In-vivo estimates for the uptake of caesium-137 by cattle grazing contaminated pasture around the Esk and Irt estuaries, Cumbria, U.K.

The Science of the Total Environment, 22 ( 1 9 8 1 ) 3 9 - - 5 0 Elsevier Scientific P u b l i s h i n g C o m p a n y , A m s t e r d a m - - P r i n t e d in T h e N e t h e r l a n d s

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IN-VIVO ESTIMATES FOR THE UPTAKE OF CAESIUM-137 BY CATTLE GRAZING CONTAMINATED PASTURE A R O U N D THE ESK A N D IRT ESTUARIES, CUMBRIA, U.K.

T. J. SUMERLING

National Radiological Protection Board, Chilton, Oxon (United Kingdom) ( R e c e i v e d J u n e 1 9 t h , 1981 ; a c c e p t e d in final f o r m J u l y 2 6 t h , 1 9 8 1 )

ABSTRACT Sea w a t e r c o n t a m i n a t e d w i t h d i l u t e d r a d i o a c t i v e e f f l u e n t f r o m t h e Windscale n u c l e a r c o m p l e x in C u m b r i a periodically floods low-lying grazing p a s t u r e a r o u n d t h e estuaries o f t h e rivers Esk, I r t a n d Mite n e a r Ravenglass. D u r i n g 1 9 7 9 , a n e x p e r i m e n t was carried o u t t o m e a s u r e t h e t r a n s f e r o f c a e s i u m - 1 3 7 f r o m grass t o m u s c l e in cows grazing t h e s e pastures. Grass samples were t a k e n a n d in vivo e x t e r n a l g a m m a - r a y m e a s u r e m e n t s were m a d e o n cattle. A very l o w t r a n s f e r c o e f f i c i e n t was f o u n d , less t h a n 9 × 10 -4 days kg -1 w i t h a best e s t i m a t e o f 4 X 10 -4 days kg -1 , c o m p a r e d w i t h a m o r e usual value o f a r o u n d 3 × 10 -2 days kg -1 . T h e l o w t r a n s f e r seems t o o c c u r because t h e b u l k o f t h e c a e s i u m - 1 3 7 o n t h e grass is b o u n d t o r e s u s p e n d e d e s t u a r i n e surface s e d i m e n t d e p o s i t e d d u r i n g flooding. In this f o r m , t h e c a e s i u m - 1 3 7 is o n l y p o o r l y a b s o r b e d across t h e g u t o f t h e grazing cattle.

INTRODUCTION

In assessing doses to man from routine and accidental releases from nuclear facilities, caesium-137 is one of the more important radionuclides to consider because of its relatively 10ng half-life (tlz2 --~ 30 years) and comparative mobility, which results in its incorporation into the f o o d chain [1]. The major dietary sources of caesium-137 from fallout are meat and milk. Between 1961 and 1963 in the United Kingdom, the consumption of milk and all types of meat accounted for 33% and 43% respectively of the total estimated daffy adult intake of caesium-137, approximately 30% of the intake from meat being attributable to beef [2]. Therefore, coefficients of transfer for caesium in cattle both from feed to milk and from feed to muscle (which forms the bulk of edible tissue) are important parameters in predictions of doses arising from releases of activity to the environment. The coefficient of transfer for caesium to milk is usually defined as the quotient: caesium activity concentration in milk average daffy caesium activity intake 0048-9697/81/0000~0000/$02.50

