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241Am and 137Cs activity in fine grained saltmarsh sediments from parts of the N.E. Irish sea shoreline
J. Environ. Radioactivity 19 (1993) 1-24
Z4tAm and 137CsActivity in Fine Grained Saitmarsh Sediments From Parts of the N.E. Irish Sea Shoreline
F. Oldfield, a N. Richardson," P. G. Appleby b "Department of Geography. t'Department of Applied Mathematics & Theoretical Physics. University of Liverpool, PO Box 147, Liverpool L69 3BX. UK
& L. Yu Department of Geography, East China Normal University, Shanghai. People's Republic of China (Received 16 April 1991; revised version received 12 November 1991 ; accepted 25 November 1991 }
ABSTRACT 2¢tAm and tWCs activities in surface and core samph,s o['saltmarsh sediment from parts of the Galloway and south Cumbria coastlines are reported. Ttle study concentrates on areas of saltmarsh which have been colonised hy Spartina anglica Hubbard., in view of its ability to trap lble grained sediments and to colonise previously bare mudflats bt3"ond the outer edge 0[" pre-existing marsh. At each site downprofile variations oJ " Am and ISrCs are presented, and most appear to show a more or less comph'te record of the history o f discharges from Sellafield. The total #tventories of the radionuclides are calculated for core samples and compared with published results from previous studies of intertidal sediments in the immediate vicinity. The relationship between particle size and radionuclule activity and the use of magnetic measurements as a relativel.v simple way o f normalizing]or this effect are examined. • ~4!
I
J. Environ• Radioactivity 0265--931X/93/$06-00 © 1993 Elsevier Publishers Ltd, England. Printed in Great Britain.
2
F Oldlh'ld, ,~. Richard.~on. P. G. Applebv, L. Yu
1 INTRODUCTION A wide range of publications and reports provide a substantial database for the evaluation of Sellafield-derived radioactivity in the northern Irish Sea and along its shores. These papers deal with the measurement and modelling of activity in offshore marine sediments (e.g. Kershaw et at., 1984, 1988: Gurbutt et al., 1988: Malcolm et al., 1990), with coastal activity in mudflats, saltmarshes and suspended sediments (e.g., Aston & Stanners; 1981, 1982: Aston et al., 1981, 1985; Stanners & Aston, 1981' Clifton & Hamilton, 1982; Baxter et al., 1987; Eakins et al., 1987: Hunt, 1987: Bonnett et al., 1988: Garland et aL, 1989a, b: Carpenter et al., 1991) and with the effects of windblown material in coastal environments landward of the shoreline (e.g. Pattenden et al., 1980; Eakins et al., 1987). Many of the measurements made reinforce the well-documented dependence ot sediment radioactivity on particle size. Although the presence of hot particles (Hamilton. 1981 Kershaw et al., 1986; Baxter et al., 1987) will account tk~r some of the noted variations in activity, and a range of transport and post-depositional processes will modify activity-particle size relationships (Aston et al., 1985), it is ncvcrthclcss apparent that these latter lie at the root of many of the temporal variation in activity at any given site (e.g. Mitchell, 1989) as well as many of the discrepancies betwcen peak measured activities m fine suspended sediments and those measured in adjacent sands and nmdflats (e.g. G:irland ('t al., 1989a). in the present report we focus on 24tAm and Iw('s activity in saltmarsh sediments. Saltmarsh vegetation tends to trap fine suspended particles. It is therefore reasonable to expect that saltmarsh areas are the ones most likely to contain peak activities and inventories and to retain a consistent stratigraphic record of radioisotope deposition. Whereas Bonnett et at. (1988) sampled and meast.red activity in sections cut mostly in areas covered by the grasses Puccinellia m a r i t i m a and Festuca ruhra, the present study has concentrated wherever possible on areas of colonization b) S p a r t i n a anglica Hubbard., in view of its capacity to trap fine grained sediments and to colonize previously bare mudflats beyond the outer edge of preexisting marsh, thereby often forming a monospecific stand (Goodman et al., 1959; Whiteside, 1987). In such environments, a high capacity for fine sediment accumulation is combined with long periods of tidal inundation. In a smaller number of cases, where mature S p a r t i n a swards were absent, strands of t l a l i m i o n e portuhtcoi~h,s (L.) Aellan were sampled, since this species is also a frequently inundated, effective finesediment trap. This stage of the study concentrates on results from parts of the Galloway and South Cumbria coastlines and includes several sites where
24t,4m and t3ZCs activity in saltmarsh sediments
3
Spartina swards have extended during the last few decades (Goodman et al., 1959). Although the Galloway coastline has been the subject of several studies, those published offer conflicting evidence with regard to levels of radioactive contamination along the coast. For example, in Garland et al. (1989a), a comparison between peak activities in suspended sediments and surface mud samples shows a threefold difference for 241Am and a ninefold difference for 137Cs. Mitchell (1989), using published sources, documents wide variations from year to year in 24tAm and 137Csactivity in surface silt from Garlieston, whilst Hunt (1987) records values for surface activity in the Kippford-Palnackie embayment well in excess of any published for the region by Garland et al. (1989a) or Baxter et al. (1987), and comparable to the suspended sediment values quoted by Garland et al. (1989a). The aims here have been to:
(i) reconcile the apparent disparity between suspended sediment and sediment surface values, (ii) quantify more fully the effcct of particle size variations on 137Cs and 24tAm radioactivity in the coastal arcas sampled, and look for cheap and simple ways of normalizing vahtes for this effect, (iii) Make a very preliminary assessment of the role of Spartina anglica in trapping 24tAm and 137Cs adsorbed to fine particles. The results present here come from the Dee estuary above and bclow Kirkcudbright and from the Kippford area of thc Urr estuary (Fig. I). In the Dec, the samplcs come from bare mudflats within the tidal channel. In the Urr the results are based on three cores from arcas of Spart#la marsh. The sccond area considered herc is that between Askam in the Duddon Estuary, and the marsh by the Foulncy Island causeway close to Rampside (Fig. 2). It includes two cores I¥om extensive saltmarsh areas within the Waincy Channel. In this instance the main concern has been to make comparisons with published results from previous studies in the immediate vicinity (Eakins et al., 1987, Bonnett et al., 1988; Carpenter et al., 1991) since, as in the case of Galloway, they present widely varying indications of surface activities and inventories. Comparisons are also drawn with published results from the Ravenglass area much closer to Sellafield, and with both published and unpublished results from the Lune and Wyre further away. 2 T H E S A M P L E SITES In the Dee (Kirkcudbright) estuary, intertidal muds were sampled at low tide in January and May 1989 and January 1990. The samples form part of
J
Bay
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Klrkcudbt/ght
k~
KIRKCUDBRIGHT
21
b
/
th Glen West (2) Glen South
Fig. 1. l,~catlon of surface sediment samples from the Dee (Kirkcudbright) e~tuar', and ~t" ct~re sites in Spartma anglica saltmar~h ira the L~rrKippl'ord. embayment, Gallo;~ax, S?,V, Scotland. C~re sites: SG S~uth Glen; K - Klppford
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:4tAm and t3~Cs activity in saltmarsh sediments
5
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IN FURNESS
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Location of core sites from Askam Marsh in the Duddon Valley and from the Walney Channel salt m a r s h e s . A = A s k a m ; W N = Walney North; W Y = W y l o c k ; F = Foulney. F i g . 2.
a larger set used in a study of magnetic property variations (Yu and Oldfield, unpublished data). Samples B, A, D & G are sandy muds from the western shoreline south of Kirkcudbright. samples L and M come from close to the head of the tidal reach, N, P, R, S, T and V from the eastern bank of the tidal reach of the river above Kirkcudbright, and sample I is from the tidal embayment to the south of Kirkcudbright. In all cases the surfaces sampled were judged to be accreting and to have received
6
Y OldlieAi. ),. Richardson. P. G Appleb). L. Yu
sediments during a recent tidal cycle. Within the sinuous upper part of the tidal channel, care was taken to avoid sampling the scoured outer bank o| each meander loop. In the Urr estuary, three cores were taken during the same visits, two from mature swards of Spartina on the western side of the main channel near South Glen. and one from the local Spartina-covered infill of a channel less than 10 m from the slipway at Kipptbrd. At Askam, the core measured was taken from a stand of Halimione portulacoides within a channel fill and at Walney North End, sediments under the same species were sampled within the extensive saltmarsh area to the north east of the airport. The Wylock Marsh and Foulney Marsh cores were taken from close to the landward edge of extensive Spartina swards. All the S. Cumbria samples were taken in April 1990 as were the samples from Spartina marshes near Sunderland Point and Glasson Dock near the mouth of the Lune Estuary and at Burrows Marsh and Stanah within the Wyre.
3 M ETHODS With the exceptions of the surface samples from the Dee near Kirkcudbright, which were scraped from the top 20 mm of intertidal sediments, most field samples were taken as cores in plastic drainpipes (usually 100 mm diameter) driven into the marsh surface then dug out. An auger core was taken from South Glen near Kippford in Galloway, using a posthole auger. The drainpipe cores were cut into 20 mm thick slices and dried at 40'C. In most cases, two adjacent slices were combined (but not mixed together) in Marinelli beakers for gamma measurement. Measurements were carri,,xl out on an Ortec GMX series coaxial germanium gamma detector. Bulk samples were placed in 350 ml Marinelli beakers: smaller particle-size samples were measured in a planar configuration. The detector was calibrated using an NPL mixed radionuclide standard solution. Measurements were corrected for self- absorption using methods based on those described in Debertin & Ren (1989). Only rarely do standard errors exceed 5 percent of the plotted values for either '37Cs or 241Am and in most cases they are less than 1 per cent. Following the well documented links between particle size and radioactivity in Irish Sea sediments (Aston et al., 1985: Baxter et al., 1987), the demonstration by Bonnett et al. (1988) that the magnetic properties of saltmarsh sediments in several of the estuaries between Galloway and N. Wales closely reflected particle size, and current work by Oldfield et al, (in press, and Fig. 9 in this paper), two magnetic properties; low field susceptibility (X) and anhysteretic remanent magnetization (ARM), were
:*lAin and tW Cs activity in saltmarsh sediments
7
measured on each sample. ;( was measured using the low frequency (0.47 KHz) circuit of a Bartington dual frequency sensor and associated Meter. ARMs were grown using a modified Molspin AF-demagnetizer, and measured using a Minispin low speed fluxgate magnetometer. The peak AF field used was 100mT and the steady DC biasing field was 0-04mT. These measurements were chosen because in the Irish Sea samples, both A R M and ARM;';( are closely related to particle size, despite differences in coastal lithology and in the relative importance of heavy mineral components (Oldfield et al., in press). This is largely because of the extent to which A R M reflects the concentration of fine grained magnetite ( < 0.1 /~m) in the clay fraction (~f. Maher, 1988), whereas Z is most strongly influenced by coarser multi-domain magnetite ( > 1.0 F~m) in the sands and coarse silts. In the present study our approach has been to measure both the magnetic and radiometric properties of a series of particle sized samples in order to explore the extent to which the latter can be normalized for particle size by means of the magnetic measurements. Particle sized subsamples were measured from 5 samples from the Galloway coastline, 4 from surface material in the Dec and Urr and I from the level of peak 2~lAm activity in a core from the Urr. They were obtained by a combination of wet sieving and pipetting as described in Yu (1989). 4 RESULTS Figures I and 2 show the location of till the sites sampled. Figures 3 (Galloway) and 4 (S. Cumbria) plot 24~Am and 137Csconcentrations versus depth tk)r all the cores takcn from the main study regions. Figure 5 plots results for the Lune and Wyre for comparison, in most cases the cores include most if not all of the significantly radioactive material at the site; only the Askam (Fig. 4) and Wyre Marsh-Stanah (Fig. 5) cores are too short to include a more or less complete inventory for each radioisotope. The results from surface samples within the Dee (Kirkcudbright) channel are plotted in Table 1. Table 2 summarises a range of features for each profile. Figures 6-9 and Table 3 summarise the rclationship between particle size, 137Csand 241Am activity, and both ARM and ARM/z. 5 ACTIVITIES A N D INVENTORIES
5.1 Galloway: surface activities In the Dee (Kirkcudbright) samples, peak values for both 241Amand 137Cs occur in the finest sediments from the upper tidal reaches. For 24~Am the values reach 690 Bq kg "l and for 137Cs over 900 Bq kg "l. The surface
F Oldlietd, IV. Richardson. P. G. Appleby. L. Yu S O U T H GLEN
1
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Fig. 3. Activity/depth profiles for J~TCsand 241Am in the Galloway salt marsh cores (sec Fig. 1 ).
