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Mercury concentrations in soil, grass, earthworms and small mammals near an industrial emission source
MERCURY CONCENTRATIONS IN SOIL, GRASS, EARTHWORMS AND SMALL MAMMALS NEAR AN INDUSTRIAL EMISSION SOURCE
K. R. BULL*, R. D. ROBERTSt, M. J. INSKIP & G. T. GOODMAN
Department of Applied Biology, Biological Sciences Group, Chelsea College, University of London, London, Great Britain
ABSTRACT
Scarcity of data describing mercury concentrations in the biota of an environment subjected to mercury fallout prompted this study around a chlor-alkali works. Atomic absorption spectrometric analysis of top-soils, grass (Festuca rubra L.), earthworms (Lumbricus terrestris L.), and atmospheric fallout, within 0-5 km and 10-30 km of the works, showed that mercury levels were significantly higher near the works. Woodmice (Apodemus sylvaticus L.) and bank voles (Clethrionomys glareolus Schr.) collected near the works had significantly greater concentrations of total mercury in brain, kidney, liver and hair than control animals. The presence of methylmercury in the mammals and L. terrestris is evidence for methylation of the inorganic mercury
fallout. INTRODUCTION
With the increasing concern about mercury pollution, much work has been carried out in the aquatic environment but few workers have studied terrestrial ecosystems affected by atmospheric emissions of the metal. Weiss et al. (1971) have shown by the analysis of the Greenland ice-sheet that the global aerial burden of mercury increased a few decades ago and has remained elevated. The increase has been attributed partly to the evaporation of naturally occurring mercury following increased agricultural activity, and partly to emissions resulting from some industrial activities and the burning of fossil fuels (Jernelrv, 1975). It is known that * Present address: Institute of Terrestrial Ecology, Monks Wood Experimental Station, Abbots Ripton, Huntingdon, Great Britain. t Present address: Botany Department, University of Liverpool, Great Britain.
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Environ. Pollut. (12) (1977)--© Applied Science Publishers Ltd, England, 1977 Printed in Great Britain
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K. R. BULL, R. D. ROBERTS, M. J. INSKIP, G. T. GOODMAN
significantly elevated mercury concentrations in air can occur around local sources and such sources are perhaps much more common than is often realised (Jepson, 1973), but only limited data are available to assess the impact of local mercury fallout on environmental burdens. The chlor-alkali process (Twigg, 1966) is recognised as a potential industrial emission source of mercury (Flewelling, 1971), and it has been shown that some of the mercury emitted to the air, albeit a small percentage, falls out within a few kilometres, whilst outside this zone the fallout is not significantly elevated (Jernelrv & Wallin, 1973). A study centred on a chlor-alkali works was therefore initiated in order to measure the mercury concentrations in the biota within the local fallout zone. The works chosen for the study was situated in open farmland and isolated from other industrial workings. It was known that losses to the air were mainly from the electrolytic cell-room. This paper describes elevated mercury concentrations in soils, grass, Festuca rubra L., and earthworms, Lumbricus terrestris L., and in tissues of the bank vole, Clethrionomys glareolus Schr., and the woodmouse, Apodemus sylvaticus L., from around the chlor-alkali works.
