materials, it is desimble to develop a model capable of predicting .... accumulation thjs would generate an exponential .... app.opriate to the year of deposition, and activity ralios at ..... Kershaw, P.J., Swift, D.J., Peotreath, R.J. and Lovcn, .... IOS. Râ¬port No. t 10, pp 87. Institutc of. Occanographic Sciences, Codalrhing, UK.
Environmental Ceochenistr,' and Health 199315(2/3)pape173
in lrishSea Sellafieldwasteradionuclides intertidaland salt marshsediments Angus B. MacKenzie and Roger D. Scott ScoftishUnivesities Researchand ReactorCentre,East Kilbide, GlasgowG75 jQU, Scotland Abstract plantin norlh Low levelliquidradioaclive wasledischarges kom the Sellafieldnuclearfuel reprocessinq 137c",6--e inventories about 3 ^ tot6 Bq oi x lola Bq of west Enqlandhad aeneratedenvironmental ol 239'2a0pr] 241Am 23924Pu 241Am ana a.s x-t014Bo of bv 1990.Mostof the and and aboutl0% ot the 137Cshas beenretainedin a depositof line marinesedimentcloseto the dischargepoint.The quantilies dischargedannuallyf.om Sellatielddecreasedby hvo ordersol magnitudefrom the of radionuclides mid-1970s to 1990bui estimaledcrilicalgroupinternalandextemalexposuredecreased by lessthanone order ol magniludeover this period.This indicatesthal during the pedod ol reduceddischarges, radionuclides lrompreviousreleases continued to contribute to thecriticalgroup alreadyin the environmenl processes ol ihe exposureand highlights the needto understand conlrolling the environmental distribution radionuclides. Redisiribution marinesedimenlis potentially ol majorsigniJicance in this context, of the contaminated particular in if it resultsin kansportol radionuclides to intertidalareas,wherecontactwilh the human population is relativelylikely. A reviewis presented workrelating1oSellafield wasteradionuclides in lrishSeasediments. ol published Dataon temporaland spatialtrendsin radionuclide concenlralions and aclivityratiosare collatedfroma numberol sourcesto showthat lhe dominantmechanism of radionuclide supplylo inlertidalareasis by of supplyforlrends redistribution of thecontaminated ma nesediment. Theimplications ot thismechanism in criticalgroupradialionexposureare considered.
consideredin conjunctionwith populationhabits surveysin order to identify the'critical gmup' andthis Since1952,the BritishNucledrFuelsLtd. (BNFL) receivingthemaximumradiationexposure, plantat Sellafieldin north i n f o r m a t i o ni s u s e d i n c o n j u n c t i o nw r t h nuclearfuel reprocessing west England (Figure 1) has, within authorised InternationalCommissionfor Radiological (ICRP)guidelines to establish discharge limits,discharged low levelliquidradioactive waite Protection procedure, limits. ln this it is necessary to evaluate into the Irish Sea.The quantitiesof radionuclides from both extemal exposure to radiation emitted dischargedannually have varied considerably the environment and internal radionuclides in with pronounced duringthistime, a maximumin the early 1970s,as indicatedin Figure 2 for total exposuredue to ing€stion or inhalation of materials.The intemaland extemal discharges of alphaandbetaemittiDgradionuclides. contamiDated criticalgroupsdo notnecessarily comprise A genenlly similar pattem applies to releasesof exposure ofthe population andthe groups individualradionuclides, includingthe isotopesof thesamemembers to variationsin caesium,plutoniumand americiumdiscussed may changewith time in response population the dischage composition or in habits. beloq with a decrease in dischargelevelsby two or morefrom themid-1970s to ordersof magnitude andFood TheMinistryofAgricultureFisheries 1990(BNFL, 1977-1990;Cambray,1982;NRPB, (MAFn carriesout suchassessments in the UK 1984). (MAFF,1971-1989), andtemporalvariations in the resulting estimated exl€mal and internal exposures It is importantto evaluatethe radiological .ignificanceof ruch r di\chdrgeof radioactive from lhe releaseof liquid radioactivewastefrom in Figure3, alongwith