Factors contributing to radiocaesium variability in upland sheep flocks in west Cumbria (United Kingdom)

Journal of Environmental Radioactivity 98 (2007) 50e68 www.elsevier.com/locate/jenvrad

Factors contributing to radiocaesium variability in upland sheep flocks in west Cumbria (United Kingdom) N.A. Beresford a,*, C.L. Barnett a, S.M. Wright a, B.J. Howard a, N.M.J. Crout b a

Centre for Ecology & Hydrology, CEH-Lancaster, Lancaster Environment Centre, Library Avenue, Bailrigg, Lancaster LAI 4AP, UK b University of Nottingham, School of Biosciences, Biology Building, University Park, Nottingham NG7 2RD, UK Received 19 March 2007; received in revised form 25 May 2007; accepted 26 May 2007 Available online 27 August 2007

Abstract Following the Chernobyl accident in 1986, restrictions were placed on the movement and slaughter of sheep within upland areas of the UK because radiocaesium activity concentrations in their meat exceeded 1000 Bq kg1 fresh weight. Some farms remain under restriction in 2007. From 1991 to 1993 detailed studies were conducted on three sheep farms within the restricted area of west Cumbria to systematically assess the various parameters which may contribute to the observed variability in radiocaesium activity concentrations within sheep flocks. This paper reports the spatial variation in soil and vegetation activity concentrations across the grazed areas at these farms and determines the influence of grazing behaviour on variability in 137Cs activity concentrations between individual sheep within the flocks. Together with previously reported results, these new data are used to draw conclusions on the factors determining variability within the three flocks. However, the factors are too site specific to be able to generalise the findings to other farms within the restricted areas of the UK. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Chernobyl accident;

137

Cs; Sheep; Grazing behaviour; Upland pastures

* Corresponding author. Tel.: þ44 1524 595856; fax: þ44 1524 61536. E-mail address: [email protected] (N.A. Beresford). 0265-931X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jenvrad.2007.05.009

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1. Introduction 1.1. Post Chernobyl restricted areas in the United Kingdom Following the Chernobyl accident in 1986, large areas of uplands in the UK were contaminated by radiocaesium. The grazing of free-ranging sheep is the predominant land use in these areas, which are characterised by nutrient-poor, highly organic acidic soils. Subsequently, the level of radiocaesium in the tissues of some sheep exceeded the limit, adopted within the UK following the Chernobyl accident of 1000 Bq kg1 fresh weight (FW), above which meat cannot enter the food chain. In 1986, restrictions were placed on the movement and slaughter of sheep in areas of west Cumbria (1670 farms), north Wales (5100 farms) and Scotland (2144 farms) with 122 farms in Northern Ireland being added in 1987 (Food Standards Agency (FSA), 2006). Whilst the number of farms under restriction reduced rapidly, there were still more than 700 farms restricted in 1991 (including more than 140 in Cumbria) when the work described in this paper began. A farm remains under restriction whilst any sheep within its flock is estimated to have more than 1000 Bq kg1 of radiocaesium in its muscle or if there is no stock-proof boundary separating the grazing area of its flock from those of farms still under restriction (Nisbet and Woodman, 2000). In 2006, these restrictions remained in place on 9 Cumbrian, 355 Welsh and 10 Scottish farms (or part farms) (Environment Agency et al., 2006). The mechanism of enforcement adopted by the UK authorities within the restricted areas is referred to as the ‘Mark and Release scheme’ (McDonough and Segal, 1991) under which farmers must notify the appropriate authorities when they want to remove sheep from areas under restriction. Before removal, the radiocaesium activity concentration in the hind-leg muscle of sheep is estimated by live-monitoring (Meredith et al., 1988). Although the actual limit is 1000 Bq Cs kg1 FW, a safety margin is applied such that sheep with an activity concentration of circa 730 Bq kg1 FW in their muscle were classed as ‘failing’. Nisbet and Woodman (2000) define this value as the working action level1; the 95% confidence level for an estimated activity concentration of 730 Bq kg1 being 1000 Bq kg1. Sheep above this limit can be removed from the restricted areas but, they are identified using paint and ear-tags, and cannot go directly for slaughter (McDonough and Segal, 1991). However, most lambs are first ‘fattened’ on improved pastures with comparatively low radiocaesium activity concentrations in vegetation. Therefore, the Mark and Release scheme did not necessitate large changes to the normal management of upland sheep and has generally been considered acceptable to affected farmers (Nisbet and Woodman, 2000). Although initially there were some issues of poor communication (Wynne, 1989) public confidence in lamb meat from these upland areas has been maintained.