values from the Urr estuary cores range from 433 Bq kg ~ to 983 Bq kg :1 for 241Am and from 706 Bq kg l to 1706 Bq kg t for 137Cs. The 241Am values lie consistently above those quoted for intertidal sediments in Garland et al. (1989a) and Baxter et ai. (1987). These values are within or beyond the upper range of values quoted for suspended sediments by Garland et al. (1989a) and Baxter et al. (1987). These values are within
:4lAin and tSTCs activity in saltmarsh sediments
9
Askam Marsh
AM-241 Concentrat;on (Bq kg "1)
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Fig. 4. Activity/depth profiles for t37Cs and Z4tAm in the salt marsh cores from the S. Cumbrian Coast (see Fig. 2).
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F Oldlqeld. N Rwhurdson P. G. Applebv. L. Yu Sunderland Potnt 2 AM-241 Concentration (Bq kg -1) 0 500 1000 1500
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Fig. 5. Activity/depth profiles for 1 3 7 C s and :*tAm in representative salt marsh cores from the Lune (Sunderland Point and Glasson) and the Wyre (Burrows Marsh and Stanah) for comparison with Figs 3 and 4.
2"tAm and 13rCs activity in saltmarsh sediments
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TABLE !
t37Cs and 24tAm C o n c e n t r a t i o n s
in Bulk Surface Samples from the Tidal Channel o f the Dee and the Tidal Flats o f Kircudbright Bay (see Fig. 1 for locationst
Sample
13rCs ( Bq kg "t )
:~t A m ( Bq kg "t )
A D G ! L M N P R S T V
354 358 455 407 619 622 899 406 919 588 498 483 180
254 263 335 239 315 298 633 273 649 417 367 353 144
B
,4 RA!
56-0 99-0 112.0 80.0 . . 127.0 290.0 -286-0 . . 162-0 -17-6
.4 R.$I Z
2.25 4.10 5.10 1.90 .
. 4.70 7.20 -6.90
.
.
. 4.80 1.60
Correlation coefficients for samples for which magnetic measurements arc available: A R M versus 137Cs r = 0-967 (n = 9) A R M versus -'41Am r = 0.986 (n = 9) A R M / z versus 1~TCs r = 0-898 (n = 9) A R M / z versus '41Am r = 0.913 (n = 9)
or beyond the upper range of values quoted for suspcnded sediments by Garland et al. (1989a) and for muds and marsh sediments by Hunt (1987). The 137Cs values also exceed those quoted by Garland et al. (1989a) and Baxter et al. (1987) for intertidal samples from these estuaries, though they fall within the range quoted by Hunt (1987). 5.2 Galloway: peak activities
In all three c o r e s 2 4 1 A m and 137Cs activities peak in the top 0.3 m of the core. For 24~Am the values range from 1082 Bq kg z to 2166 Bq kg ~, and for 137Cs between 2216 Bq kg "l and 5159 Bq kg l . These values all greatly exceed those recorded in the publications quoted with the exception of the highest values given for Kippford samples in Hunt's 1986 survey published in 1987. 5.3 Galloway: core inventories
For 24~Am these range from 239 to 434 kBq m 2 and to 1038 kBq m "2. These figures may be compared 75 kBq m 2 and 122-221 kBq m -, respectively, for the Baxter et al. (1987). Pentreath et al. (1986), on the
for 137Cs from 576 with ones of 33uppermost 0-3 m in basis of studies on
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F. Otdtieid, 3, Rtchardson. P G. Appleby. L. Yu
"fABLE 2 Surface Concentrations, Maximum Concentrations and Total Inventories for 2't~Am and t~'Cs in cores from the Galloway Coast. South Cumbrian Coast. Lune and Wyre Estuaries: (see Figs 1 and 2 for locations). The data are compared with earlier results from the Dee Estuary, Wirral {Bonnett et aL. 1988)
Surface activitie,~ Bq k"g-t ~
~,:rr Estuary South Glen I South Glen 2 Kippford
Maximum activitw~ c Bq k e i
:*/,4m
(s
:~l,4m
t¢'C~
766 933 433
I 474 1 706 766
1 567 2 166 I 082
3 523 5 159 2 216
Inventories (kBqm :j t3"Cs
Mean
Duddon Estuary and 14"alnev Channel Askam 845 845 Walney Is N I 902 3 797 Biggar Sands 3 {)94 6 340 Wylock Marsh I I 322 I 383 Foulney 607 1 074
5 047 5 687
5 833 8 497
4 572 5 689
5 17{) 7 263 Mean
Lune and 14~vrc Estuaries Sunderland 2 445 Glasson 371 Burrows Marsh 361 Stanah 5(1{I
I 664 1 575 I 205 I 656
796 742 1 655 I 106
4 4 5 6
778 355 438 311 Mean
DeeEstuao'(~Trral) Denhall A Denhall B Flint
38 42 162
392 4{)3 1 231
75 186 352
850 2 103 4 380 Mean
'*lAm
Ratio
748 1 038 576
343 434 239
2.2 24 2.4
787
339
2.3
> I 155 525 751 971 699
> 783 310 396 711 477
I-5 1.7 1.9 1.4 1-5
8211
535
I-5
657 714 822 > 1 000
122 142 2{)2 > 189
54 5-{) 4.1 5.3
798
164
4.9
670 691 670
61 53 56
11.0 13.1 11.7
677
57
11.9
submerged sediments, map estimated 241Am inventories for the whole of the northern Irish Sea and indicate values between l0 and 30 kBq m 2 for that part of the sea bed adjacent to the Galloway coast. The values presented here are thus 5 to 10 times those from previous studies and over an order of magnitude larger than those estimated from measurements made offshore. 5.4 Galloway: sediment activities and Generalized Derived Limits (GDLs) The question of suitable Generalized Derived Limits (GDLs) for relatively accessible salt marshes is considered briefly in Section 8. At this stage a
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Fig. 6. lSTCs and 24tAm activity in surface samples from the Dee (Kirkcudbright) Estuary plotted against the magnetic properties ARM (10 6 Am 2 kg d) and ARM/z (10 .4 Aml). The location labels are as shown on Fig. I.
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Fil~. 7. + ~'(._'+ and +~IAm a~.-ttx it+', m parted,: sl.,'c fralu+on,, frum ~,urf;+lue and c~+r~: ',ample,+, from the Gaih),.*,a) t.'t+.a~,t h,,:tv+een K+rk,+udb+ight i+~,, and K ippford plotted agam:.t the magnetic propertie., .-\RM 110 +++A m 2 kg+tl and A R M +5C(10 '+ A m +!), The 8 phi .,ample |'rc, m :+,ample R ha', anomalously high -"+tAm co|}centrations and nl~t) ,,:c,rttam +J +hot' particle The particle diameters for each phi size used are apprt)>.mlatel+y as follo'~s: 4 phi, 62.5 3125 lJm; 5 phi. 3 1 2 5 156 ,.m; 6 phi, 156 ~+8 +m~: 7 phi. 3.8 3 9 / + m : 8 phi, 4 2 ,£+m: ~ Itt phi, less that; 2 /+m
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:~l,4rn and I~: Cs ac'tivitv in saltmar~h sediments 1(3'O
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Fill. 8. Normalised ~rCs and 'alAm activities plotted against normalised ARM. For each suite of particle size fractions from a single sample, the peak activity ~r ARM value = 100 and other values are normalised to that. The c;tption tt) Fig. 7 explains the phi scale. Values l\~r the 8 phi fraction from sample R have been omitted (scc Fig. 7).
•
Scotland
•
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o
Wales
estuat~ F
9•
O
ee 6-
n-
3
•
o
O e•
0
2c~ O
0,
Fine %
Fig. 9. ARM/x ( l O 4 A m l ) versus sediment fraction less than 31.25/am ( = ' F i n e percentage') for a suite of samples taken from the Irish Sea shoreline in S. Scotland, the Dee Estuary S.W. of Liverpool and the Anglesey Coast. The linear relationship between magnetic property quotient and particle size is evident despite the major differences in coastal lithology between the areas sampled.
t0
F. Oldtietd, N Richardson, P. G. Applebv, L. Yu
TABLE 3 Pearson Correlations Coefficients (r) betv,een Magnetic Properties and Radioisotope Concentrations for Parucle Size Fractions ..
(A) Non-norrnalised values ARM versus ~37Cs ARM,.';( versus +~'Cs ARM versus "'*tAm ARM Z versus :'*lAm (B) Normalised values ARM versus ~TCs ARM;( versus I~-Cs A R M versus z'*~Am ARM';( versus z'*lAm
All poin!:~
8 phi omitted
0.878 0-898 0.802 0.791
0-919 0-935 0-952 0.912
0-900 0.902 0.817 0.805 (n = 19)
0.966 0.956 0.946 0-918 (n = 18)
In part A all the values used are non-normalised. In part B. both the magnetic property values and the radioisotope concentrations are normali,~d for each sample to the maximum value for that sample. In all cases the 10 and 9 phi measurements are combined and the mean value used. comparison of values within the most recently published limits for marine sediments and well mixed soils is considered (Haywood, 1987). Surface americium values are just over 3% and subsurface peak values over 7% of the marine sediment G D L ; They are around 100% and 200%, respectively, of the well-mixed soil G D L . Surface caesium values arc bctwen 10% and 25% and peak values are between 3% and 74'% of the marine sediment G D L , and arc 75% to 190%, and 240°A. to 580% respectively of the well mixed soil G D L .
