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The transport to and deposition of americium in intertidal sediments of the ravenglass estuary and its relationship to plutonium
Environmental Pollution (Series B) 3(1982) 1 9
THE TRANSPORT TO A N D DEPOSITION OF AMERICIUM IN INTERTIDAL SEDIMENTS OF THE RAVENGLASS ESTUARY A N D ITS RELATIONSHIP TO P L U T O N I U M
SIMON R. ASTON* t~ DAVID A. STANNERSt
Department o f Environmental Sciences, University of Lancaster, Lancaster LA 1 4YQ, Great Britain
ABSTRACT
The distributions of' 241Am in Jbur cores o/ intertidal sedimentji'om the Ravenglass estuary near to the Windscale nuclear juel reprocessing Jctcility, northwest England, are reported. Maximum 241Am activities (up to ~ 200pCi g - 1) arejound at depth in the sediments, and this pattern of accumulation is compared with annual discharges to the sea Ji'om Windscale. The results indicate a slow transit o]americium to the depositing sediments when the sedimentation rates derived ji'om two independent methods are applied to the cores. Thisjeature is interpreted, andjurther confirmed~ by consideration of plutonium:americium isotopic ratios, as the result of a lag-time oJ transit ji'om Windscale to the sediments oJ ~ 2.5 years. This slow movement is consistent with earlier theoretical and field data which indicate that americium is rapidO' removed to sediments on release, and subsequent O' transported with bed-loads in the coastal environment.
INTRODUCTION
The distribution of americium (241Am, tl/2 = 435 years) in intertidal sediments from the Ravenglass estuary, Cumbria, England. has recently been reported (Aston & Stanners, 198 la). The Ravenglass estuary is of radioecological interest because it is a site of sediment accumulation close to the Windscale nuclear fuel reprocessing facility (Fig. 1). Americium and plutonium are both effectively removed to marine sedimentary * Present address: International Laboratory of Marine Radioactivity, Musee Oceanographique, Monaco. + Present address: Radiochemistry Division, J.R.C. Ispra, 21020, Varese, Italy.
1 Era'iron. Pollut. Ser. B. 0143-148X/82/0003-0001/$02.75 Printed in Great Britain
~;) Applied Science Publishers Ltd, England, 1982
2
SIMON R. ASTON, DAVID A. STANNERS
N 'Windscale
I Sea,scale
IRISH
SEA
Newbi~in
Fig. 1. The location of the Windscale nuclear fuel reprocessing facility, the Ravenglass estuary, and Newbiggin (54°20'N, 3 °24'W), Scale 1:75000.
materials after discharge from Windscale to the eastern Irish Sea (Hetherington et al., 1976; Livingston & Bowen, 1977). In the present paper the deposition of 241Am in accumulating intertidal sediments is discussed and the relationship of this pollutant to the deposition of plutonium in the same sediments (Aston & Stanners, 1981 b) is considered. MATERIALS AND METHODS
Field sampling and anah,sis Intertidal sediment cores were collected in mid-1978 from the silt banks of the Newbiggin area of the tidal reaches of the River Esk, Ravenglass estuary (Fig. 1). This locality has been studied by various workers (e.g. Hetherington, 1978; Aston & Stanners, 1979, 1981b) and is a site of active sedimentation. The cores were obtained by inserting plastic core liners (7.5 cm diameter) into the sediments at low-water, digging out and transferring them to the laboratory for extrusion. The cores were carefully segmented on extrusion, dried at 60°C, and stored for radiochemical analysis. The activities of Z41Am in the core segments were determined by the method of Stanners & Aston (1980). Errors shown in the present study are ___1 tr counting statistics.