1.2. Radiocaesium variability within sheep flocks Relatively few sheep live-monitored under the Mark and Release scheme actually fail; within Cumbria, less than 1% have failed from 1988 onwards (McDonough and Segal, 1991; Howard and Beresford, 1994; Nisbet and Woodman, 2000). However, considerable variability has been observed between the radiocaesium activity concentrations in the muscle of individual sheep within flocks in the UK (Walters, 1988; Coughtrey et al., 1989; Pearce et al., 1990; 1

The working action level had subsequently been decreased to 645 Bq kg1 FW at the time of writing.

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McDonough and Segal, 1991; Howard and Beresford, 1994) and other countries (Pearce et al., 1990; McGee et al., 1994; Mehli, 1996). Between spring 1991 and the end of 1993 detailed studies were conducted on three upland sheep farms within the restricted area of west Cumbria to systematically assess the various parameters which may contribute to the observed radiocaesium variability within sheep flocks. Some of this work has already been described (Beresford et al., 1996, 1998, 2000) and is only briefly reviewed below. Variation in 137Cs activity concentrations between individuals within study flocks ranged by up to an order of magnitude (Beresford et al., 1996). Beresford et al. (1996) also showed that certain sheep always had higher or lower 137Cs activity concentrations compared with others within a flock. Consequently, the few sheep with estimated radiocaesium activity concentrations in excess of the UK intervention limit (<1 to 5% within the study flocks) on any given farm are likely to be the same individuals over time. Furthermore, as a relationship between the 137Cs activity concentration of ewes and their pre-weaned lambs was also observed, it is likely that the same ewes are producing lambs (for the human food chain) in excess of the intervention limit in subsequent years (Beresford et al., 1996). In agreement with previous observations (e.g. Colgan, 1992; Howard and Beresford, 1994) 137Cs activity concentrations were higher in the summer months, when there was also a larger range of individual 137Cs activity concentrations (Beresford et al., 1996). Under controlled conditions, variability in the transfer of radiocaesium from the diet to the muscle of sheep was determined by dry matter intake and live-weight change (Beresford et al., 1998). When the 137Cs transfer to ewes was determined under field conditions, using a faecal marker approach to estimate dietary 137Cs intake, the variation between individuals was thought to be due to herbage selection and differences between individuals in the transfer of 137Cs from the diet to muscle (Beresford et al., 2000). In this paper we report the results of spatial variation in soil and vegetation activity concentrations across the grazed areas at these three study farms and determine the influence of grazing behaviour on variability in 137Cs activity concentrations between individual sheep within the flocks. Together with the previously reported results, the data are used to draw conclusions on the factors determining variability within the three flocks. 2. Materials and methods 2.1. Study sites The study farms were selected with the aid of local Ministry of Agriculture Fisheries and Food officers in Spring 1991. All three farms were located within the south of the restricted area of west Cumbria and were known to have a comparatively high proportion of sheep which failed the Mark and Release scheme. The farm codes (A, B and C) used here are consistent with those used in previous publications (Beresford et al., 1996, 1998, 2000; Beresford, 2002). For the majority of the year sheep at the study farms graze over large upland areas of unimproved natural vegetation (referred to as ‘fell’) and only graze on improved pastures during mating and lambing or for shorter periods to enable shearing, parasite control and separation of lambs (see Howard and Beresford, 1989, 1994). At Farm A, the flock grazed a large (3 km2) unimproved enclosure; the flocks at Farms B and C graze over unenclosed (as is more typical for the region) areas of circa 7 and 3 km2 respectively. Soil maps (1:25 000 scale) were prepared for these grazing areas by soil surveyors who visited the sites and examined the soils along a series of transects using auger borings and small pits. The data from the transects were extrapolated to the rest of the grazing area using aerial photographs (ADAS 1:10 000