5.5 South Cumbria: surface activities Americium values at the surface of the four cores analysed in detail range from 607 to 1902 Bq kg "~. These are comparable to those o f Bonnett et al. (1988) from their Duddon and Walney Channel sample sites. They are significantly higher than either the mean value of 440 Bq kg ~ quoted by Hunt (1987), or the range of values for the area (15-154 Bq kg'l) given by Carpenter e t al. (1991), as well as an order o f magnitude higher than the range of 33 Bq kg 1 to 114 Bq kg n quoted by Eakins et al. (1987) for their shoreline sediment samples from more exposed sites in the same general area. The highest quoted value by the last authors, 306 Bq kg "n at Braystones, only 3 km from SeUafield, is still well below the values for surface material from the sheltered areas sampled in the present study. In the present survey, the highest surface value o f 3094 Bq kg "l was recorded
:'1,4m and t3r Cs activity in saltmarsh sediment~"
17
in the core from Biggar Sands in the Walney Channel, though it should be noted that this core was sampled only in 10 cm slices. Caesium surface activities in the cores plotted range from 845 Bq kg ~ to 3797 Bq kg ~, well within the full range of measurements quoted by Bonnett et al. (1988) but higher than the mean value of 620 Bq kg t quoted by Hunt (1987) or the range ofvalues (112-468 Bq kg -I) given in Carpenter et al. (1991) for the area between Sellafield and the Kent Estuary. The top 10 cm of the Biggar Sands core had an activity of 6340 Bq kg 1. 5.6 South Cumbria: peak activities
In the Walney Channel cores, peak americium values all lie within the top 0.3 m. That in the Askam core is at the base and may not reflect the highest activities at the site. The peak values all lie between 4572 and 5689 Bq kg ~. Such values are comparable with some of the highest measured in the latc 1970s in the lrt and Esk estuaries, much nearer to Scllafield (Aston & Stanners 1981; Stanners & Aston, 1981; Clifton & tlamilton, 1982) and much closer in time to the period of peak discharge. They also lie within the range of measurements for 'entrained silt" quoted by Eakins et al. (1987), though in their samples, the silt fraction is ncvcr more than 2-5°i, of the total bulk. The peak values measured in our saltmarsh cores arc around an order of magnitude higher than the highest value quoted for offshore cores from the mud patch only 10 km from thc Scllafield discharge pipe (Kershaw et al., 1984). Peak caesium values range from 5170 to 8497 Bq kg t . Once more, these compare well with the values in the cores from the region measured by Bonnett et al. (1988), and lie within the range recorded by Stanners and Aston (1981), Aston and Stanners (1981, 1982) and Clifton and Hamilton (1982) in the Esk and lrt Estuaries in 1978. 5.7 South Cumbria: core inventories
Americium inventories range from c. 3 1 0 k B q m 2 to more than 783 kBq m "2. These greatly exceed the Duddon and Walney values quoted by Eakins et ai. (1987) and the range for the area given by Carpenter et al. (1991), all of which lie between 28 and 150 kBq m 2. Even at Drigg, only 6 km south of Sellafield, the inventory of 718 kBq m 2 quoted by Eakins et al. (1987) is significantly less than the incomplete inventory from the saltmarsh core at Askam. For this coastline, as for that offshore from Galloway, Pentreath et al. (1986) map estimated inventories from 10 kBq m "2 to 30 kBq m "2.
18
i" Otdlield. N. Richardson. P. G. Applebv. L. Yu
Caesium inventories range from 525 kBq m-" to over 1155 kBq m : . The lowest value overlaps the range for the area in Carpenter et aL (1991). 5.8 South Cumbria: sediment activities and GDLs
Surface activities for 24tAm in the sub-sampled cores are between 2% and 6% of the marine sediment G D L and between 50% and 150% of the well mixed soil GDL. Peak values, which lie between 0-06 m and 0.28 m below the present day surface, are around 18% to 20% of the marine sediment G D L and 500% of the well mixed soil GDL. Surface caesium activities range from 12% to 55% of the marine sediment G D L and from 94% to 424% of the well mixed soil GDL. Peak caesium activities are between 74% and 122% of the marine sediment G D L and between 575% and 950% of the well mixed soil GDL.
6 SALTMARSH SEDIMENT STRATIGRAPHY The majority of thc cores appear to show a more or less complete record of the history of dischargcs from Sellafield. The rapid increase in ~~TCs discharges during the early 1970s leading to peak values during the period 1974 -78 (McColl, 1989) is recorded in the shape of the 137Csactivity versus depth profilcs in six out of the eight Galloway and S. Cumbria cores (Figs 3 and 4), as is thc marked decline in dischargcs since 1980. The distribution of -'a~Am is complicated by the fact that it arises both from direct discharges and by ingrowth from 24ZPu, The net record, decay corrected to 1989, should however give peak values in 1973-74 followed by a very sharp decline during the period 1975-77, slightly earlier than for IWCs. This displacement of the record of 24tAm vis-a-vis ~37Cs is evident in all cores except those where the sub-sampling was too coarse to resolve the separation. Broadly similar stratigraphic patterns are recorded, albeit less clearly, in the Lune and Wyre cores 60-70 km from Seilafield (Fig. 5). Similar results were obtained at some sites further south in the Dee Estuary, Wirral (Bonnett et al., 1988). Though preliminary results from some other S. Lancs and N, Wales sites indicate that disruptions to the record may occur as a result of irregularities in the process of sedimention, we infer from the above observations that, in many cases, cores from saltmarsh and tidal mud environments provide internally consistent records of radionuclide deposition and storage. These stratigraphic records may be of some chronological value in determining recent sediment accumulation rates, though possible delays in radioisotope
24t A m and taZCs activity in saltmarsh sediments
19
transfer from source to sediment site mean that for the present these values should be accepted with caution. Calculations of the cumulative discharge of 137Cs and 24~Am from Sellafield, corrected for decay and ingrowth from 24~Pu to the end of 1989, give a t37Cs/241Am ratio of 35 : !. Since the K o value for "4~Am is 2-3 orders of magnitude higher than for ~37Cs, sediments close to Sellafield will be relatively enriched in 241Am. For these sediments, the 137Cs/241Am ratio may be expected to be greatly reduced from the nominal value. This is confirmed by the calculation of the t37Cs/24tAm inventory ratios shown in Table 2. Also shown are revised calculations of earlier results from the Dee Estuary, Wirral (Bonnett et al., 1988). Of particular note is the similarity of the 137Cs/'ZatAm ratio among the cores from each region. Lowest values occur in the group ofcores from the Walney Channel and Duddon Estuary, nearest to Sellafield. There is a 50% increase in the ratio for cores in the Urr Estuary, but a 300% increase for cores in the Lune and Wyre Estuaries, though both sites are more or less equidistant from Sellafield. The higher gradient with distance south appears to be confirmed by the results given in Carpenter et al. (1991). in their results, which are from a range of environments, the significance of grain size is reflected in the disproportionately depleted '41Am inventories in cores from coarse grained sandy deposits.
7 RADIOACTIVITY AND MAGNETIC PROPERTIES Since particle size exercises a strong control over some magnetic properties (Yu et al. in press, Oldfield et al., in press) and over 24~Am and 137Cs concentrations in Irish Sea sediments (Baxter et al. 1987; Eakins et al., 1987; Bonnett et ai., 1988), the relationship between the two has been explained in several ways. in Fig. 6 the relationship between 1~7Cs and 24tAm is plotted on the one hand and both A R M and ARM/~ on the other for the set of surface samples from the Dee (Kirkcudbright) intertidal muds (Table 1). The magnetic properties and radioisotope concentrations are, save for sample M at the very head of the tidal reaches, not only linearly correlated with correlation coefficients between 0.898 and 0.986, but sequentially ordered in relation to distance from the open sea. Within each of the estuaries studied, there is a strong direct relationship between radioactive concentrations and magnetic properties in bulk sediment samples. The basis for this relationship has been explored further using particle size fractions from some of the Dee and Urr estuary samples. Table 3 summarises magnetic property: radioisotope concentration correlation coefficients for the samples and particle sizes measured. Figures 7 and 8
20
f" OldJield. N. Rtchardzon, P. G Apptehy. L. Yu
plot both A R M , ARM Z, :41Am and 13~Cs versus particle size. In this case, by pooling the results from all four samples, and despite their different locations and bulk concentrations, correlations between the two sets of measurements are still apparent, with correlation coefficients between r = 0.791 and r = 0.966. These may be compared with Bonnett et a/.'s (1988) values for IWCs versus A R M of r = 0 , 8 6 5 and r = 0 . 9 7 8 for measurements on fractions of individual samples from the Ribble and lrt estuaries respectively, and o f r = 0.851 and r = 0.966 for 24~Am vs. ARM. From the above summary it can be concluded that on both a local and a regional basis (tJ~ Fig. 9) there is considerable scope for normalising 241Am and 137Cs activities by means of rapid, simple, non-destructive measurements which can be carried out in the field or laboratory on a dry mass or fresh volume basis and on sample masses as low as 0.5 g (dry). In Fig. t0, the down-profile '41Am and 137Csconcentrations are plotted for three of the south Lake District cores as normalised values using both ARM and ARM/z. In the case of these cores from consistently fine grained sediments, the normalisation has negligible effect. This reinforces the view that the protilcs reflect sequcntial deposition rather than scdimentological variation through time.
IMPLICATIONS Using geomorphoiogical and ecological experience in field sampling, it is possible to predict and subsequently conlirm the location of sites where contemporary deposition of line grained sediments is taking place within tidal channels, and of sites within saitmarshes where sustained recent fine grained sediment accumulation has taken place and is continuing. Such sites along the Galloway and S. Cumbria coastlines have surface and peak concentrations and inventories of 137Csand 241Am considerably in excess of most of those recorded in other publications on intertidal sediment radioactivity in the same general areas. The most nearly comparable published results are those quoted for surface samples and cores of fine grained silts and muds (not saltmarshes) in the Wyre and Ribble estuaries by Carpenter et al. ( 1991 ), though they fail to document fully either the type of field contexts sampled or the stratigraphic record of concentration changes in each "inventory' core. The present records have major implications for total inventory calculations for several areas analysed by recent DoE reports (e.g., Eakins et al., 1987; Garland et al., 1989) and they may therefore modify current perspectives on the implications of radioactivity in these areas. The normal procedures and calculations used to evaluate the potential human
:~l,4rn and t3~Cs activity in saltrnarsh sediments 600O
21
6O00
SO00 4000
4000
.f_ 3 ~
3000
2000
2000 1000
0
0
0
10
20
0
30
10
Depth (cm)
~
6OOO 5OOO 4000
2OOO 1~ 0
10
20
30
9000 8000 7000 6000 •~ sooo 4o00 30O0 20O0 1000 0
)
0 0
20
Del)th (em)
30
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Depth Icrn)
6OOO
80O0
5OOO 600O 5O00 ,ooo 3O0O
40OO
2OOO
2OOO 1000
0
0
0
10
20
Depth (cml
30
0
10
20
30
Depth (cm)
Fig. 10 Normalised and non-normalised activities in three saltmarsh cores from the South Cumbrian Coast. The profiles are from (a) Askam, ""Am; (b) Askam, 137Cs; (c) Walney, 241Am; (d) Walney, I-aTCs;(e) Foulney, -'41Am and (f') Foulney, 1~TCs. The solid diamond symbol plots the non-normalised activities in Bq kg l . Values plotted as open squares have been normalised by A R M measurements; values plotted as asterisks have been normalised by ARM/x. After normalisation all the values have been scaled so that the maximum normalised value for each radioisotope in each core corresponds with its non-normalised activity.
implications o f levels o f radioactivity in different environmental contexts appear potentially controversial in relation to saltmarsh sediments. They are not marine sediments sensu stricto since most are more often exposed
22
I;'. Oid/ield, N Rtchardson. P. G. Appleby. L. ~u
than inundated; moreover some lie close to human habitation and activity, At the same time, the',' are not well-mixed soils under agricultural management, though virtually all the ones studied here have specific activities which, integrated over the top, 0-3 m (i.e. a typical plough-layer) would be well in excess of those at the surface. Moreover the potential for redeposition, further sorting, incremental accumulation and for inventory increases arising solely from the ingrowth of _,alAm from ,-4mPu, add special dimensions requiring further evaluation at these sites. In addition, account must be taken of the ability of Spartina anglica to colonise and form mature swards over other areas, thus trapping further quantities of fine grained radioactive sediments, and of the future implications of rising sea-level for the resuspension and redeposition of the most radioactive sediments presently between 0.05 and 0.30 m of the surface at most of the sites studied. Finally, it should be borne in mind that saltmarshes are frequently grazed between tidal incursions ztnd may subsequently be drained, dyked and reclaimed to t~rm agricultural land.