MATERIALS AND METHODS
Sample collection Initially, samples of surface soils (0-2 cm), the grass Festuca rubra L. and earthworms Lumbricus terrestris L. were collected during April 1974 from two groups of sites, one within a radius of 0.5 kin, the study area, and the other 10-30 km around the works. A number of moss bags, as used by Goodman & Roberts (1971), were exposed for seven weeks at the same sites. This technique, which involves the suspension of acid-washed moss (Sphagnum acutifolium agg. L.) in nylon mesh bags, has proved to be extremely useful in the monitoring of atmospherically-borne heavy metals (Goodman et al., 1974). Subsequently, during April, May and June 1974 a number of bank voles C. glareolus, and woodmice, A. sylvaticus, were caught using Longworth traps from the study area. Control animals were collected from rural sites unaffected by industrial emissions. Populations were chosen so that foraging territories did not include agricultural land and mercury concentrations in these animals were therefore unlikely to be affected by mercurydressed grain, as reported by Jefferies et al. (1973). The animals were killed using ether, deep-frozen and returned to the laboratory. After thawing, the brain, hair, kidney, liver and muscle were removed and sub-samples analysed for total mercury and methylmercury. Total mercury analysis Special attention was paid to the development and standardisation of wet acid
MERCURY IN SOIL, GRASS AND ANIMALS
137
oxidation and atomic absorption techniques for total mercury determinations. Soils were passed through a 2 mm sieve in order to remove stones and pieces of vegetation. F. rubra samples were sorted, chopped into 1 cm lengths and mixed thoroughly. Sub-samples of soils and F. rubra were digested without drying and the wet/dry weight ratio determined by drying further sub-samples at 40°C. L. terrestris were killed by deep-freezing and, after thawing, their gut contents removed prior to digestion. Digestions were carried out in Kjeldahl flasks using a concentrated 'Analar' nitric/perchloric acid mixture (4:1). Initial digestion in the cold followed by careful heating was found to prevent any evaporation losses of mercury. Digests were analysed for total mercury with a Varian AA4 atomic absorption spectrophotometer fitted with a 20 cm open-ended glass cell for flameless operation. Reduction to metallic mercury was accomplished with a 20°~ stannous chloride in 4M hydrochloric acid solution, and the mercury vapour flushed from the digest solution by an argon carrier gas. The mercury vapour/argon mixture was introduced into the absorption cell via a drying tube of anhydrous magnesium perchlorate. The apparatus described enabled quantities of mercury as small as 1 ng in 5 ml of digest solution to be measured. Methylmercury analysis Methylmercury was extracted from tissue using the method of West66 (1967). A Pye 104 gas chromatograph fitted with a Ni 63 electron capture detector (operated at 200°C) and a glass 1.5 m × 4 mm column packed with 2 ~o (w/w) neopentylglycol adipate on 80-100mesh acid-washed DMCS treated Chromasorb G (operated at 190°C) was used for the analysis. BOC Ltd high purity nitrogen was used as carrier gas at 40 ml min- x and high purity nitrogen/carbon dioxide (3 o/ /O CO2) was employed as a quench/purge gas at 20 ml min-1. The methylmercury dicyandiamide standard solutions were subjected to the same extraction procedures as the tissue to be analysed. The sensitivity of the gas chromatograph was such that an injection of 1 ng of mercury as methylmercury gave a chromatographic peak height of ~ 70 mm, enabli.ng analyses to be performed on samples of less than 1 g containing only nanogram quantities of methylmercury.
RESULTS AND DISCUSSION
Samples of soil, F. rubra and L. terrestris from sites close to the works were found to have mercury concentrations significantly higher than those further away (Table 1), and the moss-bag figures show that the current atmospheric mercury deposition is higher closer to the works. The mean concentrations of mercury in soils, F. rubra, L. terrestris, and moss-bags within the study area are more than an order of magnitude greater than those 10-30 km distant and the latter are typical of concentrations we have found in uncontaminated areas elsewhere.
138
K. R. BULL, R. D. ROBERTS, M. J. INSKIP, G. T. GOODMAN TABLE I
MERCURY CONCENTRATIONS IN SOIL AND F. rubra (IN UG/G DRY WEIGHT) AND L. terrestris (IN /./G/G FRESH WEIGHT), DEPOSITION ONTO MOSS-BAGS (IN NG/DM2/DAY) (EXPRESSED AS MEANS ± STANDARD ERROR; RANGE IN BRACKETS)
Study area <0.5 km from works
10-30 km from works
