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Plutonium in sheep faeces as an indicator of deposition on vegetation
J. Environ. Radioactivity 1 (1984)87-i05
Plutonium in Sheep Faeces as an Indicator of Deposition on Vegetation
J. D. Eakins, A. E. Lally, R. S. Cambray, D. Kilworth, R. T. Morrison and F. Pratley Environmental and Medical Sciences Division, AERE Harwell, Oxon. OXII 0RA, UK
ABSTRACT In order to study the deposition on vegetation ojplutonium arising from the testing of nuclear weapons and discharges from nuclear facilities, sheep faeces were collected from the South-west and North of England. The plutonium content of faeces has been shown to be a reproducible and sensitive indicator of deposition on herbage and, with this technique, plutonium discharged from the Sellafield Works of British Nuclear Fuels Limited can be detected 60 km from the site. On moorland pastures the concentration oJ" 239 + 24Opu in faeces, expressed as activity per gram of ash, is roughly equal numerically to the deposit of plutonium on 1 m' oJ" vegetation. Some of the plutonium ingested by a grazing sheep is absorbed and the highest concentrations are found in liver. The liver of a sheep grazing on pasture where the 239+24°pu concentration in faeces was about 2 pCi (74 mBq) g- 1 of ash, was found to contain < 1% of ~he Generalised Derived Limit Jbr mutton and offal, and the meat <0.001%.
INTRODUCTION The analysis of herbage is sometimes used to detect and identify radionuclides deposited from the atmosphere {Garner, 1963; Johnson et al., 1966; Jackson et al., 1981). However, when the rate of deposition is very slow a large area of vegetation needs to be sampled in order to o b t a i n sufficient activity for analysis. Such samples a r e n o t always easy 87 ~. UKAEA, 1984.
88
J.D. Eakins
et al.
to obtain, especially, in moorland areas, and, for the detection of plutonium, faeces from grazing animals were considered as an alternative. Plutonium is very poorly absorbed by the mammalian gut (Harrison, 1983), so that virtually all that is ingested by an animal will appear in its faeces. Furthermore, if the animal obtains its food entirely by grazing, then the ratio 238pu:239"z4"°Pu in plutonium separated from faeces should be the same as that in the vegetation on which it feeds. Animals which can be used for this purpose are cows, sheep and rabbits. Cambray & Eakins (1980) reported levels of 239 + 2.,0pu in faeces collected from all these species in West Cumbria in 1977. Values ranged from 0-08 to 7.4 pCi (3-270 mBq) g - t of ash. Rabbits are probably the best species for sampling herbage as they graze all the year round, but the collection of sufficient faeces for plutonium analysis is time-consuming. Cows are unsatisfactory as their diet is often supplemented by stored fodder and imported cereals; in addition, they do not graze on the moorland characteristic of large areas of Northern England. Sheep are fairly ubiquitous, however, capable of living off very poor pasture, and their diet is normally supplemented only in the winter and early spring. Quantities of sheep faeces, adequate for the accurate determination of plutonium, can be collected quite quickly. This paper describes measurements of plutonium in sheep faeces collected in the vicinity of the British Nuclear Fuels Limited (BNFL) reprocessing plant at Sellafield in Northern England. The results are compared with values obtained in areas remote from nuclear installations in North Devon and Yorkshire. The results are also considered in relation to the plutonium concentration in soil, herbage, rainwater and in sheep meat and offal.
METHODS
Collection and analysis of soil Soil samples were taken with a 3.8 cm diameter corer to a depth of 15 cm. The samples were dried and ground in a Pascall disc grinder, including any stones. Plutonium was determined in 70 g aliquots of the prepared soil by an acid leaching technique followed by anion exchange (Lally & Eakins, 1978) using 242pu as an internal yield tracer. The electrodeposited sources were counted by alpha spectrometry and the amounts of 238Pu and 239.24Opu determined.
Plutonium in sheep faeces as an indicator o/" deposition on regetation
89
Collection and analysis of herbage Most of the sites at which herbage samples were taken were on open moorland. All vegetation (mainly grass) was clipped by shears from an area of 5 m 2 to within 1-2 cm of the soil. The vegetation was ashed and analysed for plutonium by the same procedure used for soil.
Collection and analysis of sheep faeces Wherever possible, fresh samples of sheep faeces were collected on open moorland where the vegetation was closely cropped. The purpose of this was to ensure, as far as possible, that sheep were grazing pasture with a similar fodder weight per unit area. Samples (generally 1-2 kg) were collected in polythene bags. After ashing at 450-500 °C, 50 g aliquots of ash were taken for plutonium analysis using the procedure used for soil.
Analysis of sheep tissues Samples of tissue were ashed at 450-500°C and the ash analysed by radiochemical techniques similar to those used for the analysis of soil.
Collection and analysis of rainwater Rainwater was collected in a 25 litre polythene bottle containing hydrochloric acid and 2*2pu tracer, using a funnel of diameter 40 cm. The water was concentrated by evaporation and plutonium determined by anion exchange, electrodeposition and alpha spectrometry.
P L U T O N I U M IN SHEEP FAECES
Concentrations in samples from West Cumbria and Northern England BNFL Sellafield have discharged about 18000 Ci (670 TBq) of alphaemitting plutonium (Cambray, 1982) from their pipeline into the Irish Sea since 1957. It has been shown (Eakins et al., 1982) that a very small fraction of this activity has been transported in seaspray back to the land, and maritime-derived plutonium from this source has been detected up to 10 km inland by the analysis of soil samples (Cambray & Eakins, 1982).
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Plutonium in sheep faeces as an indicator of deposition on te~eration
Samples of sheep faeces have been collected on four occasions along a transect running inland due east from St. Bees in West Cumbria towards Whitby. The position of this transect is shown in Fig. 1, together with other sampling sites in Northern England. In Fig. 2 the results of analysis of these samples for 239+24°pu, expressed as femtocuries per gram of faecal ash, are plotted against distance from St. Bees. On two occasions, in October 1977 and June 1978, samples were collected from St. Bees to Whitby (i.e. from coast to coast). However, in July 1979 and December 1981, samples were only collected to a distance of about 70 km east of St. Bees. The concentration of 2 3 9 + 2 4 ° p u in faeces at St. Bees was on average about a factor of 20 above the asymptotic or baseline value of 50 fCi (1.9 mBq) g - t although the baseline does not appear to have been reached in the 1981 transect. In the 1977 and 1978 transects, it was evident that the concentration of 239+24°pu at Shap, 60 km from St. Bees, was about double the baseline concentration, and this was probably the case on the 1981 transect. The results in Fig. 2 have not been modified to take account of
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239+24"°Puin sheep faeces collected between St. Bees and Whitby.
92
J.D. Eakins et al.
variations in rainfall across the country. The deposition of radionuclides introduced into the atmosphere at high altitudes as a result of the testing of nuclear weapons is dependent, among other factors, on rainfall (Peirson & Salmon, 1959). However, plutonium introduced into the atmosphere in seaspray at low altitude will have a low washout factor (Peirson et aI., 1973) and rainfall is unlikely to play a significant role in its deposition. Furthermore, if plutonium in sheep faeces represents deposition over a relatively short period of time, the application of long-term average rainfall values would not be relevant. A good indicator of the source of deposited plutonium is the ratio 238pu:239 + 24°pu. In the cumulative deposit of weapon fallout in soil, this ratio is ~0.04 (Hardy et al., 1973). The ratio was not measured in sea discharges from Sellafield until 1978, when its value was 0"27 (Atherton, 1979), increasing to 0-32 in 1979 (Atherton, 1980), and 0.34 in 1980 (Atherton, 1981). However, from the analysis of surface sediments, Hetherington (1978) has estimated that the ratio ranged from 0-05 in 1966 to 024 in 1975. In Fig. 3, the 238pu:239 +24°pu ratios obtained from samples of sheep faeces are plotted against distance from St. Bees; the broken lines represent the value of the cumulative ratio from weapon fallout. In the 1977, t979 and 1981 transects, all the ratios were above that characteristic of fallout by about a factor of six at St. Bees falling to a 030--
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Plutonium in sheep faeces as an indicator of deposition on vegetation
93
factor of two at Shap. However, in 1978, although the ratio at St. Bees was similar to that on the other occasions, it d r o p p e d rapidly to a value similar to that in accumulated weapon fallout by 30 kin. In order to determine how far north and south of Sellafield plutonium derived from B N F L could be detected, a further series of faeces samples was collected in July 1979. These were taken from sites between 10 and 24 km from the sea, on a line roughly parallel to the coast, proceeding in an approximately southerly direction from a site about 16 km south-west of Carlisle to one 20 km east of the River Lune estuary between Lancaster and Fleetwood. The results are presented in Table 1, together with distances from the sea and from the Sellafield Works. The position of each site and the 238pu:239+2~°pu ratio in each sample are included in Fig. 1. Although the concentration of 239 + 2,,0pu at a few of these sites was at, or near to, the baseline values observed on the St. Bees-Whitby transects (see Fig. 2) the 238pu:239+2"*°Pu ratio was at least a factor of two above the fallout value at all sites. It is evident from these results that plutonium from Sellafield can be detected in sheep faeces at considerable distances in both a northerly and southerly as well as an easterly direction, and that the technique is more sensitive than the soil sampling method (Peirson et al., 1982). TABLE 1 Plutonium in Sheep Faeces from Northern England, North-South Survey, July 1979
Site identifier
l 2 3 4 5 6 7 8 9 l0 11
Grid rej'erence
NY NY NY NY NY NY NY SD SD SD SD
318 203 092 159 108 150 203 268 394 572 627
413 370 234 209 157 052 008 934 880 822 528
Distance (kin) From sea
From Sellafield
24 13 11 19 14 12 15 19 10 14 20
48 38 21 22 15 13 19 27 38 59 79
2~Spu (fCi g- i of ash)
4+_ 1 6_+2 10+2 12_+2 23+_3 70_+5 27_+2 11_+2 7-+ l 5+_ 1 10_+2
239- ,_.~Opu
(fCi g- l of ash)
42+2 55+5 87_+7 106+_6 147+13 447-+22 190_+9 90+7 69_+6 49+_5 76_+6
The uncertainties quoted are 2a and refer only to counting statistics.