© 1981 Elsevier Scientific P u b l i s h i n g C o m p a n y

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This coefficient has been determined in controlled feeding trials and oral dosing experiments and found to have values between 3 x 10 -3 and 1 × 10 -2 day 1-1 [3--5]. The coefficient of transfer for caesium to muscle is usually defined as: caesium activity concentration in muscle average daily caesium activity intake where the levels of caesium are in equilibrium. Since the secretion of caesium to milk appears to follow that of potassium [3], the caesium transfer to muscle may be inferred from the caesium transfer to milk and the normal distribution of potassium between muscle and milk, which yields estimates for the coefficient of transfer to muscle of 1 × 10 -2 to 5 x 10 -2 days k g - ' . Experimental determinations have yielded results between 7 x 10 -3 and 9 x 10 -2 days kg -1 with an average of 2 × 10 -2 days kg-' [6]. A widely accepted value for the coefficient has been 4 x 10 -2 days kg -1 [7, 8]. The values currently used in dose assessment studies in the U.K. for both coefficients are 7 x 10 -3 days 1-1 to milk and 3 x 10 -2 days kg -1 to muscle [9]. The above values for the coefficients have been derived from oral dosing experiments and the feeding of stalled animals with artificially contaminated fodder. The purpose of this investigation was to determine a transfer coefficient for cattle grazing in an unconstrained manner for comparison with values from controlled experiments.

CONTAMINATION AND PASTURE USE ON THE ESTUARIES

The mouths of the Esk and Irt join at Ravenglass (see Fig. 1) approximately ten kilometres south along the Cumbrian coast from the nuclear fuel reprocessing plant at Windscale. The estuarine silts show an affinity for many of the radioactive products, particularly isotopes of caesium, that are released into the sea from the plant and consequently the silts are contaminated at an unusually high level [10]. At spring high tides, and at other times when a strong onshore wind is blowing, low-lying pasture around the estuary is inundated with sea water. The water carries diluted radioactive effluent and a suspension of contaminated marine particulates which may be deposited as the tide recedes, thus contaminating the pasture. Adjacent higher pasture is contaminated to a lesser extent by resuspehsion of surface water and dried silt and by translocation of activity by animals. The affected pasture, of approximately 300 hectares, is generally of poor quality b u t is used by local farmers during the summer to graze sheep, beef cattle and dairy cattle, although lactating dairy cattle are usually taken to higher pasture, where the grazing is better. The levels of fission products and actinides in a slaughtered c o w from this area have been measured and found to be well below levels which need give rise to concern even with the most cautious assumptions concerning local eating habits [11, 20]. The activity concentrations measured in liver were

41

B

C RAVENGLASS

"\

\

.

~:~ Pasture grazed by ~:~ animals measured

Scale

I 0

: kilometres

I 1

I 2

I 3

Fig. 1. The estuarial system around Ravenglass.

approximately 10 Bq kg -1 of caesium-137 and 0.6 Bq kg -1 of all isotopes of plutonium. The concentrations in all other tissues were lower. A c o m m i t t e d effective dose equivalent of less than 4 pSv y-I can be calculated for a person consuming meat only from this source at the U.K. average annual consumption rate. However, the enhanced activity levels in the environment present an opportunity to investigate radionuclide transfer from pasture to animals in uncontrived conditions. In particular, the behaviour of caesium137 may be readily investigated since this nuclide may be measured at low levels (around 10 Bq kg -~ ) in live animals by external gamma-ray counting. For the investigation the cooperation of four cattle farmers, whose animals graze sea-washed pasture on the estuary, was enlisted. One of the farms, Farm A, maintained a herd of Galloway beef cattle. The other farms, B, C and D maintained Friesian dairy herds. The location of the areas grazed by the animals from each farm is marked on Fig. 1. External gamma-ray measurements were made on cattle; grass, soil and water samples were collected on each farm, and transfer coefficients for caesium to muscle were calculated all as described below.

