- Email: [email protected]
Neptunium in intertidal coastal and estuarine sediments in the Irish Sea
J. Environ. Radioactivity 14 (1991) 135--145
Neptunium in Intertidal Coastal and Estuarine Sediments in the Irish Sea D. J. Assinder," M. Yamamoto,b C. K. Kim,C R. Seki, ~ Y. Takaku, d Y. Y a m a u c h i , b K. K o m u r a , b K. U e n o b & G . S. B o u r n e a "Schoolof Ocean Sciences, University College of North Wales. Menai Bridge, Anglesey, Gwynedd LL595EY, UK bLow Level Radioactivity Laboratory. Kanazawa University, Tatsunokuchi, l,~hikawa 923-12, Japan 'Department of Chemistry, University ofTsukttba, Tsukuba, lbaraki, 3(15Japan 'tMarubun Co., 8"-IOdenma-cho, Nihonbashi. Tokyo 103, Japan (Received 19 December 1989; revised version received 5 March 190(I; accepted 14 May ItP'~O)
ABSTRACT A survey has been carried out of 2+TNp activities in sediments of t/re Irish Sea. Two independent methods of analysis were used (ICI'-MS attd alpha-spectrometry) to enable tire intercomparison of tire results produced. The 2JrNp content of the samples ranged from 0.01-4,5 Bq/kg ]br all areas apart from the Ribble Estuary. In the Ribble, a source of 2~7Np other than Sellafield has been found which supports 2.¢7Np activities in sediments up to !.1.3 Bq/kg. This anomaly is verified by comparing '-¢7Np with tire 2 ~°'24°pu content of tire sediments. These are believed to be the highest environmental 2rZNp value~ yet recorded.
INTRODUCTION The Irish Sea receives controlled discharges of effluents from the Scllaticld (Windscalc) nuclear fuel reprocessing plant in Cumbria. E n h a n c e d levels of transuranic elements such as plutonium and americium have been 135 J. Environ. Radioactivity 0265-931X/91/$03.50 (~) 1~.)1 Elsevier Science Publishers Ltd, England. Printed in Great Britain
136
D.J.A.s.s'tnder et al.
reported in a variety of environmental media (e.g. Hunt, 1980-1989: Aston et al., 1985). However, data on -'-~VNpare sparse (e.g. Pentreath & Harvey, 1981: Byrne, 1986), despite Np being important from both radiological and high or intermediate level waste disposal viewpoints (Harvey, 1981; Cohen, 1982). In particular, the proportion of'--~TNp in the alpha discharges from Seilafield is increasing as a consequence of 237Np levels showing little, if any, decrease over the last ten years, in contrast to the other alpha-emitters whose levels have dropped substantially (BNFL, 198(b-1989). In this study, intertidal coastal and estuarine sediments were collected from predominantly muddy sites around the British coast of the lrish Sea. A suite of radionuclides were measured independently by two laboratories and the results for 237Np and its relationship to 23t~'24~lPuare presented here. Other results for 23Spu, 241pu. 2~lAm and 137Cs are presented elsewhere (Assinder et al., in prep.). This is the first survey of 237Np activities in surface sediment in the Irish Sea because, although this radionuclidc is monitored in the Ministry of Agriculture, Fisheries and Food annual surveys, only a limited number of fish and shelitish are examined (e.g. tlunt,'1989).
MET! IODS Sediment sa,mples were collected in July 1988 at the 24 intertidal sites shown in Fig. I. A later survey of the Ribble [£stuary was carried out in September 1989 (Fig. 2). At all sites, the top l cm of sediment was carefully removed at low-water, placed in plastic sample bags and quickly transported in a cooi-tmx to the laboratory where the samples were frozen until required. After defrosting, the samples were thoroughly homogenised before separation into sub-samples for the laboratories involved. All sediments were then dried at 60°C before Np and Pu analyses were performed. Pu measurements at both laboratories (Tsukuba and Kanazawa) and Np measurements at Kanazawa were carried out using conventional radiochemical separations and alpha-spectrometry (Yamato, 1982; Yamamoto et al., 1989). For the Np analyses, a 0-5-10 g sample was used with 23'JNp as a yield tracer. Full details of the analytical method have been reported elsewhere together with its precision (average value = 6.5%) and accuracy (average value - 2.9%) (Yamamoto et al., 1989). Np measurements at Tsukuba were performed by ICP-MS analysis using a VG Elemental (Plasmaquad PO2 Plus) system at Marubun Co. Full details of the sediment extraction method used and instrument
Neptunium in intertidal coauttal and estuarine sediments
137
S5°
ond ~
' " :: : :~ :I :il
[.
}:
~
N. Irel
~
/
k
/,.,.
. ~
England _S,tt,.fi,td
~,,,.gt,ss;~ sit
I
I[stu*r~es/
ISlS
slt.
'*",-W ) St,°
Irish Seo
ST
~
prest*f
• Ribble [sfuary/~ N
: CHGnchester~
Woles 6°
S"
t,"
3"
Fig. I. l,ocati~m m a p of the n o r t h e r n Irish Sea. ~ i t h sites of intertidal s e d i m e n t samples.
operating conditions are given elsewhere (Kim et al., It)Sg). This technique requires approximately ten times less sediment to achieve the same precision and its detection limit (3tr standard deviation of the background) of 0.02 mBq/ml of sample solution is about one order of magnitude lower than that for NAA and alpha-spectrometry. The results therefore provide an opportunity to compare both systems of measurement.