23spu / 2~9 - 2.tOpu
0-10 0-11 0-1! 0-11 0"16 0-16 0-14 0-12 0-10 0-i0 0"13
94
J.D. Eakins
et al.
Concentrations in samples following a minor release of plutonium by direct emission from Sellafield Works There was a minor release of radioactivity from the Sellafield Works on 11 September 1979 (BNFL, 1979) when the wind was southwesterly. A sample of sheep faeces collected on 25 September 1979 near Wastwater, 13 km ENE of the works and 12 km from the sea contained 1400 fCi (52 mBq) of 239+2"t°pu g-1 of ash. A further sample, taken 6 km to the north-east of the works on Cold Fell, contained 2200 fCi .(80 mBq) g- L. The Wastwater sample was from site 6 on the north-south series of samples collected in July 1979 (Table 1), when the concentration of 239+2a'°Pu in faecal ash was 447 fCi (17 mBq) g - i . A sample collected at the same site in July 1978 contained 280 fCi (10 mBq) g-~ and in May 1977 the concentration was 200 fCi (7 mBq) g-1 (Cambray & Eakins, 1980). From the July 1979 W - E transect shown in Fig. 2, which runs a few kilometres north of Wastwater, a concentration of - 3 0 0 fCi (11 mBq) g - I of ash would be expected. Thus there is ample evidence that the concentration of 239+2a°pu in sheep faeces ash at Wastwater is normally in the range 200-500 fCi (7-19 mBq) g - l . Similarly from the July 1979 data (Fig.-2) a 239+2"*°Pu concentration of about 500fCi (19 mBq) g-~ at Cold Fell would be expected, instead of the observed value of 2200 fCi (80 mBq) g-1. These measurements demonstrate the sensitivity of the technique for detecting minor releases to the atmosphere of radioactivity containing plutonium. Concentrations in samples from areas remote from nuclear installations In order to compare concentrations of plutonium in sheep faeces collected in the North and North-west of England with those in areas remote from nuclear installations, five samples were taken in July 1979 along a transect running inland for 18 km from the coast of Somerset. This area is 360 km to the south of Sellafield. The results are shown in Table 2, from which it is evident that the plutonium concentrations were independent of distance from the sea. The mean value was 31 fCi (1"1 mBq) g - t ash. The mean 238pu:239+2a°pu ratio was 004 which corresponds to that in fallout from nuclear weapons testing. To establish the reproducibility of sampling, concentrations of 239+2"t'°Pu were measured in seven samples of sheep faeces, collected from a triangular area of about 30 k m 2 in the vicinity of Tan Hill (NGR
Plutonium in sheep faeces as an indicator of deposition on vegetation
95
TABLE 2 Plutonium in Sheep Faeces from North Somerset, July 1979
Site
Porlock Lucott M o o r Chetsford Water Withypool Ansty Gate
GriLl reference
SS SS SS SS SS
866 846 849 847 835
461 445 420 339 298
Distance from sea /kin; 16 4.3 6"0 13'9 18"2
The uncertainties quoted are 2~ and refer
only to
23'~pu (Jgi g - ~ ojash) I+ 1± 1± 2± 1±
I 1 l l 1
23'~+ 2a'°Pu fjCi g - 1 of ash)
z3Spu/ ,_39 - "--~opu
24+5 32±3 30±4 40+4 29±3
0.04 0.03 0'03 0"05 0"03
counting statistics.
NY 896 067), North Yorkshire in July 1979. This region is about 90 km east of the Sellafield Works of BNFL and sufficiently remote not to be affected by it significantly (see Fig. 2). The concentration of 239 + 2-~0pu in the samples varied between 32 and 67 fCi (1-2-2'5 mBq) g- t of ash with a mean of 45 fCi (1"7 mBq) g - t. The coefficient of variation was 25%. The ratio of 2 3 8 p u : 2 3 9 + 2 a ° P u w a s 005 at all sites, characteristic of that in fallout. The results indicate that the concentration of plutonium in samples of sheep faeces collected over a wide area is reproducible, within a factor of about two, providing the grazing is uniform. R E L A T I O N S H I P BETWEEN C O N C E N T R A T I O N OF P L U T O N I U M IN S H E E P FAECES AND IN VEGETATION The root uptake of plutonium by plants from soil is very poor (Cawse & Turner, 1982) and most of the activity associated with the aerial parts of plants represents direct foliar deposition. The plutonium content of herbage will depend upon many factors. During the winter months, when there is little growth, plutonium will accumulate on vegetation but there will be a sudden fall in concentration coinciding with the flush of new growth in the spring. It is also known that sheep ingest significant quantities of soil when grazing and in winter, when the pasture is poor, soil can constitute 10-14% of the dry matter intake (Field & Purves, 1964; Arnold et al., 1966; Healy, 1967). In the summer months, under more favourable grazing conditions, the concentration of plutonium in sheep faeces will approximate more closely to that in herbage as the ingestion of soil will be minimised.
96
J.D. Eakins et al.
To compare the relationship between plutonium in ashed sheep faeces and in vegetation, samples of each were collected from seven sites in England during 1978. Measurements were also made of plutonium in soil samples taken at the same sites to a depth of 15 cm. The results of these measurements are given in Table 3. There was a good correlation (r=0-8 I: p <0"05) between concentrations of plutonium in sheep faeces and in vegetation but the correlation with levels of plutonium in soil was not significant (r =0"48; p<0"10). In a study of the vertical distribution in soil of 239 + 2"~°Pu at Tan Hill (Eakins el al., 1980 and unpublished work) it was shown that the concentration in the top 2 cm of soil was three times the average from 0-15 cm. Thus the 239~-2~°pu concentrations in soil in Table 3 almost certainly underestimate the concentrations in surface soil ingested by sheep. The concentration of 239 +-,,,Opu in ashed sheep faeces was higher than in ashed vegetation, the mean value for this ratio being 2.2 _+0"8 (SD). As some of the mineral content of vegetation will be absorbed during passage through the gastrointestinal tract of the sheep, a value for this ratio exceeding unity would be expected but, to account for a factor of at least two, it seems likely that there must be some ingestion of surface soil Scandium, which is a ubiquitous element in soil, can be used to estimate the soil contamination of vegetation as it is not translocated by TABLE 3
239+24°pu and Scandium in Sheep Faeces, Vegetation and Soil Grid reference
NZ 098 035 NY401 124 NY401 109 NY235 136 NY 972 105 SD 590 670 SS 788 298
Vegetation collected (5 m 2) Dry wt (g)
Ash wt (g)
68.5 93-2 139-7 174.2 134.3 336.1 92-0
6.30 6.91 11-2 14.1 14.0 16.2 10.0
z39+2~Opu (JCi g- t of ash) Sheep faeces
63+6 134+_9 149_+7 497+_28 734_+47 133_+8 21+2
Vegeration
Scandium concentration (rag kg-1 of ash) Soil
31+_9 130+11 61_+16 46+_6 116+_12 42_+5 2104- 19 482+_16 166+_18 109_+4 120_+14 100_+4 9+_3 33_+4
Sheep faeces
v 2 5 3 2 3 --
The uncertainties quoted are 2a and refer only to counting statistics.
Vegeration
Soil
<2 3 5 ~ <2 2 --
5 4 19 17 14 3 --
Plutonium in sheep faeces as an indicator of deposition on vegetation
97
plants to a significant extent. According to Bowen (1979) levels of scandium in dry soil range from 0"5 to 55 mg kg- 1 with a median value of about 7 mg kg -~. According to the same author, the maximum concentration of scandium in dry vegetation is 0-2 mg kg- 1. To obtain some idea of the extent of soil contamination of vegetation (and consequently of sheep faeces) samples from the first six sites listed in Table 3 were analysed for scandium by optical emission spectroscopy and the results are included in that table. The mean ash weight/dry weight ratio of the vegetation samples was 008, hence scandium concentrations in the ash should not exceed 2 mg kg-~. This value coincided with the minimum detectable level of scandium for the analytical method used. However, it can be seen from Table 3 that concentrations of scandium in samples of vegetation ash were of the order of 2 mg kg-1. The levels in ashed faeces were invariably quite similar to those in vegetation but, in some cases, much lower than in soil. This suggests that most of the material in sheep faeces is derived from vegetation with only a minor contribution from ingested soil. As pointed out above, most of the plutonium associated with vegetation represents material deposited directly on the aerial parts of plants. It is to be expected, therefore, that the concentration of plutonium in herbage, and also in sheep faeces, will depend primarily upon its recent deposition rather than on the cumulative deposit. To examine the relationship between concentrations of plutonium in sheep faeces and in rain, monthly samples of rainwater were collected from a hill farm on Exmoor in North Devon during the period April-November 1979. Samples of sheep faeces were collected once each month from the same farm. The results of these measurements are given in Table 4. The concentrations of 239+24"°Pu in rainwater during this period were very low and that of 23Spu was below the limit of detection of 20 fCi (0.7 mBq) per sample. However, 238pu was detectable in sheep faeces and the 238pH:239+24°pu ratios approximated to the value expected from nuclear weapon fallout. As shown in Fig. 4, there was a good correlation (r = 0"88; p < 0-01) between the concentration of 239 + 24Opu in ashed sheep faeces and in the corresponding rainwater samples, expressed as fCi litre-1. However, when the concentration in faeces was compared with the total activity in monthly rainwater samples the correlation was not significant (r = 0-49; p > 0-20). In Fig. 4 the positive intercept on the ),-axis probably represents plutonium in faeces derived from the ingestion of soil and also from dry
98
J.D.