3
AMERICIUM DEPOSITION IN INTERTIDALSEDIMENTS RESULTS AND DISCUSSION
Table 1 presents the results of the analyses of 24.1Am in four cores collected from the Newbiggin area at Ravenglass. The surface segments ranged in 24"~Am activity from 48.8 + 2-8 to 60-2_+ 2 . 8 p C i g - 1 (dry weight) a n d were similar to the surface activities of 24"1Am reported earlier for this intertidal area (Aston & Stanners, 1981a). In all four cores, greater 24"~Am activities were f o u n d at depth a n d in the buried sediments. The m a x i m u m activities (pCi g - 1 dry weight) a n d their respective depths were 197-7 ± 8.4 (3.0 3.5 cm), 98-4 +_ 9-5 ( 4 - 6 c m ) . 159.2 _+ 8.6 (6-7 cm) a n d 135.2 _+ 7.8 ( 4 - 5 c m ) for cores E - l , E-2, E-4 a n d E-5. As the depth in each core increased, the 24.1Am activities decreased steadily to values of a r o u n d 10 pCi g - 1 or less for the deepest segments obtained. The vertical distributions of 24"~Am activities in the four cores are illustrated in Fig. 2, a n d they clearly display similar patterns of TABLE 1 24~Am ACTIVITIES (pCi g-~, dry weight) IN
SEGMENTS OF INTERTIDAL SEDIMENT CORES COLLECTED AT NEWBIGGIN, RAVENGLASS ESTUARY
Core Depth (em) 0-0.5 0"5 1-0 1"1~1-5 1"5 2.0 2"ff-2"5 2.5 3.0 3'1~3"5 3'5 4.0 4.0 4.5 4-5 5.0 5-(~5-5 5"5 6"0 6'(~6'5 6'5 7.0 7.0 7.5 7.5 8"0 8"(~ 8.5 8"5 9.0 9.0 9"5 9"5 10"0 10-(~ 10.5 10"5 11.0 11"0 11.5 11"5 1 2 ' 0 12"0 12"5 12.5 13"0 13.0 13.5 13"5 14"0 14.0-14.5 14"5 15-0
E-1
E-2 241Am
Depth (cm)
241Am
60-2+2-8 0-2 24 83"5 + 3.6 4-6 107.9+4.7 (~8 129"0-+5.5 8-10 161"8-+6"9 169.7-+6.7 1(~12 12 14 197"7 -+8.4 167"7 _+7"6 14-16 156.5+_6.7 144.9 _ 6"2 118-4_+5.1 105.7+_4.6 85"7 -+3.7 74.5 +_3"2 56"0 +_2"4 45.5 ± 2"0 41 "8 +_1.8 34"9 +_1.5 29"1 + 1-3 24.3 + 1.1 23"3 + 1"0 20"1 +_0.9 14"3_+0-6 13"8+_0"6 6"3+_0.3 6"3_+0'3 13"8+_0-6 12"7+_0"6 10.1 _+0.4 11"6+_0.5
59.2 -+6.0 83.3 _+8.3 98"4 _+9.5 62.1 -+5.8 22.6+_2.7 11.4+_1.7 5"5 +_1.2 2"8 _+1.0
E-4 Depth (cm) 0 1
1-2 2 3 34 4-5 5 6 (~7 7-8 8 9 9 10 10 11 11 12 12 13 13 14 14 15 15 16 16 17
E-5 241Am
54.4+2.8
64.3 ± 2-7 80.9 + 3.3 97-1 _+3.9 112-3+5.4 135.3+_6.5 159-2+_8.6 134'1_+6"5 120.7+_6.1 83"6_+4.3 68"6_+3"29 45.3+_2.3 32.7-+1.7 24.4+_1.0 18.9-+1.0 13"9+_0.7 7.2+_0.4
Depth (crn)
241Am
0 1
48.8+2.8
1-2 2-3 34 4-5 5-6 6-7 7 8 89 9-10 10 11 11 12
63.0 + 3.7 86.7 + 4.7 110.5+6"0 135.2+7-8 123.2+7-1 89.7+_5-3 45.9_+2.5 22.9+_1.3 13.8+0"8 11-1+0.6 4.9+_0.3
p C c j -1
~8o
o c n3
10,
/
/
\
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\
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/
./ core
E-4
Core
E-5
15
P C g -I
io,o
/
/
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180 m
24 IArn
10¸
'kk
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,£o
cm
2~,Am
\
0
0
i
crn
\.
AJ
A/
AJ J
/
/ /
/ core
E-2
core
E-1
t Fig. 2. The vertical distribution of 241Am (pCi g- 1 dry weight) in intertidal sediment cores collected at Newbiggin, Ravenglass estuary.