RESULTS Irish Sea survey
The activities of 237Np and 23'~'241)pudetermined by both laboratories in the 24 surface samples are given in Table 1. There was no significant difference between the activities measured in the two laboratories when analysed by
D.J. Assinder e t al+
138
~
,.i ~
~ ~
o
.E
-g
t~
o
m
rv"
t-v'
,.¢ 'D e-,
!
e',
r. o.al
z
E t~
,
++ J
i
Neptunium in intertidal coastal and estuarine sediments
~
=.
+1 +I
-'. - / - ' .
--~. .
.
.
.
.
.
.
.
.
.
.
.
.
] +1 +t +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 I +~ +1 +1 +1
-~,~--,~'+~3~-'.~-=h
~,~
=
e~.,=.
139
=~.,,..--
+1 +1 +1 +1 +1 +1 +1 +1 +t +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 m~,+ '
+1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1
e.l r,,,-
.Z Z~
~* ,';, ~ -'I" -4" ~* r.- ,,'n ~', ~,", -- ~, -- ~ ~'* ,,", ~, ~ ~", -- ~-, ,-,n ~ '-~ 4"I +I +I 4-I +I +I +I +I +I +I 4-I +I +I 4-I +I @I +I +I 4-I +I 4-I +I +I @I
P..
~z +I Z
+I
,-.,'' " '
e-
- ~ ,., ~.
.
.= .
.- , - ~ .c .
.
.
.
.
,,~
e-
E +I +I +I +I +I +I +I +I ~I +I +I +I +I +I +I +I ~I +I +I +I +I +I +I +I
rt
z
A
_
.
_
.~
.._~
~
.~
-
~.~
_~,
~.
+t
.
~.
it
<
N
Z
~j
.y-, ~
~
~
~
~
~
.
.
.
.
~
.
. .
<
140
D.J. Asstnder
et at.
blocked analysis of variance (Np: F = 3.87, df 1,21; Pu: F = 1.72, df 1,21). Variations are well within the normal range encountered due to sample heterogeneity, as found during I A E A intercalibration exercises (e.g. I A E A , 1984). As expected, the limit of detection for 237Np is lower for the ICP-MS technique, as shown by samples $3 and $20 and, even with the smaller sample sizes employed, the precision of the method is equal to. or greater than, alpha-spectrometric techniques. The z3'~'2~*lPuactivities generally decrease with distance from Sellafield, This was shown by a similar survey carried out in 1979-1980 (Aston et a l . , 1985) where it was concluded that this geographical trend was refined by the effect of sediment grain-size, i.e. a coastal sand near to Sellaficld will have a lower activity than a mud at great distance from the source but, when similar size sediments are considered, a general decreasing trend is observed. Due to this difficulty in comparing sediments from similar sites, it is hard to compare directly activities of Pu between 1979 and 1988, I lowcvcr, in general, it appears that 23'~'2al~l~u activities in surface sediments have decreased in the estuaries close to the source (e.g. the Esk) whilst those at greater distances show similar values to the previous survey. This aspect will tic discussed further with reference to the full suite of radionuclidcs in a later paper (Assindcr et al., in prep.). The '37Np activities ('i'~d~lc I) show a similar trend to '~"'"~"Pu for all samples except $7 and $8 in the Ribblc Estuary. l lore, the average -'37Np/2+'~'2auPu activity quotient fl~r other samples of =1)-27% is increased by an order of magnitude. Possible interferences in both methods from known high U, Th and Pa isotope activities and their decay products in sediments from this estuary (e.g. l lunt, 1989) have bccn investigated and rejected. The anomalously high 237Np activities and -';TNp/-'3'~'"tq~Pu ratio give evidence for a previously unrccogniscd sourcc of "-37Np in the Ribblc Estuary area. This produces sediment 237Np activities at least three times higher than those recorded in Esk Estu:lry sediments. This linding prompted a more detailed look at this estuary.
Ribble Estuary survey The activities of 237Np and 23tJ'24*~Pumeasured in the Kanazawa laboratory in the 9 surface samples are given in Table 2. The Z37Np activities recorded in these sediments show several features. Firstly, in samples well beyond the tidal limit in the Dcepdalc Brook tributary of the Savick Brook (samples R8 and R9), the Np activities and the 237Np/23'J'24~Jpu ratios are high and the 23'~'2"U)Puactivities are very low, indicating that the original source of their Np is not from marine-derived Scllafield material.
Neptunium in intertidal coastal and estuarine sediments
141
TABLE 2 Activities of 237Np and z3~'zU)Pu and the -'37Np/Z-~'<-"U)PuRatio in Sediments of the Ribble Estuary and Selected Tributaries
Site no. RI R2 R3 R4 ( = $ 7 ) R5 ( = $ 8 ) R6 R7 R8 R9
23rNp (Bq/kg d~. wt) 2-02 1-94 2-15 5-95 12-1 11-1 3.62 5.09 8.48
4- 0- l I 4. 0.13 4. 0" 12 + 0.30 4. 0.4 +0.5 + 0.23 + 0.26 + I).34
-'-'°'-'4*'Pu (Bq/kg d ~ wt)
:3r Np/"-'°'" ~' Pu (%)
203 + 16 223 4- 13 235 4. 13 179 4. 11 140 __ 7 2104.8 86 4. 4 2.2 4. 0-1 3.5 4. 0.2
0-77 4. 0-06 0.87 _+ 0.08 0-91 + 0-07 3'32 +_ 0.26 ,~-67 ___0.56 5-31 4-0.33 4-19 4. 0-33 228 4. 17 246 + 16
Secondly. following its dispersion in the marine environment, the distribt, tion of Np. although affected by the varying grain-size distributions of the samples collected, appears to show increasing values towards the ttpstream ends of Savick Brook and Dcepdale Brook with a maxinmm value of 12-1 Bq/kg in the vcry line scdimcnts at R5(=$8. which from Table 1 was mcasurcd as 13.3 Bq/kg by the Tsukt, ba laboratory).