E a k i n s et al.
TABLE 4 C o m p a r i s o n o f P l u t o n i u m in R a i n w a t e r with P l u t o n i u m in S h e e p Faeces from N o r t h Devon 3,[onth <1979)
Rainjhll
Sheep .laeces
Rainwater
( t h i n ,)
,_3Spu
,_3, - 2aOpu
"->*'--~Opu
(jCi,ff -~ o f ash)
2 3 0 - -'aO p U
qCi
dCi
qUi
23Spu
"-3'~-'-'-Opu (fCi g - l ); :3~ - ,.~Opu in deposit /fCi m-Z2
month - I ) litre -L) m - z )
April May June July August September October November
89 147 69 9l 198 81 127 206
130+30 124-3 1070 180___60 10+_3 1470 70+_20 8+_2 550 1004-40 9+_4 820 130+_40 5+_2 990 60+_30 6+_3 490 50+_20 3+_I 380 50+_20 2+_ 1 410
z 3~ - ,_.~op u m sheep jaeces
<0-2 <0.1 <0-3 <0.2 <0.2 <03 <0-4 <0.4
30_+3 20+_2 24+_3 214-2 17+_3 14+_2 13___2 14+_2
0-04 0-05 0-03 0-04 0-06 0-06 0-04 0.05
Mean
0.029 0.014 0-043 0-026 0-016 0.029 0.033 0.035 0-028 _+ 0.010 (SD)
T h e uncertainties q u o t e d Ere 2a a n d refer only to c o u n t i n g statistics. 3/-
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C o r r e l a t i o n b e t w e e n 239÷2*°Pu in rain a n d s h e e p faeces.
Plutonium in sheep faeces as an indicator of deposition on vegetation
99
deposition on vegetation. The sum of these two factors at this site ranged from 30 to 60%. The dry deposition component in nuclear weapon fallout in the United Kingdom has been shown by Cambray et al. (1970) to be about 20%, suggesting that the contribution to plutonium in faeces from soil at this North Devon site was between I0 and 40%. The annual deposit of plutonium from nuclear weapon fallout in 1979 was < 1 % of the maximum annual deposit which occurred in 1963 (Cambray et al.. 1980) and the concentration in soil would not have altered significantly throughout the year. It should be reiterated that the correlation between rainwater and sheep faeces is only likely to apply in areas such as North Devon which are exposed only to plutonium deposition from the testing of nuclear weapons. In West Cumbria where the major source is at a low altitude, it will not necessarily apply.
RELATIONSHIP BETWEEN C O N C E N T R A T I O N OF P L U T O N I U M IN SHEEP FAECES AND IN SHEEP MEAT AND OFFAL To relate the concentration of plutonium in sheep faeces to that in sheep meat and offal, tissues from a ewe which had grazed on pasture near the sea at Drigg in West Cumbria were analysed for plutonium. The sheep, which was about 7 years old, had grazed for 3 years (from 1978-81) on pasture which runs down to sand dunes adjacent to the sea and is exposed to plutonium-bearing seaspray. It did not have access to the nearby salt marshes. The area grazed is about NGR NY 050 985, which is approximately 5 km SSE of the Sellafield Works. A sample of sheep faeces collected from this pasture on 9 December 1981, during the week the ewe was killed, contained 2800 fCi (104 mBq) 239+24Opu g-1 of ash. The 238pu:239+24"°Pu ratio was 0-29. A sample collected at the same site on 17 June 1981, contained 2300 fCi (85 mBq) g-1 of ash. From Fig. 2, it appears that the concentration of 239,2.,Opu in sheep faeces collected near the Cumbrian coast has been fairly' constant, within a factor of two, over the past few years and it is reasonable to assume that the concentration at Drigg over the period 1978-81 has been about 2000 fCi (74 m B q ) g - i The results of analysis of the sheep tissues for plutonium are presented in Table 5 expressed as picocuries per kilogram wet weight in the case of
100
J. D. Eakins et al. TABLE 5
Plutonium in Tissues of a Sheep from Drigg. West Cumbria Tissue
Lung Liver Muscle Bone (vertebra) Bone (rib) Kidney Heart
_,3s Pu
-"39- _'~oPu
(pCi kg- 1)
gpCi kg- t)
0-27 _+005 9.98 4-0.85 0-005 4-0005 0"95 4-0"13 066__.0-I 1 0.104-0.10 0.22 4-0"1l
0.99 4-0-08 37-3_- 2-3 0-024 + 0-009 3-74 ± 0"26 2'16+_0"19 0-244-0'13 0"364-0.12
The uncertainties quoted are 2e7 and refer only to counting statistics. the soft tissues. Bone was dried at 105 °C before analysis and the results are expressed as pCi kg-~ dry weight. The greatest concentration of plutonium was found in liver followed by bone. The 23Spu:239+2'*°Pu ratios in the tissues were generally similar to that in sheep faeces, confirming that the material was derived mainly from the Sellafield Works and not from-nuclear weapon fallout. It appears from this study that a concentration of 239+24"°Pu in sheep faeces of 1 pCi (37 m B q ) g corresponds to a concentration in sheep meat of about 0.01 pCi (0'4 mBq) k g - ~ and in liver of a b o u t 20 pCi (740 mBq) k g - ~. Levels of 238 + 239 +240pu in the livers of sheep which had grazed in other areas of northern England have been reported by Popplewell et al. (1981). They found that the liver of a sheep which had grazed near Penrith, 55 km to the north-east of Sellafield, contained 0-23 pCi k g - ~. On the basis of the above relationship, this corresponds to a concentration of 80 fCi g - x of ashed sheep faeces, which is almost certainly correct to within a factor of two. Generalised Derived Limits (GDLs) for plutonium in a variety of foodstuffs and environmental materials have been published by the National Radiological Protection Board (Simmonds et al., 1982). The G D L for 239+24°pu in mutton and offal is 5-4 x 103 pCi (200 Bq) k g The concentrations of 239+2'~°Pu found in sheep liver and meat in the present study correspond to only 0-7% and < 0.001% respectively of the G D L . It is reasonable to assume, therefore, that if sheep graze in an area in which the 239+2'*°Pu content of herbage is such that the resulting concentrations in faeces are relatively constant at about 2000 fCi k g - t ,
Plutonium in sheep faeces as an indicator of deposition on regetation
101
then liver from these sheep will contain less than 1% of the GDL. It must be emphasised that these concentrations of 239-- 24.0pu are only encountered where sheep graze close to the Cumbrian coast and they will decrease quite rapidly with distance inland.
DISCUSSION A comparison of the concentration of plutonium in ashed sheep faeces and in ashed vegetation from the corresponding sites showed that the concentration in faeces was approximately twice that in vegetation. This difference is attributed to (a) the absorption of nutrients from herbage in the gastrointestinal tract of the sheep, (b) the ingestion of surface soil by the sheep, and (c) possibly also to the selective grazing habits of the sheep. The result indicated that, at least in summer, plutonium in sheep faeces is derived predominantly from vegetation. Using the data in Table 3, it can be shown that the concentration of 239+24°pu in femtocuries per gram ashed sheep faeces is approximately equal, numerically, to the number of femtocuries associated with 1 m 2 of vegetation. The mean vaiue for the faeces/herbage ratio at the seven sites listed in Table 3 was 1"1 ___0-5 (SD). However, it should be stressed that this relationship probably only applies to open moorland, and on more productive pastures the concentration in sheep faeces could well be less than that associated with vegetation from 1 m 2. A sheep ingests about 1-1"5 kg dry weight of fodder per day (Gunn, 1981). In these studies the weight of faecal ash taken for determination of plutonium was equivalent to about 600 g dry weight of faeces, or about half the daily dry weight intake. The mean dry weight of vegetation collected from the moorland sites listed in Table 3 was 30 g m-2 and in such areas it will be necessary for sheep to graze the equivalent of 40 m e per day. Thus the quantity of sheep faeces taken for analysis represents about 20m 2 of vegetation which demonstrates the sensitivity of the technique and also the excellent averaging effect of the sheep as a sampler of vegetation. As the plutonium content of herbage is due almost entirely to direct foliar deposition, its concentration in sheep faeces will reflect current deposition rather than the cumulative deposit. The data from North Devon (see Table 4) showed a good correlation, on a monthly basis, between the concentration of 239 + 2 4 0 p u in sheep faeces and in rainwater,
102
J.D. Eakins
et al.