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and 1:18 000 scale) of the grazing areas. Soils were classified according to Avery (1980) (Figs. 1e3); complete descriptions of the soils and associated vegetation at the three sites can be found in Beresford (2002). 2.2. Study sheep The flocks at Farms A, B and C comprised of approximately 400, 1200 and 500 ewes, respectively, and contained predominantly two upland breeds of sheep (Herdwick and Swaledale). The lambing percentages at Farms A and B over the period 1991e1993 were approximately 100% (i.e. each ewe producing one lamb) whilst that at Farm C was considerably lower at approximately 50%. After weaning, male lambs were sold (for subsequent slaughter) and most female lambs removed to lowland pastures until the following spring when they were returned to the flock as future breeding stock. During the winter at Farms A and C the flocks had access to improved pasture and some supplementary feedstuffs. Groups of 40 ewes were selected at each farm and were each fitted with uniquely numbered ear-tags. Each year, as soon as possible after lambing, newly born lambs were paired to their dams and also given numbered ear-tags. The study groups were maintained at between 40 and 50 ewes throughout the 3 years; female lambs, when returned to the farms in the spring after they were born were returned to the study group. The study animals were live-monitored to determine their 137Cs activity concentration within less than 2 days of being collected from the unimproved grazing areas for routine farming practices. The 137Cs activity concentration in hind-leg muscle was determined using a hand-held, lead-shielded 44.5 mm NaI detector linked to a single channel analyser (Meredith et al., 1988; Beresford et al., 1996). The live-monitoring results have been presented previously (Beresford et al., 1996) and are not repeated here although they are used in the subsequent analyses of the influence of grazing behaviour.

Fig. 1. Soil map for Farm A. Contour lines are at 10 m altitude intervals.

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Fig. 2. Soil map for Farm B. Contour lines are at 10 m altitude intervals.

2.2.1. Grazing observations Each study ewe was also fitted with an individually numbered/coloured collar and painted (using lanolin based sheep marker fluid) with unique coloured markings on the head and/or back. This enabled identification of each ewe whilst on the fell from a distance of circa 200 m with the aid of binoculars. Whilst on the fell, observations of where individuals grazed were made; observation days were targeted towards the periods either side (2e4 weeks) of live-monitoring dates. 2.3. Soil and vegetation sampling At Farms A and B soil and vegetation sampling was conducted during July/August 1992; sampling at Farm C was conducted in early September 1992. Eighty sites selected randomly from a 100 m grid based on Ordnance Survey national grid squares were sampled over the grazing areas of Farms A and C; whereas, over the larger grazing area of Farm B, 100 randomly selected sites were sampled. Sites were located using landmarks, Ordnance Survey 1:25 000 scale maps and compass bearings. At each sampling site, the vegetation species present were noted. Vegetation was then collected, using hand shears, from an area of 1 m2, to a height of circa 1 cm above the soil surface. Soil samples were collected down to 30 cm or the bedrock/stony layer if present, using a spade. After being trimmed to a uniform

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Fig. 3. Soil map for Farm C. Contour lines are at 10 m altitude intervals.

rectangle, samples were divided into two layers, 0e4 cm and >4 cm. A soil category was attributed to each sampling site on the basis of the soil maps (Figs. 1e3) and observations of soil characteristics made during sampling. 2.3.1. Sample preparation and analyses Graminaceous species (i.e. grasses and sedges such as Nardus stricta, Agrostis spp., Eriophorum vaginatum, Festuca ovina, Molinia caerulea and Tricophorum cespitosum), which were the dominant component of collected samples, were separated from the samples and retained for analyses. Where sufficient quantities of non-graminaceous species were present, these were sub-divided into component species for subsequent analyses. At Farm C there were four sample sites where no graminaceous species were present; three were dominated by Calluna vulgaris (heather) and one by Pteridium aquilinum (bracken). The bulk density was determined for both soil layers by sub-sampling a known volume of soil, drying at 80  C and weighing. The percentage loss on ignition (LOI) of the soils was determined by ashing a subsample (1 g) of oven-dried material for 2 h at 500  C. Soil and vegetation samples were dried at 80  C; the total weights of dried vegetation samples were recorded. Dried samples were ground and accurately weighed into 150 ml plastic containers for subsequent gamma-analyses.