9 R E C O M M E N D A T I O N S FOR F U T U R E RESEARCH ( I ) There should be a thorough evaluation of the present and potential role of Spartina anglica its a trapping agent for fine particleassocia, tcd radioactivity in the northern Irish Sea. This should embrace empirical measurements, surveys of current distribution and predictions of future spread. (2) Other more locally dominant vegetation types require similar study, notably stands of llalimione portuh.'oides and Phragmites commuhis, The latter reed colonises areas at, as well as beyond, the upper limit of tidal influence and is frequently rooted in very fine grained organic sediments. It is widely distributed in sheltered tidal channels along the Galloway shoreline. (3) The finest sediments accumulating below stands of Spartina especially can be highly reduced. The effect of this on chemical speciation, mobility and biological uptake of radionuclides requires further study. (4) Completion of !. 2 and 3 would provide a database from which revised assessments of the human implications of present and nearfuture levels of coastal radioactivity should then be derived in the light of the points made above and of future research. (5) The information provided by 1 and 2 should be incorporated in model simulations of the likely effects of channel changes and of rising sea-level on the redistribution of fine sediments in the coastal areas of greatest concern and vulnerability.
'~1,4m and I~'Cs activity in saltmarsh sediments
23
(6) Magnetic measurements ( A R M and A R M z ) should be used alongside radiometric measurements on existing and future samples in order to normalise values where appropriate and with further calibration, and to shed light on the processes such as transport and redistribution which give rise to changes in the relationship between particle size assemblages and radioactivity levels.
REFERENCES Aston, S. R. & Stanners, D. A. (1981). Americium in intertidal sediments from the coastal environs of Windscale. Marine Poll. Bull.. 12, 149-53. Aston, S. R. & Stanners, D. A. (1982). Spatial variability in the distribution of Windscale fusion products in estuarine sediments. Marine Poll. Bull.. 14, 16774. Aston, S. R., Assinder, D. J. & Kelly, M. (1985). Plutonium in intertidal coastal and estuarine sediments in the Northern Irish Sea. Est. Coast. Slw(] S~'i., 20, 761-71. Aston, S. R., Assindcr, D. J. , Stanncrs, D. A. &Rac, J. E. (1981). Plutonium occurrence and phase distribution in sediments of the Wyrc Estuary, Northwest England. Marine Poll. Bull.. 12, 308 14. Baxter, M. S., Cook, G. T. & McDonald, P. (1987). An assessment of artificial radionuclide transfer from Scllaticld to the South West of Scotland, Report to the DOE, DOE Report Number D()E/RW/'89/127. Bonnett, P. J. P., Applcby; P. G. & Oldficld, [:. (1988). Radionuclidcs in coastal and estuarine sediments from Wirral and Lancashire. Sci. Total Environ... 711, 215-36. Citrpcnter, R. C., Burton, P. J., Strange, L. P. & Pratlcy, |:. W. (1991). Radionuclides in intertidal sands and sediments from Morccambc Bay to the Dec Estuary, AERE-R 13803, HMSO, London. Clifton, R. J. & Hamilton, E. (1982). The application of Radioisotopes in the study of estuarine sedimentary processes. Est. Coast. ShelfSt'i.. 14, 433 46. Debertin, K. & Ren, J. (1989). Measurement of the activity of radioactive samples in Marinelli Beakers. Nucl. Inst. Meth. Phys. Rex.. A278, 541-9. Eakins, J. D., Morgan, A., Baston, G. M. N., Pratley, F. A., Yarnold, L. P. & Burton, P. J. (1987). Studies ofenvironmental radioactivity in Cumbria, Part 8. Plutonium and americium in the intertidal sands of north west England, AERER 12061, HMSO, London. Garland, J. A., McKay, W. A., Cambray, R. S. & Burton, P. J. (1989a). Manmade radionuclides in the environment of Dumfries and Galloway. Nud. Energy. 28, 369-92. Garland, J. A., Cambray, R. S., Burton, P. J. & McKay, W. A. (1989h). Artificial radioactivity on the coasts of Wales, AERE G5307, HMSO, London. Goodman, P. J., Braybrooks, E. M. & Lambert, J. M. (1959). Investigations into dieback in Sparthul townsendii. The present status of Spartbta townsendii in Britain. J. Ecol.. 47, 651-77. Gurbutt, P. A., Kershaw, P. J. & Durance, J. A. (1988). Modelling the distribution of soluble and particle absorbed radionuclides in the Irish Sea. In Radioactivity
24
t, OhtlieM A Rwhard~~,n. P, G. Appleh~, L. Yu
and Oceanograph~ .... ,4 lo~d tor Oceanoeraphy. ed. J. C. Guary, P. Gueguema~ & R. J. Pentreath. Elsevier, Amsterdam. Hamilton, E. I. (1981). Alpha particle radioactivity of hot particles from the Esk estuary, Nature. 290, 69t)- 93. Haywood, S. M. (19871. Revised generalised derived hmits for radioisotopes ol strontium, iodine, caesium, plutonium, americium and curium, National Radiological Protection Board, Chilton, NRPB-GS8. Hunt. G. J. (1987). Aquatic environment monitoring report No. 18. Radioactivity m surface and coastal waters of the British Isles, 1986, M A F F , 1545N 0142-2499, Kershaw, P. J., S~ift, D, J,, Pcntreath. R. J. & Lovett, M. B. (1984). The incorporation of plutonium, americium and curium into the Irish Sea seabed b~ biological activit3 S~,i. 75,tal Environ.. 411, 61 81. Kershav,, P. J., Brcarlc~. J. W., Woodhcad. D. S. & Lovctt, M. B., (19s6). Alpha emitting, hot particles in Irish Sea sediments. Sci. Total Environ., 53, 77-87, Kersha~, P. J., Pentreath. R. J., Gurbutt. P. A., Woodhead. D. S., Durance, J. A, & Camplin, W. C. (1988). Modelling the behaviour of long-lived radionuclides in the Irish Sea comparison of model predictions with field observations, in Ah,thods liar ,.Is.~'.~'s#tg thc Reliahilitv of Environnwntal Tran,v/~,r Mo~h't Predictions, ed. (;. Dcsmct. Elsevier, Amsterdam, pp. 241 9 . Maher, B. A. (It)SS). Magnetic propertics of some synthetic sub-micron magnetites. J, Gc~ph)'~. Rc~., 94. ,"g3 96. Malcolm, S. J., Kcrshaw, P. J,, Lovctt, M. B. & |iarvcy, B. R. (19901. The interstitial water chemistry of '~'~ '4'~Pu and z'~tAm in the sediments of the north-east Irish Sea. G(,o('him. ('osttlochbn. A~'/tl, ~ , 29 35. McColI, N. (1989). Sources of radioactivity in the north east Atlantic: fuel processing plants and past sea dumping opcrz.tions. In The Radioh~gi~'al l"?xposttre ~] tht" Popukttion o.f the' European Comvnunit.v .from Radioactivity in North b:urolwan Alarim" Water, Proceedings of a seminar on the ('EC Project "MARINA', Brugcs. 1989, pp. 95 III. Mitchell, P. [1989). Radionuclidc monitoring in the Irish Sca. In Tht, Radiologic'al ['sxIIosurc r?/"ttw P~lmh~ti,n ~,/ the European ('O/?ltHlUlig)' ]~'Ottt Radioactivity in North Ettropean Ahtrine ~|'ater, Proceedings of a seminar on the CEC Projcct ' M A R I N A ' , Brugc~, 198tLpp. 185 288. Pattenden, N. J., Cambray, R. S., Playfi~rd, K., Eakins, J. D. & Fisher, E. M. R. (1981)). Studies of environmental radioactivity in Cumbria. Part 3. Measurcments of radionuclides in airborne and deposited material, AERE R4857, HMSO, London, Pentreath, R. J., Woodhead, D. S., Kcrshaw, P. J., Jeffries, D. F. & Lovett, M. B. (1986). The behaviour of plutonium and americium in the Irish Sea. Rapp. p.w'. Cons, int. E.wh~r. Ah'r., 186, 60-9. Stanners, D. A. & Aston, S. R. (1981). 1~4Cs : 137Cs and t°6Ru : 137Cs Ratios in intertidal scdimcnts from the Cumbria and Lancashire Coasts, England. Est, Coast Shel['Sci.. 13, 409-17, Whitesidc, M. (1987). Spartina Colonization. In Morecamhe Bay." An Assessment of Present Ecological knowledge, ed. N. A. Robinson & A. W. Pringle. Centre lbr North-West Regional Studies/Morecambe Bay study group, pp. 118-29. Yu, L. (1989). Environmental applications of mineral magnetic measurements: towards a quantitative approach. PhD thesis, University of Liverpool, Liverpool.
Z4tAm and 137CsActivity in Fine Grained Saitmarsh Sediments From Parts of the N.E. Irish Sea Shoreline
F. Oldfield, a N. Richardson," P. G. Appleby b "Department of Geography. t'Department of Applied Mathematics & Theoretical Physics. University of Liverpool, PO Box 147, Liverpool L69 3BX. UK
& L. Yu Department of Geography, East China Normal University, Shanghai. People's Republic of China (Received 16 April 1991; revised version received 12 November 1991 ; accepted 25 November 1991 }
ABSTRACT 2¢tAm and tWCs activities in surface and core samph,s o['saltmarsh sediment from parts of the Galloway and south Cumbria coastlines are reported. Ttle study concentrates on areas of saltmarsh which have been colonised hy Spartina anglica Hubbard., in view of its ability to trap lble grained sediments and to colonise previously bare mudflats bt3"ond the outer edge 0[" pre-existing marsh. At each site downprofile variations oJ " Am and ISrCs are presented, and most appear to show a more or less comph'te record of the history o f discharges from Sellafield. The total #tventories of the radionuclides are calculated for core samples and compared with published results from previous studies of intertidal sediments in the immediate vicinity. The relationship between particle size and radionuclule activity and the use of magnetic measurements as a relativel.v simple way o f normalizing]or this effect are examined. • ~4!