materialto theenvironment andthisis achieved by Sellafieldare summarised by theICRP Criticalgroup identifi€ation of the 'critical pathways'leadingto thelimitsrecommended from fell by anorderofmagnitude maximum human exposureto radiationas a intemalexposure (Pentreath, consequence of the discharge 1980).In a maximumof 2.3 msv in 1981(3.45msv if this appmach,radionuclidemonitoringresultsare enhancedgut uptate of a€tinidesis assumed)to gut uptake 0.19msv in 1989(0.4msv if enhanced of actinides is assumed), while extemal exposure To whom conespondenceshould be addressed. decreased from a maximumof0.55 msv in 1980to Introduction
174
SeLlalield wasIe ndiohuc Lides
0.079mSv in 1989(MAFF, 197l-1989).These reduclionsin critical groupexposurearc small in comparisonwilh the two orders of magnitude decrease in the Sellafielddischarge, implyinglhat period during the of reducedeffluent releases, radionuclidespresentin the environmentfrom previousdischarges continuedto makea significant contributionto the criticalgroupexposure. While the actual evaluationof critical group exposureis basedupon regular measurem€nts of
radionuclideconcentrations in environmenlal materials,it is desimbleto developa modelcapable of predictingturure rends in exposure,and the aboveobservationsindicatethat, in order to do so, it is necessary to identifyandquantifytheprocesses governingthe environrhental dispersionof the radionuclides.Of particular importancein this contextis characterisation of processes whichresult in transportof the wasteradionuclidesto flood plains, beachesand intertidal areaswhere conlact wilh lhe humanpopulation is rclativelylikely. We presentbelow a review of selected literaturerelatingto Sellafieldwarteradionuclides in lrish Sea sedimentsand collate resultsfrom various sourcesto show that redistributionof contaminated silt is the dominantmechanismof supplyof Sellafieldwasteradionuclides to intertidal sediments in thisarea.Theeffectsof thismechanism of tBnsporton projectedfends of radionuclide concentrationsin intertidal sedimenlsand the resultantimplicationsfor critical group extemal arcconsloereo, exposure Inventories and Concenhations of Sellaffeld Waste Radionuclidesin Idsh S€aSediments plutoniumand americiumare Radiocaesium, responsiblefor a significantpart of the human radiationexposureresultingfrom Sellafieldwaste discharges, andthedecaychamcteristics ofthe main isotopesof these elementsin the waste are summarised in Table L It shouldbe notedthat ahr 2{Pu and concentralionsare normallv ouoted as a 23924oPu valuesinceit is nbt oossibleto comoosite resoivetheenergies ofthe alphaparticleiemitted in t h e d e c a y o f t h e s e t w o i s o t o p e sb y a l p h a spectroscopy and theirenvironmental behaviourrs effectivelvidentical.Also.in additionto its intrinsic 24lPudecaysto contributignto radiationexl)osure, 24lAm t4lPu produce so rhat dischareed from provrdes Sellafield anin situsource ot lalLm in the
FiglJra I Map oI the hiJh Sea showinq the position oJ te senafietd nucleat fuel rcprccessins plant.
(o) 2s0
12000 AUTHORISED OISCI]ARGE UITIT
^
200
E 8o0o 150
a
{ oooo I
q
q
I
50
-
2000
o Fo
YFAR
YEAR
FlE fe2 Total annMl guantitiesoJ@) alpho ennti,ts rd.tir,rrlules onn (b) blto.nitrnt Ircn SettaJield(MAFF. l97l 1989)
ndionuclides rcte6ed
t75
A. B. MacKenzie andR. D. Sco Tsble I Decay chanctetistics of sebcted
Half life Nuclide (yeart
rYct
B7.t4 2-4r x t04 6.51x 103 14.35 432.2
Energy Exposure (MeV) risk
Weight Nuclidc
2.(a
r3?cs 3o.r'l 238Pu 23ePu ':opu 24lPu 2atA^
Deay
Table 2 Estimatedem,ircnnentalinventoriesoJ radionucLd.s fton (^) Setweld, (b) atnosphenc nucleal weaponstestsand (c) Chernobl.
"l '{ .I
0 ,|
0.662 lnternal 0.605 External 0.796 Ext€rnal 0.511 Intemal 0.662 Extemal 5.49 lntcmal 5.15? Inlemal 5.168 Inlemal 0.021 Intemal 5.486 Inlemal 0.060 Extemal
(bq)
(kc)
lal Estimatedem)ircnmentalinventoriesof Sellalied wosrerclonuelidesin 19 |BNFL 1977 1989 Canbmf,l982; NRPB,l9U).