AMERICIUM D E P O S I T I O N IN I N T E R T I D A L SEDIMENTS
3
activity distribution. The reasons for the maxima in 241Am activity at depth in the cores may be due to one or both of the following factors: (i) the changing temporal discharge of 241Am from Windscale, with an accompanying response in the contamination of sediments deposited at different times and (ii) the postdepositional migration of 241Am mediated by physical, chemical or biological processes which gives rise to americium translocation within the profile. The rates of net sedimentation for the cores described here have been determined by the application of a fission product deposition technique (Aston & Stanners, 1979) and for cores E- 1, E-2 and E-5 by the use of the 238 Pu :239 + 240 Pu ratio (Aston & Stanners, 1981 b). The results of the two independent methods are in rather good agreement (Table 2), considering the difficulties associated with the determination of TABLE 2 ESTIMATES OF THE NET SEDIMENTATION RATES ( c m y e a r - ~ ) FOR CORES E-l. E-2 AND E-5 AT NEWBIGGIN, RAVENGLASSESTUARY
Core
Fission products method
Plutonium isotopes method
Mean value
E-I E-2 E-4 E-5
1"6 1.6 2" 1 1.6
1.3 1.0 -1.0
1-45 1.3 -1.3
sedimentation rates in coastal and estuarine environments. The sedimentation rates derived from fission product deposition were found to be slightly greater than those predicted from the plutonium isotopic ratio approach. The two techniques differ from each other in that the latter uses the temporal changes in the plutonium isotopic composition of surface sediments at the actual Newbiggin site, whilst the fission product approach employs the annual discharge variations in quantities and isotopic composition of the individual radionuclides released from the Windscale pipeline. For these reasons, the mean derived n e t sedimentation rates are now used for 241Am deposition interpretation. If the sediments at Newbiggin respond to fluctuations in the annual discharges of 24tAm from Windscale, and assuming the mean sedimentation rates already discussed, the peak 24tAm activities should occur at 7-25, 6.5 and 6.5cm in cores E-l, E-2 and E-5, respectively when no allowance is made for a finite transport time (lag-time) from the pipeline to the Ravenglass estuary. The maximum activities are found at 3.5, 5.0 and 4.5 cm, respectively, which implies that either there is a finite transit time for americium to reach the sediment sites, or a process of postdepositional upward translocation is operating. The transit times which must be invoked to match the discharge and deposition peak activities at these sedimentation rates are 2.6, 1-15 and 1.5 years, respectively. These results have a mean of 1.75 years, but it must be noted that the errors in the estimates of the sedimentation rates
O
SIMON R. ASTON, D A V I D A. STANNERS
and americium transit times are likely to be greater for cores E-2 and E-5 because rather large sediment segments were taken for analysis. Core E-i, with its detailed (0.5cm) segments, showed the best agreement between the results of two sedimentation rate measurements (1.3, 1.6cm year-1), and also allowed the peak 241Am activity depth to be much more accurately determined. For this reason the transit time of americium to Newbiggin from Windscale is probably nearer to 2.6 years than the lower estimates which are based on less certain estimation of the 241Am maximum activity depth. Aston & Stanners (1981 b) have concluded that the transit time of plutonium to the Newbiggin site is 2.3 to 3 years, and suggested that this is in good agreement with the observation that plutonium is rapidly and effectively removed to bottom sediments after release to the Irish Sea (Hetherington, 1976), whereafter it will be mainly transported by the slow movement of bottom sediments in residual currents. These observations are also compatible with theoretical calculations which predict that a highly insoluble Pu(OH)4 form of plutonium occurs in sea water (Aston, 1980), which has been found to be associated with the non-detrital iron/manganese oxide phases in the sediments (Aston & Stanners, 1981b). It appears from the present data that americium may be transported in the sediment phase at about the same rate as plutonium in the eastern Irish Sea. Further, more conclusive evidence that the positions of the americium activity peaks in the cores are a result of a lag-time in transit, rather than processes of translocation within the deposited sediment, may be obtained. The temporal variations in the 238pu + 239+24°pu:241Am ratio of Windscale discharges are known (Table 3). If there is a measurable and essentially equal transit time of these two transuranics from the pipeline to Newbiggin, this should result in a predictable upward displacement of the temporal variations in the ratio deposited in cores of known accumulation rates. The 238pu + 239+ 24Opu:241Am activity ratio profile for core E-I is shown in Fig. 3, together with the recent annual pattern of this ratio in Windscale effluents. The effluent ratios are plotted to show their predicted distributions in the cores on the basis of(i) the assumed mean sedimentation rate of 1.45 cm year- 1 and (ii) the assumed mean sedimentation rate and the lag-time of 2-6 years evaluated above for 241Am in this core. It is apparent that without any lag-time TABLE 3 RATIOS OF 238pu d- 239+24°pu:241Am ACTIVITIES DISCHARGED ANNUALLY FROM THE WINDSCALE PIPELINE
Year
Ratio
Year
Ratio
1968 1969 1970 1971 1972 1973
1.43 2.26 1.73 1.11 0-71 0.60
1974 1975 1976 1977 1978
0.39 1.22 3.94 9.94 7.14
AMERICIUM DEPOSITION IN INTERTIDAL SEDIMENTS
7
238pu e239*239pu : 241Am 5
10
o 'O
cm
5-
10. core E-
Fig. 3. The 238pu Jr 239 + 240pl.l:241Am ratio profile for core E-l, and the temporal pattern of this ratio in annual discharges from Windscale. The discharge ratios are shown as predicted by a sedimentation rate of 1.4cm year-1 and with a transit lag-time of 2.6 years. I-1, Core data: O, discharge no lag-time; A , discharge 2.6 year lag.