which suggests that plutonium in sheep faeces reflects only recent deposition on herbage. As shown in Table 4, the mean value for the ratio of 239 + 2.,Opu in sheep faeces to the monthly deposition of e39- e.*Opu in rainwater was 0-028--+0-010 (SD). If it is assumed that the activity per gram of ashed sheep faeces is numerically equal to the activity per square metre of vegetation and that the activity in vegetation is accumulated on average over a period of about a month, then the results in the final column of Table 4 suggest that about ,~ ~o'/o of the plutonium deposited in rain is retained by the leaf surfaces. The Normalised Specific Activity (NSA) is a quantity which relates the concentration of a contaminant per unit mass of vegetation to its daily rate of ground deposition. NSA =
Activity per kilogram dr)' matter on vegetation Activity deposited per day per square metre of ground
If it is assumed that the ash weight of faeces is approximately equal to the ash weight of the vegetation ingested it is possible to calculate NSA values from the North Devon data. In order to do this a conversion factor of 12-6 (derived from Table 3) for the ratio of dry weight to ash weight has been assumed. The mean value obtained is 68 m 2 day kg- i which is similar to that of 73 m 2 day k g - I derived by Chamberlain (1970) for 9°Sr on herbage. The results of measurements of _,3o+ ,_.topu in sheep faeces collected on transects originating on the Cumbrian coast and extending across the North of England (see Fig. 2) showed that the concentrations fell off rapidly with distance from the coast. However, the penetration inland varied with time certainly on an annual basis, and probably on a much shorter timescale, reflecting changing weather conditions and, in particular, wind speed and direction. The variable "baseline' values observed in North Devon (19 fCi g - i ) , Somerset (31 fCi g-l), Tan Hill (45 fCi g-') and the asymptotic values in the transects in Northern England ( ~ 5 0 f C i g - t ) are accounted for by the same factors. The variable concentrations of plutonium in sheep faeces at the same sampling site on different occasions provide further evidence that such measurements are influenced more by recent deposition on vegetation rather than on the cumulative deposit in soil. A comparison of Figs 2 and 3 shows that the ratio 238pu:239 + 2'a°Pu is the more sensitive indicator of plutonium in faeces in excess of that expected from nuclear weapon fallout. It is important therefore that this
Plutonium in sheep faeces as an indicator of deposition on cegetation
103
ratio should be determined with adequate accuracy and precision. The main difficulty arises in the determination of :3Spu at low levels, when interference from 22STh, a natural radioelement with a similar :e energy to 238pu, can be a problem. These difficulties can be minimised by taking a large sample of faecal ash for analysis and repeating the thorium removal steps in the analytical procedure. CONCLUSIONS It has been shown that plutonium in sheep faeces is a sensitive and reproducible indicator of concentrations of plutonium recently deposited on vegetation. The 23Spu:239+24°pu ratio in sheep faeces also reflects that in recently deposited plutonium. The analysis of sheep faeces collected in Northern England has enabled the distribution of plutonium discharged from the Sellafield Works of BNFL to be delineated. Such material can sometimes be detected as far as Shap Fell, 60 km to the east of Sellafield and at similar distances to the north and south. Most of this material represents plutonium originally dis_charged to the Irish Sea but transported back to land in seaspray, although there will be a contribution from direct discharges into the atmosphere. It was also demonstrated that the analysis of sheep faeces can detect small accidental releases of plutonium to the atmosphere, provided that information is available on pre-existing concentrations. It is shown that, at least in summer, plutonium in sheep faeces is derived mainly from vegetation. In winter the contribution from plutonium ingested in surface soil will increase. On moorland sheep runs, the concentration of 239 + 2~.0pu in faeces, expressed as femtocuries per gram of ash, approximates numerically to the amount associated with vegetation sampled from 1 m 2. It has also been established that where sheep graze on pastures such that the concentration of 239 + 2 a 0 p u in their faeces is 2 pCi (74 m B q ) g - 1 ash then its concentration in sheep liver and in sheep meat will be < 1% and <0-001% respectively of the Generalised Derived Limit. ACKNOWLEDGEMENTS The authors would like to thank Dr A. Morgan, Dr D. H. Peirson and Dr A. C. Chamberlain of Environmental and Medical Sciences Division,
104
J. D. Eakins et al.
AERE, for their general advice and encouragement. They would also like to thank Dr R. S. Atherton and Mr H. Longley of British Nuclear Fuels Limited for their helpful comments during the preparation of this paper.
REFERENCES Arnold, G. W., McManus, W. R. & Bush, I. G. (1966). Studies in the wool production of grazing sheep. 5. Observations on teeth wear and carry over effects. Australian Journal of Experimental Agriculture and Animal Husbandry, 6, 10l. Atherton, R. S. (1979). Annual report on radioactive discharges and monitoring of the environment, 1978, British Nuclear Fuels Limited, Health and Safety Directorate. Atherton, R. S. (1980). Annual report on radioactive discharges and monitoring of the environment, 1979, British Nuclear Fuels Limited, Health and Safety Directorate. Atherton, R. S. (1981). Annual report on radioactive discharges and monitoring of the environment, 1980, British Nuclear Fuels Limited, Health and Safety Directorate. BNFL (1979). Press Releases, 11 September 1979 and 5 October 1979. Bowen, H. J. M. (1979). Environmental chemistry of the elements, New York, Academic Press. Cambray, R. S. (1982). Annual discharges of certain long-lived radionuclides to the sea and to the atmosphere from the Sellc~eId Works, Cambria, 1957-1981, AERE-M 3269. London, HMSO. Cambray, R. S. & Eakins, J. D. (1980). Studies ojem, ironmental radioactivity in Cumbria. Part 1. Concentrations of plutonium and caesium-137 in environmental samples from West Cumbria and a possible maritime effect, AERE-R 9807. London, HMSO. Cambray, R. S. & Eakins, J. D. (1982), Pu, 2"~iAm and 13VCs in soil in West Cumbria and a maritime effect, Nature, 300, 46. Cambray, R. S., Fisher, Miss E. M. R., Brooks, W. L. & Peirson, D. H. (1970). Radioactive fallout in air and rain: results to the middle of 1970, AERE-R 6556. London, HMSO. Cambray, R. S., Fisher, Miss E. M. R., Playford, K., Eakins, J. D. & Peirson, D. H. (1980). Radioactive fallout in air and rain: results to the end of 1979, AERE-R 9672. London, HMSO. Cawse, P. A. & Turner, G. S. (1982). The uptake of radionuclides by plants: a review of recent literature, AERE-R 9887. London, HMSO. Chamberlain, A. C. (1970). Interception and retention of radioactive aerosols by vegetation, Atmospheric Environment, 4, 57. Eakins, J. D., Lally, A. E. & Kilworth, D. (1980). In Environmental & Medical Sciences Divisional Progress Report, January-December 1979, AERE-PR EMS 7. London, HMSO.
Plutonium in sheep faeces as an indicator of deposition on ~'egetation
105
Eakins. J. D., Lally, A. E., Burton, P. J., Kilworth, D. R. & Pratley, F. A. (1982). Studies of environmental radioactivity m Cumbria. Part 5. The mag'nitude and mechanism of enrichment of sea spray with actinides in West Cumbria, AERE-R 10127. London, HMSO. Field, A. C. & Purves, D. (1964). The intake of soil by' grazing sheep, Proceedings of Nutrition Society, 23, XXIV-V. Garner, R. J. (1963). Environmental contamination and grazing animals, Health Physics, 6, 597. Gunn, R. G. (Ed.) (1981). The Hill Farming Research Organisation. Biennial Report 1979-1981. Hardy, E. P., Krey, P. W. & Volchok, H. L. (1973). Global inventory and distribution of fallout plutonium, Nature, 241,444. Harrison, J. D. (1983). The gastrointestinal absorption of plutonium, americium and curium. Radiation Protection Dosimetry, 5 (I), 19. Healy, W. B. (1967). Ingestion of soil by sheep. Proceedings of New Zealand Society of Animal Production, 27, 109. Hetherington, J. A. (1978). The uptake of plutonium nuclides by marine sediments, Marine Science Communications, 4 (3), 239. Jackson, E. M., Noakes, J. E. & Spaulding, J. D. (1981). Forage: a sensitive indicator of airborne radioactivity, Health Physics, 40, 84. Johnson, J. E., Ward, G. M. & Stewart, H. E (1966). Interpretation of gamma-ray spectra of environmental forage samples, Health Physics, 12, 37. Lally, A. E. & Eakins, J. D. (1978). Some recent advances in environmental analysis at AERE Harwell. Symposium on the determination of radionuclides in environmental and biological materials. Central Electricity Generating Board, Sudbury House, London 9-10 October 1978. Peirson, D. H. & Salmon, L. (1959). Gamma radiation from deposited fallout, Nature, 184, 1678. Peirson, D. H., Cawse, R A., Salmon, L. & Cambray, R. S. (1973). Trace elements in the atmospheric environment, Nature, 241, 252. Peirson, D. H., Cambray, R. S., Cawse, E A., Eakins, J. D. & Pattenden, N. J. (1982). Environmental radioactivity in Cumbria, Nature, 300, 27. Popplewell, D. S., Ham, G. J., Savory, T. E. & Bradford, W. R. (1981). Radionuclide concentrations in some cattle and sheep from West Cumbria. National Radiological Protection Board Bulletin No. 41. Simmonds, J. R., Harrison, N. T. & Linsley, G. S. (1982). Generalised Derived Limits Jbr radioisotopes of plutonium. National Radiological Protection Board Report NRPB-DL5.