F?cs 239.240Pu
3.ox 1016 6.8 x lola 8.9x l0ta
9.4 253.0 '7.0
(h) Estinated input of rcdionuclidesto the environmentftom atnosphericnuclear h'eaponstests (Jotepher al.,l97l: HadJ et al, 1973).
ll?cs 1.3x lor8 406 (Day andCross,1981). environment 239Pu 8.9 x lol5 3,900 the total environmental Table2 summarises l o x I0' 24" that hadbeen inventories of selectedradionuclides generatedby effluent releasesfrom Sellafieldup to Thble2 also includesthe (c) htinuted quantitiesof radionuclidesrcLeosed 1990.For comparison, global inventoriesof radionuclidesarisingfrom dutingthe Chernobrlrcocto.accidentin 1986(NRPB, in the 1950s t986). almospheric testingof nuclearweapons and l960s and falloutfrom the Chemobylnuclear reactoraccidentin Aoril 1986.The scaleof the Total 2 x t0l8 radionuclidecontaminationfrom the sellafield 12.5 4 x lo16 dischargeis apparentfrom the fact thqt it has r37cs generatJdan environmentalinventory of tlTcs that is approximatelyequalto tbe quantityreleasedin the o This fisure repr€sentsthe estimatedenvironmental Chernobylaccident.Moreover,althoughthe inv€ntoryof "'Am In 1990producedb) decayor radionuclideinventoriesresulting from the "'Pu released dunngaunospheric nuclearweapons Sellafield dischargeare one to two orders of magnirudelower thanthosefrom weaponstesting fallout. the ooint sourcenatureof-theeffluentrelease m e a n st h ; t i t h a s g e n e r a t e dr a d i o n u c l i d e concentrations that are locally much higherthan dominantly concenlraled in theareaof finesediment globally. lying off the Cumbriancoast(Smith etal., 1980, thoseftom fallout,whlchwasdispersed Most of the radiocaesiumdischargedfrom Penteathet al., 1984).Maximum inventoriesof out rbout 106Bq m 2, correspondinq Sellafieldremainsin solutionandis tmnsported to concenlrations year of the lrish Seaon a timescaleof lessthana of theorderof tor Bq kgtl, of 2lo2ohu and zalAm (Jefferies et aI., 1973;Baxrereral , I 9?9;McKinley were observedin this sedimentin the mid-1970s is, (Pentreath a1..1981 a,b).About 107.of theradiocaesium ?t etal., 1984)and little redissolulionof however.incorDoratedin the sedimentsof the area, theseactinideshas subsequendy been observed mainly by clty mineralsin a depositof fine (HuntandKershaw,1990). point(Smitheral., closeto the discharge sediment As a result of the high radionuclide 1980;Miller etal., 1982;Pentreath€tar., 1984; and total inventoriesin the fine concentrations ofup to about Joneser al, 1986)andconcentrations research sediment closeto Sellafield,a considemble in thissediment in I.3 x l(F Bq kgt wereobserved of devoted to characterisation effort has been themid-1970sandearly1980s(Miller e, al., 1982; dating processes the deposit. Radioca6on affecting lones et aI.. 1985). Recent observationshave (Korshaq 1986:Kershawet dL, l9E8)hasrevealed indicated that, with the presentrelatively low in Irish Seawater, lhat lhe contaminatedsilt deposit is subjectto concentrations of mdiocaesium wilh a maximum al presenl, accumulalion hastaken negliSible of radiocaesium significantredissolution cm ya'. It is, sedimentation rate of 0.02-{.08 (Hunt and Dlacefrom the surfacesediments to intense bioturbative mixrng however, subject kershaq 1990:Mccanney€. a/., 1993;MacKenzie penetration waste which in ofSellafield has resuhed ?ra/.,1993).In conrasLover90 oftheamencrum m in the to a depth of about 0.5 radionuclides and Dlutoniumreleasedfrom Sellafieldhas been (Kershaw€t al., 1983,1984). in the sediments of the lrish Seaand, sediment incorporated as with the radiocaesium, thesenuclideshavebeen Contaminationof intertidal and floodplain
176
SeUaf eld wastercdionuclides
(b) ICRP RECOMI1ENDED I.]MIT
RAVENETASS SAU,ION I WHIIEMVEN/CUMBRIAN NSHERME! IFISHERUEN ICRP RECOMMENDED UMIT
E
E
72 t-----
z
-
F
YEAR
YEAR
Figufe3 MAFF estinates for (a) ctiti.aL Broup internal erposurc (o) atd inte/nal erposurc assuninS an enhonced But uptdke factorJor actini.les (L), and (b) critical grcup ertemrl eryosure to .adiation 6 a rcsuh oJ the SellafrekrdischarSe(MAFF, I97l 1989).