ACTIVITY MEASUREMENTS ON THE CATTLE

In early May 1979, five or six cattle on each of the four farms were measured by external gamma-ray counting. These were all mature cattle except for three yearlings measured at farm A. Most of the cattle had win-

42

tered at their respective farms away from the contaminated pasture. In midNovember the cows were remeasured after grazing sea-washed pasture during the summer. These measurements were made at least two days after the cattle had been taken off the contaminated pasture to allow time for clearance of activity from the gut [12]. As far as possible the same animals as those measured in May were chosen. Suitable replacements were f o u n d for cows that did n o t graze sea-washed pasture during the summer or that had been slaughtered. The measurements were made using a side-shielded sodium iodide crystal of 13 cm diameter and 1 0 c m depth. The detector was placed as close as possible (0--2 cm) to the large triangular muscle mass between femur and tibia on each animal; this position was selected since the region has the greatest concentration of muscle and edible tissue in the cow. For this purpose, the animals were secured in a byre or milking parlour while the gamma-ray spectrum was accumulated for a period of 10 to 20 min. The background c o u n t in the caesium-137 photo-peak region was assessed from the immediately adjacent higher energy region. Calibration measurements were carried out in the laboratory using large containers filled with a caesium-137 solution in an appropriate configuration.

RESULTS OF ANIMAL MEASUREMENTS

The estimated specific activity of caesium-137 in muscle of the cattle in May is given in Table 1. The cattle at Farm A had grazed sea-washed pasture and the foreshore until mid-February and had then been brought to the farm TABLE 1 Specific a c t i v i t y o f the muscle of cattle, May m e a s u r e m e n t s

Farm

Animals

Specific activity (Bq kg -1 o f 137Cs)

A

2 a d u l t cows 3 o n e year old heifers

AND. ~ 7 40--70 a

B

6 cows

AND.

,

~ 7

C

6cows

AND.

,

~11

D

3 cows 3 cows

AND. ( 11 2 0 - - 4 0 a'b

A N D = activity n o t d e t e c t e d , f o l l o w e d b y t h e 95% d e t e c t i o n activity. ( T h e 95% detect i o n a c t i v i t y is t h a t activity w h i c h it is e s t i m a t e d w o u l d b e d e t e c t e d o n 95% o f occasions; it c o r r e s p o n d s t o a p p r o x i m a t e l y 3.3 s t a n d a r d d e v i a t i o n s o f the net c o u n t . ) a C o m b i n e d r a n d o m a n d s y s t e m a t i c u n c e r t a i n t i e s are o f the order of +50%. b C o r r e c t e d for activity in g u t (see text).

43

where they had fed on uncontaminated fodder originating outside West Cumbria. The three yearlings measured all showed clear evidence for the presence of caesium-137 although the two adult cows that had grazed alongside them did not. The cattle at Farm B and C and three of the animals at Farm D had wintered at their respective farms and none of these animals showed any evidence for the presence of caesium-137. However, the gamma-ray spectra from three animals at Farm D, which had been brought directly from contaminated pasture, clearly indicated the presence of ruthenium-106, zirconium-95 and caesium-137. Since the uptake of ruthenium and zirconium from the gut is low [6, 1 3 ] , it was assumed that these nuclides were located in the gut together with a fraction of the caesium-137. The fraction of caesium-137 accompanying the ruthenium-106 and zirconium-95 was determined from the analysis of grass samples from the pasture. The tabulated estimates of activity in muscle were calculated with allowance for the presence of the interfering activity in the gut. Table 2 shows the results of the November measurements on cattle. All four cattle measured at Farm A showed clearly enhanced caesium-137 levels; the calves apparently had slightly higher levels than their dams although this difference may be an artifact due to differences in geometry. These animals had been removed from the pasture t w o days earlier. Animals measured at Farms B, C and D had been removed from the sea-washed pasture by the farmers for 30, 8 and 80 days respectively prior to the measurement. None of the cattle at Farm B showed any evidence for the presence of caesium137. L o w levels of caesium-137, near the detection limit of the system, were measured in one animal at Farm C and one at Farm D.

TABLE 2

Specific a c t i v i t y o f the muscle o f c a t t l e , November measurements

Farm

Animals

A

2 adult cows 2 six month old c a l v e s

B

6 cows

AND.