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The 134Cs and 137Cs activity concentrations in soil and vegetation samples were determined using hyper-pure germanium semiconductor detectors with relative efficiencies ranging from 25% to 40%. Resultant spectra were analysed using the Canberra ‘Apogee’ software package; analysis times (ranging from 1.4 to 47 h depending upon sample activity) were sufficient to achieve a counting error of less than 5% of the reported result for 137Cs at 1.65 s. The detectors were calibrated using mixed gamma standards (National Physical Laboratory, Teddington, UK) incorporated into appropriate matrices (including different soil densities) and volumes. 2.4. Statistical analyses and data manipulation Regression analyses and some within site comparisons (paired T-tests) were performed using the analysis tools available within Microsoft Excel. Comparisons between sites, and also within sites between soil types and vegetation groups, were performed using the generalised linear model (PROC GLM) procedures of the SAS statistical software package. The study area had received significant depositions of 137Cs prior to the Chernobyl accident as a result of both the 1957 Windscale accident and above ground nuclear weapons testing (both sources are hereafter referred to as aged radiocaesium). To determine the contribution of these ‘aged’ sources the total 137Cs activities determined in soil and vegetation samples a 137Cs:134Cs ratio of 1.7 in Chernobyl fallout in west Cumbria as derived by Wright et al. (2003) for Cumbria was assumed. At the time of sampling this would result in a Chernobyl 137Cs:134Cs ratio of circa 12 as a result of physical decay (the exact ratio varying between farms depending upon sampling time). Wright et al. (2003) derived this radiocaesium ratio from 156 measurements of cow milk collected within Cumbria in the aftermath of the Chernobyl accident (MAFF/WO, 1987); the 137Cs:134Cs ratios ranged from 1.08 to 2.54 in individual samples and the value used is in agreement with other estimates from more restricted data sets for Cumbria (see Wright et al., 2003 for comparison). 2.4.1. Spatial analyses Spatial interpolation of the data sets (log transformed) was conducted using block (10  10 m) kriging (Karssenberg and Burrough, 1996) using ESRI’s ARCInfo geographical information system software.

3. Results and discussion 3.1. Radiocaesium deposit Caesium-137 was detectable within all soil samples. Levels of 134Cs were below the minimum detectable activity in some samples, largely the deeper soil layer (i.e. >4 cm). The radiocaesium deposit (i.e. radiocaesium activity concentration per metre square at the time of sampling) at each sampling location was estimated using the bulk densities determined for each soil layer. The deposit of both 134Cs and 137Cs was spatially heterogeneous across all three grazing areas; variation between sampling sites approached an order of magnitude across each area (see Table 1). Semivariograms plotted for the interpolation of the deposition using block kriging demonstrated no spatial trends to the measured variation in deposition. There was no significant difference in the deposit of either radiocaesium isotope on the basis of sampling site soil type at any of the three farms ( p > 0.05). 3.1.1. Estimation of pre-Chernobyl deposit A comparison of the 137Cs and 134Cs deposits demonstrates a ratio in excess of that expected from the Chernobyl accident at virtually all sampling sites (Fig. 4). The 137Cs activities attributable to aged sources and the Chernobyl accident are shown in Table 1. Estimation of the

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Table 1 Summary of radiocaesium deposition data for each grazing area as determined during the summer of 1992 Farm A

Farm B

Farm C

Cs, Bq m2 Mean  SE Range n

15 440  810 5430e35 490 80

15 580  750 5320e44 090 100

11 500  720 4060e36 870 80

Cs, Bq m2 Mean  SE Range n

790  38 276e1990 51

970  44 371e2800 87

680  40 260e2270 49

21  0.7 11e37 51

16  0.2 12e23 87

19  0.47 13e28 49

137 Cs deposit due to aged sources Mean  SE 41  1.9 Range 8.4e66 n 47

25  0.9 7.2e46 84

32  1.3 4.4e55 49

% Aged 137Cs in 0e4 cm soil layer Mean  SE 43  3.3 Range 8.1e92 n 47

41  2.3 3.2e92 84

44  3.9 5.9e94 49

% Chernobyl 137Cs in 0e4 cm soil layer Mean  SE 56  2.7 Range 19e97 n 47

52  2.0 16e95 84

42  3.1 4.6e89 49

137

134

137

Cs:134Cs Mean  SE Range n

%

amount of aged 137Cs was not attempted at sampling sites where 134Cs activities were below the minimum detectable activity in either soil layer. There were a few (n ¼ 6) sampling sites where the amount of aged 137Cs in one of the soil layers was estimated to be negative (ranging to  1200 Bq m2 for a peat soil sampling site at Farm A which had a 137Cs:134Cs ratio of 11.1 compared to the value of 12.0 used to estimate the contribution of aged sources at this farm). Negative values were not used in the derivation of summary statistics or during statistical analyses. The estimated percentage of the total 137Cs deposit which was attributable to aged sources varied significantly ( p < 0.05) between sampling areas in the order, Farm A (41%) > Farm C (32%) > Farm B (25%). The Chernobyl and aged 137Cs deposits at Farm A were less correlated (R2 ¼ 0.13) than at Farm C (R2 ¼ 0.33) and Farm B (R2 ¼ 0.40). There was a relationship between soil bulk density and the 137Cs:134Cs ratio in the deposit at all three sampling areas; sites with a low soil bulk density having a lower 137Cs:134Cs ratio (Fig. 4). The lower 137:134Cs ratio at sampling sites with more organic soils suggests a loss of pre-Chernobyl-derived radiocaesium from soils with increasing organic matter content (as demonstrated by a low soil bulk density). Cawse and Baker (1990) have previously reported that circa 50% of 137Cs deposited from above ground nuclear weapons tests was lost from peat soils at sites across the UK over a period of 20 years as a consequence of water flow and lack of clay minerals. At all three study farms, the percentage of Chernobyl-derived