I
J. Environ• Radioactivity 0265--931X/93/$06-00 © 1993 Elsevier Publishers Ltd, England. Printed in Great Britain.
2
F Oldlh'ld, ,~. Richard.~on. P. G. Applebv, L. Yu
1 INTRODUCTION A wide range of publications and reports provide a substantial database for the evaluation of Sellafield-derived radioactivity in the northern Irish Sea and along its shores. These papers deal with the measurement and modelling of activity in offshore marine sediments (e.g. Kershaw et at., 1984, 1988: Gurbutt et al., 1988: Malcolm et al., 1990), with coastal activity in mudflats, saltmarshes and suspended sediments (e.g., Aston & Stanners; 1981, 1982: Aston et al., 1981, 1985; Stanners & Aston, 1981' Clifton & Hamilton, 1982; Baxter et al., 1987; Eakins et al., 1987: Hunt, 1987: Bonnett et al., 1988: Garland et aL, 1989a, b: Carpenter et al., 1991) and with the effects of windblown material in coastal environments landward of the shoreline (e.g. Pattenden et al., 1980; Eakins et al., 1987). Many of the measurements made reinforce the well-documented dependence ot sediment radioactivity on particle size. Although the presence of hot particles (Hamilton. 1981 Kershaw et al., 1986; Baxter et al., 1987) will account tk~r some of the noted variations in activity, and a range of transport and post-depositional processes will modify activity-particle size relationships (Aston et al., 1985), it is ncvcrthclcss apparent that these latter lie at the root of many of the temporal variation in activity at any given site (e.g. Mitchell, 1989) as well as many of the discrepancies betwcen peak measured activities m fine suspended sediments and those measured in adjacent sands and nmdflats (e.g. G:irland ('t al., 1989a). in the present report we focus on 24tAm and Iw('s activity in saltmarsh sediments. Saltmarsh vegetation tends to trap fine suspended particles. It is therefore reasonable to expect that saltmarsh areas are the ones most likely to contain peak activities and inventories and to retain a consistent stratigraphic record of radioisotope deposition. Whereas Bonnett et at. (1988) sampled and meast.red activity in sections cut mostly in areas covered by the grasses Puccinellia m a r i t i m a and Festuca ruhra, the present study has concentrated wherever possible on areas of colonization b) S p a r t i n a anglica Hubbard., in view of its capacity to trap fine grained sediments and to colonize previously bare mudflats beyond the outer edge of preexisting marsh, thereby often forming a monospecific stand (Goodman et al., 1959; Whiteside, 1987). In such environments, a high capacity for fine sediment accumulation is combined with long periods of tidal inundation. In a smaller number of cases, where mature S p a r t i n a swards were absent, strands of t l a l i m i o n e portuhtcoi~h,s (L.) Aellan were sampled, since this species is also a frequently inundated, effective finesediment trap. This stage of the study concentrates on results from parts of the Galloway and South Cumbria coastlines and includes several sites where
24t,4m and t3ZCs activity in saltmarsh sediments
3
Spartina swards have extended during the last few decades (Goodman et al., 1959). Although the Galloway coastline has been the subject of several studies, those published offer conflicting evidence with regard to levels of radioactive contamination along the coast. For example, in Garland et al. (1989a), a comparison between peak activities in suspended sediments and surface mud samples shows a threefold difference for 241Am and a ninefold difference for 137Cs. Mitchell (1989), using published sources, documents wide variations from year to year in 24tAm and 137Csactivity in surface silt from Garlieston, whilst Hunt (1987) records values for surface activity in the Kippford-Palnackie embayment well in excess of any published for the region by Garland et al. (1989a) or Baxter et al. (1987), and comparable to the suspended sediment values quoted by Garland et al. (1989a). The aims here have been to:
(i) reconcile the apparent disparity between suspended sediment and sediment surface values, (ii) quantify more fully the effcct of particle size variations on 137Cs and 24tAm radioactivity in the coastal arcas sampled, and look for cheap and simple ways of normalizing vahtes for this effect, (iii) Make a very preliminary assessment of the role of Spartina anglica in trapping 24tAm and 137Cs adsorbed to fine particles. The results present here come from the Dee estuary above and bclow Kirkcudbright and from the Kippford area of thc Urr estuary (Fig. I). In the Dec, the samplcs come from bare mudflats within the tidal channel. In the Urr the results are based on three cores from arcas of Spart#la marsh. The sccond area considered herc is that between Askam in the Duddon Estuary, and the marsh by the Foulncy Island causeway close to Rampside (Fig. 2). It includes two cores I¥om extensive saltmarsh areas within the Waincy Channel. In this instance the main concern has been to make comparisons with published results from previous studies in the immediate vicinity (Eakins et al., 1987, Bonnett et al., 1988; Carpenter et al., 1991) since, as in the case of Galloway, they present widely varying indications of surface activities and inventories. Comparisons are also drawn with published results from the Ravenglass area much closer to Sellafield, and with both published and unpublished results from the Lune and Wyre further away. 2 T H E S A M P L E SITES In the Dee (Kirkcudbright) estuary, intertidal muds were sampled at low tide in January and May 1989 and January 1990. The samples form part of
J
Bay
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Klrkcudbt/ght
k~
KIRKCUDBRIGHT
21
b
/
th Glen West (2) Glen South
Fig. 1. l,~catlon of surface sediment samples from the Dee (Kirkcudbright) e~tuar', and ~t" ct~re sites in Spartma anglica saltmar~h ira the L~rrKippl'ord. embayment, Gallo;~ax, S?,V, Scotland. C~re sites: SG S~uth Glen; K - Klppford
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:4tAm and t3~Cs activity in saltmarsh sediments
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W~
F 0 l
-.~ * ">
Foulney
,
km ,
,
4 |
Location of core sites from Askam Marsh in the Duddon Valley and from the Walney Channel salt m a r s h e s . A = A s k a m ; W N = Walney North; W Y = W y l o c k ; F = Foulney. F i g . 2.
a larger set used in a study of magnetic property variations (Yu and Oldfield, unpublished data). Samples B, A, D & G are sandy muds from the western shoreline south of Kirkcudbright. samples L and M come from close to the head of the tidal reach, N, P, R, S, T and V from the eastern bank of the tidal reach of the river above Kirkcudbright, and sample I is from the tidal embayment to the south of Kirkcudbright. In all cases the surfaces sampled were judged to be accreting and to have received
6
Y OldlieAi. ),. Richardson. P. G Appleb). L. Yu
sediments during a recent tidal cycle. Within the sinuous upper part of the tidal channel, care was taken to avoid sampling the scoured outer bank o| each meander loop. In the Urr estuary, three cores were taken during the same visits, two from mature swards of Spartina on the western side of the main channel near South Glen. and one from the local Spartina-covered infill of a channel less than 10 m from the slipway at Kipptbrd. At Askam, the core measured was taken from a stand of Halimione portulacoides within a channel fill and at Walney North End, sediments under the same species were sampled within the extensive saltmarsh area to the north east of the airport. The Wylock Marsh and Foulney Marsh cores were taken from close to the landward edge of extensive Spartina swards. All the S. Cumbria samples were taken in April 1990 as were the samples from Spartina marshes near Sunderland Point and Glasson Dock near the mouth of the Lune Estuary and at Burrows Marsh and Stanah within the Wyre.
3 M ETHODS With the exceptions of the surface samples from the Dee near Kirkcudbright, which were scraped from the top 20 mm of intertidal sediments, most field samples were taken as cores in plastic drainpipes (usually 100 mm diameter) driven into the marsh surface then dug out. An auger core was taken from South Glen near Kippford in Galloway, using a posthole auger. The drainpipe cores were cut into 20 mm thick slices and dried at 40'C. In most cases, two adjacent slices were combined (but not mixed together) in Marinelli beakers for gamma measurement. Measurements were carri,,xl out on an Ortec GMX series coaxial germanium gamma detector. Bulk samples were placed in 350 ml Marinelli beakers: smaller particle-size samples were measured in a planar configuration. The detector was calibrated using an NPL mixed radionuclide standard solution. Measurements were corrected for self- absorption using methods based on those described in Debertin & Ren (1989). Only rarely do standard errors exceed 5 percent of the plotted values for either '37Cs or 241Am and in most cases they are less than 1 per cent. Following the well documented links between particle size and radioactivity in Irish Sea sediments (Aston et al., 1985: Baxter et al., 1987), the demonstration by Bonnett et al. (1988) that the magnetic properties of saltmarsh sediments in several of the estuaries between Galloway and N. Wales closely reflected particle size, and current work by Oldfield et al, (in press, and Fig. 9 in this paper), two magnetic properties; low field susceptibility (X) and anhysteretic remanent magnetization (ARM), were
:*lAin and tW Cs activity in saltmarsh sediments
7
measured on each sample. ;( was measured using the low frequency (0.47 KHz) circuit of a Bartington dual frequency sensor and associated Meter. ARMs were grown using a modified Molspin AF-demagnetizer, and measured using a Minispin low speed fluxgate magnetometer. The peak AF field used was 100mT and the steady DC biasing field was 0-04mT. These measurements were chosen because in the Irish Sea samples, both A R M and ARM;';( are closely related to particle size, despite differences in coastal lithology and in the relative importance of heavy mineral components (Oldfield et al., in press). This is largely because of the extent to which A R M reflects the concentration of fine grained magnetite ( < 0.1 /~m) in the clay fraction (~f. Maher, 1988), whereas Z is most strongly influenced by coarser multi-domain magnetite ( > 1.0 F~m) in the sands and coarse silts. In the present study our approach has been to measure both the magnetic and radiometric properties of a series of particle sized samples in order to explore the extent to which the latter can be normalized for particle size by means of the magnetic measurements. Particle sized subsamples were measured from 5 samples from the Galloway coastline, 4 from surface material in the Dec and Urr and I from the level of peak 2~lAm activity in a core from the Urr. They were obtained by a combination of wet sieving and pipetting as described in Yu (1989). 4 RESULTS Figures I and 2 show the location of till the sites sampled. Figures 3 (Galloway) and 4 (S. Cumbria) plot 24~Am and 137Csconcentrations versus depth tk)r all the cores takcn from the main study regions. Figure 5 plots results for the Lune and Wyre for comparison, in most cases the cores include most if not all of the significantly radioactive material at the site; only the Askam (Fig. 4) and Wyre Marsh-Stanah (Fig. 5) cores are too short to include a more or less complete inventory for each radioisotope. The results from surface samples within the Dee (Kirkcudbright) channel are plotted in Table 1. Table 2 summarises a range of features for each profile. Figures 6-9 and Table 3 summarise the rclationship between particle size, 137Csand 241Am activity, and both ARM and ARM/z. 5 ACTIVITIES A N D INVENTORIES
5.1 Galloway: surface activities In the Dee (Kirkcudbright) samples, peak values for both 241Amand 137Cs occur in the finest sediments from the upper tidal reaches. For 24~Am the values reach 690 Bq kg "l and for 137Cs over 900 Bq kg "l. The surface
F Oldlietd, IV. Richardson. P. G. Appleby. L. Yu S O U T H GLEN
1
AM-24 t Concentration 500
1000
;SOO
( B q kg "1)
2000
C S - 137' C o n c e n t r a t i o n tSO0
2500
( B Q kg -I )
3000
4500
6O00 I
0
u
2O
2O
40
40 .