,

~ 5

C

1 cow 5 cows

AND.

5a ,

< 7

1 cow 3 cows

AND.

13 a ,

< 7

D

Specific activity ( B q k g -1 o f 137Cs) 15--18 a 19--24 a

A N D ----activity not detected f o l l o w e d b y t h e 9 5 % detection activity. a Combined random and systematic uncertainties a r e o f the order o f -+50%.

44 THE GAMMA-RAY SURVEY AND COLLECTION OF SAMPLES

The fields grazed by the cattle were surveyed using sodium iodide scintillation dose rate meters. Generally, the survey was made by traversing parallel lines 20--40 m apart and taking readings at 1 m above the ground every 20 m. On certain bigger pastures representative lines at distances greater than 40 m apart were traversed. Nine separate fields or pastures were surveyed covering almost eighty hectares. There was a clear inverse relation between exposure rate and elevation presumably due to a direct link between level of contamination and frequency of flooding. Furthermore, it was often possible to distinguish areas with different levels of contamination by the appearance of the grass. The lower-lying, more contaminated pasture included grass varieties adapted to a saline environment and was more heavily covered by fine silt. On the basis of the gamma-ray survey, sampling sites were chosen at points representative of the different levels of contamination within a field. Usually three sampling points were chosen in each field except where the field showed small variation in dose rate levels. At each point, grass was cut with shears to a height of 1--2 cm above ground over an area of approximately 1 m 2 . A 1 0 ¢ m diameter soil core of approximately fifteen centimetres depth was also taken. The top two centimetres of the core were separated from the rest of the core for separate measurement. The samples, grass, topsoil and subsoil were dried and the activity of gamma-ray emitting nuclides measured with a lithium-drifted germanium detector. A breakdown of the area surveyed by exposure rate and typical caesium-137 activities measured in grass and topsoil from those areas is given in Table 3. Samples of the water drunk by the cattle were also taken. The water was, in all cases, from pipe-fed troughs or fresh flowing streams and measurements showed t h a t in no case did the activity concentration exceed 10 Bq 1-1

TABLE 3 Breakdown of area surveyed by exposure rate, with typical caesium-137 specific activities in grass and topsoil samples taken from those areas Exposure rate (pGy h -1 in air)

< 0.1 0.1--0.2 0.2--0.5 0.5--1.0 1.0--2.0 :> 2.0

Area (hectares)

5.8 5.8 20.9 18.3 19.8 8.1

Specific activity of 137Cs (Bq kg -1 dry weight) Grass

Topsoil (to 2 em depth)

i00-- 200 100-- 200 400--2000 1000--4000 2000--6000 6000--9000

40-100 100-200 2000--10,000 5000--15,000 12,000--30,000 15.000--30,000

45

of caesium-137 and in most cases did n o t exceed the detection limit of less than 1 Bq 1-1 of caesium-137. Intake of water therefore does n o t contribute significantly to the activity intake of the cattle.

ESTIMATION OF DAILY CAESIUM INTAKE AND TRANSFER COEFFICIENT

The grazed pastures were divided into regions of high, medium and low contamination on the basis of the results of the gamma-ray survey. The boundaries between the high and medium contamination regions and the medium and low contamination regions were generally, though n o t always, the 1.0 p G y h -1 and 0.5 p G y h -1 dose rate contours respectively. The area of each region was calculated and it was assumed that within each region the level of caesium-137 contamination on the grass was constant and equal to the specific caesium-137 activity measured in the grass sample from that region. Three estimates for the possible average specific activity of grass consumed by the cattle on each pasture were then made. Firstly maximum activity intake animal was postulated, that is, one grazing throughout the field. Three regions of area A l, A: and A3 were defined and assigned a uniform specific activity Q1, Q2 and Q3. The average specific activity of the grass consumed by a maximum intake c o w was assumed to be:

A1Q2 + A 2 Q 2 + A a Q 3 A1 + A 2 + A 3 This is probably an overestimate of the average activity of grass consumed, since the higher, least contaminated, ground is generally better pasture supporting more, and better quality, grass. Furthermore, it is reasonable to assume that although cattle are partial to salt in small quantities they do in general prefer the grass of the higher pasture to the salt- and silt-covered grass of the lower pasture. Next an intermediate intake animal was postulated, that is, one showing a preference for the higher pasture (A l) and not grazing the lowest pasture (A3) to any significant extent. The estimated average specific activity of grass consumed by this animal was taken as: 2AIQ1 +A2Q2

2A 1 + A : Finally, a minimum activity intake animal was postulated, that is, one grazing only the best grass available on the higher pasture so that the specific activity of its intake was Q1. Where the cattle grazed more than one field, the specific activity or intake values were further modified by occupancy factors, which is an estimate of the proportion of the animals' time spent in each field. This information was supplied by the farmers. It is known that cattle consume some topsoil as they graze, the quantity consumed depending on the type and condition of the pasture. An average figure is 4% by dry weight of the daffy intake [9]. The ratio of the specific

46 TABLE 4 Postulated specific activity of grass c o n s u m e d by the cattle Farm

A B C D

Postulated specific activity (Bq kg -1 dry weight) Maximum

Intermediate

Minimum

3700 1000 1100 1700

2800 650 680 710

1800 470 420 630

TABLE 5 F r a c t i o n of daily intake of caesium-137 per kilogram of muscle at equilibrium (transfer coefficient) estimated for cattle at each farm using the ' i n t e r m e d i a t e ' assumption for the specific activity of grass and a daily intake of 15 kg of dry feed

Location

Transfer coefficient (day kg -1 )

Basis for estimate

Farm A

4 X 10 -4

2 cows (Galloway)

Farm B

<:9 X 10 -4

6 cows (Friesian)

Farm C

6 × 10 -4 <:9 X 10 -4

1 cow (Friesian) 5 cows (Friesian)

Farm D

8 x 10 -3 <:4 × 10 -3

1 cow (Friesian) 3 cows (Friesian)

caesium-137 topsoil activity divided by the specific caesium-137 grass activity at each measurement site was found to vary from a b o u t one on the higher pastures to a maximum of ten on the most contaminated pastures. For a value of 4% and median values of the specific soil activity, it seems probable that the intake of soil could not increase the total activity intake by more than 10%. Since the length of sward and moisture content varied considerably over the pastures monitored, the 4% value is unlikely to be applicable to all the pastures. Therefore the effect on activity intake of the consumption of soil has been neglected as being small in relation to other uncertainties in the investigation. Table 4 gives the postulated average specific activity of grass consumed by the cattle under the three intake assumptions. A mature c o w consumes approximately 15 kg of dry matter per day [14]. Therefore, average dally caesium-137 intakes for the cows can be obtained by multiplying the values

47 in the table by this factor. It is n o t possible to estimate daily intakes for the calves at Farm A since they were still suckling at the time of measurement in November. The measured activities in muscle in November were used to estimate the activities in muscle at the time of each cow's removal from the contaminated pasture, when it was assumed that equilibrium existed between caesium-137 intake and concentration in muscle. A value of 30 days [3] was used in this calculation for the biological half-life of caesium in muscle. Values for the fractional daily intake of activity per kilogram of meat at equilibrium were then calculated using the intermediate assumption for specific activity of grass consumed. Table 5 shows the values obtained for the animals at each farm.