the core and discharge ratios are not compatible; the recent high 238pu + 239 + 24Opu:241Am ratios in the discharges of 1976 1978 are not yet found at the top of the core. However, if the previously estimated lag-time of 2-6 years is applied to the data, the ratio profile expected from the delayed arrival of the discharge at Newbiggin is a very good match to the actual core ratio profile. This confirms that there is a transport time of about two-and-a-half years for 241Am, 238pu and 239 + 2 4 0 p u to this site. Similar calculations have been successfully applied to the other three cores. Theoretical arguments based on the application of the Nernst Equation to americium redox potential data for alkaline conditions have been used to evaluate the chemical forms of this element in the marine environment (Aston et al., 1981). The conclusion is that Am(OH)4 and Am(OH)3 species will exist, and these exceedingly insoluble forms should, in parallel with Pu(OH)4, be removed to sediments. This is compatible with the observed removal of 241Am to the Irish Sea bed (Hetherington et al., 1976). The present results are also in agreement with the
8
SIMON R. ASTON, DAVID A. STANNERS
conclusions of these workers, since the 241Am (and plutonium isotopes) are found to be moved at a slow rate to the Ravenglass estuary. This transport is consistent with a transport in association with slow moving bed-loads rather than with the water masses of the coastal environment.
CONCLUSIONS
The main conclusions may be stated as the following series of points. (1) Americium (:41Am) derived from low-level liquid effluents discharged to the northeastern Irish Sea is present in intertidal sediments at Ravenglass estuary. (2) The activities of 24tAm in vertical profiles of the sediments range from ~ 10 to ~ 200 pCi g - t (dry weight). Maximum activities occur at a few centimetres depth in the sediments. (3) The occurrence of 241Am peak activities at lesser depths than predicted in the sediments has been interpreted as the result of a finite lag-time of transport from Windscale to the Ravenglass estuary. (4) The best estimate of this lag-time is ~ 2.5 years, and this is consistent with data on the 238 Pu + 239 + 240 pu :24~Am ratio observed in the sediment profile and annual discharge from Windscale. (5) The slow transport of americium agrees well with earlier field observations of the rapid removal of this pollutant to seabed sediments upon release at Windscale and theoretical predictions that americium should be rapidly removed from sea water to sediments in highly insoluble forms. Americium, in parallel with plutonium, seems to be mainly transported to Ravenglass estuary by sediment bedload movements and not with more rapid water-mass movements.
ACKNOWLEDGEMENTS
We are grateful to S. Benner for technical assistance in this study. This work was financially supported by the Natural Environment Research Council for which we are grateful.
REFERENCES
ASTON,S. R. (1980). Evaluation of the chemicalformsof plutonium in sea water. Mar. Chem., 8, 319 25.
ASTON,S. R. & STAN~EgS,D. A. (1979). The determination of estuarine sedimentation rates by 134.137Cs and other artificial radionuclide profiles. Estuar. & Coastal Mar, Sci., 9, 529-42. ASTON,S. R. & S'rANNERS,D. A. ( 1981a). Americiumin intertidal sediments from the coastal environsof Windscale. Mar. Pollut. Bull., 12, 149-53. AS'tON, S. R. & STANNERS,D. A. (1981b). Plutonium transport to, deposition and immobility in, intertidal sediments from the Irish Sea. Nature, Lond., 289, 581-2.
AMERICIUM DEPOSITION IN INTERTIDAL SEDIMENTS
9
ASTON, S. R., AVOGADRO,A., MURRAY,C. N. & STANNERS, D. A. (1981). Theoretical and practical problems in the evaluation of physicochemical forms of transuranics in the marine environment. In Proc. Syrup. Impacts oJ Radionuclide Releases to the Marine Environment. Vienna, IAEA (in press). HETHERINGTON, J. A. (1976). The behaviour of plutonium nuclides in the Irish Sea. In Environmental toxicity of aquatic radionuclides--Models and mechanisms, ed. by M. W. Miller and J. N. Stannard, 81-106. Ann Arbor, Ann Arbor Sciences Inc. HETHERINGTON, J. A. (1978). The uptake of plutonium nuclides by marine sediments. Mar. Sci. Commun., 4, 239-74. HETHERINGTON, J. A., JEFFER1ES,D. F., MITCHELL,N. T., PENTREATH,R. J. & WOODHEAD,D. S. (1976). Environmental and public health consequences of the controlled discharge of transuranic elements to the marine environment. In Proc. Syrup. Transuranium Nuclides in the Marine Em, ironment, 81 106. Vienna, IAEA. LIVINGSTON,H. D. & BOWEN,V. T. (1977). Windscale effluents in the waters and sediments of the M inch. Nature, Lond., 269, 586-8. STANNERS,D. A. & ASTON, S. R. (1980). The rapid determination of americium in sediments by NaI(T1) gamma spectroscopy. Ent'iron. Technol. Lett., !, 333 8.