~ 6o
60~
80
I00
100
SOUTH GLEN O
SO0
1OOO
2 1500
2000
2500
1500
3000
4500
60O0 !
0
u
2O
20
40
40
6o
e0
80
80
100
100 -
£ Q
KIPPFORD 0 0
2S0 ~
SO0 i
750 i
10OO J
125o a
750 0
20
20 -
30
30-
l
I
1500 l
1
2250 l
,J
Fig. 3. Activity/depth profiles for J~TCsand 241Am in the Galloway salt marsh cores (sec Fig. 1 ).
values from the Urr estuary cores range from 433 Bq kg ~ to 983 Bq kg :1 for 241Am and from 706 Bq kg l to 1706 Bq kg t for 137Cs. The 241Am values lie consistently above those quoted for intertidal sediments in Garland et al. (1989a) and Baxter et ai. (1987). These values are within or beyond the upper range of values quoted for suspended sediments by Garland et al. (1989a) and Baxter et al. (1987). These values are within
:4lAin and tSTCs activity in saltmarsh sediments
9
Askam Marsh
AM-241 Concentrat;on (Bq kg "1)
0
0
2000
4000
6000
E 10
~2o]
0
i
CS137 Concentration (8q kg -1) 1500 3 0 0 0 4500 6000 75OO i
i
I
~
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i
i
J
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u
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Walney Island 0 20O0 0
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=
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3000
6000
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Q
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Wylock Marsh 0 20oo
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~ 10-~
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a
4ooo
75,oo
' 01 ' 20
3O
ot ....
3 0 0 0 45,0o 6ooo
10-
''''
Foulney Marsh 0 20~
~600
30
4000
6000
0
3000
6000
9000
ot .......
~'20
20
30
30
Fig. 4. Activity/depth profiles for t37Cs and Z4tAm in the salt marsh cores from the S. Cumbrian Coast (see Fig. 2).
I )
F Oldlqeld. N Rwhurdson P. G. Applebv. L. Yu Sunderland Potnt 2 AM-241 Concentration (Bq kg -1) 0 500 1000 1500
0 t
0
,
u
0
CS-137 Conce¢~tration (Bq v,g-~) 1500 3 0 0 0 4 5 0 0 6 0 0 0 7500
1
~,,,
~
. . . . . .
,o!
~
2oi o
~ 30 ~° t
~
Glasson 0
1000
500
L
~
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o
1500
,~o
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°t 20 ¸
20
30
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o
Burrows
Marsh
0 O|
lOO0 ~
2000
3000
0
~'
10
20
g~ 30
~ 10
>
30 /
Stanah 1 0
,3,opo, 6ooo, 9o9o
i
,
2000
0
3000
,3,00o
i
6o00 i 9o00 •
10
u ¢-
E
2O
20
30
30
Fig. 5. Activity/depth profiles for 1 3 7 C s and :*tAm in representative salt marsh cores from the Lune (Sunderland Point and Glasson) and the Wyre (Burrows Marsh and Stanah) for comparison with Figs 3 and 4.
2"tAm and 13rCs activity in saltmarsh sediments
11
TABLE !
t37Cs and 24tAm C o n c e n t r a t i o n s
in Bulk Surface Samples from the Tidal Channel o f the Dee and the Tidal Flats o f Kircudbright Bay (see Fig. 1 for locationst
Sample
13rCs ( Bq kg "t )
:~t A m ( Bq kg "t )
A D G ! L M N P R S T V
354 358 455 407 619 622 899 406 919 588 498 483 180
254 263 335 239 315 298 633 273 649 417 367 353 144
B
,4 RA!
56-0 99-0 112.0 80.0 . . 127.0 290.0 -286-0 . . 162-0 -17-6
.4 R.$I Z
2.25 4.10 5.10 1.90 .
. 4.70 7.20 -6.90
.
.
. 4.80 1.60
Correlation coefficients for samples for which magnetic measurements arc available: A R M versus 137Cs r = 0-967 (n = 9) A R M versus -'41Am r = 0.986 (n = 9) A R M / z versus 1~TCs r = 0-898 (n = 9) A R M / z versus '41Am r = 0.913 (n = 9)
or beyond the upper range of values quoted for suspcnded sediments by Garland et al. (1989a) and for muds and marsh sediments by Hunt (1987). The 137Cs values also exceed those quoted by Garland et al. (1989a) and Baxter et al. (1987) for intertidal samples from these estuaries, though they fall within the range quoted by Hunt (1987). 5.2 Galloway: peak activities
In all three c o r e s 2 4 1 A m and 137Cs activities peak in the top 0.3 m of the core. For 24~Am the values range from 1082 Bq kg z to 2166 Bq kg ~, and for 137Cs between 2216 Bq kg "l and 5159 Bq kg l . These values all greatly exceed those recorded in the publications quoted with the exception of the highest values given for Kippford samples in Hunt's 1986 survey published in 1987. 5.3 Galloway: core inventories
For 24~Am these range from 239 to 434 kBq m 2 and to 1038 kBq m "2. These figures may be compared 75 kBq m 2 and 122-221 kBq m -, respectively, for the Baxter et al. (1987). Pentreath et al. (1986), on the
for 137Cs from 576 with ones of 33uppermost 0-3 m in basis of studies on
12
F. Otdtieid, 3, Rtchardson. P G. Appleby. L. Yu
"fABLE 2 Surface Concentrations, Maximum Concentrations and Total Inventories for 2't~Am and t~'Cs in cores from the Galloway Coast. South Cumbrian Coast. Lune and Wyre Estuaries: (see Figs 1 and 2 for locations). The data are compared with earlier results from the Dee Estuary, Wirral {Bonnett et aL. 1988)
Surface activitie,~ Bq k"g-t ~
~,:rr Estuary South Glen I South Glen 2 Kippford
Maximum activitw~ c Bq k e i
:*/,4m
(s
:~l,4m
t¢'C~
766 933 433
I 474 1 706 766
1 567 2 166 I 082
3 523 5 159 2 216
Inventories (kBqm :j t3"Cs
Mean
Duddon Estuary and 14"alnev Channel Askam 845 845 Walney Is N I 902 3 797 Biggar Sands 3 {)94 6 340 Wylock Marsh I I 322 I 383 Foulney 607 1 074
5 047 5 687
5 833 8 497
4 572 5 689
5 17{) 7 263 Mean
Lune and 14~vrc Estuaries Sunderland 2 445 Glasson 371 Burrows Marsh 361 Stanah 5(1{I
I 664 1 575 I 205 I 656
796 742 1 655 I 106
4 4 5 6
778 355 438 311 Mean
DeeEstuao'(~Trral) Denhall A Denhall B Flint
38 42 162
392 4{)3 1 231
75 186 352
850 2 103 4 380 Mean
'*lAm
Ratio
748 1 038 576
343 434 239
2.2 24 2.4
787
339
2.3
> I 155 525 751 971 699
> 783 310 396 711 477
I-5 1.7 1.9 1.4 1-5
8211
535
I-5
657 714 822 > 1 000
122 142 2{)2 > 189
54 5-{) 4.1 5.3
798
164
4.9
670 691 670
61 53 56
11.0 13.1 11.7
677
57
11.9
submerged sediments, map estimated 241Am inventories for the whole of the northern Irish Sea and indicate values between l0 and 30 kBq m 2 for that part of the sea bed adjacent to the Galloway coast. The values presented here are thus 5 to 10 times those from previous studies and over an order of magnitude larger than those estimated from measurements made offshore. 5.4 Galloway: sediment activities and Generalized Derived Limits (GDLs) The question of suitable Generalized Derived Limits (GDLs) for relatively accessible salt marshes is considered briefly in Section 8. At this stage a
500
60O
700
100
0
t
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T
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I
0
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200
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250
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ARM
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M
T
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(3
T
M
I
6
!
6
A~IX
RN
ARM/X
RN
I
8
!
9
Fig. 6. lSTCs and 24tAm activity in surface samples from the Dee (Kirkcudbright) Estuary plotted against the magnetic properties ARM (10 6 Am 2 kg d) and ARM/z (10 .4 Aml). The location labels are as shown on Fig. I.
100
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ZI ~0 0
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400 ARM
500
600
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Am 24! tB~.KO) 1500
E]
+
400
500
1500
ilkl/Kgl 1,5OO
/x
i 300
2500 2000
0
A
~%
+
5
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A A
ca
A
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ARM/X
AHM,X
15
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i
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Fil~. 7. + ~'(._'+ and +~IAm a~.-ttx it+', m parted,: sl.,'c fralu+on,, frum ~,urf;+lue and c~+r~: ',ample,+, from the Gaih),.*,a) t.'t+.a~,t h,,:tv+een K+rk,+udb+ight i+~,, and K ippford plotted agam:.t the magnetic propertie., .-\RM 110 +++A m 2 kg+tl and A R M +5C(10 '+ A m +!), The 8 phi .,ample |'rc, m :+,ample R ha', anomalously high -"+tAm co|}centrations and nl~t) ,,:c,rttam +J +hot' particle The particle diameters for each phi size used are apprt)>.mlatel+y as follo'~s: 4 phi, 62.5 3125 lJm; 5 phi. 3 1 2 5 156 ,.m; 6 phi, 156 ~+8 +m~: 7 phi. 3.8 3 9 / + m : 8 phi, 4 2 ,£+m: ~ Itt phi, less that; 2 /+m
Am+241
: 2C~
CI 137 I ~ g )
200O
tOO
~),.X ;
o
ul
3OOO
3500
2OOO
0
~A
o
o
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e~.
A
0
SO0
+000
+SO0
20~)
2500
1"3
4OOO
450,0
2500
9 ~0 pr~
IB~'KOl
•
Cs 137
3O00
3500
40¢0
4500
g
+~
15
:~l,4rn and I~: Cs ac'tivitv in saltmar~h sediments 1(3'O
80
0 D
~o
<>
~o
Norm A~241
te~.m C~137 40
40
A
2o
N
o o
I
I
I
I
I
20
40
60
80
100
;K ~
0 0
Norm ARM
i
i
I
I
I
20
40
6O
80
1O0
Norm ARM
•
9 - 1 0 M'd
ep~ <> 7p~ I7
X
sp~ 4p~
Fill. 8. Normalised ~rCs and 'alAm activities plotted against normalised ARM. For each suite of particle size fractions from a single sample, the peak activity ~r ARM value = 100 and other values are normalised to that. The c;tption tt) Fig. 7 explains the phi scale. Values l\~r the 8 phi fraction from sample R have been omitted (scc Fig. 7).
•
Scotland
•
Dee
o
Wales
estuat~ F
9•
O
ee 6-
n-
3
•
o
O e•
0
2c~ O
0,
Fine %
Fig. 9. ARM/x ( l O 4 A m l ) versus sediment fraction less than 31.25/am ( = ' F i n e percentage') for a suite of samples taken from the Irish Sea shoreline in S. Scotland, the Dee Estuary S.W. of Liverpool and the Anglesey Coast. The linear relationship between magnetic property quotient and particle size is evident despite the major differences in coastal lithology between the areas sampled.
t0
F. Oldtietd, N Richardson, P. G. Applebv, L. Yu
TABLE 3 Pearson Correlations Coefficients (r) betv,een Magnetic Properties and Radioisotope Concentrations for Parucle Size Fractions ..