DISCUSSION OF RESULTS The specific activities in the cattle in May (Table 1 ) w e r e generally below the limit of detection, since most of the animals had wintered away from the contaminated pasture. The activities in grass measured in July (Table 3) were in agreement with other surveys conducted in the area in previous summers [21]. In view of the caesium-137 activities measured in grass, the specific activity measurements in muscle made in November (Table 2) are low, although the higher levels of activity in grass measured on Farm A (Table 4) correspond with a higher level of activity measured in the animals from t h a t farm. The difference between the postulated specific activity of grass consumed by the cattle under m a x i m u m and m i n i m u m assumptions is less than a factor of three. This suggests that, even though there was variation in contamination over the pastures measured, the average daily intakes c o m p u t e d for the adult cows using the intermediate intake assumption is unlikely to be in error by more than a factor of two. The values for the transfer coefficient of caesium to meat for cattle from different farms (Table 5} are remarkably consistent except for the result for one cow from Farm D. However, the results from this farm are suspect since the animals had been removed from the contaminated pasture eighty days, that is, almost three biological half-life periods before the external gammaray measurements were made. The value from Farm A must be regarded as most reliable since it was at this farm that the activity in the animals was highest and therefore least subject to uncertainty. If the m a x i m u m and m i n i m u m postulated specific activities of grass consumed are used, estimates for the caesium transfer coefficient of 3 x 10 -4 days kg -1 and 6 x 10 -4 days kg -1 may be obtained for the two adult cattle at Farm A. The best estimate for the caesium transfer coefficient for these animals, obtained using the intermediate postulate of average specific grass activity, is 4 × 10 -4 days kg -I . This value is based on only two animals, but is consistent with the limiting value of less than 9 x 10 -4 days kg -1 , which is based on eleven animals at Farms B and C.

48 It is n o t impossible that there was some residual gut activity in the animals when measured at Farm A. However, if significant interfering activity were present the true tissue activity would be lower than recorded and the calculated transfer coefficients even smaller than those given above. The unusually low transfer coefficient determined in this experiment strongly suggests that the bulk of the caesium-137 on the estuary pastures is n o t available for uptake by the cattle. An investigation of caesium-137 distribution in freshwater silts and clays has shown that the major part of the activity (over 80%) may be expected to be b o u n d to very small particles, of less than t w o micron diameters [15]. Caesium is particularly strongly sorbed by some clay minerals, notably illite, in which the interlattice spacing is the optimum distance to permit trapping of caesium ions [16]. The particle size and mineral composition of the Ravenglass deposits vary throughout the estuary, being in some places mainly sand (quartz) and in other places more fine silt and clay material containing illite, kaolinite, calcite, chlorite and iron flocs [17, 18]. It has been shown that the bulk of the caesium-137 attached to the Ravenglass material is very strongly b o u n d and n o t significantly removed without treatment likely to destroy the physical structure of the clay mineral particles [17]. The mixing of downstream freshwater flow with incoming tide at Ravenglass provides good conditions in which to form a suspension in water of contaminated silt from the estuary mudbanks. The suspended material is then deposited in low-lying pastures during flooding. The very small particles, with which the bulk of the caesium-137 is probably associated, may be retained on herbage for considerable periods, trapped amongst hairs, in crevices, or on sticky glandular secretions. Simple experiments have been performed on fresh grass samples from West Cumbria. In samples taken from two sea-washed pastures grazed by the cattle, it was found that between 1 and 20% only of the caesium-137 activity in or on the grass was soluble in distilled water. In samples taken from inland sites, where the caesium-137 activity arises mainly from air discharges from t h e Windscale plant, 70 to 90% of the caesium-137 was soluble under the same conditions. Although this experiment was n o t in any way intended to mimic the behaviour of the caesium-137 in the gut of the grazing animals, it does demonstrate a difference in the form of the caesium contamination between the two sites. On the basis of the above discussion, it seems highly likely that most of the caesium-137 activity on the grass of the estuary pastures that is consumed by the grazing cattle is attached to fine silt particles or associated with ferruginous material on the surface of the grass and in either case is n o t available for uptake by the cattle. The higher levels of caesium-137 measured in the Farm A cattle in May deserve noting. Between the previous November and February, these animals had been grazing rough pasture adjoining the seashore to the northwest of the estuary (A' in Fig. 1). Grass contamination levels here are lower than on the estuary pastures, around 100 Bq kg -1 dry weight in t w o samples taken in July. However, the contamination is most probably due to resuspended sea-

49

water deposited as spray, and the soil is predominantly sandy. It seems likely therefore, that the caesium-137 on these foreshore pastures is in a form more readily absorbed through the gut of grazing animals. However the pastures are not economically important, since they are rarely grazed and then by very few animals.