3.81 ± 0'95 (0.69-12.6) 4.01 ± 0.65 (1.30-9.41) 1.29 ± 0"32 (0.27-3.27) 63'0 ± 9.9 (37-124)
0-106 ± 0.009 (0.04-0-19) 0.103 ± 0"008 (0.07-0'17) 0.041 ± 0'006 (0.031-0.048) 3.50 ± 0.71 (0--6.7)
Surface soil ( < 2 cm) (n = 42) F. rubra (n = 34) L. terrestris (n = 18) Moss-bags (n = 21)
Degree of significance *** *** ** ***
Note: ** and *** denote statistical significance at P < 0.01 and 0.001, respectively as shown by Student's two tailed t-tests for unpaired data with unequal variance, n = Number in sample. M e r c u r y c o n c e n t r a t i o n s in tissues o f small m a m m a l s w i t h i n the s t u d y a r e a are significantly h i g h e r t h a n c o n c e n t r a t i o n s in c o n t r o l a n i m a l s e x c e p t f o r m u s c l e tissue o f A. sylvaticus ( T a b l e 2). It w o u l d a p p e a r , t h e r e f o r e , t h a t s m a l l m a m m a l s l i v i n g in a n a r e a s u b j e c t e d to e l e v a t e d m e r c u r y d e p o s i t i o n c a n a c c u m u l a t e s o m e o f this m e r c u r y in t h e i r tissues. I n a d d i t i o n to the a n i m a l s d e s c r i b e d in the table, o n e s p e c i m e n o f A. sylvaticus c o l l e c t e d in early A p r i l h a d t h e f o l l o w i n g m e r c u r y levels (in/~g H g / g fresh w e i g h t ) : b r a i n 22-6, h a i r 3.2, k i d n e y 1.76, liver 1.76, m u s c l e 9"18. A s these h i g h c o n c e n t r a t i o n s d o n o t fall statistically w i t h i n the A. sylvaticus p o p u l a t i o n r a n g e the figures w e r e o m i t t e d f r o m the t a b u l a t e d d a t a . L a r g e v a r i a t i o n s in tissue c o n c e n t r a t i o n s w i t h i n a n a r e a are n o t a l t o g e t h e r u n e x p e c t e d , a n d m a y be d u e to differences in age, diet, m e t a b o l i s m a n d t e r r i t o r i a l a c t i v i t y ( S o u t h e r n , 1964), as well as differences in f a l l o u t resulting f r o m v a r i a t i o n s in w i n d direction, topography and distance from the works. TABLE 2 MERCURY CONCENTRATIONS(IN /./G/G FRESH WEIGHT) IN TISSUESOF C. glareolus AND A. sylvaticus (EXPRESSEDAS MEAN ± STANDARDERROR; RANGEIN BRACKETS)
Tissue Brain Hair Kidney Liver Muscle
C. glareolus Study area Control (n = 7) (n = 6) 0.13 ± 0.02 (0'07-0.20) 0.91 ± 0"24 (0"40-2.15) 0.35 4- 0.10 (0" 14-0.75) 0-15 ± 0'04 (0.06-0-34) 0.28 ± 0-08 (0.08-0.66)
0"05 4- 0"01"** (0.03-0.08) 0.18 ± 0.04** (0.12-0.35) 0.08 ± 0'02* (0.02-0.17) 0.06 ± 0"02* (0.03-0.13) 0.06 ± 0.01" (0.04-0.11 )
A. sylvaticus Study area Control (n = 6) (n = 10) 0.55 -- 0.28 (0.09-1.88) 0-78 ± 0.12 (0.50-1.36) 0.52 ± 0.16 (0.17-1.29) 0.23 ± 0.07 (0-09-0'53) 0-98 ± 0.73 (0.06-4.59)
0.06 ± 0-01" (0.03-4). 13) 0.12 ± 0.01"** (0.05-0'19) 0.12 ± 0.02** (0.05-0.27) 0"04 ± 0.01"* (0.01-0.07) 0.07 ± 0.01 NS (0.03-0.13)
Note." *, ** and *** denote statistical significance at P < 0.025; 0.01 and 0.001, respectively. (NS = not statistically significant) as shown by Student's one-tailed t-tests for unpaired data with unequal variance, n = Number in sample.
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139
The concentrations of methylmercury are low ( < 10 ~o of the total mercury) but tend to be higher in the animals closer to the works (Table 3), notably in kidney and muscle tissue. Emissions from the works do not include organo-mercury compounds and the area has no known history of methylmercury biocide usage. However, in addition to the methylmercury in the small mammals, L. terrestris from within the study area were found to have 8-13 ~o of their mercury in the methylated form. Although methylation in aquatic systems is well documented (Jensen & Jernel6v, 1969) it has only recently been suggested that methylation can occur in agricultural soils (Beckert et al,, 1974). Work is continuing to study the dynamics of the methylation and accumulation in biota in the area. TABLE 3 CONCENTRATION OF MERCURY (NG/G FRESH WEIGHT) AS METHYLMERCURY IN TISSUES OF C. AND A. sylvaticus (MEAN VALUES; RANGE IN BRACKETS)
Brain Kidney Liver Muscle
glareolus
Study area*
Mean °//oHg as MeHg
Control*
Mean % Hg as MeHg
6.2 (2.5-11.9) 13.3 (10.6-17.0) 10.1 (4.8-14.0) 10-6 (4.6-19-0)
03 0'4 0.5 8-5
Not detectable 2.3 (1.0-3.7) 6-9 (4-0-13.4) 2.9 (1.3-4.4)
-2.5 2.0 5.0
*n~5. Although differences between the mercury concentrations in the mammals from the study and control areas are not as marked as those for vegetation and soil, it is known that the half-life retention of mercury by small mammals is only a few days (Nordberg & Skerfving, 1972) and this limits their use as indicator organisms. The body burden of mercury in small mammals therefore may not reflect concentrations in other species with slower excretion rates.