DISCUSSION Several features concerning the sources and gcoehcnlistry of Np in the Irish Sea can be examined using this data set. In the Ribble Estuary, the anomalous 237Np values measured suggest that there is a source of Np located in the area of Deepdale Brook and Saviek Brook. If the anomalous values were only located in tidal areas then some unusual, and unknown, focusing mechanism for very recent Sellafield discharges with higher 237Np/239"24°pu ratios (Table 3) may be hypothesised. However, due to the levels noted in riverine sediments well upstream from the tidal limit, the Np source is probably connected with the BNFL Springfields site or the adjacent Northern Research Laboratories--Springfields of the U K A E A . Although several nuclides are regularly measured by BNFL and MAFF, any 237Np activity present in discharges from these plants is not routinely monitored or reported (e.g. BNFL, 1989; Hunt, 1989). The BNFL Springfields Works converts uranium ore concentrates into uranium hexafluoride, uranium metal and uranium oxide for subsequent manufacture of fuel elements for nuclear reactors. A significant proportion of the uranium processed contains reactor-uranium which has been
D. J, Assmder et al.
142
TABLE 3 '37Np and '3u'2a°Pu Activities and the ~7Np/23~'24~Pu Ratio Discharged from Sellafield for the Period 1979--1988 (BNFL, 1980--1989)
Year
: ~;Np discharge (TBq)
:"~:a'Pu discharge (TBq)
1979 19811 1981 1982 1983 1984
0-33 0,f~7 0,41 0-3 0-3 0.3
37 20 15 16 8-7 8-3
Total alpha Pu discharge (TBq) 1985 198~1 1987
! 988
11,2 0.4 0.23 0- 28
3.4 2-7 1.3 1.4
"L¢ZNp/2-*~'2a'ptt ratio (% 0-89 3--I 2-7 I-9 3-4 3-6
"rNp/l~,tal alpha Pu ratio (%) 5.9 15 IS 20
recycled via Sellafield (D. Pomfret, BNFL tieaith and Safety Directorate, 19t,~l, pets. c o m m . ) . Plutonium nuclides are separated and removed during reprocessing but a fraction of the Np remains largely with the ur,'mium compounds. 237Np may therefore be a waste product of tile Springfiekls conversion process. It is interesting to note that these operations support sediment activities three times higher than those near Sellafield. When considering the Irish Sea data as a whole, most information can be derived from a consideration of the -'37Np/-'3"'-'4"pu ratio. The -'37Np/ -'3"'24°pu activity ratios for samples other than those in the Ribble area were in the range 0.18-4).42% (Table 1). in the Sellatield discharges, this ratio has varied between 0-89 and 20% between 1979 and 1988 (Table 3) (BNFL, 198~1989), with an average value weighted by total discharge activity of 237Np and -'3u'24"puof 3.1%. This relative deficiency of -'37Np in the sediments supports previous work (e.g. Harvey, 1981; Germain et al., 1987) indicating that plutonium is adsorbed more efficiently than neptunium by particulate material. in weil-oxidised seawater, neptunium is considered to be present mainly as the soluble pentavalent species of Np(V), NpO2 + (e.g. Allard, 1982), ai though Np( i V) (Harvey, 1981 ) and other species also occu r (Germain et al., 1987). Consequently, it has a lower distribution coefficient (Kd) for
Neptunium in intertidal coastal and estuarine sediments
143
uptake by particulate materials when compared with Am(III) and Pu(IV) (Bondietti & Tamura, 1980; Harvey & Kershaw, 1984). Pentreath and Harvey (1981) measured Kd values for Z37Np in the Irish Sea using the particulate fraction and filtrate of surface water samples. This showed a tendency for the Z37Np Kj value to decrease with increasing distance from Sellafield possibly due to a change in the oxidation state of neptunium from IV to V. If this finding is reflected in deposited sediments and assuming the Kd for plutonium is not subject to similar variations, the Z37Np/239"24°pu ratio in the surface sediments should decrease with increasing distance from Sellafield. Table 1 shows that no such trend is observed. There are several possible reasons for this discrepancy: (i) The variation in Kd values found by Pentreath and Harvey (1981) may have been a reflection, partly or wholly, of processes other than change in oxidation state. Factors such as the grain-size distribution and composition of the suspended sediments are known to cause Ka variations (e.g. Edgington & Nelson, 1984). (ii) The variations.observed in suspended sediments may not be reflected in deposited sediments. Deposited sediments represent a thoroughly mixed sediment grain population in terms of grain size, composition, date of initial contamination and subsequent history. As noted for Pu (Aston et al., 1985), this tends to obscure activity trends in the Irish Sea. (iii) The assumption that the K,t for Pu is not subject to similar variations may bc incorrect. Dclinition of the spatial variation of the relative proportions of Pu (ill + J.V) imd Pu (V + VI) in seawater and sediments in the Irish Sea is still an area in which few dclinitc conclusions have been drawn, although the details arc being tackled by some groups (e.g. Mudgc et al., 1988). The most realistic conclusion to draw from this data set is that these surface sediments appear to bc an unreliable medium in which to investigate this effect due to the complications caused by the variation with time of the 237Np/23~'24°pu discharge ratio, possible changes of oxidation states of both Np and Pu in the environment after discharge and the sedimentary processes mixing all sediments in the Irish Sea as detailed for plutonium by Aston et al. (1985). llowever, now that the 237Np distribution has bccn examined initially, the Ribble Estuary may provide a location where Np analyses can be successfully carried out in more detail due to the higher activities present and details of its marine geochemistry can then be clarilicd.