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58 24

R2=0.45; p<0.01

137Cs:134Cs

in deposit

22 20 18 16 14 12 10 0

100

200

300

400

500

600

700

800

900

1000

Soil bulk density kg m-3 (DM) Fig. 4. An example of the relationship between the isotopic ratio of radiocaesium deposit and soil bulk density (horizontal dashed line denotes Chernobyl 137Cs:134Cs ratio of 1.7 decay corrected to time of sampling); data for Farm B. Relationships at Farms A and C yielded R2 values of 0.33 ( p < 0.01) and 0.24 ( p < 0.01), respectively.

radiocaesium deposit within the 0e4 cm soil layer of sampling sites on peat soils was significantly less than for other soil types ( p < 0.05). The higher contribution of aged radiocaesium at Farm A compared to the other two sites is consistent with the pattern of deposition from the 1957 Windscale accident (Chamberlain, 1959; Cawse, 1980; Jackson and Jones, 1991).

3.2. Radiocaesium activity concentrations in vegetation Caesium-137 was detectable in all graminaceous vegetation samples (Table 2); 134Cs was below the limits of detection in circa 10 samples from each farm. The radiocaesium ratio in vegetation samples indicated significant contributions from aged radiocaesium sources (Table 2). Radiocaesium activity concentrations determined in graminaceous vegetation were

Table 2 Summary of radiocaesium activity concentrations in graminaceous vegetation Farm A

Farm B

Farm C

Cs Bq kg1 (DM) Mean  SE Range

650  58 46e2070

650  50 16e2690

480  42 50e1870

Cs Bq kg1 (DM) Mean  SE Range

43  3.6 4.7e129

48  3.5 5.4e195

30  2.8 3.4e113

17  0.21 14e22

15  0.09 13e19

17  0.23 12e20

137

134

137

Cs:134Cs Mean  SE Range

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considerably more variable than radiocaesium deposits (compare Tables 1 and 2). Variation in 137 Cs activity concentrations between different (non-graminaceous) species of vegetation from the same sampling site was less than that observed between graminaceous vegetation samples (see Table 3). At all farms, there was a spatial pattern (as determined by interpolation) to the variation in radiocaesium activity concentrations within graminaceous vegetation:  Radiocaesium activity concentrations tended to increase with altitude at Farm A (Fig. 5).  At Farm B vegetation collected from the south of the sampling area had the lowest radiocaesium activity concentrations (Fig. 6).  Samples collected from the south-east of the sampling area at Farm C had the highest radiocaesium activity concentrations (Fig. 7).

3.2.1. Transfer of radiocaesium from soil to vegetation Over all three grazing areas there was no relationship between the radiocaesium activity concentration in vegetation and the radiocaesium deposit (R2 < 0.08). As an example, Fig. 8 compares the 137Cs activity concentration in vegetation with the estimated 137Cs deposit at sampling sites within the grazing area of Farm A.

Table 3 Caesium-137 activity concentrations determined in separated non-graminaceous species. Activity concentrations for the specific species are also presented as a ratio to that of the graminaceous species from which they were sub-sampled Cs Bq kg1 DM

Cs Bq kg1 DM in species:137Cs Bq kg1 DM in graminaceous sample

137

137

Mean  SDa

Mean  SDa

Farm A Juncus effucus Pteridium aquilium Polytrichum spp. Mossesb

560 20e180 790e860 410e1610

0.4 0.4e1.4 0.8e2.8 0.7e1.4

1 2 2 2

Farm B Myritilus gayle Polytrichum spp.

430  133 770

0.6  0.23 1.4

4 1

Farm C Calluna vulgarisc M. gayle Vaccinium oxycoccus P. aquilium Polytrichum spp. Mossesb Cladonia spp.

870  549 990  150 840 310  344 1010  264 650  411 1160

1.2  0.39 1.1  0.51 1.0 0.7  0.48 1.6  0.40 1.5  0.47 3.7

11 5 1 9 3 20 1

Species

a

n

Where n ¼ 2 range is presented. Mosses is used to describe a mixture of small ground lying mosses (often Hypnum spp. and Rhytidium spp.) and does not include Sphagnum spp. or Polytricum spp. c Sample comprised shoots and woody material as insufficient sample to analyse separately. An additional sample was separated and 137Cs activity concentrations of 1690 and 480 Bq kg1 DM determined in the shoots and woody material, respectively. b

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Fig. 5. Spatial pattern to the variation in radiocaesium activity concentrations within graminaceous vegetation at Farm A. The overlaid grid displays UK Ordnance Survey National Grid co-ordinates; grid intervals are 500 m.