ACKNOWLEDGMENTS Two of us, K. R. Bull and M. J. Inskip, were supported in this work by a grant from the Natural Environment Research Council, and we are grateful to the Royal Society for the gas chromatograph.
REFERENCES
BECKERT,W. F., MOGHISSI,A. A., Au, F. H. F., BRETTHAER,E. E. & MCFARLANE,J. C. (1974). Formation of methylmercury in a terrestrial environment. Nature, Lond., 249, 674-5. FLEWELLING)F. J. (1971). Loss of mercury to the environment from chlor-alkali plants. In Mercury in man's environment, Proc. Special Symp. R. Soc. Can., 34-9. GOODMAN,G. R. & ROBERTS,T. M. (1971). Plants and soils as indicators of metals in the air. Nature, Lond., 231,287-92. GOODMAN,G. T., SMITH,S., PARRY,G. D. R. ~¢. INSKIP,M. J. (1974). The use of moss-bags as deposition gauges for airborne metals. Proc. Conf. nam. Soc. clean Air, 41st, 1-16.
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K. R. BULL~ R. D. ROBERTS~ M. J. 1NSKIP, G. T. GOODMAN
JEFFERIE8, D. J., STAINSBY, B. & FRENCH, M. C. (1973). The ecology of small mammals in arable fields drilled with winter wheat and the increase in their dieldrin and mercury residues. J. Zool., 171, 513-39. JENSEN, S. & JERNELOV, A. (1969). Biological methylation of mercury in aquatic organisms. Nature, Lond., 223, 753-4. JEPSON, A. F. (1973). Measurement of mercury vapour in the atmosphere. In Trace elements in the environment, ed. E. L. Kothny, 81-95. Washington, D.C., American Chemical Society. JERNEL6V, A. & WALLIN,T. 0973). Air-borne mercury fall-out on snow around five Swedish chlor-alkali plants. Atmos. Environ., 7, 209-14. JERNEL()V, A. (1975). Cycling of mercury in the environment. Syrup. Br. Ecol. Soc., 15th, 49-55. NORDBERG, G. R. & SKERFVING,S. (1972). Mercury in the environment, ed. by L. T. Friberg & J. J. Vostal, 29-91. Cleveland, CRC Press. SOUTHERN, H. N. (1964). The handbook of British mammals. Oxford, Blackwell. TWIGG, G. D. (1966). Chlorine. Sch. Sci. Rev., 162, 352-407. WEISS, H. V., KOIDE, M. & GOLDBERG, E. D. (1971). Mercury in a Greenland ice sheet: Evidence of recent input by man. Science, N. Y., 174, 692-4. WEST60, G. (1967). Determination of methylmercury compounds in foodstuffs. Acta chem. scand., 21, 1790-800.