sedimenls of the Irish Sea with Sellafield waste continuousremoval of someof the radionuclidesto gives rise 10 a major contribution to the critical the sediments.Assumingthe kineticsof the removal group exlernal exposure and, in some areas pro€es\ are lalt relalile to the rate of disper\ion, c o n l a in I n g l r n e - g r r r n e di n l e r t i d a ls e d i m e n ( s . different radionuclides will be removed from radionuclideconcentrationsapproachthose of the solution with different efficiencies, according to o f f s h o r e s i l t ( M A F F , l 9 7 l 1 9 8 9 ) . T h u s , theh Kn values (Ka = solid phase concenfiation concenrralionc of up lo L2 ^ 104Bq L8 | of ll-c.. d i v i d e d b j a q u e o u sp h a s e c o n c e n t r a t i o na t 4.5 x 103 Bq kg_l of 23e.2a0Pu x and 3.3 103 equilibrium). Bq kg I of alem ha\e beenreponedfor locarionc It is reasonableto assume that the rate of such as Whitehaven and Newbiggin (MAFF, removal of a given radionuclidefrom solution will 1971-1989).Lower concentrationsare observedin depend upon its aqueous phase concentration and coarser grained sediments, with those of sands that, in a uniform system,the processwill follow typically being about an order of magninrdelower flrstorder kiletics. This would result in removal of than concentrationsin silts. a constantfraction of the amountofthe radionuclide from solution over unit path length, giving an exponenLial decreasein mdionuclideconcentrations Rddionuclide Transport to Intertidal Sediments in the water with increasing distance from the The transporl of Sellafield waite radionuclidesto discharge point. Mixing with less contaminated sea intertidai sediments of the Irish Sea can be water will result in dilution ol the radionuclide envisaged to occur either by movement of the concentrationsas the water moves away fiom the radionuclides in solution in s€awateruntil they dischargepoint and,in a uniform system,this would interact with the sedimentat the site of deposition also result in a reductionin concentrationswith the (solution transport) or by initial uptake of the rateofreduction againfollowing first orderkinetics. radionuclidesin seabedsedimentsand subsequent Thus. the net result of dilution and removal to the transler of some fract;on of this contaminated sedimenl during solution transport would be an decrease in radionuclide concentratron sedimenl to the intertidal areas (particulale exponenlial in the wat€r with increasing distance from Sellafield t r a n s p o r t )( M a c K e n z i e e t a l , 1 9 8 7 ) . T h e s e \^ produce and this ould a correrponding decrease in altemative mechanismsof transpon will result in markedly different spatial and temporalpattemsof radionuclidesupply to the underlying sediment.In radionuclide sLrpplyto intertidal areas, with a uniform sediment with a constant rate of correspond;nglydifferent implicationsfor the long accumulationthjs would generatean exponential term radiological significanceof the Sellafield decrease in radionuclide concentrations in the surfacematerial with increasingdistancefrom the discharge. ln solution transporf, radionuclides are dischargepoint, i.e.: dispersed away from the discharge point in the dissolvedstate in accordancewith the dominantly whereCo is rhe sedimenrradionuclideconcenrrarion northward sea-watcr circulation pattem (Jefferies at the dischargepoint, C* is the concentrationat etal., 1973) and, as the water moves. there is di\tance\, and k is a conslant.The decreasein
t78
Sc l lali c ld waste ndio nuclides
Table 3 Radionuclide concenratio,sil |rsh S.a suiace ledinen\ in 1987(McDoMld er ., 1990). Distance (km) fron Sellafield t4 20 29 49 63
ll9.l,10Pu
24lAm
sedrment
lbq kg I
(Bqktr)
(Bqkc-l)
13..r 93
596X 26 229! tO 6 91 4 262!24 462! 22
2O5=26 3813 233! 14
205: 14
I l.l l3.i
236! 15 3 0 9 11 8