(A) Non-norrnalised values ARM versus ~37Cs ARM,.';( versus +~'Cs ARM versus "'*tAm ARM Z versus :'*lAm (B) Normalised values ARM versus ~TCs ARM;( versus I~-Cs A R M versus z'*~Am ARM';( versus z'*lAm
All poin!:~
8 phi omitted
0.878 0-898 0.802 0.791
0-919 0-935 0-952 0.912
0-900 0.902 0.817 0.805 (n = 19)
0.966 0.956 0.946 0-918 (n = 18)
In part A all the values used are non-normalised. In part B. both the magnetic property values and the radioisotope concentrations are normali,~d for each sample to the maximum value for that sample. In all cases the 10 and 9 phi measurements are combined and the mean value used. comparison of values within the most recently published limits for marine sediments and well mixed soils is considered (Haywood, 1987). Surface americium values are just over 3% and subsurface peak values over 7% of the marine sediment G D L ; They are around 100% and 200%, respectively, of the well-mixed soil G D L . Surface caesium values arc bctwen 10% and 25% and peak values are between 3% and 74'% of the marine sediment G D L , and arc 75% to 190%, and 240°A. to 580% respectively of the well mixed soil G D L .
5.5 South Cumbria: surface activities Americium values at the surface of the four cores analysed in detail range from 607 to 1902 Bq kg "~. These are comparable to those o f Bonnett et al. (1988) from their Duddon and Walney Channel sample sites. They are significantly higher than either the mean value of 440 Bq kg ~ quoted by Hunt (1987), or the range of values for the area (15-154 Bq kg'l) given by Carpenter e t al. (1991), as well as an order o f magnitude higher than the range of 33 Bq kg 1 to 114 Bq kg n quoted by Eakins et al. (1987) for their shoreline sediment samples from more exposed sites in the same general area. The highest quoted value by the last authors, 306 Bq kg "n at Braystones, only 3 km from SeUafield, is still well below the values for surface material from the sheltered areas sampled in the present study. In the present survey, the highest surface value o f 3094 Bq kg "l was recorded
:'1,4m and t3r Cs activity in saltmarsh sediment~"
17
in the core from Biggar Sands in the Walney Channel, though it should be noted that this core was sampled only in 10 cm slices. Caesium surface activities in the cores plotted range from 845 Bq kg ~ to 3797 Bq kg ~, well within the full range of measurements quoted by Bonnett et al. (1988) but higher than the mean value of 620 Bq kg t quoted by Hunt (1987) or the range ofvalues (112-468 Bq kg -I) given in Carpenter et al. (1991) for the area between Sellafield and the Kent Estuary. The top 10 cm of the Biggar Sands core had an activity of 6340 Bq kg 1. 5.6 South Cumbria: peak activities
In the Walney Channel cores, peak americium values all lie within the top 0.3 m. That in the Askam core is at the base and may not reflect the highest activities at the site. The peak values all lie between 4572 and 5689 Bq kg ~. Such values are comparable with some of the highest measured in the latc 1970s in the lrt and Esk estuaries, much nearer to Scllafield (Aston & Stanners 1981; Stanners & Aston, 1981; Clifton & tlamilton, 1982) and much closer in time to the period of peak discharge. They also lie within the range of measurements for 'entrained silt" quoted by Eakins et al. (1987), though in their samples, the silt fraction is ncvcr more than 2-5°i, of the total bulk. The peak values measured in our saltmarsh cores arc around an order of magnitude higher than the highest value quoted for offshore cores from the mud patch only 10 km from thc Scllafield discharge pipe (Kershaw et al., 1984). Peak caesium values range from 5170 to 8497 Bq kg t . Once more, these compare well with the values in the cores from the region measured by Bonnett et al. (1988), and lie within the range recorded by Stanners and Aston (1981), Aston and Stanners (1981, 1982) and Clifton and Hamilton (1982) in the Esk and lrt Estuaries in 1978. 5.7 South Cumbria: core inventories
Americium inventories range from c. 3 1 0 k B q m 2 to more than 783 kBq m "2. These greatly exceed the Duddon and Walney values quoted by Eakins et ai. (1987) and the range for the area given by Carpenter et al. (1991), all of which lie between 28 and 150 kBq m 2. Even at Drigg, only 6 km south of Sellafield, the inventory of 718 kBq m 2 quoted by Eakins et al. (1987) is significantly less than the incomplete inventory from the saltmarsh core at Askam. For this coastline, as for that offshore from Galloway, Pentreath et al. (1986) map estimated inventories from 10 kBq m "2 to 30 kBq m "2.
18
i" Otdlield. N. Richardson. P. G. Applebv. L. Yu
Caesium inventories range from 525 kBq m-" to over 1155 kBq m : . The lowest value overlaps the range for the area in Carpenter et aL (1991). 5.8 South Cumbria: sediment activities and GDLs
Surface activities for 24tAm in the sub-sampled cores are between 2% and 6% of the marine sediment G D L and between 50% and 150% of the well mixed soil GDL. Peak values, which lie between 0-06 m and 0.28 m below the present day surface, are around 18% to 20% of the marine sediment G D L and 500% of the well mixed soil GDL. Surface caesium activities range from 12% to 55% of the marine sediment G D L and from 94% to 424% of the well mixed soil GDL. Peak caesium activities are between 74% and 122% of the marine sediment G D L and between 575% and 950% of the well mixed soil GDL.
6 SALTMARSH SEDIMENT STRATIGRAPHY The majority of thc cores appear to show a more or less complete record of the history of dischargcs from Sellafield. The rapid increase in ~~TCs discharges during the early 1970s leading to peak values during the period 1974 -78 (McColl, 1989) is recorded in the shape of the 137Csactivity versus depth profilcs in six out of the eight Galloway and S. Cumbria cores (Figs 3 and 4), as is thc marked decline in dischargcs since 1980. The distribution of -'a~Am is complicated by the fact that it arises both from direct discharges and by ingrowth from 24ZPu, The net record, decay corrected to 1989, should however give peak values in 1973-74 followed by a very sharp decline during the period 1975-77, slightly earlier than for IWCs. This displacement of the record of 24tAm vis-a-vis ~37Cs is evident in all cores except those where the sub-sampling was too coarse to resolve the separation. Broadly similar stratigraphic patterns are recorded, albeit less clearly, in the Lune and Wyre cores 60-70 km from Seilafield (Fig. 5). Similar results were obtained at some sites further south in the Dee Estuary, Wirral (Bonnett et al., 1988). Though preliminary results from some other S. Lancs and N, Wales sites indicate that disruptions to the record may occur as a result of irregularities in the process of sedimention, we infer from the above observations that, in many cases, cores from saltmarsh and tidal mud environments provide internally consistent records of radionuclide deposition and storage. These stratigraphic records may be of some chronological value in determining recent sediment accumulation rates, though possible delays in radioisotope
24t A m and taZCs activity in saltmarsh sediments
19
transfer from source to sediment site mean that for the present these values should be accepted with caution. Calculations of the cumulative discharge of 137Cs and 24~Am from Sellafield, corrected for decay and ingrowth from 24~Pu to the end of 1989, give a t37Cs/241Am ratio of 35 : !. Since the K o value for "4~Am is 2-3 orders of magnitude higher than for ~37Cs, sediments close to Sellafield will be relatively enriched in 241Am. For these sediments, the 137Cs/241Am ratio may be expected to be greatly reduced from the nominal value. This is confirmed by the calculation of the t37Cs/24tAm inventory ratios shown in Table 2. Also shown are revised calculations of earlier results from the Dee Estuary, Wirral (Bonnett et al., 1988). Of particular note is the similarity of the 137Cs/'ZatAm ratio among the cores from each region. Lowest values occur in the group ofcores from the Walney Channel and Duddon Estuary, nearest to Sellafield. There is a 50% increase in the ratio for cores in the Urr Estuary, but a 300% increase for cores in the Lune and Wyre Estuaries, though both sites are more or less equidistant from Sellafield. The higher gradient with distance south appears to be confirmed by the results given in Carpenter et al. (1991). in their results, which are from a range of environments, the significance of grain size is reflected in the disproportionately depleted '41Am inventories in cores from coarse grained sandy deposits.
7 RADIOACTIVITY AND MAGNETIC PROPERTIES Since particle size exercises a strong control over some magnetic properties (Yu et al. in press, Oldfield et al., in press) and over 24~Am and 137Cs concentrations in Irish Sea sediments (Baxter et al. 1987; Eakins et al., 1987; Bonnett et ai., 1988), the relationship between the two has been explained in several ways. in Fig. 6 the relationship between 1~7Cs and 24tAm is plotted on the one hand and both A R M and ARM/~ on the other for the set of surface samples from the Dee (Kirkcudbright) intertidal muds (Table 1). The magnetic properties and radioisotope concentrations are, save for sample M at the very head of the tidal reaches, not only linearly correlated with correlation coefficients between 0.898 and 0.986, but sequentially ordered in relation to distance from the open sea. Within each of the estuaries studied, there is a strong direct relationship between radioactive concentrations and magnetic properties in bulk sediment samples. The basis for this relationship has been explored further using particle size fractions from some of the Dee and Urr estuary samples. Table 3 summarises magnetic property: radioisotope concentration correlation coefficients for the samples and particle sizes measured. Figures 7 and 8
20
f" OldJield. N. Rtchardzon, P. G Apptehy. L. Yu
plot both A R M , ARM Z, :41Am and 13~Cs versus particle size. In this case, by pooling the results from all four samples, and despite their different locations and bulk concentrations, correlations between the two sets of measurements are still apparent, with correlation coefficients between r = 0.791 and r = 0.966. These may be compared with Bonnett et a/.'s (1988) values for IWCs versus A R M of r = 0 , 8 6 5 and r = 0 . 9 7 8 for measurements on fractions of individual samples from the Ribble and lrt estuaries respectively, and o f r = 0.851 and r = 0.966 for 24~Am vs. ARM. From the above summary it can be concluded that on both a local and a regional basis (tJ~ Fig. 9) there is considerable scope for normalising 241Am and 137Cs activities by means of rapid, simple, non-destructive measurements which can be carried out in the field or laboratory on a dry mass or fresh volume basis and on sample masses as low as 0.5 g (dry). In Fig. t0, the down-profile '41Am and 137Csconcentrations are plotted for three of the south Lake District cores as normalised values using both ARM and ARM/z. In the case of these cores from consistently fine grained sediments, the normalisation has negligible effect. This reinforces the view that the protilcs reflect sequcntial deposition rather than scdimentological variation through time.