K. R. BULL*, R. D. ROBERTSt, M. J. INSKIP & G. T. GOODMAN
Department of Applied Biology, Biological Sciences Group, Chelsea College, University of London, London, Great Britain
ABSTRACT
Scarcity of data describing mercury concentrations in the biota of an environment subjected to mercury fallout prompted this study around a chlor-alkali works. Atomic absorption spectrometric analysis of top-soils, grass (Festuca rubra L.), earthworms (Lumbricus terrestris L.), and atmospheric fallout, within 0-5 km and 10-30 km of the works, showed that mercury levels were significantly higher near the works. Woodmice (Apodemus sylvaticus L.) and bank voles (Clethrionomys glareolus Schr.) collected near the works had significantly greater concentrations of total mercury in brain, kidney, liver and hair than control animals. The presence of methylmercury in the mammals and L. terrestris is evidence for methylation of the inorganic mercury
fallout. INTRODUCTION
With the increasing concern about mercury pollution, much work has been carried out in the aquatic environment but few workers have studied terrestrial ecosystems affected by atmospheric emissions of the metal. Weiss et al. (1971) have shown by the analysis of the Greenland ice-sheet that the global aerial burden of mercury increased a few decades ago and has remained elevated. The increase has been attributed partly to the evaporation of naturally occurring mercury following increased agricultural activity, and partly to emissions resulting from some industrial activities and the burning of fossil fuels (Jernelrv, 1975). It is known that * Present address: Institute of Terrestrial Ecology, Monks Wood Experimental Station, Abbots Ripton, Huntingdon, Great Britain. t Present address: Botany Department, University of Liverpool, Great Britain.
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Environ. Pollut. (12) (1977)--© Applied Science Publishers Ltd, England, 1977 Printed in Great Britain
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K. R. BULL, R. D. ROBERTS, M. J. INSKIP, G. T. GOODMAN
significantly elevated mercury concentrations in air can occur around local sources and such sources are perhaps much more common than is often realised (Jepson, 1973), but only limited data are available to assess the impact of local mercury fallout on environmental burdens. The chlor-alkali process (Twigg, 1966) is recognised as a potential industrial emission source of mercury (Flewelling, 1971), and it has been shown that some of the mercury emitted to the air, albeit a small percentage, falls out within a few kilometres, whilst outside this zone the fallout is not significantly elevated (Jernelrv & Wallin, 1973). A study centred on a chlor-alkali works was therefore initiated in order to measure the mercury concentrations in the biota within the local fallout zone. The works chosen for the study was situated in open farmland and isolated from other industrial workings. It was known that losses to the air were mainly from the electrolytic cell-room. This paper describes elevated mercury concentrations in soils, grass, Festuca rubra L., and earthworms, Lumbricus terrestris L., and in tissues of the bank vole, Clethrionomys glareolus Schr., and the woodmouse, Apodemus sylvaticus L., from around the chlor-alkali works.
MATERIALS AND METHODS
Sample collection Initially, samples of surface soils (0-2 cm), the grass Festuca rubra L. and earthworms Lumbricus terrestris L. were collected during April 1974 from two groups of sites, one within a radius of 0.5 kin, the study area, and the other 10-30 km around the works. A number of moss bags, as used by Goodman & Roberts (1971), were exposed for seven weeks at the same sites. This technique, which involves the suspension of acid-washed moss (Sphagnum acutifolium agg. L.) in nylon mesh bags, has proved to be extremely useful in the monitoring of atmospherically-borne heavy metals (Goodman et al., 1974). Subsequently, during April, May and June 1974 a number of bank voles C. glareolus, and woodmice, A. sylvaticus, were caught using Longworth traps from the study area. Control animals were collected from rural sites unaffected by industrial emissions. Populations were chosen so that foraging territories did not include agricultural land and mercury concentrations in these animals were therefore unlikely to be affected by mercurydressed grain, as reported by Jefferies et al. (1973). The animals were killed using ether, deep-frozen and returned to the laboratory. After thawing, the brain, hair, kidney, liver and muscle were removed and sub-samples analysed for total mercury and methylmercury. Total mercury analysis Special attention was paid to the development and standardisation of wet acid