changesin the rate of radionuclidesupply,since pipeline.fie observedincre:Ise in the l3?cs/2rlAm radionuclides in the sedimentwill contrnue!o activity ratio over the approximately100 km contribute to theexposureunlil theyareburiedto a distanceflom the pipelineto the entranceof the depthsufficientto reducelhe transmission of the Nolti Chalnells clearlymuchsmallerthanthe six radiationsemittedduring their decayto negligible ofdersofmagnitudeincrease tha!wouldbeexpected p.oponions. Thus,even in lhe caseof solution if solurionnarsponwerethe dominantmechanism transport, a proportionalresponse betweenaxtemal of radionuclideEansportlo lhe sedimentsof the exposureaDddischargerate from Sellafieldwould arca.Thus,even in theseearly observations there not beexpected, However,solutiontransportwould was evidencethat solution transpori could not g i v e a n e s s e n t i a l l yi m m e d i a t er e d u c t r o nr n adequately accountfor the observeddistributionof radionuclide tupply in response to anyreductron ln radionuclides in the sediments. the dischargewhereasin the caseof partlculare Surveysof radionuclideconcentrations iD trmsPort,the supply would continuefo. a much intettidalsediments ofthe Scoftishcoastof theLish longerperiod,definedby sedimentredisaributionSeacarriedour durins 1985and 1986revealedthat processes. Therefore,althoughlhe mteof reduclion Ii?cs, 2i9.:4oPu andf,alAm concentrations va ed of externalexposureresultiDgfrom solution a c c o r d i n gt o s e d i m e n tr y p e , w i t h h i g h e r ransponwould not be in direct proportionto any concentrations in finer sediments, but did not decrease in the discharge,it would nevenheless be decreasesystematicallywith increasingdistance muchmorerapidthanthat which wouldresultfrom fromSellafield(Reid€t al., 1986:MacKenzleet al., panrculate transpon. 1987).Signiflcantly,tbe concen[rations of these nuclideswerefound to be linearlycorrelatedover the approximately 100 km extentof the sndy, as Radionuclidesin Irish SeaSediment i l l u s t r a t e di n F i g u r e 4 , a s i t u a t i o nc l e a r l y i n c o m p a t i b lw e i t h s o l u t i o nt r a D s p o rot f t h e L.ateruld.isttibutionof rad.ionuclid.es in l sh Sea which would have generated radionuclides, an variationin activity.atiowith distance. exponential In 197?/78, shonly after rhe penodof maJorinpur More recently. McDonalderdl. (1990)aLso of Fdionuclid€sto the lrish Sea from Sellafield. l3?cs.2lelao!! ard lalAm (har. observed in i987. Peritreath er aL (1934)observedthat 2392ahu and concenradons were linearlyco.relatedin offshorc 2alAminventoriisin sudacesedimentsdecreased surfacesedimentscollectedalong a transect 2 from about 106Bq m in lhe vicinity of lhe extendingfrom the silt depositnearthe SellaJield Sellafieldpipelineto abour3 x l0l Bq m-l at fie pipelineto KirkcudbrightBay on theScottishcoast, enrancelo.theNonh Channel,some100km awty. andthatconcenlrations variedwith rhenanreof tie Sinc-l3q:roPu and:alAmexhrbitsenerally simrlar sedimentbut not with increasingdistancefrom geochemical behaviout the obseied disiribution Sellafield(Table3). Once more, this provides couldbeinrcrpreted as beingcompatiblewith either evidencethat particularerather than soluiion solutiontransponor the early sragesof paniculate lransportdominatesradionuclidesupply to these l r a n s p o rw t ith limited redistribution o f t h e sediments, Moreover.the maakedcontrastbetween contaminated sediment. t h e s u r v e y si n r h e 1 9 8 0 s( R e i d e t a l , 1 9 8 6 ; However,. e, al., 1987;McDonald?t al, 1990),in -Miller er al. (1982\ also .eponed lvlacKenzie thar.in l9?8. r'Cs concenrrarrons decrersedfrorn w h i ch r e l a t i v e l y u n i f o r m r a d i o 0 u c l i d e about4 x l0I Bq kal closeto Sellafieldro abour concentralions wereobservedin similars€diments 40 Bq kg-r ar rheenrmnce ro.theNorthChrnnel. over wide arcas,and thoseof the 1970s(Miller mc r i v i r yr J t t o e ta L . , 1 9 8 2 i n d i c a t i n gt h t r t h e r r / C s / r 4 r A a P,e n t r e a t sht a l , 1 9 8 4 ) ,i n w h i c h increased overthisdistance, butonlyby anorderof concentrations decreesed raprdlywith Increasrng ftagnitude.The distributionof Sellafieldwaste distancefrom Sellafield.imoliesthat a substantial radionuclidesin lrish Sea sedimenrsin 1977/?8 degreeof larcialredistributidn of rhecontaminated cpproximated well to rhe hypothericxl srruition sediment look placebetweenthe rhid-1970s andrhe considered aboveinvolvrngretention of 80?oof (hc mid-1980s, consistent with paniculate transpon ihe '"'lm enrl l09cof rhe "'Cs wirhinI0 km of rhe
A. B. MacK.^zie and R. D Scot
G) r,oo
(b)'50
i
& loo F aoo
e o
3
coNCENlMnoN (Bq
(Bq CoNcENIRAnoN
Figtft 4 Corrclalionolfu)t37Cs anrt24tAnconcentrations, anct(b)2392a0Pu and2ltAm concenrrattons in inteftidalsedinentsof the Scoxishcoastol the |rishSea(Reidet ol., 1986:MocKenie et oI. 1987).