CONCLUSION

The transfer coefficient for caesium-137 from grass to the muscle of cows grazing sea-washed pasture around the Ravenglass Estuary was found to be approximately two orders of magnitude less than commonly-accepted values. The low transfer is apparently related to the binding of the caesium contamination to fine grain silt particles. The significance of the result obtained in this investigation is twofold. In general, it demonstrates the danger of applying standard values of nuclide transfer in situations where all enhancing or inhibiting mechanisms of transfer have not first been identified and taken into account. In particular, it demonstrates the importance of the physical form of the caesium-137 in the Ravenglass Estuary area. It has been suggested that inland movement of radionuclides from the estuary may occur by the suspension in air of dried silt [19]. However, from this investigation, it appears that if such inland transport takes place the radiocaesium may be in a form which is unavailable or only marginally available for uptake by plants and animals.

ACKNOWLEDGEMENT

The author would like to thank Mr J. Stuart of Ministry of Agriculture, Fisheries and Food, Cockermouth, for his advice and assistance with this study.

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50 from radionuclides in agricultural products. IAEA-SM-237/54, 1979. 7 L. Fredriksson, R. J. Garner and R. Scott Russell, in R. Scott Russell (Editor), Radioactivity in Human Diet, Ch. 15, Pergamon Press, Oxford, 1966. 8 R. J. Garner, Transfer of radioactive materials from the terrestrial environment to animals and man. CRC Crit. Rev. Environ. Control, 2 (3) (1971) 356. 9 S.M. Haywood, J. R. Simmonds and G. S. Linsley, The development of models for the transfer of 137Cs and 9°Sr in the pasture--cow--milk pathway using fallout data. N R P B - R l l 0 , HMSO, 1980. 10 N. T. Mitchell, Radioactivity in surface and coastal waters of the British Isles 1972-73. Technical Report of the Fisheries Radiobiology Laboratory, FRL 10, 1975. 11 D. S. Popplewell, G. J. Ham, T. E. Savory and W. R. Bradford, Radionuclide concentrations in some cattle and sheep from West Cumbria. Radiological Protection Bulletin No. 41, July 1981. 12 S. L. Hood and C. L. Comar, Metabolism of caesium-137 in rats and farm animals. Arch. Biochem. Biophys., 45 (1953) 423--433. 13 ICRP Publication 2, Report of Committee II on Permissible Dose for Internal Radiation, Pergamon Press, 1959. 14 C. L. Comar, in R. Scott Russell (Editor), Radioactivity and Human Diet, Ch. 7, Pergamon Press, Oxford, 1966. 15 T. F. Lominick and D. A. Gardiner, The occurrence and retention of radionuclides in the sediments of White Oak Lake. Health Phys., 11 (1965) 567--577. 16 T. Tamura and D. G. Jacobs, Structural implications in caesium sorption. Health Phys., 2 (1960) 391--398. 17 D. Charles, The desorption behaviour of artificial radionuclides sorbed onto estuarine silt. MSc Thesis, University of Manchester, January 1981. 18 E.I. Hamilton, Personal communication, 1981. 19 Royal Commission on Environmental Pollution. Sixth Report, Nuclear Power and the Environment. HMSO, London, 1976, p. 135. 20 J. R. Simmonds and G. A. M. Webb, The choice of food consumption rates for radiation dose assessments. Radiological Protection Bulletin No. 42, September 1981. 21 A. Knight, Personal communication, 1981.