MERCURY IN SOIL, GRASS AND ANIMALS
137
oxidation and atomic absorption techniques for total mercury determinations. Soils were passed through a 2 mm sieve in order to remove stones and pieces of vegetation. F. rubra samples were sorted, chopped into 1 cm lengths and mixed thoroughly. Sub-samples of soils and F. rubra were digested without drying and the wet/dry weight ratio determined by drying further sub-samples at 40°C. L. terrestris were killed by deep-freezing and, after thawing, their gut contents removed prior to digestion. Digestions were carried out in Kjeldahl flasks using a concentrated 'Analar' nitric/perchloric acid mixture (4:1). Initial digestion in the cold followed by careful heating was found to prevent any evaporation losses of mercury. Digests were analysed for total mercury with a Varian AA4 atomic absorption spectrophotometer fitted with a 20 cm open-ended glass cell for flameless operation. Reduction to metallic mercury was accomplished with a 20°~ stannous chloride in 4M hydrochloric acid solution, and the mercury vapour flushed from the digest solution by an argon carrier gas. The mercury vapour/argon mixture was introduced into the absorption cell via a drying tube of anhydrous magnesium perchlorate. The apparatus described enabled quantities of mercury as small as 1 ng in 5 ml of digest solution to be measured. Methylmercury analysis Methylmercury was extracted from tissue using the method of West66 (1967). A Pye 104 gas chromatograph fitted with a Ni 63 electron capture detector (operated at 200°C) and a glass 1.5 m × 4 mm column packed with 2 ~o (w/w) neopentylglycol adipate on 80-100mesh acid-washed DMCS treated Chromasorb G (operated at 190°C) was used for the analysis. BOC Ltd high purity nitrogen was used as carrier gas at 40 ml min- x and high purity nitrogen/carbon dioxide (3 o/ /O CO2) was employed as a quench/purge gas at 20 ml min-1. The methylmercury dicyandiamide standard solutions were subjected to the same extraction procedures as the tissue to be analysed. The sensitivity of the gas chromatograph was such that an injection of 1 ng of mercury as methylmercury gave a chromatographic peak height of ~ 70 mm, enabli.ng analyses to be performed on samples of less than 1 g containing only nanogram quantities of methylmercury.
RESULTS AND DISCUSSION
Samples of soil, F. rubra and L. terrestris from sites close to the works were found to have mercury concentrations significantly higher than those further away (Table 1), and the moss-bag figures show that the current atmospheric mercury deposition is higher closer to the works. The mean concentrations of mercury in soils, F. rubra, L. terrestris, and moss-bags within the study area are more than an order of magnitude greater than those 10-30 km distant and the latter are typical of concentrations we have found in uncontaminated areas elsewhere.
138
K. R. BULL, R. D. ROBERTS, M. J. INSKIP, G. T. GOODMAN TABLE I
MERCURY CONCENTRATIONS IN SOIL AND F. rubra (IN UG/G DRY WEIGHT) AND L. terrestris (IN /./G/G FRESH WEIGHT), DEPOSITION ONTO MOSS-BAGS (IN NG/DM2/DAY) (EXPRESSED AS MEANS ± STANDARD ERROR; RANGE IN BRACKETS)
Study area <0.5 km from works
10-30 km from works
3.81 ± 0'95 (0.69-12.6) 4.01 ± 0.65 (1.30-9.41) 1.29 ± 0"32 (0.27-3.27) 63'0 ± 9.9 (37-124)
0-106 ± 0.009 (0.04-0-19) 0.103 ± 0"008 (0.07-0'17) 0.041 ± 0'006 (0.031-0.048) 3.50 ± 0.71 (0--6.7)
Surface soil ( < 2 cm) (n = 42) F. rubra (n = 34) L. terrestris (n = 18) Moss-bags (n = 21)
Degree of significance *** *** ** ***
