioecologists. IV report of the workgroup on soil-to-plant transfer factors. Bilthoven, The Netherlands, pp. 28-50. Dahlmann, R.C. and K.W. McLeod, 1977. Foliar and root pathways of plutonium contamination of vegetation. In: M.G. White and P.B. Dunaway (Eds), Transuranics in the Environment. Energy Research & Development Administration, Las Vegas, pp. 305320. Davis, J.J., 1963. Cesium and its relationships to potassium in ecology. In: V. Schultz and A.W. Klement (Eds), Radioecology. Chapman & Hall, Ltd, London, pp. 539-556. Frissel, M.J., 1985. IVth Report of the working group on soil-to-plant transfer factors. Union Int. des Radioecologistes, Bilthoven.

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