IMPLICATIONS Using geomorphoiogical and ecological experience in field sampling, it is possible to predict and subsequently conlirm the location of sites where contemporary deposition of line grained sediments is taking place within tidal channels, and of sites within saitmarshes where sustained recent fine grained sediment accumulation has taken place and is continuing. Such sites along the Galloway and S. Cumbria coastlines have surface and peak concentrations and inventories of 137Csand 241Am considerably in excess of most of those recorded in other publications on intertidal sediment radioactivity in the same general areas. The most nearly comparable published results are those quoted for surface samples and cores of fine grained silts and muds (not saltmarshes) in the Wyre and Ribble estuaries by Carpenter et al. ( 1991 ), though they fail to document fully either the type of field contexts sampled or the stratigraphic record of concentration changes in each "inventory' core. The present records have major implications for total inventory calculations for several areas analysed by recent DoE reports (e.g., Eakins et al., 1987; Garland et al., 1989) and they may therefore modify current perspectives on the implications of radioactivity in these areas. The normal procedures and calculations used to evaluate the potential human
:~l,4rn and t3~Cs activity in saltrnarsh sediments 600O
21
6O00
SO00 4000
4000
.f_ 3 ~
3000
2000
2000 1000
0
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2OOO 1~ 0
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9000 8000 7000 6000 •~ sooo 4o00 30O0 20O0 1000 0
)
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Fig. 10 Normalised and non-normalised activities in three saltmarsh cores from the South Cumbrian Coast. The profiles are from (a) Askam, ""Am; (b) Askam, 137Cs; (c) Walney, 241Am; (d) Walney, I-aTCs;(e) Foulney, -'41Am and (f') Foulney, 1~TCs. The solid diamond symbol plots the non-normalised activities in Bq kg l . Values plotted as open squares have been normalised by A R M measurements; values plotted as asterisks have been normalised by ARM/x. After normalisation all the values have been scaled so that the maximum normalised value for each radioisotope in each core corresponds with its non-normalised activity.
implications o f levels o f radioactivity in different environmental contexts appear potentially controversial in relation to saltmarsh sediments. They are not marine sediments sensu stricto since most are more often exposed
22
I;'. Oid/ield, N Rtchardson. P. G. Appleby. L. ~u
than inundated; moreover some lie close to human habitation and activity, At the same time, the',' are not well-mixed soils under agricultural management, though virtually all the ones studied here have specific activities which, integrated over the top, 0-3 m (i.e. a typical plough-layer) would be well in excess of those at the surface. Moreover the potential for redeposition, further sorting, incremental accumulation and for inventory increases arising solely from the ingrowth of _,alAm from ,-4mPu, add special dimensions requiring further evaluation at these sites. In addition, account must be taken of the ability of Spartina anglica to colonise and form mature swards over other areas, thus trapping further quantities of fine grained radioactive sediments, and of the future implications of rising sea-level for the resuspension and redeposition of the most radioactive sediments presently between 0.05 and 0.30 m of the surface at most of the sites studied. Finally, it should be borne in mind that saltmarshes are frequently grazed between tidal incursions ztnd may subsequently be drained, dyked and reclaimed to t~rm agricultural land.
9 R E C O M M E N D A T I O N S FOR F U T U R E RESEARCH ( I ) There should be a thorough evaluation of the present and potential role of Spartina anglica its a trapping agent for fine particleassocia, tcd radioactivity in the northern Irish Sea. This should embrace empirical measurements, surveys of current distribution and predictions of future spread. (2) Other more locally dominant vegetation types require similar study, notably stands of llalimione portuh.'oides and Phragmites commuhis, The latter reed colonises areas at, as well as beyond, the upper limit of tidal influence and is frequently rooted in very fine grained organic sediments. It is widely distributed in sheltered tidal channels along the Galloway shoreline. (3) The finest sediments accumulating below stands of Spartina especially can be highly reduced. The effect of this on chemical speciation, mobility and biological uptake of radionuclides requires further study. (4) Completion of !. 2 and 3 would provide a database from which revised assessments of the human implications of present and nearfuture levels of coastal radioactivity should then be derived in the light of the points made above and of future research. (5) The information provided by 1 and 2 should be incorporated in model simulations of the likely effects of channel changes and of rising sea-level on the redistribution of fine sediments in the coastal areas of greatest concern and vulnerability.
'~1,4m and I~'Cs activity in saltmarsh sediments
23
(6) Magnetic measurements ( A R M and A R M z ) should be used alongside radiometric measurements on existing and future samples in order to normalise values where appropriate and with further calibration, and to shed light on the processes such as transport and redistribution which give rise to changes in the relationship between particle size assemblages and radioactivity levels.
REFERENCES Aston, S. R. & Stanners, D. A. (1981). Americium in intertidal sediments from the coastal environs of Windscale. Marine Poll. Bull.. 12, 149-53. Aston, S. R. & Stanners, D. A. (1982). Spatial variability in the distribution of Windscale fusion products in estuarine sediments. Marine Poll. Bull.. 14, 16774. Aston, S. R., Assinder, D. J. & Kelly, M. (1985). Plutonium in intertidal coastal and estuarine sediments in the Northern Irish Sea. Est. Coast. Slw(] S~'i., 20, 761-71. Aston, S. R., Assindcr, D. J. , Stanncrs, D. A. &Rac, J. E. (1981). Plutonium occurrence and phase distribution in sediments of the Wyrc Estuary, Northwest England. Marine Poll. Bull.. 12, 308 14. Baxter, M. S., Cook, G. T. & McDonald, P. (1987). An assessment of artificial radionuclide transfer from Scllaticld to the South West of Scotland, Report to the DOE, DOE Report Number D()E/RW/'89/127. Bonnett, P. J. P., Applcby; P. G. & Oldficld, [:. (1988). Radionuclidcs in coastal and estuarine sediments from Wirral and Lancashire. Sci. Total Environ... 711, 215-36. Citrpcnter, R. C., Burton, P. J., Strange, L. P. & Pratlcy, |:. W. (1991). Radionuclides in intertidal sands and sediments from Morccambc Bay to the Dec Estuary, AERE-R 13803, HMSO, London. Clifton, R. J. & Hamilton, E. (1982). The application of Radioisotopes in the study of estuarine sedimentary processes. Est. Coast. ShelfSt'i.. 14, 433 46. Debertin, K. & Ren, J. (1989). Measurement of the activity of radioactive samples in Marinelli Beakers. Nucl. Inst. Meth. Phys. Rex.. A278, 541-9. Eakins, J. D., Morgan, A., Baston, G. M. N., Pratley, F. A., Yarnold, L. P. & Burton, P. J. (1987). Studies ofenvironmental radioactivity in Cumbria, Part 8. Plutonium and americium in the intertidal sands of north west England, AERER 12061, HMSO, London. Garland, J. A., McKay, W. A., Cambray, R. S. & Burton, P. J. (1989a). Manmade radionuclides in the environment of Dumfries and Galloway. Nud. Energy. 28, 369-92. Garland, J. A., Cambray, R. S., Burton, P. J. & McKay, W. A. (1989h). Artificial radioactivity on the coasts of Wales, AERE G5307, HMSO, London. Goodman, P. J., Braybrooks, E. M. & Lambert, J. M. (1959). Investigations into dieback in Sparthul townsendii. The present status of Spartbta townsendii in Britain. J. Ecol.. 47, 651-77. Gurbutt, P. A., Kershaw, P. J. & Durance, J. A. (1988). Modelling the distribution of soluble and particle absorbed radionuclides in the Irish Sea. In Radioactivity
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t, OhtlieM A Rwhard~~,n. P, G. Appleh~, L. Yu
and Oceanograph~ .... ,4 lo~d tor Oceanoeraphy. ed. J. C. Guary, P. Gueguema~ & R. J. Pentreath. Elsevier, Amsterdam. Hamilton, E. I. (1981). Alpha particle radioactivity of hot particles from the Esk estuary, Nature. 290, 69t)- 93. Haywood, S. M. (19871. Revised generalised derived hmits for radioisotopes ol strontium, iodine, caesium, plutonium, americium and curium, National Radiological Protection Board, Chilton, NRPB-GS8. Hunt. G. J. (1987). Aquatic environment monitoring report No. 18. Radioactivity m surface and coastal waters of the British Isles, 1986, M A F F , 1545N 0142-2499, Kershaw, P. J., S~ift, D, J,, Pcntreath. R. J. & Lovett, M. B. (1984). The incorporation of plutonium, americium and curium into the Irish Sea seabed b~ biological activit3 S~,i. 75,tal Environ.. 411, 61 81. Kershav,, P. J., Brcarlc~. J. W., Woodhcad. D. S. & Lovctt, M. B., (19s6). Alpha emitting, hot particles in Irish Sea sediments. Sci. Total Environ., 53, 77-87, Kersha~, P. J., Pentreath. R. J., Gurbutt. P. A., Woodhead. D. S., Durance, J. A, & Camplin, W. C. (1988). Modelling the behaviour of long-lived radionuclides in the Irish Sea comparison of model predictions with field observations, in Ah,thods liar ,.Is.~'.~'s#tg thc Reliahilitv of Environnwntal Tran,v/~,r Mo~h't Predictions, ed. (;. Dcsmct. Elsevier, Amsterdam, pp. 241 9 . Maher, B. A. (It)SS). Magnetic propertics of some synthetic sub-micron magnetites. J, Gc~ph)'~. Rc~., 94. ,"g3 96. Malcolm, S. J., Kcrshaw, P. J,, Lovctt, M. B. & |iarvcy, B. R. (19901. The interstitial water chemistry of '~'~ '4'~Pu and z'~tAm in the sediments of the north-east Irish Sea. G(,o('him. ('osttlochbn. A~'/tl, ~ , 29 35. McColI, N. (1989). Sources of radioactivity in the north east Atlantic: fuel processing plants and past sea dumping opcrz.tions. In The Radioh~gi~'al l"?xposttre ~] tht" Popukttion o.f the' European Comvnunit.v .from Radioactivity in North b:urolwan Alarim" Water, Proceedings of a seminar on the ('EC Project "MARINA', Brugcs. 1989, pp. 95 III. Mitchell, P. [1989). Radionuclidc monitoring in the Irish Sca. In Tht, Radiologic'al ['sxIIosurc r?/"ttw P~lmh~ti,n ~,/ the European ('O/?ltHlUlig)' ]~'Ottt Radioactivity in North Ettropean Ahtrine ~|'ater, Proceedings of a seminar on the CEC Projcct ' M A R I N A ' , Brugc~, 198tLpp. 185 288. Pattenden, N. J., Cambray, R. S., Playfi~rd, K., Eakins, J. D. & Fisher, E. M. R. (1981)). Studies of environmental radioactivity in Cumbria. Part 3. Measurcments of radionuclides in airborne and deposited material, AERE R4857, HMSO, London, Pentreath, R. J., Woodhead, D. S., Kcrshaw, P. J., Jeffries, D. F. & Lovett, M. B. (1986). The behaviour of plutonium and americium in the Irish Sea. Rapp. p.w'. Cons, int. E.wh~r. Ah'r., 186, 60-9. Stanners, D. A. & Aston, S. R. (1981). 1~4Cs : 137Cs and t°6Ru : 137Cs Ratios in intertidal scdimcnts from the Cumbria and Lancashire Coasts, England. Est, Coast Shel['Sci.. 13, 409-17, Whitesidc, M. (1987). Spartina Colonization. In Morecamhe Bay." An Assessment of Present Ecological knowledge, ed. N. A. Robinson & A. W. Pringle. Centre lbr North-West Regional Studies/Morecambe Bay study group, pp. 118-29. Yu, L. (1989). Environmental applications of mineral magnetic measurements: towards a quantitative approach. PhD thesis, University of Liverpool, Liverpool.