THE TRANSPORT TO A N D DEPOSITION OF AMERICIUM IN INTERTIDAL SEDIMENTS OF THE RAVENGLASS ESTUARY A N D ITS RELATIONSHIP TO P L U T O N I U M
SIMON R. ASTON* t~ DAVID A. STANNERSt
Department o f Environmental Sciences, University of Lancaster, Lancaster LA 1 4YQ, Great Britain
ABSTRACT
The distributions of' 241Am in Jbur cores o/ intertidal sedimentji'om the Ravenglass estuary near to the Windscale nuclear juel reprocessing Jctcility, northwest England, are reported. Maximum 241Am activities (up to ~ 200pCi g - 1) arejound at depth in the sediments, and this pattern of accumulation is compared with annual discharges to the sea Ji'om Windscale. The results indicate a slow transit o]americium to the depositing sediments when the sedimentation rates derived ji'om two independent methods are applied to the cores. Thisjeature is interpreted, andjurther confirmed~ by consideration of plutonium:americium isotopic ratios, as the result of a lag-time oJ transit ji'om Windscale to the sediments oJ ~ 2.5 years. This slow movement is consistent with earlier theoretical and field data which indicate that americium is rapidO' removed to sediments on release, and subsequent O' transported with bed-loads in the coastal environment.
INTRODUCTION
The distribution of americium (241Am, tl/2 = 435 years) in intertidal sediments from the Ravenglass estuary, Cumbria, England. has recently been reported (Aston & Stanners, 198 la). The Ravenglass estuary is of radioecological interest because it is a site of sediment accumulation close to the Windscale nuclear fuel reprocessing facility (Fig. 1). Americium and plutonium are both effectively removed to marine sedimentary * Present address: International Laboratory of Marine Radioactivity, Musee Oceanographique, Monaco. + Present address: Radiochemistry Division, J.R.C. Ispra, 21020, Varese, Italy.
1 Era'iron. Pollut. Ser. B. 0143-148X/82/0003-0001/$02.75 Printed in Great Britain
~;) Applied Science Publishers Ltd, England, 1982
2
SIMON R. ASTON, DAVID A. STANNERS
N 'Windscale
I Sea,scale
IRISH
SEA
Newbi~in
Fig. 1. The location of the Windscale nuclear fuel reprocessing facility, the Ravenglass estuary, and Newbiggin (54°20'N, 3 °24'W), Scale 1:75000.
materials after discharge from Windscale to the eastern Irish Sea (Hetherington et al., 1976; Livingston & Bowen, 1977). In the present paper the deposition of 241Am in accumulating intertidal sediments is discussed and the relationship of this pollutant to the deposition of plutonium in the same sediments (Aston & Stanners, 1981 b) is considered. MATERIALS AND METHODS
Field sampling and anah,sis Intertidal sediment cores were collected in mid-1978 from the silt banks of the Newbiggin area of the tidal reaches of the River Esk, Ravenglass estuary (Fig. 1). This locality has been studied by various workers (e.g. Hetherington, 1978; Aston & Stanners, 1979, 1981b) and is a site of active sedimentation. The cores were obtained by inserting plastic core liners (7.5 cm diameter) into the sediments at low-water, digging out and transferring them to the laboratory for extrusion. The cores were carefully segmented on extrusion, dried at 60°C, and stored for radiochemical analysis. The activities of Z41Am in the core segments were determined by the method of Stanners & Aston (1980). Errors shown in the present study are ___1 tr counting statistics.