L4.4
D. J. Assinder et al.
CONCLUSIONS Previous studies have indicated the occurrence in the intertidal sediments around the Irish Sea of neptunium discharged in effluent from Sellafieid. This study shows in more detail the distribution of 237Np activities and illustrates the intercomparabitity of ICP-MS and alpha-spectrometric techniques for its measurement. A hitherto unreported source of neptunium in the Ribble Estuary of north-west England has been revealed. This source produces higher Z37Np activities in present-day intertidal surface sediments in the Ribble than in similar deposits near Sellafield. 237Np in the Ribble Estuary, although trivial in radioiogical terms, should in future provide a valuable site for an analysis of the dispersion of Np from a riverine source, through an estuarine system into the marine environment.
ACKNOWLEDGEMENTS This work was supported by a Grant in Aid for Scientific Research, Ministry of Education, Science and Culture in Japan, 1989. The authors arc grateful to the Royal Society of London for a travel award to Japan fl~r D.J.A. and to Sara tlarvey for the drafting of the figures. The authors' thanks also go to the three referees h~r many helpful comments on the manuscript.
REFERI£NCES Allard, B. (1982). Solubilities of actinides in neutral or basic'solutions. In
Actinides in Perspective, ed. N. E. Edelstein. Pergamon Press, Oxfl~rd, pp.
553--79. Assinder, D. J., Yamamoto, M., Kim, C. K., Seki, R., Takaku, Y., Yamauchi, Y., Komura, K. & Ueno, K. (in prep.) IaTCs, 23K,z.~,,.2-,t. 24tpu and "~41Amin intertidal coastal and estuarine sediments in the northern Irish Sea (to be submitted to J. Environ. Radioactivity). Aston, S. R., Assinder, D. J. & Kelly, M. (1985). Plutonium in intertidal coastal and estuarine sediments in the northern Irish Sea. Est. Coastal and ShelfSci., 20, 761-7 I. BNFL (198(g-1989). Annual:report on radioactive discharges and monitoring of the environment, 1979--1988. BNFL, Risley, Warrington. Bondietti, E. A. & Tamura, T. (19811). Physico-ehemical associations of plutonium and other actinides in soil. in Transuranic Elements in the Environment, ed. W, C. Hanson. US Department of Energy, DOE/TIC-22 8(10, pp. 145--64. Byrne, A. R. (1986). Determination of -'37Np in Cumbrian ~diments by neutron activation analysis: preliminary results. J. Environ. Radioactivity, 4, 133--44.
Neptunium in intertidal coastal and estuarine sediments
145
Cohen, B. L. (1982). Effects of ICRP Publication 30 and the 1980 BEIR report on hazard assessments of high-level waste. Health Physics, 42, 13_'~4Y Edgington, D. N. & Nelson, D. M. (1984). The chemical behaviour of long-lived radionuclides in the marine environment, In International Symposium on the Behaviour of Long-Lived Radionuclides in the Marine Environment. ed. A. Cigna & C. Myttenaere, CEC, Luxembourg. Eur 9214EN, pp. 19-66. Germain, P., Gandon, R., Masson, M. & Guegu6niat. P. (1987). Experimental studies of the transfer of neptunium from sea water to sediments and organisms (annelids and molluscs). J, Environ. Radioactivity, 5, 37-55. Harvey. B. R. (1981). Potential for post-depositional migration of neptunium in Irish Sea sediments. In Impacts of Radionuclide Release in the Marine Environment, IAEA, Vienna, IAEA-SM-248, pp. 97~IOY Harvey, B. R. & Kershaw, P. J. (1984). Physico-chemical interactions of long-lived radionuclides in coastal marine sediments and some comparisons with deep sea environments. Eur. Rep. Comm. Eur. Commun., EUR-9214, pp. 131-41. Hunt, G. J. ( 198(I--1989). Radioactivity in surface and coastal waters of the British Isles, 1978--1988. Aquatic Enviromnent Monitoring Reports, MAFF, Lowesto ft. I AEA (19S4). intcrcomparison of artificial radiont,clidc measurements on marine sediment SD-N-I/I. Report No. 21, IAEA. Vicuna. Kim, C. K., Takaku, Y.,'Yamamoto, M., Kawamura, i I., Shiraishi, K., lgarashi, Y., Igarashi, S., Takayama, i I. & Ikeda, M. (1989). I)etermination of : ~TNp m environmental samples using inductively coupled plasma mass spectrometry. J. Radioanal. Nm'l. ('hem., 132, 131-7. Mudgc, S., ! lamilton-Taylor, J., Kelly, M. & Br,ldshaw. K. (1988). I,aboratory studies of thc chemical behaviour of plutonium associated with contaminated estt,a,rine sedimeuts. J. Env. Radioact., 8,217-38. Pentreath, R. J. & ! larvey, B. R. (1981). The presence of-' ~TNp in tile Irish Scat. Mar. l-col. Prog. Ser., 6,243-7. Yanlanmto, M., ('hatani, K., Komura, K. & Ueno, K. (It-~g<)). l)evelopment of alpha ray spectrometric techniques for the measurement of low-level -' ~TNp in environmental soil and sediment. Radiochim. Acta, 47, 63-8. Yamato, A. (1982). An anion exchange method for the determination of "~lAm and plutonium in environmental and biological samples. J. Radioanal. (Twin., 7 5 , 265-73.