The transfer of radiocaesium from soil to graminaceous vegetation has been estimated as the aggregated transfer factor [Tag; the ratio of the radiocaesium activity concentration in vegetation (Bq kg1 dry matter (DM)) to the radiocaesium deposit (Bq m2)]. Aggregated transfer values were estimated for total 137Cs, aged 137Cs and 134Cs (which can be taken to be representative of Chernobyl-derived radiocaesium) (Table 4). Although mean Tag values estimated for Chernobyl-derived radiocaesium were higher than those derived for aged 137Cs at all three sites, differences were not significant. There were no significant between-farm differences in estimated Tag values. In a study conducted during late 1989, on two sampling areas of predominately humic ranker soil close to the grazing area of Farm A, significantly higher Tag values were derived for Chernobyl radiocaesium compared with aged deposits (Beresford et al., 1992). In the earlier study, circa 80% of Chernobyl-derived radiocaesium was within the top 0e4 cm layer compared to 42e56% in this work (Table 1). The comparative difference in transfer between the two studies is therefore likely to be due to a greater proportion of Chernobyl-derived radiocaesium being lower in the soil profile, and hence beneath the main plant rooting zone, in the work reported here.

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Fig. 6. Spatial pattern to the variation in radiocaesium activity concentrations within graminaceous vegetation at Farm B. The overlaid grid displays UK Ordnance Survey National Grid co-ordinates; grid intervals are 500 m.

There were significant spatial patterns in estimated Tag values across the three sampling areas which were broadly consistent with those described above for the radiocaesium activity concentrations in vegetation. Spatial variation in Tag followed similar spatial patterns for aged and Chernobyl-derived radiocaesium. Significantly higher Tag values for the total 137Cs and Chernobyl-derived radiocaesium deposits were estimated for samples from peat soils compared with those from brown earths and ferric stagnopodzols at Farm A ( p < 0.05) (see Fig. 1 for soil map). There were no soil-dependent significant differences between Tag values derived for aged radiocaesium. At Farm B, Tag values derived for the total 137Cs deposit were significantly higher for sampling sites on peat soils than on ranker-stagnopodzol soil and the stagnopodzol soil ( p < 0.05) (see Fig. 2 for soil map). These differences are consistent with understanding of the mobility of Cs in different soils types (Livens and Loveland, 1988). There were no significant differences on the basis of soil type for either aged or Chernobyl-derived radiocaesium (note that there are fewer sites at which Tag values for Chernobyl and aged radiocaesium were derived

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Fig. 7. Spatial pattern to the variation in radiocaesium activity concentrations within graminaceous vegetation at Farm C. The overlaid grid displays UK Ordnance Survey National Grid co-ordinates; grid intervals are 500 m.

compared with the number of Tag estimates for the total 137Cs deposit). No significant soildependent differences in the transfer of radiocaesium to vegetation were observed at Farm C.

3.3. Grazing observations On every occasion a sheep was identified its position was recorded (to approximately the nearest 50 m) using landmarks, compass bearings and Ordnance Survey maps. The total numbers of recorded observations of study ewes were 149 at Farm A, 538 at Farm B and 172 at Farm C; a total of 19 observation days were conducted for each grazing area. The number of times an individual was observed ranged from once to more than 20 times. At all three farms, individual sheep were often observed in a similar location. Changes in grazing behaviour were noted at all three farms during the winter: (i) animals congregating in areas supplementary feeds were located; (ii) at Farms A and C the sheep had access to adjacent improved fields; (iii) waterlogged areas were avoided.

137Cs

activity concentration in vegetation (Bq kg-1 DM)

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2500

2000

1500

1000

500

0 0

5000

10000

15000

20000 137Cs

25000

30000

35000

40000

Bq m-2

Fig. 8. An example of the lack of a relationship between radiocaesium deposition and the radiocaesium activity concentration in vegetation; 137Cs data from Farm A.