3
AMERICIUM DEPOSITION IN INTERTIDALSEDIMENTS RESULTS AND DISCUSSION
Table 1 presents the results of the analyses of 24.1Am in four cores collected from the Newbiggin area at Ravenglass. The surface segments ranged in 24"~Am activity from 48.8 + 2-8 to 60-2_+ 2 . 8 p C i g - 1 (dry weight) a n d were similar to the surface activities of 24"1Am reported earlier for this intertidal area (Aston & Stanners, 1981a). In all four cores, greater 24"~Am activities were f o u n d at depth a n d in the buried sediments. The m a x i m u m activities (pCi g - 1 dry weight) a n d their respective depths were 197-7 ± 8.4 (3.0 3.5 cm), 98-4 +_ 9-5 ( 4 - 6 c m ) . 159.2 _+ 8.6 (6-7 cm) a n d 135.2 _+ 7.8 ( 4 - 5 c m ) for cores E - l , E-2, E-4 a n d E-5. As the depth in each core increased, the 24.1Am activities decreased steadily to values of a r o u n d 10 pCi g - 1 or less for the deepest segments obtained. The vertical distributions of 24"~Am activities in the four cores are illustrated in Fig. 2, a n d they clearly display similar patterns of TABLE 1 24~Am ACTIVITIES (pCi g-~, dry weight) IN
SEGMENTS OF INTERTIDAL SEDIMENT CORES COLLECTED AT NEWBIGGIN, RAVENGLASS ESTUARY
Core Depth (em) 0-0.5 0"5 1-0 1"1~1-5 1"5 2.0 2"ff-2"5 2.5 3.0 3'1~3"5 3'5 4.0 4.0 4.5 4-5 5.0 5-(~5-5 5"5 6"0 6'(~6'5 6'5 7.0 7.0 7.5 7.5 8"0 8"(~ 8.5 8"5 9.0 9.0 9"5 9"5 10"0 10-(~ 10.5 10"5 11.0 11"0 11.5 11"5 1 2 ' 0 12"0 12"5 12.5 13"0 13.0 13.5 13"5 14"0 14.0-14.5 14"5 15-0
E-1
E-2 241Am
Depth (cm)
241Am
60-2+2-8 0-2 24 83"5 + 3.6 4-6 107.9+4.7 (~8 129"0-+5.5 8-10 161"8-+6"9 169.7-+6.7 1(~12 12 14 197"7 -+8.4 167"7 _+7"6 14-16 156.5+_6.7 144.9 _ 6"2 118-4_+5.1 105.7+_4.6 85"7 -+3.7 74.5 +_3"2 56"0 +_2"4 45.5 ± 2"0 41 "8 +_1.8 34"9 +_1.5 29"1 + 1-3 24.3 + 1.1 23"3 + 1"0 20"1 +_0.9 14"3_+0-6 13"8+_0"6 6"3+_0.3 6"3_+0'3 13"8+_0-6 12"7+_0"6 10.1 _+0.4 11"6+_0.5
59.2 -+6.0 83.3 _+8.3 98"4 _+9.5 62.1 -+5.8 22.6+_2.7 11.4+_1.7 5"5 +_1.2 2"8 _+1.0
E-4 Depth (cm) 0 1
1-2 2 3 34 4-5 5 6 (~7 7-8 8 9 9 10 10 11 11 12 12 13 13 14 14 15 15 16 16 17
E-5 241Am
54.4+2.8
64.3 ± 2-7 80.9 + 3.3 97-1 _+3.9 112-3+5.4 135.3+_6.5 159-2+_8.6 134'1_+6"5 120.7+_6.1 83"6_+4.3 68"6_+3"29 45.3+_2.3 32.7-+1.7 24.4+_1.0 18.9-+1.0 13"9+_0.7 7.2+_0.4
Depth (crn)
241Am
0 1
48.8+2.8
1-2 2-3 34 4-5 5-6 6-7 7 8 89 9-10 10 11 11 12
63.0 + 3.7 86.7 + 4.7 110.5+6"0 135.2+7-8 123.2+7-1 89.7+_5-3 45.9_+2.5 22.9+_1.3 13.8+0"8 11-1+0.6 4.9+_0.3
p C c j -1
~8o
o c n3
10,
/
/
\
2~o
\
!
10-
/
./ core
E-4
Core
E-5
15
P C g -I
io,o
/
/
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180 m
24 IArn
10¸
'kk
pC g"
,£o
cm
2~,Am
\
0
0
i
crn
\.
AJ
A/
AJ J
/
/ /
/ core
E-2
core
E-1
t Fig. 2. The vertical distribution of 241Am (pCi g- 1 dry weight) in intertidal sediment cores collected at Newbiggin, Ravenglass estuary.