Plutonium in Sheep Faeces as an Indicator of Deposition on Vegetation
J. D. Eakins, A. E. Lally, R. S. Cambray, D. Kilworth, R. T. Morrison and F. Pratley Environmental and Medical Sciences Division, AERE Harwell, Oxon. OXII 0RA, UK
ABSTRACT In order to study the deposition on vegetation ojplutonium arising from the testing of nuclear weapons and discharges from nuclear facilities, sheep faeces were collected from the South-west and North of England. The plutonium content of faeces has been shown to be a reproducible and sensitive indicator of deposition on herbage and, with this technique, plutonium discharged from the Sellafield Works of British Nuclear Fuels Limited can be detected 60 km from the site. On moorland pastures the concentration oJ" 239 + 24Opu in faeces, expressed as activity per gram of ash, is roughly equal numerically to the deposit of plutonium on 1 m' oJ" vegetation. Some of the plutonium ingested by a grazing sheep is absorbed and the highest concentrations are found in liver. The liver of a sheep grazing on pasture where the 239+24°pu concentration in faeces was about 2 pCi (74 mBq) g- 1 of ash, was found to contain < 1% of ~he Generalised Derived Limit Jbr mutton and offal, and the meat <0.001%.
INTRODUCTION The analysis of herbage is sometimes used to detect and identify radionuclides deposited from the atmosphere {Garner, 1963; Johnson et al., 1966; Jackson et al., 1981). However, when the rate of deposition is very slow a large area of vegetation needs to be sampled in order to o b t a i n sufficient activity for analysis. Such samples a r e n o t always easy 87 ~. UKAEA, 1984.
88
J.D. Eakins
et al.
to obtain, especially, in moorland areas, and, for the detection of plutonium, faeces from grazing animals were considered as an alternative. Plutonium is very poorly absorbed by the mammalian gut (Harrison, 1983), so that virtually all that is ingested by an animal will appear in its faeces. Furthermore, if the animal obtains its food entirely by grazing, then the ratio 238pu:239"z4"°Pu in plutonium separated from faeces should be the same as that in the vegetation on which it feeds. Animals which can be used for this purpose are cows, sheep and rabbits. Cambray & Eakins (1980) reported levels of 239 + 2.,0pu in faeces collected from all these species in West Cumbria in 1977. Values ranged from 0-08 to 7.4 pCi (3-270 mBq) g - t of ash. Rabbits are probably the best species for sampling herbage as they graze all the year round, but the collection of sufficient faeces for plutonium analysis is time-consuming. Cows are unsatisfactory as their diet is often supplemented by stored fodder and imported cereals; in addition, they do not graze on the moorland characteristic of large areas of Northern England. Sheep are fairly ubiquitous, however, capable of living off very poor pasture, and their diet is normally supplemented only in the winter and early spring. Quantities of sheep faeces, adequate for the accurate determination of plutonium, can be collected quite quickly. This paper describes measurements of plutonium in sheep faeces collected in the vicinity of the British Nuclear Fuels Limited (BNFL) reprocessing plant at Sellafield in Northern England. The results are compared with values obtained in areas remote from nuclear installations in North Devon and Yorkshire. The results are also considered in relation to the plutonium concentration in soil, herbage, rainwater and in sheep meat and offal.
METHODS
Collection and analysis of soil Soil samples were taken with a 3.8 cm diameter corer to a depth of 15 cm. The samples were dried and ground in a Pascall disc grinder, including any stones. Plutonium was determined in 70 g aliquots of the prepared soil by an acid leaching technique followed by anion exchange (Lally & Eakins, 1978) using 242pu as an internal yield tracer. The electrodeposited sources were counted by alpha spectrometry and the amounts of 238Pu and 239.24Opu determined.
Plutonium in sheep faeces as an indicator o/" deposition on regetation
89
Collection and analysis of herbage Most of the sites at which herbage samples were taken were on open moorland. All vegetation (mainly grass) was clipped by shears from an area of 5 m 2 to within 1-2 cm of the soil. The vegetation was ashed and analysed for plutonium by the same procedure used for soil.
Collection and analysis of sheep faeces Wherever possible, fresh samples of sheep faeces were collected on open moorland where the vegetation was closely cropped. The purpose of this was to ensure, as far as possible, that sheep were grazing pasture with a similar fodder weight per unit area. Samples (generally 1-2 kg) were collected in polythene bags. After ashing at 450-500 °C, 50 g aliquots of ash were taken for plutonium analysis using the procedure used for soil.
Analysis of sheep tissues Samples of tissue were ashed at 450-500°C and the ash analysed by radiochemical techniques similar to those used for the analysis of soil.
Collection and analysis of rainwater Rainwater was collected in a 25 litre polythene bottle containing hydrochloric acid and 2*2pu tracer, using a funnel of diameter 40 cm. The water was concentrated by evaporation and plutonium determined by anion exchange, electrodeposition and alpha spectrometry.
P L U T O N I U M IN SHEEP FAECES
Concentrations in samples from West Cumbria and Northern England BNFL Sellafield have discharged about 18000 Ci (670 TBq) of alphaemitting plutonium (Cambray, 1982) from their pipeline into the Irish Sea since 1957. It has been shown (Eakins et al., 1982) that a very small fraction of this activity has been transported in seaspray back to the land, and maritime-derived plutonium from this source has been detected up to 10 km inland by the analysis of soil samples (Cambray & Eakins, 1982).
KEY
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91
Plutonium in sheep faeces as an indicator of deposition on te~eration
Samples of sheep faeces have been collected on four occasions along a transect running inland due east from St. Bees in West Cumbria towards Whitby. The position of this transect is shown in Fig. 1, together with other sampling sites in Northern England. In Fig. 2 the results of analysis of these samples for 239+24°pu, expressed as femtocuries per gram of faecal ash, are plotted against distance from St. Bees. On two occasions, in October 1977 and June 1978, samples were collected from St. Bees to Whitby (i.e. from coast to coast). However, in July 1979 and December 1981, samples were only collected to a distance of about 70 km east of St. Bees. The concentration of 2 3 9 + 2 4 ° p u in faeces at St. Bees was on average about a factor of 20 above the asymptotic or baseline value of 50 fCi (1.9 mBq) g - t although the baseline does not appear to have been reached in the 1981 transect. In the 1977 and 1978 transects, it was evident that the concentration of 239+24°pu at Shap, 60 km from St. Bees, was about double the baseline concentration, and this was probably the case on the 1981 transect. The results in Fig. 2 have not been modified to take account of
o
-~ u ~S T
1600-1400 1200-1000 800 600
October 1977
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July
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WhRby
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239+24"°Puin sheep faeces collected between St. Bees and Whitby.
92
J.D. Eakins et al.
variations in rainfall across the country. The deposition of radionuclides introduced into the atmosphere at high altitudes as a result of the testing of nuclear weapons is dependent, among other factors, on rainfall (Peirson & Salmon, 1959). However, plutonium introduced into the atmosphere in seaspray at low altitude will have a low washout factor (Peirson et aI., 1973) and rainfall is unlikely to play a significant role in its deposition. Furthermore, if plutonium in sheep faeces represents deposition over a relatively short period of time, the application of long-term average rainfall values would not be relevant. A good indicator of the source of deposited plutonium is the ratio 238pu:239 + 24°pu. In the cumulative deposit of weapon fallout in soil, this ratio is ~0.04 (Hardy et al., 1973). The ratio was not measured in sea discharges from Sellafield until 1978, when its value was 0"27 (Atherton, 1979), increasing to 0-32 in 1979 (Atherton, 1980), and 0.34 in 1980 (Atherton, 1981). However, from the analysis of surface sediments, Hetherington (1978) has estimated that the ratio ranged from 0-05 in 1966 to 024 in 1975. In Fig. 3, the 238pu:239 +24°pu ratios obtained from samples of sheep faeces are plotted against distance from St. Bees; the broken lines represent the value of the cumulative ratio from weapon fallout. In the 1977, t979 and 1981 transects, all the ratios were above that characteristic of fallout by about a factor of six at St. Bees falling to a 030--
025
October 1977
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July 1979
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.
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Ratio of 238pu to 2~9+"4°Pu in sheep faeces collected between St. Bees and Whitby.
Plutonium in sheep faeces as an indicator of deposition on vegetation
93
factor of two at Shap. However, in 1978, although the ratio at St. Bees was similar to that on the other occasions, it d r o p p e d rapidly to a value similar to that in accumulated weapon fallout by 30 kin. In order to determine how far north and south of Sellafield plutonium derived from B N F L could be detected, a further series of faeces samples was collected in July 1979. These were taken from sites between 10 and 24 km from the sea, on a line roughly parallel to the coast, proceeding in an approximately southerly direction from a site about 16 km south-west of Carlisle to one 20 km east of the River Lune estuary between Lancaster and Fleetwood. The results are presented in Table 1, together with distances from the sea and from the Sellafield Works. The position of each site and the 238pu:239+2~°pu ratio in each sample are included in Fig. 1. Although the concentration of 239 + 2,,0pu at a few of these sites was at, or near to, the baseline values observed on the St. Bees-Whitby transects (see Fig. 2) the 238pu:239+2"*°Pu ratio was at least a factor of two above the fallout value at all sites. It is evident from these results that plutonium from Sellafield can be detected in sheep faeces at considerable distances in both a northerly and southerly as well as an easterly direction, and that the technique is more sensitive than the soil sampling method (Peirson et al., 1982). TABLE 1 Plutonium in Sheep Faeces from Northern England, North-South Survey, July 1979
Site identifier
l 2 3 4 5 6 7 8 9 l0 11
Grid rej'erence
NY NY NY NY NY NY NY SD SD SD SD
318 203 092 159 108 150 203 268 394 572 627
413 370 234 209 157 052 008 934 880 822 528
Distance (kin) From sea
From Sellafield
24 13 11 19 14 12 15 19 10 14 20
48 38 21 22 15 13 19 27 38 59 79
2~Spu (fCi g- i of ash)
4+_ 1 6_+2 10+2 12_+2 23+_3 70_+5 27_+2 11_+2 7-+ l 5+_ 1 10_+2
239- ,_.~Opu
(fCi g- l of ash)
42+2 55+5 87_+7 106+_6 147+13 447-+22 190_+9 90+7 69_+6 49+_5 76_+6
The uncertainties quoted are 2a and refer only to counting statistics.