Neptunium in Intertidal Coastal and Estuarine Sediments in the Irish Sea D. J. Assinder," M. Yamamoto,b C. K. Kim,C R. Seki, ~ Y. Takaku, d Y. Y a m a u c h i , b K. K o m u r a , b K. U e n o b & G . S. B o u r n e a "Schoolof Ocean Sciences, University College of North Wales. Menai Bridge, Anglesey, Gwynedd LL595EY, UK bLow Level Radioactivity Laboratory. Kanazawa University, Tatsunokuchi, l,~hikawa 923-12, Japan 'Department of Chemistry, University ofTsukttba, Tsukuba, lbaraki, 3(15Japan 'tMarubun Co., 8"-IOdenma-cho, Nihonbashi. Tokyo 103, Japan (Received 19 December 1989; revised version received 5 March 190(I; accepted 14 May ItP'~O)
ABSTRACT A survey has been carried out of 2+TNp activities in sediments of t/re Irish Sea. Two independent methods of analysis were used (ICI'-MS attd alpha-spectrometry) to enable tire intercomparison of tire results produced. The 2JrNp content of the samples ranged from 0.01-4,5 Bq/kg ]br all areas apart from the Ribble Estuary. In the Ribble, a source of 2~7Np other than Sellafield has been found which supports 2.¢7Np activities in sediments up to !.1.3 Bq/kg. This anomaly is verified by comparing '-¢7Np with tire 2 ~°'24°pu content of tire sediments. These are believed to be the highest environmental 2rZNp value~ yet recorded.
INTRODUCTION The Irish Sea receives controlled discharges of effluents from the Scllaticld (Windscalc) nuclear fuel reprocessing plant in Cumbria. E n h a n c e d levels of transuranic elements such as plutonium and americium have been 135 J. Environ. Radioactivity 0265-931X/91/$03.50 (~) 1~.)1 Elsevier Science Publishers Ltd, England. Printed in Great Britain
136
D.J.A.s.s'tnder et al.
reported in a variety of environmental media (e.g. Hunt, 1980-1989: Aston et al., 1985). However, data on -'-~VNpare sparse (e.g. Pentreath & Harvey, 1981: Byrne, 1986), despite Np being important from both radiological and high or intermediate level waste disposal viewpoints (Harvey, 1981; Cohen, 1982). In particular, the proportion of'--~TNp in the alpha discharges from Seilafield is increasing as a consequence of 237Np levels showing little, if any, decrease over the last ten years, in contrast to the other alpha-emitters whose levels have dropped substantially (BNFL, 198(b-1989). In this study, intertidal coastal and estuarine sediments were collected from predominantly muddy sites around the British coast of the lrish Sea. A suite of radionuclides were measured independently by two laboratories and the results for 237Np and its relationship to 23t~'24~lPuare presented here. Other results for 23Spu, 241pu. 2~lAm and 137Cs are presented elsewhere (Assinder et al., in prep.). This is the first survey of 237Np activities in surface sediment in the Irish Sea because, although this radionuclidc is monitored in the Ministry of Agriculture, Fisheries and Food annual surveys, only a limited number of fish and shelitish are examined (e.g. tlunt,'1989).
MET! IODS Sediment sa,mples were collected in July 1988 at the 24 intertidal sites shown in Fig. I. A later survey of the Ribble [£stuary was carried out in September 1989 (Fig. 2). At all sites, the top l cm of sediment was carefully removed at low-water, placed in plastic sample bags and quickly transported in a cooi-tmx to the laboratory where the samples were frozen until required. After defrosting, the samples were thoroughly homogenised before separation into sub-samples for the laboratories involved. All sediments were then dried at 60°C before Np and Pu analyses were performed. Pu measurements at both laboratories (Tsukuba and Kanazawa) and Np measurements at Kanazawa were carried out using conventional radiochemical separations and alpha-spectrometry (Yamato, 1982; Yamamoto et al., 1989). For the Np analyses, a 0-5-10 g sample was used with 23'JNp as a yield tracer. Full details of the analytical method have been reported elsewhere together with its precision (average value = 6.5%) and accuracy (average value - 2.9%) (Yamamoto et al., 1989). Np measurements at Tsukuba were performed by ICP-MS analysis using a VG Elemental (Plasmaquad PO2 Plus) system at Marubun Co. Full details of the sediment extraction method used and instrument
Neptunium in intertidal coauttal and estuarine sediments
137
S5°
ond ~
' " :: : :~ :I :il
[.
}:
~
N. Irel
~
/
k
/,.,.
. ~
England _S,tt,.fi,td
~,,,.gt,ss;~ sit
I
I[stu*r~es/
ISlS
slt.
'*",-W ) St,°
Irish Seo
ST
~
prest*f
• Ribble [sfuary/~ N
: CHGnchester~
Woles 6°
S"
t,"
3"
Fig. I. l,ocati~m m a p of the n o r t h e r n Irish Sea. ~ i t h sites of intertidal s e d i m e n t samples.
operating conditions are given elsewhere (Kim et al., It)Sg). This technique requires approximately ten times less sediment to achieve the same precision and its detection limit (3tr standard deviation of the background) of 0.02 mBq/ml of sample solution is about one order of magnitude lower than that for NAA and alpha-spectrometry. The results therefore provide an opportunity to compare both systems of measurement.