AMERICIUM D E P O S I T I O N IN I N T E R T I D A L SEDIMENTS
3
activity distribution. The reasons for the maxima in 241Am activity at depth in the cores may be due to one or both of the following factors: (i) the changing temporal discharge of 241Am from Windscale, with an accompanying response in the contamination of sediments deposited at different times and (ii) the postdepositional migration of 241Am mediated by physical, chemical or biological processes which gives rise to americium translocation within the profile. The rates of net sedimentation for the cores described here have been determined by the application of a fission product deposition technique (Aston & Stanners, 1979) and for cores E- 1, E-2 and E-5 by the use of the 238 Pu :239 + 240 Pu ratio (Aston & Stanners, 1981 b). The results of the two independent methods are in rather good agreement (Table 2), considering the difficulties associated with the determination of TABLE 2 ESTIMATES OF THE NET SEDIMENTATION RATES ( c m y e a r - ~ ) FOR CORES E-l. E-2 AND E-5 AT NEWBIGGIN, RAVENGLASSESTUARY
Core
Fission products method
Plutonium isotopes method
Mean value
E-I E-2 E-4 E-5
1"6 1.6 2" 1 1.6
1.3 1.0 -1.0
1-45 1.3 -1.3
sedimentation rates in coastal and estuarine environments. The sedimentation rates derived from fission product deposition were found to be slightly greater than those predicted from the plutonium isotopic ratio approach. The two techniques differ from each other in that the latter uses the temporal changes in the plutonium isotopic composition of surface sediments at the actual Newbiggin site, whilst the fission product approach employs the annual discharge variations in quantities and isotopic composition of the individual radionuclides released from the Windscale pipeline. For these reasons, the mean derived n e t sedimentation rates are now used for 241Am deposition interpretation. If the sediments at Newbiggin respond to fluctuations in the annual discharges of 24tAm from Windscale, and assuming the mean sedimentation rates already discussed, the peak 24tAm activities should occur at 7-25, 6.5 and 6.5cm in cores E-l, E-2 and E-5, respectively when no allowance is made for a finite transport time (lag-time) from the pipeline to the Ravenglass estuary. The maximum activities are found at 3.5, 5.0 and 4.5 cm, respectively, which implies that either there is a finite transit time for americium to reach the sediment sites, or a process of postdepositional upward translocation is operating. The transit times which must be invoked to match the discharge and deposition peak activities at these sedimentation rates are 2.6, 1-15 and 1.5 years, respectively. These results have a mean of 1.75 years, but it must be noted that the errors in the estimates of the sedimentation rates
O
SIMON R. ASTON, D A V I D A. STANNERS
and americium transit times are likely to be greater for cores E-2 and E-5 because rather large sediment segments were taken for analysis. Core E-i, with its detailed (0.5cm) segments, showed the best agreement between the results of two sedimentation rate measurements (1.3, 1.6cm year-1), and also allowed the peak 241Am activity depth to be much more accurately determined. For this reason the transit time of americium to Newbiggin from Windscale is probably nearer to 2.6 years than the lower estimates which are based on less certain estimation of the 241Am maximum activity depth. Aston & Stanners (1981 b) have concluded that the transit time of plutonium to the Newbiggin site is 2.3 to 3 years, and suggested that this is in good agreement with the observation that plutonium is rapidly and effectively removed to bottom sediments after release to the Irish Sea (Hetherington, 1976), whereafter it will be mainly transported by the slow movement of bottom sediments in residual currents. These observations are also compatible with theoretical calculations which predict that a highly insoluble Pu(OH)4 form of plutonium occurs in sea water (Aston, 1980), which has been found to be associated with the non-detrital iron/manganese oxide phases in the sediments (Aston & Stanners, 1981b). It appears from the present data that americium may be transported in the sediment phase at about the same rate as plutonium in the eastern Irish Sea. Further, more conclusive evidence that the positions of the americium activity peaks in the cores are a result of a lag-time in transit, rather than processes of translocation within the deposited sediment, may be obtained. The temporal variations in the 238pu + 239+24°pu:241Am ratio of Windscale discharges are known (Table 3). If there is a measurable and essentially equal transit time of these two transuranics from the pipeline to Newbiggin, this should result in a predictable upward displacement of the temporal variations in the ratio deposited in cores of known accumulation rates. The 238pu + 239+ 24Opu:241Am activity ratio profile for core E-I is shown in Fig. 3, together with the recent annual pattern of this ratio in Windscale effluents. The effluent ratios are plotted to show their predicted distributions in the cores on the basis of(i) the assumed mean sedimentation rate of 1.45 cm year- 1 and (ii) the assumed mean sedimentation rate and the lag-time of 2-6 years evaluated above for 241Am in this core. It is apparent that without any lag-time TABLE 3 RATIOS OF 238pu d- 239+24°pu:241Am ACTIVITIES DISCHARGED ANNUALLY FROM THE WINDSCALE PIPELINE
Year
Ratio
Year
Ratio
1968 1969 1970 1971 1972 1973
1.43 2.26 1.73 1.11 0-71 0.60
1974 1975 1976 1977 1978
0.39 1.22 3.94 9.94 7.14
AMERICIUM DEPOSITION IN INTERTIDAL SEDIMENTS
7
238pu e239*239pu : 241Am 5
10
o 'O
cm
5-
10. core E-
Fig. 3. The 238pu Jr 239 + 240pl.l:241Am ratio profile for core E-l, and the temporal pattern of this ratio in annual discharges from Windscale. The discharge ratios are shown as predicted by a sedimentation rate of 1.4cm year-1 and with a transit lag-time of 2.6 years. I-1, Core data: O, discharge no lag-time; A , discharge 2.6 year lag.