concentration wouldthereforebecharact€rised by a the sedimentwouldbe a constantfractionof those regularhalvingdistance(calculated asln 2./k)which of thedischarge for the yearof deposition. would be small for radionuclideswith high K,l In the contrastingmechanismof particle valuesand large for radionuclideswith low Kd transport,dispersioI of the radionuclidesis values.The activity mtio of radionuclideswith primarily controlledby redistributionprocesses contrastingK! valueswould, therefore,also vaq/ affectingthecontaminated sedirnent in the vicinity exponentiallywith distancefrom the discharge of the pipeline.This sedimentis subjectto rapid, ooint.suchthat: intensiveverticalmixing(Kershaw,1986;Kershaw R" = R"e*t+z)r etal., 1983,1984,1988)and this will effect the whereRo is the activityratioat thedischarge point. mdionuclideconcenftations and activity ratios.If Rr is the activityratioat distancex, andkl and k2 theaDDroximation is madelhat thesedimentwhich are the respectiveconstanls characterising the rate is aviiiablefor redistributionis subj€cttorapid,total o f d e c r e a s eo f t h e i n d i v i d u a lr a d i o n u c l i d e v e r t i c a l m i x i n g , t h e n t h e r a d i o n u c l i d e concentmtions. For example,if in sucha system concentrations in lhesedimentwouldbc determined SoEaof the 2a\Amand'ljvo of the l37Cs'were by the time-integrated discharge, allowingfor retainedwithin lo.km of the discharge poiot, lhen r a d i o a c t i v ed € c a y a n d i n g r o w t h , a n d t h e values of0.16km-rand0.01Ikm-rareobrained for radionuclide activityratioswouldbe equalto those the valuesof k for 2alAmand llTCs respectively. of thetime-integrated discharge. If redistribution of Continuation of solutiontmnsoortoveradistanceof such contaminatedsedimentwere the dominant 100km (theapproximate distance fromSellafieldto mechanismof waste transportto more remote the Nonh Channel)with continuous removalto the s e d i m e n tw s i t h i n t h e I r i s h - S e ar,a d i o n u c l i d e sediment accordineto thesevaluesofk wouldresult concentrationsthroughoutthe area would vary in an increase in thi llTCV?alAm ratioby six orders accordingto the extent of dispersionof the of magnitude. contaminated materialand temporalvaiahons In s u r [ a c e r a d i o n u c l i d ec o n c e n l r a l i o n si n a n T h e o p e r a t i o no f s l c h a m e c h a n i s m of depositwould be determinedby radionuclidedispersionwould meanthat any accumulating v a r i a t i o n s i n t h e t i m e - i n t e g r a t eSde l l a f i e l d variation in the discharsewould result in a i s c h a r g e . d C o n s e q u e n t l y , r a di o n u cl i d e proportional variationin the-supply of radionuclides conc€ntralions at differentdeDthswould also to