3.3.1. Geostatistical analyses In order to try to determine if the grazing observations could be used to try to explain any of the variability in 137Cs activity concentrations between individuals within the flocks observed the surfaces of 137Cs activity concentrations in vegetation described above have been used to predict the 137Cs activity concentration of graminaceous vegetation in areas grazed by study ewes. Concurrent to the work described in this study, sheep at Farm C were used in a field test of a global position satellite (GPS) tracking unit (Rutter et al., 1997). During August/September 1993 three ewes at the farm were fitted with the GPS units and released to the fell with the flock and recording made for up to 8 days. The ewes had a comparatively small grazing area over the measurement period ranging between circa 25 and 30 ha. These observations are in agreement Table 4 Aggregated transfer factor values derived to describe the transfer of radiocaesium to graminaceous vegetation samples collected in summer 1992. Tag values are presented for the total 137Cs, aged and Chernobyl-derived radiocaesium deposits Tag (m2 kg1)

n

Mean  SE

Range

Farm A Total 137Cs Aged 137Cs Chernobyl Cs

(5.98  0.74)  102 (5.44  1.15)  102 (6.31  0.78)  102

(0.26e30)  102 (0.06e33)  102 (0.39e20)  102

80 38 42

Farm B Total 137Cs Aged 137Cs Chernobyl Cs

(5.22  0.46)  102 (5.33  0.57)  102 (5.84  0.51)  102

(0.10e24)  102 (0.15e22)  102 (0.25e22)  102

100 76 79

Farm C Total 137Cs Aged 137Cs Chernobyl Cs

(5.04  0.57)  102 (4.50  1.11)  102 (5.09  0.72)  102

(0.49e29)  102 (0.87e29)  102 (0.55e24)  102

75 35 38

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with previous studies which report the home range of upland sheep within the UK to be in the range 3e50 ha (Hunter, 1964; Hewson and Wilson, 1979; Lawrence and Wood-Gush, 1988). On the basis of the grazing observations, and knowledge of the home range of upland sheep, the 137Cs activity concentration in grazed vegetation for each sheep was estimated, from the kriged surfaces (Figs. 5e7), as the mean vegetation activity concentration from 100 m2 pixels falling within a 250 m radius circle centred on each observation point. Because of the changes in grazing behaviour during winter months (see above) and the observed seasonal changes in the 137Cs activity concentrations in the muscle of study ewes (Beresford et al., 1996), only those observations made between the end of May and mid-October each year were used in this analyses. The number of observation points falling between these dates was 144 representing 37 sheep at Farm A; 453 representing 50 sheep at Farm B; 158 representing 61 sheep at Farm C. For the purposes of comparison, the mean 137Cs activity concentrations of muscle (from Beresford et al., 1996) and predicted vegetation were calculated for every ewe; measurements and predictions for all observations being averaged. At Farms A and B, predicted vegetation activity concentrations explained 33% ( p < 0.01) and 52% ( p < 0.01), respectively, of the variability in the 137Cs activity concentration of the study sheep (see Fig. 9 which present the comparison at Farm B as an example). However, at Farm C the R2 value obtained was only 0.08. The linear relationships fitted to the mean comparisons at Farms A and B yielded similar expressions, both having a ratio between the 137Cs activity concentration (FW) in muscle and the 137Cs activity concentration (DM) in vegetation in the range 0.72e0.75 suggesting a similar concentration ratio between 137Cs activity concentrations in sheep and vegetation at these farms. Hove et al. (1994) report comparable concentration ratios in the range 0.6e0.7 for sheep in a number of Nordic countries. Similarity with the previous concentration ratios suggests that the methodology adopted provides a realistic estimate of the mean 137Cs activity concentration in vegetation being grazed by individuals. Linear expressions fitted to the data intercept the y-axis at a value of circa 200 Bq kg1 which is the approximate detection limit of the NaI detector. 1600 137

-1

137

137Cs

activity concentration in muscle (Bq kg-1 FW)

Cs (Bq kg FW) in sheep muscle = 0.72*(

-1

Cs activity concentration in vegetation (Bq kg DM)) + 242

2

1400

R = 0.52

1200 1000 800 600 400 200 0 0

200 137Cs

400

600

800

1000

1200

activity concentration in gaminaceaous vegetation (Bq kg-1 DM)

Fig. 9. Comparison of the mean 137Cs activity concentration in the muscle of study ewes at Farm B with the mean predicted 137Cs activity concentration in vegetation in areas in which they were observed to graze.