the core and discharge ratios are not compatible; the recent high 238pu + 239 + 24Opu:241Am ratios in the discharges of 1976 1978 are not yet found at the top of the core. However, if the previously estimated lag-time of 2-6 years is applied to the data, the ratio profile expected from the delayed arrival of the discharge at Newbiggin is a very good match to the actual core ratio profile. This confirms that there is a transport time of about two-and-a-half years for 241Am, 238pu and 239 + 2 4 0 p u to this site. Similar calculations have been successfully applied to the other three cores. Theoretical arguments based on the application of the Nernst Equation to americium redox potential data for alkaline conditions have been used to evaluate the chemical forms of this element in the marine environment (Aston et al., 1981). The conclusion is that Am(OH)4 and Am(OH)3 species will exist, and these exceedingly insoluble forms should, in parallel with Pu(OH)4, be removed to sediments. This is compatible with the observed removal of 241Am to the Irish Sea bed (Hetherington et al., 1976). The present results are also in agreement with the
8
SIMON R. ASTON, DAVID A. STANNERS
conclusions of these workers, since the 241Am (and plutonium isotopes) are found to be moved at a slow rate to the Ravenglass estuary. This transport is consistent with a transport in association with slow moving bed-loads rather than with the water masses of the coastal environment.
CONCLUSIONS
The main conclusions may be stated as the following series of points. (1) Americium (:41Am) derived from low-level liquid effluents discharged to the northeastern Irish Sea is present in intertidal sediments at Ravenglass estuary. (2) The activities of 24tAm in vertical profiles of the sediments range from ~ 10 to ~ 200 pCi g - t (dry weight). Maximum activities occur at a few centimetres depth in the sediments. (3) The occurrence of 241Am peak activities at lesser depths than predicted in the sediments has been interpreted as the result of a finite lag-time of transport from Windscale to the Ravenglass estuary. (4) The best estimate of this lag-time is ~ 2.5 years, and this is consistent with data on the 238 Pu + 239 + 240 pu :24~Am ratio observed in the sediment profile and annual discharge from Windscale. (5) The slow transport of americium agrees well with earlier field observations of the rapid removal of this pollutant to seabed sediments upon release at Windscale and theoretical predictions that americium should be rapidly removed from sea water to sediments in highly insoluble forms. Americium, in parallel with plutonium, seems to be mainly transported to Ravenglass estuary by sediment bedload movements and not with more rapid water-mass movements.
ACKNOWLEDGEMENTS
We are grateful to S. Benner for technical assistance in this study. This work was financially supported by the Natural Environment Research Council for which we are grateful.
REFERENCES
ASTON,S. R. (1980). Evaluation of the chemicalformsof plutonium in sea water. Mar. Chem., 8, 319 25.
ASTON,S. R. & STAN~EgS,D. A. (1979). The determination of estuarine sedimentation rates by 134.137Cs and other artificial radionuclide profiles. Estuar. & Coastal Mar, Sci., 9, 529-42. ASTON,S. R. & S'rANNERS,D. A. ( 1981a). Americiumin intertidal sediments from the coastal environsof Windscale. Mar. Pollut. Bull., 12, 149-53. AS'tON, S. R. & STANNERS,D. A. (1981b). Plutonium transport to, deposition and immobility in, intertidal sediments from the Irish Sea. Nature, Lond., 289, 581-2.
AMERICIUM DEPOSITION IN INTERTIDAL SEDIMENTS
9
ASTON, S. R., AVOGADRO,A., MURRAY,C. N. & STANNERS, D. A. (1981). Theoretical and practical problems in the evaluation of physicochemical forms of transuranics in the marine environment. In Proc. Syrup. Impacts oJ Radionuclide Releases to the Marine Environment. Vienna, IAEA (in press). HETHERINGTON, J. A. (1976). The behaviour of plutonium nuclides in the Irish Sea. In Environmental toxicity of aquatic radionuclides--Models and mechanisms, ed. by M. W. Miller and J. N. Stannard, 81-106. Ann Arbor, Ann Arbor Sciences Inc. HETHERINGTON, J. A. (1978). The uptake of plutonium nuclides by marine sediments. Mar. Sci. Commun., 4, 239-74. HETHERINGTON, J. A., JEFFER1ES,D. F., MITCHELL,N. T., PENTREATH,R. J. & WOODHEAD,D. S. (1976). Environmental and public health consequences of the controlled discharge of transuranic elements to the marine environment. In Proc. Syrup. Transuranium Nuclides in the Marine Em, ironment, 81 106. Vienna, IAEA. LIVINGSTON,H. D. & BOWEN,V. T. (1977). Windscale effluents in the waters and sediments of the M inch. Nature, Lond., 269, 586-8. STANNERS,D. A. & ASTON, S. R. (1980). The rapid determination of americium in sediments by NaI(T1) gamma spectroscopy. Ent'iron. Technol. Lett., !, 333 8.