23spu / 2~9 - 2.tOpu
0-10 0-11 0-1! 0-11 0"16 0-16 0-14 0-12 0-10 0-i0 0"13
94
J.D. Eakins
et al.
Concentrations in samples following a minor release of plutonium by direct emission from Sellafield Works There was a minor release of radioactivity from the Sellafield Works on 11 September 1979 (BNFL, 1979) when the wind was southwesterly. A sample of sheep faeces collected on 25 September 1979 near Wastwater, 13 km ENE of the works and 12 km from the sea contained 1400 fCi (52 mBq) of 239+2"t°pu g-1 of ash. A further sample, taken 6 km to the north-east of the works on Cold Fell, contained 2200 fCi .(80 mBq) g- L. The Wastwater sample was from site 6 on the north-south series of samples collected in July 1979 (Table 1), when the concentration of 239+2a'°Pu in faecal ash was 447 fCi (17 mBq) g - i . A sample collected at the same site in July 1978 contained 280 fCi (10 mBq) g-~ and in May 1977 the concentration was 200 fCi (7 mBq) g-1 (Cambray & Eakins, 1980). From the July 1979 W - E transect shown in Fig. 2, which runs a few kilometres north of Wastwater, a concentration of - 3 0 0 fCi (11 mBq) g - I of ash would be expected. Thus there is ample evidence that the concentration of 239+2a°pu in sheep faeces ash at Wastwater is normally in the range 200-500 fCi (7-19 mBq) g - l . Similarly from the July 1979 data (Fig.-2) a 239+2"*°Pu concentration of about 500fCi (19 mBq) g-~ at Cold Fell would be expected, instead of the observed value of 2200 fCi (80 mBq) g-1. These measurements demonstrate the sensitivity of the technique for detecting minor releases to the atmosphere of radioactivity containing plutonium. Concentrations in samples from areas remote from nuclear installations In order to compare concentrations of plutonium in sheep faeces collected in the North and North-west of England with those in areas remote from nuclear installations, five samples were taken in July 1979 along a transect running inland for 18 km from the coast of Somerset. This area is 360 km to the south of Sellafield. The results are shown in Table 2, from which it is evident that the plutonium concentrations were independent of distance from the sea. The mean value was 31 fCi (1"1 mBq) g - t ash. The mean 238pu:239+2a°pu ratio was 004 which corresponds to that in fallout from nuclear weapons testing. To establish the reproducibility of sampling, concentrations of 239+2"t'°Pu were measured in seven samples of sheep faeces, collected from a triangular area of about 30 k m 2 in the vicinity of Tan Hill (NGR
Plutonium in sheep faeces as an indicator of deposition on vegetation
95
TABLE 2 Plutonium in Sheep Faeces from North Somerset, July 1979
Site
Porlock Lucott M o o r Chetsford Water Withypool Ansty Gate
GriLl reference
SS SS SS SS SS
866 846 849 847 835
461 445 420 339 298
Distance from sea /kin; 16 4.3 6"0 13'9 18"2
The uncertainties quoted are 2~ and refer
only to
23'~pu (Jgi g - ~ ojash) I+ 1± 1± 2± 1±
I 1 l l 1
23'~+ 2a'°Pu fjCi g - 1 of ash)
z3Spu/ ,_39 - "--~opu
24+5 32±3 30±4 40+4 29±3
0.04 0.03 0'03 0"05 0"03
counting statistics.
NY 896 067), North Yorkshire in July 1979. This region is about 90 km east of the Sellafield Works of BNFL and sufficiently remote not to be affected by it significantly (see Fig. 2). The concentration of 239 + 2-~0pu in the samples varied between 32 and 67 fCi (1-2-2'5 mBq) g- t of ash with a mean of 45 fCi (1"7 mBq) g - t. The coefficient of variation was 25%. The ratio of 2 3 8 p u : 2 3 9 + 2 a ° P u w a s 005 at all sites, characteristic of that in fallout. The results indicate that the concentration of plutonium in samples of sheep faeces collected over a wide area is reproducible, within a factor of about two, providing the grazing is uniform. R E L A T I O N S H I P BETWEEN C O N C E N T R A T I O N OF P L U T O N I U M IN S H E E P FAECES AND IN VEGETATION The root uptake of plutonium by plants from soil is very poor (Cawse & Turner, 1982) and most of the activity associated with the aerial parts of plants represents direct foliar deposition. The plutonium content of herbage will depend upon many factors. During the winter months, when there is little growth, plutonium will accumulate on vegetation but there will be a sudden fall in concentration coinciding with the flush of new growth in the spring. It is also known that sheep ingest significant quantities of soil when grazing and in winter, when the pasture is poor, soil can constitute 10-14% of the dry matter intake (Field & Purves, 1964; Arnold et al., 1966; Healy, 1967). In the summer months, under more favourable grazing conditions, the concentration of plutonium in sheep faeces will approximate more closely to that in herbage as the ingestion of soil will be minimised.
96
J.D. Eakins et al.
To compare the relationship between plutonium in ashed sheep faeces and in vegetation, samples of each were collected from seven sites in England during 1978. Measurements were also made of plutonium in soil samples taken at the same sites to a depth of 15 cm. The results of these measurements are given in Table 3. There was a good correlation (r=0-8 I: p <0"05) between concentrations of plutonium in sheep faeces and in vegetation but the correlation with levels of plutonium in soil was not significant (r =0"48; p<0"10). In a study of the vertical distribution in soil of 239 + 2"~°Pu at Tan Hill (Eakins el al., 1980 and unpublished work) it was shown that the concentration in the top 2 cm of soil was three times the average from 0-15 cm. Thus the 239~-2~°pu concentrations in soil in Table 3 almost certainly underestimate the concentrations in surface soil ingested by sheep. The concentration of 239 +-,,,Opu in ashed sheep faeces was higher than in ashed vegetation, the mean value for this ratio being 2.2 _+0"8 (SD). As some of the mineral content of vegetation will be absorbed during passage through the gastrointestinal tract of the sheep, a value for this ratio exceeding unity would be expected but, to account for a factor of at least two, it seems likely that there must be some ingestion of surface soil Scandium, which is a ubiquitous element in soil, can be used to estimate the soil contamination of vegetation as it is not translocated by TABLE 3
239+24°pu and Scandium in Sheep Faeces, Vegetation and Soil Grid reference
NZ 098 035 NY401 124 NY401 109 NY235 136 NY 972 105 SD 590 670 SS 788 298
Vegetation collected (5 m 2) Dry wt (g)
Ash wt (g)
68.5 93-2 139-7 174.2 134.3 336.1 92-0
6.30 6.91 11-2 14.1 14.0 16.2 10.0
z39+2~Opu (JCi g- t of ash) Sheep faeces
63+6 134+_9 149_+7 497+_28 734_+47 133_+8 21+2
Vegeration
Scandium concentration (rag kg-1 of ash) Soil
31+_9 130+11 61_+16 46+_6 116+_12 42_+5 2104- 19 482+_16 166+_18 109_+4 120_+14 100_+4 9+_3 33_+4
Sheep faeces
v 2 5 3 2 3 --
The uncertainties quoted are 2a and refer only to counting statistics.
Vegeration
Soil
<2 3 5 ~ <2 2 --
5 4 19 17 14 3 --
Plutonium in sheep faeces as an indicator of deposition on vegetation
97
plants to a significant extent. According to Bowen (1979) levels of scandium in dry soil range from 0"5 to 55 mg kg- 1 with a median value of about 7 mg kg -~. According to the same author, the maximum concentration of scandium in dry vegetation is 0-2 mg kg- 1. To obtain some idea of the extent of soil contamination of vegetation (and consequently of sheep faeces) samples from the first six sites listed in Table 3 were analysed for scandium by optical emission spectroscopy and the results are included in that table. The mean ash weight/dry weight ratio of the vegetation samples was 008, hence scandium concentrations in the ash should not exceed 2 mg kg-~. This value coincided with the minimum detectable level of scandium for the analytical method used. However, it can be seen from Table 3 that concentrations of scandium in samples of vegetation ash were of the order of 2 mg kg-1. The levels in ashed faeces were invariably quite similar to those in vegetation but, in some cases, much lower than in soil. This suggests that most of the material in sheep faeces is derived from vegetation with only a minor contribution from ingested soil. As pointed out above, most of the plutonium associated with vegetation represents material deposited directly on the aerial parts of plants. It is to be expected, therefore, that the concentration of plutonium in herbage, and also in sheep faeces, will depend primarily upon its recent deposition rather than on the cumulative deposit. To examine the relationship between concentrations of plutonium in sheep faeces and in rain, monthly samples of rainwater were collected from a hill farm on Exmoor in North Devon during the period April-November 1979. Samples of sheep faeces were collected once each month from the same farm. The results of these measurements are given in Table 4. The concentrations of 239+24"°Pu in rainwater during this period were very low and that of 23Spu was below the limit of detection of 20 fCi (0.7 mBq) per sample. However, 238pu was detectable in sheep faeces and the 238pH:239+24°pu ratios approximated to the value expected from nuclear weapon fallout. As shown in Fig. 4, there was a good correlation (r = 0"88; p < 0-01) between the concentration of 239 + 24Opu in ashed sheep faeces and in the corresponding rainwater samples, expressed as fCi litre-1. However, when the concentration in faeces was compared with the total activity in monthly rainwater samples the correlation was not significant (r = 0-49; p > 0-20). In Fig. 4 the positive intercept on the ),-axis probably represents plutonium in faeces derived from the ingestion of soil and also from dry
98
J.D.