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4. Conclusions Before the work described here a number of workers had noted considerable variability between the radiocaesium activities of individuals within sheep flocks grazing semi-natural ecosystems in areas of Europe which had received significant levels of fallout from the Chernobyl accident (Walters, 1988; Coughtrey et al., 1989; Pearce et al., 1990; McDonough and Segal, 1991; Howard and Beresford, 1994; McGee et al., 1994; Mehli, 1996). However, whilst there had been studies of some individual parameters (predominantly grazing preference; e.g. Rafferty et al., 1993; Salt et al., 1994) which may contribute to the observed variability, there had been no attempt to conduct a more complete analyses of causal effects. The work described here (and in the associated papers referred to) is, to our knowledge, the first attempt to systematically assess the various parameters which may contribute to variability in radiocaesium activity concentrations between individuals within sheep flocks. 4.1. Deposition, herbage transfer and grazing behaviour Whilst radiocaesium deposition was variable at all three study sites (approaching an order of magnitude; Table 1) there was no spatial trend to the variation. Furthermore, there was no relationship between the amount of radiocaesium deposition and the activity concentration in graminaceous vegetation at any of the three study farms. Therefore, across a single grazing area, variation in deposition alone cannot be used to predict variation in the radiocaesium activity concentrations of vegetation and sheep. There was considerably greater variation (circa two orders of magnitude) in the radiocaesium activity concentration of vegetation across each of the grazing areas and this had significant spatial patterns (see Figs. 5e7). At Farms A and B the transfer of radiocaesium from soil to vegetation could be related to soil type. A considerable component of the variability observed in the 137Cs activity concentration in muscle between individuals within a flock (typically an order of magnitude see Beresford et al., 1996) appears to be explained by differences in the spatial grazing behaviour at Farms A and B. There was apparently no influence of grazing area on the 137Cs activity concentration of sheep at Farm C. The range in predicted 137Cs activity concentrations in grazed vegetation at this farm was comparatively low compared to the other two sites; the study sheep all tended to graze in areas with 137Cs activity concentrations in vegetation between 250 and 500 Bq kg1 DM. 4.2. Factors influencing radiocaesium variability within sheep flocks The work presented here and in associated papers (Beresford et al., 1996, 1998, 2000) has evaluated the factors determining the observed variability between the 137Cs activity concentrations of individual sheep within upland sheep flocks. On all three farms the 137Cs activity concentration in the muscle of study sheep ranged over approximately an order of magnitude and on repeated monitoring occasions the same sheep tended to always have the highest 137Cs activity concentrations (Beresford et al., 1996). However, the factors contributing to this variation differed between the farms:  At Farms A and B approximately 30e50% of the variability within the study flocks could be explained by where the sheep grazed (see Section 3.3.1 above). There was no significant influence of area grazed at Farm C.

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 At Farm A there was a significant difference in the 137Cs activity concentration of the two predominant sheep breeds (Herdwick and Swaledale) within the flock (Beresford et al., 1996). This was attributable to the differing spatial grazing behaviours of the two species. No breed differences were observed at the other two farms.  At all three farms there was a positive relationship between the 137Cs activity concentration of lambs and their dams prior to weaning (Beresford et al., 1996). However, there was no relationship post-weaning.  Under controlled conditions variability in the transfer of radiocaesium from diet to muscle was correlated with live-weight change and dry matter intake rate (Beresford et al., 1998). There was evidence of individual variation in the transfer from diet to muscle contributing to observed variability in 137Cs activity concentrations between individuals within the flock at Farm B (a significant positive relationship being determined between the two parameters) when transfer was determined under field conditions (Beresford et al., 2000). However, this was not observed at Farm A (note this study was not possible for practical reasons at Farm C).  At Farms A and C there was some evidence (indicated by relationships between n-alkanes originating from plant cuticular waxes and 137Cs activity concentrations in faeces) that herbage composition contributed to the observed 137Cs variability within flocks although this could not be quantified (Beresford, 2002). This was not indicated at Farm B, although the vegetation cover at this site was more uniform than at the other two. At Farm A, a significant negative relationship between 137Cs transfer from the diet to muscle and daily 137Cs intake was observed, potentially as a consequence of differences in herbage dry matter digestibility (see Beresford et al., 2000). Studies on three study farms, described here and in associated publications, have contributed to our knowledge of the factors which may influence the observed variability in 137Cs activity concentration in upland sheep flocks. However, these appear to be too site specific to be able to generalise the findings to other farms within the restricted areas of the UK.

Acknowledgements The majority of the work described within this paper was funded by the Ministry of Agriculture Fisheries and Foods (now Food Standards Agency). Their contribution and the support of the contract officer, Caroline Morris, is gratefully acknowledged. We would also like to thank Dr. Bob Mayes (MLURI) for his contribution to those aspects of the research programme already published. The soils maps (Figs. 1e3) were provided by Andrew Hipkin (AHA, Stirling), and John Adamson and Mike Hornung of CEH.

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