the sedimentat any given point. Thus tn an correspond time-inlegiateddischarge to the a c c u m u l a t i n gs e d i m e n tw h i c h e x p e r t e n c e s year app.opriate to the of deposition,and activity negligiblemixingandis subjectto contamination in ralios different would be a constant at depths this way,radionuclide concentrations in lhe surfacc f r a c t i o n o f t h o s e o f the corresponding sediment wouldexhibittemporalvariations in direct time-integrated discharge. propo(ion to variationsin the discharge, add, allowing for radioactivedecay aod ingrowth, fie levelof extemalexposweresukingfrom conc€ntrations al differentdeDthsin the sediment conrrminalion sedrmenl will varyin oftheintenidal wouldbeaconstant func(ion oitheannualsellafieldresponse to changesin the rateof radionuclide dischargecorespondingto lhe yearof depositton. deposition,but the variationsin exposurewould Conscquently, activity ratiosat differentdepthsin almostcerta;nlynor be in drrectpronor|ionlo
179
A. B. MacKenzie andR. D. Scotl
> 0.10
e. 6000
YEAR
YEAR
(b) oao
a 9L 15oo
F,ooo
YEAR
YEAR
Fietrre6 TenDoruI wiations in (d thetJtCsy'JTC, ,atio,ina bl nc2saPt'o2a0Puo"n,,g ,otio aciiu'ry in lrkh Seainte idal sedinentsand Sellafreuliquid (MAFF, t 97t-t989). elfluent
.-
E rooo
s Tsp.ent vatiatiou.jn concennationsol(a) lig:re '"cs, (b)'"'uPu, a"d (c)'zaIAnin inte idaL sediments oJthe l.ishS4(MAFF, l97l-t989).
C a m b r a ya n d E a k i n s ( 1 9 8 0 ) o b s e r v e d enhancedconcentrations of plutoniumin coastal soils in Cumbria which. on th€ basis of the 238Pur'239 24oPu activityratio could be idendfiedas originatingfrom the SellafieldIiquid effluent discharge. Furtherwork, involvingair filteringand trappingof airbomeparticulatematerialon muslin screens, established thatthe mecha.nism of onshore transferof the radionuclides entailedinjectionof contaminated fine sedimentinto the air abovethe surf zoneduringperiodsof strongonshorewind, andit wasestimated thatbv 1982.some7.4 x 1010 Bq of 19 2ahu had beentransponedonshorein this way (Peirsonet al., 1982iEakinse, dl, 1982).A sisnificantasDeclof this work was theobservation 2o9u and uv-gal;ns it. (1982)tharrhe 238Pu/ne 2llAnx,21oraoPu "r activity ralios of rhe airborne particuiatesmatchedthose of the time-integrated
180
ScI lafi cld vas I e r oll i oruc lid cs
(o) too
\q
e 1.4
+
6 6 1,2
g 1
#1o
E B
5.0
0,4
; 0,6 5.0
5 o.1 tt 0.2
1,0
E
YEAR
i
YEAR
2at 2aaPuoctitiry ratos diuided^byrhe lml39 FigtreT Tetnpara! variatiot' in (a) lrish sca itneniaat sedi^ert correspondingratio in the ar\ual Sellafeld di:charye,and (b) lrkh Seo intertidal sedinett "'' A,nf" ""Pu actjyil ntio.\ dNid.d b:!+. .ancspotdins tatia ih IIE ti,ne itnegtuted Sella|ield discharse,aLloeingJor ingrowth of "'An frcn deca\af'"' Pr.