E a k i n s et al.
TABLE 4 C o m p a r i s o n o f P l u t o n i u m in R a i n w a t e r with P l u t o n i u m in S h e e p Faeces from N o r t h Devon 3,[onth <1979)
Rainjhll
Sheep .laeces
Rainwater
( t h i n ,)
,_3Spu
,_3, - 2aOpu
"->*'--~Opu
(jCi,ff -~ o f ash)
2 3 0 - -'aO p U
qCi
dCi
qUi
23Spu
"-3'~-'-'-Opu (fCi g - l ); :3~ - ,.~Opu in deposit /fCi m-Z2
month - I ) litre -L) m - z )
April May June July August September October November
89 147 69 9l 198 81 127 206
130+30 124-3 1070 180___60 10+_3 1470 70+_20 8+_2 550 1004-40 9+_4 820 130+_40 5+_2 990 60+_30 6+_3 490 50+_20 3+_I 380 50+_20 2+_ 1 410
z 3~ - ,_.~op u m sheep jaeces
<0-2 <0.1 <0-3 <0.2 <0.2 <03 <0-4 <0.4
30_+3 20+_2 24+_3 214-2 17+_3 14+_2 13___2 14+_2
0-04 0-05 0-03 0-04 0-06 0-06 0-04 0.05
Mean
0.029 0.014 0-043 0-026 0-016 0.029 0.033 0.035 0-028 _+ 0.010 (SD)
T h e uncertainties q u o t e d Ere 2a a n d refer only to c o u n t i n g statistics. 3/-
f
o
/
I
I
32
30 -/
25,-
2
o
'k
s
1 4--
t
2--
Fig. 4.
i
I
I
I
O
1
2
3 ~. 5 Z]9'2&OpU
t
t
I
P I
[
I
I
5 7 ~ 9 10 11 COnCentrC%lO n in rQl~
I
!
12 13 14 { fCI [ T )
15
1 ) 15
17
C o r r e l a t i o n b e t w e e n 239÷2*°Pu in rain a n d s h e e p faeces.
Plutonium in sheep faeces as an indicator of deposition on vegetation
99
deposition on vegetation. The sum of these two factors at this site ranged from 30 to 60%. The dry deposition component in nuclear weapon fallout in the United Kingdom has been shown by Cambray et al. (1970) to be about 20%, suggesting that the contribution to plutonium in faeces from soil at this North Devon site was between I0 and 40%. The annual deposit of plutonium from nuclear weapon fallout in 1979 was < 1 % of the maximum annual deposit which occurred in 1963 (Cambray et al.. 1980) and the concentration in soil would not have altered significantly throughout the year. It should be reiterated that the correlation between rainwater and sheep faeces is only likely to apply in areas such as North Devon which are exposed only to plutonium deposition from the testing of nuclear weapons. In West Cumbria where the major source is at a low altitude, it will not necessarily apply.
RELATIONSHIP BETWEEN C O N C E N T R A T I O N OF P L U T O N I U M IN SHEEP FAECES AND IN SHEEP MEAT AND OFFAL To relate the concentration of plutonium in sheep faeces to that in sheep meat and offal, tissues from a ewe which had grazed on pasture near the sea at Drigg in West Cumbria were analysed for plutonium. The sheep, which was about 7 years old, had grazed for 3 years (from 1978-81) on pasture which runs down to sand dunes adjacent to the sea and is exposed to plutonium-bearing seaspray. It did not have access to the nearby salt marshes. The area grazed is about NGR NY 050 985, which is approximately 5 km SSE of the Sellafield Works. A sample of sheep faeces collected from this pasture on 9 December 1981, during the week the ewe was killed, contained 2800 fCi (104 mBq) 239+24Opu g-1 of ash. The 238pu:239+24"°Pu ratio was 0-29. A sample collected at the same site on 17 June 1981, contained 2300 fCi (85 mBq) g-1 of ash. From Fig. 2, it appears that the concentration of 239,2.,Opu in sheep faeces collected near the Cumbrian coast has been fairly' constant, within a factor of two, over the past few years and it is reasonable to assume that the concentration at Drigg over the period 1978-81 has been about 2000 fCi (74 m B q ) g - i The results of analysis of the sheep tissues for plutonium are presented in Table 5 expressed as picocuries per kilogram wet weight in the case of
100
J. D. Eakins et al. TABLE 5
Plutonium in Tissues of a Sheep from Drigg. West Cumbria Tissue
Lung Liver Muscle Bone (vertebra) Bone (rib) Kidney Heart
_,3s Pu
-"39- _'~oPu
(pCi kg- 1)
gpCi kg- t)
0-27 _+005 9.98 4-0.85 0-005 4-0005 0"95 4-0"13 066__.0-I 1 0.104-0.10 0.22 4-0"1l
0.99 4-0-08 37-3_- 2-3 0-024 + 0-009 3-74 ± 0"26 2'16+_0"19 0-244-0'13 0"364-0.12
The uncertainties quoted are 2e7 and refer only to counting statistics. the soft tissues. Bone was dried at 105 °C before analysis and the results are expressed as pCi kg-~ dry weight. The greatest concentration of plutonium was found in liver followed by bone. The 23Spu:239+2'*°Pu ratios in the tissues were generally similar to that in sheep faeces, confirming that the material was derived mainly from the Sellafield Works and not from-nuclear weapon fallout. It appears from this study that a concentration of 239+24"°Pu in sheep faeces of 1 pCi (37 m B q ) g corresponds to a concentration in sheep meat of about 0.01 pCi (0'4 mBq) k g - ~ and in liver of a b o u t 20 pCi (740 mBq) k g - ~. Levels of 238 + 239 +240pu in the livers of sheep which had grazed in other areas of northern England have been reported by Popplewell et al. (1981). They found that the liver of a sheep which had grazed near Penrith, 55 km to the north-east of Sellafield, contained 0-23 pCi k g - ~. On the basis of the above relationship, this corresponds to a concentration of 80 fCi g - x of ashed sheep faeces, which is almost certainly correct to within a factor of two. Generalised Derived Limits (GDLs) for plutonium in a variety of foodstuffs and environmental materials have been published by the National Radiological Protection Board (Simmonds et al., 1982). The G D L for 239+24°pu in mutton and offal is 5-4 x 103 pCi (200 Bq) k g The concentrations of 239+2'~°Pu found in sheep liver and meat in the present study correspond to only 0-7% and < 0.001% respectively of the G D L . It is reasonable to assume, therefore, that if sheep graze in an area in which the 239+2'*°Pu content of herbage is such that the resulting concentrations in faeces are relatively constant at about 2000 fCi k g - t ,
Plutonium in sheep faeces as an indicator of deposition on regetation
101
then liver from these sheep will contain less than 1% of the GDL. It must be emphasised that these concentrations of 239-- 24.0pu are only encountered where sheep graze close to the Cumbrian coast and they will decrease quite rapidly with distance inland.
DISCUSSION A comparison of the concentration of plutonium in ashed sheep faeces and in ashed vegetation from the corresponding sites showed that the concentration in faeces was approximately twice that in vegetation. This difference is attributed to (a) the absorption of nutrients from herbage in the gastrointestinal tract of the sheep, (b) the ingestion of surface soil by the sheep, and (c) possibly also to the selective grazing habits of the sheep. The result indicated that, at least in summer, plutonium in sheep faeces is derived predominantly from vegetation. Using the data in Table 3, it can be shown that the concentration of 239+24°pu in femtocuries per gram ashed sheep faeces is approximately equal, numerically, to the number of femtocuries associated with 1 m 2 of vegetation. The mean vaiue for the faeces/herbage ratio at the seven sites listed in Table 3 was 1"1 ___0-5 (SD). However, it should be stressed that this relationship probably only applies to open moorland, and on more productive pastures the concentration in sheep faeces could well be less than that associated with vegetation from 1 m 2. A sheep ingests about 1-1"5 kg dry weight of fodder per day (Gunn, 1981). In these studies the weight of faecal ash taken for determination of plutonium was equivalent to about 600 g dry weight of faeces, or about half the daily dry weight intake. The mean dry weight of vegetation collected from the moorland sites listed in Table 3 was 30 g m-2 and in such areas it will be necessary for sheep to graze the equivalent of 40 m e per day. Thus the quantity of sheep faeces taken for analysis represents about 20m 2 of vegetation which demonstrates the sensitivity of the technique and also the excellent averaging effect of the sheep as a sampler of vegetation. As the plutonium content of herbage is due almost entirely to direct foliar deposition, its concentration in sheep faeces will reflect current deposition rather than the cumulative deposit. The data from North Devon (see Table 4) showed a good correlation, on a monthly basis, between the concentration of 239 + 2 4 0 p u in sheep faeces and in rainwater,
102
J.D. Eakins
et al.