RESULTS Irish Sea survey
The activities of 237Np and 23'~'241)pudetermined by both laboratories in the 24 surface samples are given in Table 1. There was no significant difference between the activities measured in the two laboratories when analysed by
D.J. Assinder e t al+
138
~
,.i ~
~ ~
o
.E
-g
t~
o
m
rv"
t-v'
,.¢ 'D e-,
!
e',
r. o.al
z
E t~
,
++ J
i
Neptunium in intertidal coastal and estuarine sediments
~
=.
+1 +I
-'. - / - ' .
--~. .
.
.
.
.
.
.
.
.
.
.
.
.
] +1 +t +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 I +~ +1 +1 +1
-~,~--,~'+~3~-'.~-=h
~,~
=
e~.,=.
139
=~.,,..--
+1 +1 +1 +1 +1 +1 +1 +1 +t +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 m~,+ '
+1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1
e.l r,,,-
.Z Z~
~* ,';, ~ -'I" -4" ~* r.- ,,'n ~', ~,", -- ~, -- ~ ~'* ,,", ~, ~ ~", -- ~-, ,-,n ~ '-~ 4"I +I +I 4-I +I +I +I +I +I +I 4-I +I +I 4-I +I @I +I +I 4-I +I 4-I +I +I @I
P..
~z +I Z
+I
,-.,'' " '
e-
- ~ ,., ~.
.
.= .
.- , - ~ .c .
.
.
.
.
,,~
e-
E +I +I +I +I +I +I +I +I ~I +I +I +I +I +I +I +I ~I +I +I +I +I +I +I +I
rt
z
A
_
.
_
.~
.._~
~
.~
-
~.~
_~,
~.
+t
.
~.
it
<
N
Z
~j
.y-, ~
~
~
~
~
~
.
.
.
.
~
.
. .
<
140
D.J. Asstnder
et at.
blocked analysis of variance (Np: F = 3.87, df 1,21; Pu: F = 1.72, df 1,21). Variations are well within the normal range encountered due to sample heterogeneity, as found during I A E A intercalibration exercises (e.g. I A E A , 1984). As expected, the limit of detection for 237Np is lower for the ICP-MS technique, as shown by samples $3 and $20 and, even with the smaller sample sizes employed, the precision of the method is equal to. or greater than, alpha-spectrometric techniques. The z3'~'2~*lPuactivities generally decrease with distance from Sellafield, This was shown by a similar survey carried out in 1979-1980 (Aston et a l . , 1985) where it was concluded that this geographical trend was refined by the effect of sediment grain-size, i.e. a coastal sand near to Sellaficld will have a lower activity than a mud at great distance from the source but, when similar size sediments are considered, a general decreasing trend is observed. Due to this difficulty in comparing sediments from similar sites, it is hard to compare directly activities of Pu between 1979 and 1988, I lowcvcr, in general, it appears that 23'~'2al~l~u activities in surface sediments have decreased in the estuaries close to the source (e.g. the Esk) whilst those at greater distances show similar values to the previous survey. This aspect will tic discussed further with reference to the full suite of radionuclidcs in a later paper (Assindcr et al., in prep.). The '37Np activities ('i'~d~lc I) show a similar trend to '~"'"~"Pu for all samples except $7 and $8 in the Ribblc Estuary. l lore, the average -'37Np/2+'~'2auPu activity quotient fl~r other samples of =1)-27% is increased by an order of magnitude. Possible interferences in both methods from known high U, Th and Pa isotope activities and their decay products in sediments from this estuary (e.g. l lunt, 1989) have bccn investigated and rejected. The anomalously high 237Np activities and -';TNp/-'3'~'"tq~Pu ratio give evidence for a previously unrccogniscd sourcc of "-37Np in the Ribblc Estuary area. This produces sediment 237Np activities at least three times higher than those recorded in Esk Estu:lry sediments. This linding prompted a more detailed look at this estuary.
Ribble Estuary survey The activities of 237Np and 23tJ'24*~Pumeasured in the Kanazawa laboratory in the 9 surface samples are given in Table 2. The Z37Np activities recorded in these sediments show several features. Firstly, in samples well beyond the tidal limit in the Dcepdalc Brook tributary of the Savick Brook (samples R8 and R9), the Np activities and the 237Np/23'J'24~Jpu ratios are high and the 23'~'2"U)Puactivities are very low, indicating that the original source of their Np is not from marine-derived Scllafield material.
Neptunium in intertidal coastal and estuarine sediments
141
TABLE 2 Activities of 237Np and z3~'zU)Pu and the -'37Np/Z-~'<-"U)PuRatio in Sediments of the Ribble Estuary and Selected Tributaries
Site no. RI R2 R3 R4 ( = $ 7 ) R5 ( = $ 8 ) R6 R7 R8 R9
23rNp (Bq/kg d~. wt) 2-02 1-94 2-15 5-95 12-1 11-1 3.62 5.09 8.48
4- 0- l I 4. 0.13 4. 0" 12 + 0.30 4. 0.4 +0.5 + 0.23 + 0.26 + I).34
-'-'°'-'4*'Pu (Bq/kg d ~ wt)
:3r Np/"-'°'" ~' Pu (%)
203 + 16 223 4- 13 235 4. 13 179 4. 11 140 __ 7 2104.8 86 4. 4 2.2 4. 0-1 3.5 4. 0.2
0-77 4. 0-06 0.87 _+ 0.08 0-91 + 0-07 3'32 +_ 0.26 ,~-67 ___0.56 5-31 4-0.33 4-19 4. 0-33 228 4. 17 246 + 16
Secondly. following its dispersion in the marine environment, the distribt, tion of Np. although affected by the varying grain-size distributions of the samples collected, appears to show increasing values towards the ttpstream ends of Savick Brook and Dcepdale Brook with a maxinmm value of 12-1 Bq/kg in the vcry line scdimcnts at R5(=$8. which from Table 1 was mcasurcd as 13.3 Bq/kg by the Tsukt, ba laboratory).