Note: ** and *** denote statistical significance at P < 0.01 and 0.001, respectively as shown by Student's two tailed t-tests for unpaired data with unequal variance, n = Number in sample. M e r c u r y c o n c e n t r a t i o n s in tissues o f small m a m m a l s w i t h i n the s t u d y a r e a are significantly h i g h e r t h a n c o n c e n t r a t i o n s in c o n t r o l a n i m a l s e x c e p t f o r m u s c l e tissue o f A. sylvaticus ( T a b l e 2). It w o u l d a p p e a r , t h e r e f o r e , t h a t s m a l l m a m m a l s l i v i n g in a n a r e a s u b j e c t e d to e l e v a t e d m e r c u r y d e p o s i t i o n c a n a c c u m u l a t e s o m e o f this m e r c u r y in t h e i r tissues. I n a d d i t i o n to the a n i m a l s d e s c r i b e d in the table, o n e s p e c i m e n o f A. sylvaticus c o l l e c t e d in early A p r i l h a d t h e f o l l o w i n g m e r c u r y levels (in/~g H g / g fresh w e i g h t ) : b r a i n 22-6, h a i r 3.2, k i d n e y 1.76, liver 1.76, m u s c l e 9"18. A s these h i g h c o n c e n t r a t i o n s d o n o t fall statistically w i t h i n the A. sylvaticus p o p u l a t i o n r a n g e the figures w e r e o m i t t e d f r o m the t a b u l a t e d d a t a . L a r g e v a r i a t i o n s in tissue c o n c e n t r a t i o n s w i t h i n a n a r e a are n o t a l t o g e t h e r u n e x p e c t e d , a n d m a y be d u e to differences in age, diet, m e t a b o l i s m a n d t e r r i t o r i a l a c t i v i t y ( S o u t h e r n , 1964), as well as differences in f a l l o u t resulting f r o m v a r i a t i o n s in w i n d direction, topography and distance from the works. TABLE 2 MERCURY CONCENTRATIONS(IN /./G/G FRESH WEIGHT) IN TISSUESOF C. glareolus AND A. sylvaticus (EXPRESSEDAS MEAN ± STANDARDERROR; RANGEIN BRACKETS)
Tissue Brain Hair Kidney Liver Muscle
C. glareolus Study area Control (n = 7) (n = 6) 0.13 ± 0.02 (0'07-0.20) 0.91 ± 0"24 (0"40-2.15) 0.35 4- 0.10 (0" 14-0.75) 0-15 ± 0'04 (0.06-0-34) 0.28 ± 0-08 (0.08-0.66)
0"05 4- 0"01"** (0.03-0.08) 0.18 ± 0.04** (0.12-0.35) 0.08 ± 0'02* (0.02-0.17) 0.06 ± 0"02* (0.03-0.13) 0.06 ± 0.01" (0.04-0.11 )
A. sylvaticus Study area Control (n = 6) (n = 10) 0.55 -- 0.28 (0.09-1.88) 0-78 ± 0.12 (0.50-1.36) 0.52 ± 0.16 (0.17-1.29) 0.23 ± 0.07 (0-09-0'53) 0-98 ± 0.73 (0.06-4.59)
0.06 ± 0-01" (0.03-4). 13) 0.12 ± 0.01"** (0.05-0'19) 0.12 ± 0.02** (0.05-0.27) 0"04 ± 0.01"* (0.01-0.07) 0.07 ± 0.01 NS (0.03-0.13)
Note." *, ** and *** denote statistical significance at P < 0.025; 0.01 and 0.001, respectively. (NS = not statistically significant) as shown by Student's one-tailed t-tests for unpaired data with unequal variance, n = Number in sample.
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139
The concentrations of methylmercury are low ( < 10 ~o of the total mercury) but tend to be higher in the animals closer to the works (Table 3), notably in kidney and muscle tissue. Emissions from the works do not include organo-mercury compounds and the area has no known history of methylmercury biocide usage. However, in addition to the methylmercury in the small mammals, L. terrestris from within the study area were found to have 8-13 ~o of their mercury in the methylated form. Although methylation in aquatic systems is well documented (Jensen & Jernel6v, 1969) it has only recently been suggested that methylation can occur in agricultural soils (Beckert et al,, 1974). Work is continuing to study the dynamics of the methylation and accumulation in biota in the area. TABLE 3 CONCENTRATION OF MERCURY (NG/G FRESH WEIGHT) AS METHYLMERCURY IN TISSUES OF C. AND A. sylvaticus (MEAN VALUES; RANGE IN BRACKETS)
Brain Kidney Liver Muscle
glareolus
Study area*
Mean °//oHg as MeHg
Control*
Mean % Hg as MeHg
6.2 (2.5-11.9) 13.3 (10.6-17.0) 10.1 (4.8-14.0) 10-6 (4.6-19-0)
03 0'4 0.5 8-5
Not detectable 2.3 (1.0-3.7) 6-9 (4-0-13.4) 2.9 (1.3-4.4)
-2.5 2.0 5.0
*n~5. Although differences between the mercury concentrations in the mammals from the study and control areas are not as marked as those for vegetation and soil, it is known that the half-life retention of mercury by small mammals is only a few days (Nordberg & Skerfving, 1972) and this limits their use as indicator organisms. The body burden of mercury in small mammals therefore may not reflect concentrations in other species with slower excretion rates.
ACKNOWLEDGMENTS Two of us, K. R. Bull and M. J. Inskip, were supported in this work by a grant from the Natural Environment Research Council, and we are grateful to the Royal Society for the gas chromatograph.
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K. R. BULL~ R. D. ROBERTS~ M. J. 1NSKIP, G. T. GOODMAN
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