which suggests that plutonium in sheep faeces reflects only recent deposition on herbage. As shown in Table 4, the mean value for the ratio of 239 + 2.,Opu in sheep faeces to the monthly deposition of e39- e.*Opu in rainwater was 0-028--+0-010 (SD). If it is assumed that the activity per gram of ashed sheep faeces is numerically equal to the activity per square metre of vegetation and that the activity in vegetation is accumulated on average over a period of about a month, then the results in the final column of Table 4 suggest that about ,~ ~o'/o of the plutonium deposited in rain is retained by the leaf surfaces. The Normalised Specific Activity (NSA) is a quantity which relates the concentration of a contaminant per unit mass of vegetation to its daily rate of ground deposition. NSA =
Activity per kilogram dr)' matter on vegetation Activity deposited per day per square metre of ground
If it is assumed that the ash weight of faeces is approximately equal to the ash weight of the vegetation ingested it is possible to calculate NSA values from the North Devon data. In order to do this a conversion factor of 12-6 (derived from Table 3) for the ratio of dry weight to ash weight has been assumed. The mean value obtained is 68 m 2 day kg- i which is similar to that of 73 m 2 day k g - I derived by Chamberlain (1970) for 9°Sr on herbage. The results of measurements of _,3o+ ,_.topu in sheep faeces collected on transects originating on the Cumbrian coast and extending across the North of England (see Fig. 2) showed that the concentrations fell off rapidly with distance from the coast. However, the penetration inland varied with time certainly on an annual basis, and probably on a much shorter timescale, reflecting changing weather conditions and, in particular, wind speed and direction. The variable "baseline' values observed in North Devon (19 fCi g - i ) , Somerset (31 fCi g-l), Tan Hill (45 fCi g-') and the asymptotic values in the transects in Northern England ( ~ 5 0 f C i g - t ) are accounted for by the same factors. The variable concentrations of plutonium in sheep faeces at the same sampling site on different occasions provide further evidence that such measurements are influenced more by recent deposition on vegetation rather than on the cumulative deposit in soil. A comparison of Figs 2 and 3 shows that the ratio 238pu:239 + 2'a°Pu is the more sensitive indicator of plutonium in faeces in excess of that expected from nuclear weapon fallout. It is important therefore that this
Plutonium in sheep faeces as an indicator of deposition on cegetation
103
ratio should be determined with adequate accuracy and precision. The main difficulty arises in the determination of :3Spu at low levels, when interference from 22STh, a natural radioelement with a similar :e energy to 238pu, can be a problem. These difficulties can be minimised by taking a large sample of faecal ash for analysis and repeating the thorium removal steps in the analytical procedure. CONCLUSIONS It has been shown that plutonium in sheep faeces is a sensitive and reproducible indicator of concentrations of plutonium recently deposited on vegetation. The 23Spu:239+24°pu ratio in sheep faeces also reflects that in recently deposited plutonium. The analysis of sheep faeces collected in Northern England has enabled the distribution of plutonium discharged from the Sellafield Works of BNFL to be delineated. Such material can sometimes be detected as far as Shap Fell, 60 km to the east of Sellafield and at similar distances to the north and south. Most of this material represents plutonium originally dis_charged to the Irish Sea but transported back to land in seaspray, although there will be a contribution from direct discharges into the atmosphere. It was also demonstrated that the analysis of sheep faeces can detect small accidental releases of plutonium to the atmosphere, provided that information is available on pre-existing concentrations. It is shown that, at least in summer, plutonium in sheep faeces is derived mainly from vegetation. In winter the contribution from plutonium ingested in surface soil will increase. On moorland sheep runs, the concentration of 239 + 2~.0pu in faeces, expressed as femtocuries per gram of ash, approximates numerically to the amount associated with vegetation sampled from 1 m 2. It has also been established that where sheep graze on pastures such that the concentration of 239 + 2 a 0 p u in their faeces is 2 pCi (74 m B q ) g - 1 ash then its concentration in sheep liver and in sheep meat will be < 1% and <0-001% respectively of the Generalised Derived Limit. ACKNOWLEDGEMENTS The authors would like to thank Dr A. Morgan, Dr D. H. Peirson and Dr A. C. Chamberlain of Environmental and Medical Sciences Division,
104
J. D. Eakins et al.
AERE, for their general advice and encouragement. They would also like to thank Dr R. S. Atherton and Mr H. Longley of British Nuclear Fuels Limited for their helpful comments during the preparation of this paper.
REFERENCES Arnold, G. W., McManus, W. R. & Bush, I. G. (1966). Studies in the wool production of grazing sheep. 5. Observations on teeth wear and carry over effects. Australian Journal of Experimental Agriculture and Animal Husbandry, 6, 10l. Atherton, R. S. (1979). Annual report on radioactive discharges and monitoring of the environment, 1978, British Nuclear Fuels Limited, Health and Safety Directorate. Atherton, R. S. (1980). Annual report on radioactive discharges and monitoring of the environment, 1979, British Nuclear Fuels Limited, Health and Safety Directorate. Atherton, R. S. (1981). Annual report on radioactive discharges and monitoring of the environment, 1980, British Nuclear Fuels Limited, Health and Safety Directorate. BNFL (1979). Press Releases, 11 September 1979 and 5 October 1979. Bowen, H. J. M. (1979). Environmental chemistry of the elements, New York, Academic Press. Cambray, R. S. (1982). Annual discharges of certain long-lived radionuclides to the sea and to the atmosphere from the Sellc~eId Works, Cambria, 1957-1981, AERE-M 3269. London, HMSO. Cambray, R. S. & Eakins, J. D. (1980). Studies ojem, ironmental radioactivity in Cumbria. Part 1. Concentrations of plutonium and caesium-137 in environmental samples from West Cumbria and a possible maritime effect, AERE-R 9807. London, HMSO. Cambray, R. S. & Eakins, J. D. (1982), Pu, 2"~iAm and 13VCs in soil in West Cumbria and a maritime effect, Nature, 300, 46. Cambray, R. S., Fisher, Miss E. M. R., Brooks, W. L. & Peirson, D. H. (1970). Radioactive fallout in air and rain: results to the middle of 1970, AERE-R 6556. London, HMSO. Cambray, R. S., Fisher, Miss E. M. R., Playford, K., Eakins, J. D. & Peirson, D. H. (1980). Radioactive fallout in air and rain: results to the end of 1979, AERE-R 9672. London, HMSO. Cawse, P. A. & Turner, G. S. (1982). The uptake of radionuclides by plants: a review of recent literature, AERE-R 9887. London, HMSO. Chamberlain, A. C. (1970). Interception and retention of radioactive aerosols by vegetation, Atmospheric Environment, 4, 57. Eakins, J. D., Lally, A. E. & Kilworth, D. (1980). In Environmental & Medical Sciences Divisional Progress Report, January-December 1979, AERE-PR EMS 7. London, HMSO.
Plutonium in sheep faeces as an indicator of deposition on ~'egetation
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Eakins. J. D., Lally, A. E., Burton, P. J., Kilworth, D. R. & Pratley, F. A. (1982). Studies of environmental radioactivity m Cumbria. Part 5. The mag'nitude and mechanism of enrichment of sea spray with actinides in West Cumbria, AERE-R 10127. London, HMSO. Field, A. C. & Purves, D. (1964). The intake of soil by' grazing sheep, Proceedings of Nutrition Society, 23, XXIV-V. Garner, R. J. (1963). Environmental contamination and grazing animals, Health Physics, 6, 597. Gunn, R. G. (Ed.) (1981). The Hill Farming Research Organisation. Biennial Report 1979-1981. Hardy, E. P., Krey, P. W. & Volchok, H. L. (1973). Global inventory and distribution of fallout plutonium, Nature, 241,444. Harrison, J. D. (1983). The gastrointestinal absorption of plutonium, americium and curium. Radiation Protection Dosimetry, 5 (I), 19. Healy, W. B. (1967). Ingestion of soil by sheep. Proceedings of New Zealand Society of Animal Production, 27, 109. Hetherington, J. A. (1978). The uptake of plutonium nuclides by marine sediments, Marine Science Communications, 4 (3), 239. Jackson, E. M., Noakes, J. E. & Spaulding, J. D. (1981). Forage: a sensitive indicator of airborne radioactivity, Health Physics, 40, 84. Johnson, J. E., Ward, G. M. & Stewart, H. E (1966). Interpretation of gamma-ray spectra of environmental forage samples, Health Physics, 12, 37. Lally, A. E. & Eakins, J. D. (1978). Some recent advances in environmental analysis at AERE Harwell. Symposium on the determination of radionuclides in environmental and biological materials. Central Electricity Generating Board, Sudbury House, London 9-10 October 1978. Peirson, D. H. & Salmon, L. (1959). Gamma radiation from deposited fallout, Nature, 184, 1678. Peirson, D. H., Cawse, R A., Salmon, L. & Cambray, R. S. (1973). Trace elements in the atmospheric environment, Nature, 241, 252. Peirson, D. H., Cambray, R. S., Cawse, E A., Eakins, J. D. & Pattenden, N. J. (1982). Environmental radioactivity in Cumbria, Nature, 300, 27. Popplewell, D. S., Ham, G. J., Savory, T. E. & Bradford, W. R. (1981). Radionuclide concentrations in some cattle and sheep from West Cumbria. National Radiological Protection Board Bulletin No. 41. Simmonds, J. R., Harrison, N. T. & Linsley, G. S. (1982). Generalised Derived Limits Jbr radioisotopes of plutonium. National Radiological Protection Board Report NRPB-DL5.