DISCUSSION Several features concerning the sources and gcoehcnlistry of Np in the Irish Sea can be examined using this data set. In the Ribble Estuary, the anomalous 237Np values measured suggest that there is a source of Np located in the area of Deepdale Brook and Saviek Brook. If the anomalous values were only located in tidal areas then some unusual, and unknown, focusing mechanism for very recent Sellafield discharges with higher 237Np/239"24°pu ratios (Table 3) may be hypothesised. However, due to the levels noted in riverine sediments well upstream from the tidal limit, the Np source is probably connected with the BNFL Springfields site or the adjacent Northern Research Laboratories--Springfields of the U K A E A . Although several nuclides are regularly measured by BNFL and MAFF, any 237Np activity present in discharges from these plants is not routinely monitored or reported (e.g. BNFL, 1989; Hunt, 1989). The BNFL Springfields Works converts uranium ore concentrates into uranium hexafluoride, uranium metal and uranium oxide for subsequent manufacture of fuel elements for nuclear reactors. A significant proportion of the uranium processed contains reactor-uranium which has been
D. J, Assmder et al.
142
TABLE 3 '37Np and '3u'2a°Pu Activities and the ~7Np/23~'24~Pu Ratio Discharged from Sellafield for the Period 1979--1988 (BNFL, 1980--1989)
Year
: ~;Np discharge (TBq)
:"~:a'Pu discharge (TBq)
1979 19811 1981 1982 1983 1984
0-33 0,f~7 0,41 0-3 0-3 0.3
37 20 15 16 8-7 8-3
Total alpha Pu discharge (TBq) 1985 198~1 1987
! 988
11,2 0.4 0.23 0- 28
3.4 2-7 1.3 1.4
"L¢ZNp/2-*~'2a'ptt ratio (% 0-89 3--I 2-7 I-9 3-4 3-6
"rNp/l~,tal alpha Pu ratio (%) 5.9 15 IS 20
recycled via Sellafield (D. Pomfret, BNFL tieaith and Safety Directorate, 19t,~l, pets. c o m m . ) . Plutonium nuclides are separated and removed during reprocessing but a fraction of the Np remains largely with the ur,'mium compounds. 237Np may therefore be a waste product of tile Springfiekls conversion process. It is interesting to note that these operations support sediment activities three times higher than those near Sellafield. When considering the Irish Sea data as a whole, most information can be derived from a consideration of the -'37Np/-'3"'-'4"pu ratio. The -'37Np/ -'3"'24°pu activity ratios for samples other than those in the Ribble area were in the range 0.18-4).42% (Table 1). in the Sellatield discharges, this ratio has varied between 0-89 and 20% between 1979 and 1988 (Table 3) (BNFL, 198~1989), with an average value weighted by total discharge activity of 237Np and -'3u'24"puof 3.1%. This relative deficiency of -'37Np in the sediments supports previous work (e.g. Harvey, 1981; Germain et al., 1987) indicating that plutonium is adsorbed more efficiently than neptunium by particulate material. in weil-oxidised seawater, neptunium is considered to be present mainly as the soluble pentavalent species of Np(V), NpO2 + (e.g. Allard, 1982), ai though Np( i V) (Harvey, 1981 ) and other species also occu r (Germain et al., 1987). Consequently, it has a lower distribution coefficient (Kd) for
Neptunium in intertidal coastal and estuarine sediments
143
uptake by particulate materials when compared with Am(III) and Pu(IV) (Bondietti & Tamura, 1980; Harvey & Kershaw, 1984). Pentreath and Harvey (1981) measured Kd values for Z37Np in the Irish Sea using the particulate fraction and filtrate of surface water samples. This showed a tendency for the Z37Np Kj value to decrease with increasing distance from Sellafield possibly due to a change in the oxidation state of neptunium from IV to V. If this finding is reflected in deposited sediments and assuming the Kd for plutonium is not subject to similar variations, the Z37Np/239"24°pu ratio in the surface sediments should decrease with increasing distance from Sellafield. Table 1 shows that no such trend is observed. There are several possible reasons for this discrepancy: (i) The variation in Kd values found by Pentreath and Harvey (1981) may have been a reflection, partly or wholly, of processes other than change in oxidation state. Factors such as the grain-size distribution and composition of the suspended sediments are known to cause Ka variations (e.g. Edgington & Nelson, 1984). (ii) The variations.observed in suspended sediments may not be reflected in deposited sediments. Deposited sediments represent a thoroughly mixed sediment grain population in terms of grain size, composition, date of initial contamination and subsequent history. As noted for Pu (Aston et al., 1985), this tends to obscure activity trends in the Irish Sea. (iii) The assumption that the K,t for Pu is not subject to similar variations may bc incorrect. Dclinition of the spatial variation of the relative proportions of Pu (ill + J.V) imd Pu (V + VI) in seawater and sediments in the Irish Sea is still an area in which few dclinitc conclusions have been drawn, although the details arc being tackled by some groups (e.g. Mudgc et al., 1988). The most realistic conclusion to draw from this data set is that these surface sediments appear to bc an unreliable medium in which to investigate this effect due to the complications caused by the variation with time of the 237Np/23~'24°pu discharge ratio, possible changes of oxidation states of both Np and Pu in the environment after discharge and the sedimentary processes mixing all sediments in the Irish Sea as detailed for plutonium by Aston et al. (1985). llowever, now that the 237Np distribution has bccn examined initially, the Ribble Estuary may provide a location where Np analyses can be successfully carried out in more detail due to the higher activities present and details of its marine geochemistry can then be clarilicd.