with the anomalythat nearbyintenidalsediments, apparentlydifferentlag times appliedto different (Aston and Stanners,1979;Astonet radioouclides andAston,1981;Hamiltonand al, 1981;Stanners Clarke,1984,Kershawet al., 1990).Difficultiesin veftical distributian of radiotluclidesin Irish Sea matchingsedimentprofileswith the discharge pattern were also recognisedto result from !tdtmenl l ixing of the sediment AccumllaliDgsedimentspreservea recordof p o s t - d e p o s i t i o n am (N{acKenzie 1982;Hamiltonandclarke, and Scott, temporalvariationsin radionuclide supplyand 1984). activity ratios at the site of deposition,provided M a c K e n z i ee t a l . ( 1 9 9 3 ) , h a v e r e c e n t l y therc is no significantposl-depositional mixing of presented a critical appraisalof the techniqueof to diagenesis, andnumerousstudieshaveatternpted matching sedimentradionuclideconcentration profilesofradionuclides in Irish rclateconcentration pattemby use profiles with the Sellafielddischarge pattem. Sea sedimentsto the Sellafielddischarge a s[udy ofradionuclide distributions of results from I n y e c t i g a l i o nosf o f f ( h o r es e d i r n e nhl sn \ e , i n andpr€viously geieral, failed to revealsedimentradionuclide in SolwayFirth saltmarshsediments profilesihai beara convincingresemblance to the publisheddata for coresfrom Marfport Harbou. dischargepattern and, as noted above,there is (Kershawz/aI., 1990).11was arguedthat the evidencethat the main areaof contaminated silt is s e d i m e n tp r o f i l e s p r e s e r v e dr e c o r d so f t h e SelJafielddischargeratherthan of subject10intensive,rapidverticalmixing(Kershaw, t;me-integrated variations radionuclides annual in the quantities-of 1986;Kershaw €rdl., 1983;1984). particulate ;ndicating rather thansolution released, Sincethe late 1970s,howevel severalstudies This radionuclides to these locations. transport of in of Sellafieldwasteradionuclide distributions primarilyuponthefollowing was conclusion based accumulating interlidaland saltmarsh sediments of the Irish Sea have been repo(edin whichthe observations: profilesin the radionuclideconcentrationprofiles exhibited (a) Radionuclideconcentratjon bore a qualitative Soiway and Maryport sediments distinctsubsurface maximaand hadshapes bearing patem, with the discharge resemblance to Sellafield to temporalchanges in the a qual;tative resemblance The v&riation bctween dislinct sub-surfacc maxirna. (Aston pattem Sellafielddischarge and StanDers, 19'79, A.sIon et a|.,1981;Stanners andAston,1981; the maximum and surfacesedimentradionuclide was,however,lessthana factorof Hamiltonand Clarke,1984,Kershawer dl., 1990). concentrations the However,a common featurethat emergedin ten whereas variationbetweenmaximumard discharges from Seilafieldwas two attemptsto matchthe shapesof suchradionuclide contemporary or more, indicatingthat orders of magnilude variations profileswith Iemporal concenlration in variations ;n sediment radionuclide concentralions was the necessity the discharge to invokea'lag to those the discharge; were not lineady related of time' of severalyears betwecnthe releaseof 233P#e2o0Pu lsaCs/r3?Cs (b) und activity ratiosat from and their deposition in radionuclides Sellafield Sellafielddischargerather than those of annual discharges,indicaiing effectively completemixing of contaminatedsediment of differenl agesbefore onshoretransfer,
t8l
A. A. MocKenzPand R. D. Sto
agesin differentsediments is a constantcomprisingth€ sum of threeseparate depthsofcorresponding weresimilarto eachotherbut weresystematically constantscharacterisingradioactive decay, lower than those of the Sellafielddischarge, dispersionof the contaminated sedimentand mixing redissolution indicatingthat the sedimenthad undergone of the radionuclides. Use of this priorto deposilion andthallhedegree o[mixingwas equationandthepublisheddischarge dataindicated similarat the variouslocationsconsidered; thatamericiumandplutoniumconcentrations in tbe alAml'?3e'Qu activityratio at different depositingsedimentare now, under conditions 1c1 The deprhsin eachof lhe \edimenl\war,sithinenor. wherethe annualinput of radionuclides from equal to that of the time-integrateddischarg€ S e l l a f i e l di s s m a l l r e l a t i v et o t h e € x i s t i n g appropriate to the year of deposition, but bore no environmental inventories, subjecttoanexponential values. fixedrelationship to the annualdischarge reduction with time as the result of Despitethe potentialcomplexityof the dispersion/dilution of the contaminated silt within particulatetransportmechanism,MacKenzieet al. thegenemlIrishSeasediment system,with halving (1993)foundthattheprocess ofradionuclide supply timesfor theconcentration in therange4 to ? years. to intertidal sedimentsin the Irish Sea can be Radiocaesium wasadditionallyfoundto be subject modelledby the simpleequation: in agreement with the to significantredissolution, dc/dr = Ir ,{cr work of HuntandKershaw(1990)andMccartney in the ul al (1993),resultingin half{ime valuesof 3 to 4 whereCt is the radionuclideconcentration depositingsedimentat time t, It is an input term yearsin this case. and ,^ Two significantconsequences wereidentified representing the annualSellafielddischarge 200
400
r