L4.4
D. J. Assinder et al.
CONCLUSIONS Previous studies have indicated the occurrence in the intertidal sediments around the Irish Sea of neptunium discharged in effluent from Sellafieid. This study shows in more detail the distribution of 237Np activities and illustrates the intercomparabitity of ICP-MS and alpha-spectrometric techniques for its measurement. A hitherto unreported source of neptunium in the Ribble Estuary of north-west England has been revealed. This source produces higher Z37Np activities in present-day intertidal surface sediments in the Ribble than in similar deposits near Sellafield. 237Np in the Ribble Estuary, although trivial in radioiogical terms, should in future provide a valuable site for an analysis of the dispersion of Np from a riverine source, through an estuarine system into the marine environment.
ACKNOWLEDGEMENTS This work was supported by a Grant in Aid for Scientific Research, Ministry of Education, Science and Culture in Japan, 1989. The authors arc grateful to the Royal Society of London for a travel award to Japan fl~r D.J.A. and to Sara tlarvey for the drafting of the figures. The authors' thanks also go to the three referees h~r many helpful comments on the manuscript.
REFERI£NCES Allard, B. (1982). Solubilities of actinides in neutral or basic'solutions. In
Actinides in Perspective, ed. N. E. Edelstein. Pergamon Press, Oxfl~rd, pp.
553--79. Assinder, D. J., Yamamoto, M., Kim, C. K., Seki, R., Takaku, Y., Yamauchi, Y., Komura, K. & Ueno, K. (in prep.) IaTCs, 23K,z.~,,.2-,t. 24tpu and "~41Amin intertidal coastal and estuarine sediments in the northern Irish Sea (to be submitted to J. Environ. Radioactivity). Aston, S. R., Assinder, D. J. & Kelly, M. (1985). Plutonium in intertidal coastal and estuarine sediments in the northern Irish Sea. Est. Coastal and ShelfSci., 20, 761-7 I. BNFL (198(g-1989). Annual:report on radioactive discharges and monitoring of the environment, 1979--1988. BNFL, Risley, Warrington. Bondietti, E. A. & Tamura, T. (19811). Physico-ehemical associations of plutonium and other actinides in soil. in Transuranic Elements in the Environment, ed. W, C. Hanson. US Department of Energy, DOE/TIC-22 8(10, pp. 145--64. Byrne, A. R. (1986). Determination of -'37Np in Cumbrian ~diments by neutron activation analysis: preliminary results. J. Environ. Radioactivity, 4, 133--44.
Neptunium in intertidal coastal and estuarine sediments
145
Cohen, B. L. (1982). Effects of ICRP Publication 30 and the 1980 BEIR report on hazard assessments of high-level waste. Health Physics, 42, 13_'~4Y Edgington, D. N. & Nelson, D. M. (1984). The chemical behaviour of long-lived radionuclides in the marine environment, In International Symposium on the Behaviour of Long-Lived Radionuclides in the Marine Environment. ed. A. Cigna & C. Myttenaere, CEC, Luxembourg. Eur 9214EN, pp. 19-66. Germain, P., Gandon, R., Masson, M. & Guegu6niat. P. (1987). Experimental studies of the transfer of neptunium from sea water to sediments and organisms (annelids and molluscs). J, Environ. Radioactivity, 5, 37-55. Harvey. B. R. (1981). Potential for post-depositional migration of neptunium in Irish Sea sediments. In Impacts of Radionuclide Release in the Marine Environment, IAEA, Vienna, IAEA-SM-248, pp. 97~IOY Harvey, B. R. & Kershaw, P. J. (1984). Physico-chemical interactions of long-lived radionuclides in coastal marine sediments and some comparisons with deep sea environments. Eur. Rep. Comm. Eur. Commun., EUR-9214, pp. 131-41. Hunt, G. J. ( 198(I--1989). Radioactivity in surface and coastal waters of the British Isles, 1978--1988. Aquatic Enviromnent Monitoring Reports, MAFF, Lowesto ft. I AEA (19S4). intcrcomparison of artificial radiont,clidc measurements on marine sediment SD-N-I/I. Report No. 21, IAEA. Vicuna. Kim, C. K., Takaku, Y.,'Yamamoto, M., Kawamura, i I., Shiraishi, K., lgarashi, Y., Igarashi, S., Takayama, i I. & Ikeda, M. (1989). I)etermination of : ~TNp m environmental samples using inductively coupled plasma mass spectrometry. J. Radioanal. Nm'l. ('hem., 132, 131-7. Mudgc, S., ! lamilton-Taylor, J., Kelly, M. & Br,ldshaw. K. (1988). I,aboratory studies of thc chemical behaviour of plutonium associated with contaminated estt,a,rine sedimeuts. J. Env. Radioact., 8,217-38. Pentreath, R. J. & ! larvey, B. R. (1981). The presence of-' ~TNp in tile Irish Scat. Mar. l-col. Prog. Ser., 6,243-7. Yanlanmto, M., ('hatani, K., Komura, K. & Ueno, K. (It-~g<)). l)evelopment of alpha ray spectrometric techniques for the measurement of low-level -' ~TNp in environmental soil and sediment. Radiochim. Acta, 47, 63-8. Yamato, A. (1982). An anion exchange method for the determination of "~lAm and plutonium in environmental and biological samples. J. Radioanal. (Twin., 7 5 , 265-73.