Uranium and thorium concentrations in an estuary affected by phosphate fertilizer processing: Experimental results and a modelling study

J. En i on. Radioac i i y, Vol. 35, No. 3, pp. 281-304, 1997 0 1997 Published by Else ie Science Limi ed All igh s ese ed. P in ed in I eland ELSEVIER PII: SO265-931X(96)00057-4 0265-931X/97 $17.00 + 0.00 U anium and Tho ium Concen a ions in an Es ua y A ec ed by Phospha e Fe ilize P ocessing: Expe imen al Resul s and a Modelling S udy R. Pe ihiiez & A. Ma inez-Agui e Dp o Fisica Aplicada, E.U. Ingenie ia Tkcnica Ag icola, Uni e sidad de Se illa, C a U e a km 1, 41014 Se illa, Spain (Recei ed 16 Feb ua y 1996; accep ed 25 June 1996) ABSTRACT The Odiel i e , in sou hwes Spain, o ms an es ua ine sys em which is uj ec ed by was e disposal om a e ilize complex. U anium and ho ium concen a ions in wa e s and suspended ma e , ac i i y a ios and dis ibu ion coe icien s, kd, ha e been measu ed along he i e du ing wo di e en idal s a es. The esul s ha e shown ha a adioac i e impac is being deli e ed o he i e , as well as a sign jiican a iabili y depending on he sampling poin and he idal s a e. Thus, a quan i a i e s udy o he dis ibu ion o adio- nuclides can be ca ied ou bes bq’ means o a ma hema ical model. The model includes he pa i ion o adio ace s be ween ou phases (wa e , suspended ma e and wo sedimen ac ions) and has been designed o non-equilib ium condi ions. Thus, adio ace ans e s a e desc ibed in e ms o kine ic ans e coe icien s ins ead o kds. The model simul aneously sol es he hyd odynamic equa ions, he suspended ma e equa ion (including depos ion and esuspension p ocesses) and he equa ions which desc ibe he ime e olu- ion o adionuclide concen a ions in each one o he ou phases. The model has yielded good esul s in p edic ing U and Th concen a ions in wa e and suspended ma e , dis ibu ion coe icien s and ThjU mass a ios. o 1997 Published by Else ie Science Limi ed. All igh s ese ed 1 INTRODUCTION The Odiel i e is loca ed in he sou hwes o Spain. A i s lowes eaches, i o ms a idal es ua ine sys em which discha ges in o he A lan ic Ocean. 281 282 R. Pe ici iez, A. Ma inez-Agui e The es ua y is close o a la ge indus ial a ea in which a phospha e e i- lize p ocessing complex ope a es. This complex p ocesses a ound 1.6 MT o phospha e ock pe yea om Mo occo, Senegal and Togo. Pa o he was e is eleased di ec ly in o he Odiel i e . I is well known (Van de Heijde e al., 1988; Laiche & Sco , 1991) ha such was es con ain signi ican amoun s o na u al adionuclides: U, Th, Ra and hei daugh e s. The p esence o 226Ra and 224Ra in wa e and suspended ma e samples collec ed om he Odiel i e has al eady been in es iga ed (Pe ia iez & Ga cia-Leon, 1993; Pe iaiiez e al., 1994a). These s udies ha e e ealed ha a adioac i e impac is being deli e ed o he i e since, o ins ance, le els up o 670mBq 1-l o 226Ra ha e been measu ed in a wa e sample collec ed close o he e ilize complex. Al hough he e a e da a on he U- and Th-iso ope con en in he Odiel i e wa e and suspended ma e (Ma inez-Agui e e al., 1994a), a mo e de ailed s udy is epo ed in his pape . The objec i e o his wo k is o unde s and he U and Th dispe sion mechanisms in dynamic aqua ic sys ems, including he dis ibu ion be ween he solid and liquid phases as well as he e ec o idal oscilla ions. Thus, wa e samples we e collec ed along he Odiel i e a wo di e en idal s a es (high and low wa e ) and he U- and Th-iso ope con en s in wa e and suspended ma e we e measu ed. The dis ibu ion coe icien s kd we e also calcula ed. A quali a i e desc ip ion o he esul s is no s aigh o wa d as he e is signi ican a iabili y in ac i i y concen a ions and kd, depending upon he sampling poin and he idal s a e. This is a logical esul since we a e dealing wi h an open sys em in which some di e en e ec s a e aking place a he same ime: non-cons an inpu om he sou ce, adso p ion o adionuclides on o suspended ma e and bo om sedimens, deso p ion, and wa e and suspended ma e mo e- men s due o ides. Thus, a quan i a i e unde s anding o he U and Th dispe sion can be achie ed bes by means o ma hema ical models. A wo-dimensional model, ha includes he ans e p ocesses be ween ou phases (wa e , suspended ma e and wo g ain size ac ions o bo om sedimen s), has been applied o he Odiel i e . The model simul- aneously sol es he hyd odynamic equa ions, he suspended ma e dynamic equa ion (including he esuspension and deposi ion p ocesses) and he equa ions ha desc ibe he ime e olu ion o adionuclide concen a ions in o each o he ou phases. Al hough he model has been p e iously applied o s udy he dispe sion o 226Ra in he Odiel i e (Pe iaiiez e al., 1996a), he e a e wo main objec i es in applying i o U and Th: o achie e a quan i a i e unde s anding o he expe imen al esul s and o p o ide ex a alida ion o he model, ha is, o alida e he model o elemen s wi h e y di e en geochemical beha iou s, such as U and Th. U anium and ho ium in an es ua y 283 In he nex sec ion, he sampling and expe imen al me hods a e desc i- bed. Nex , he expe imen al esul s a e discussed. The modelling wo k is desc ibed in Sec ion 4. 2 EXPERIMENTAL Wa e samples we e collec ed in plas ic bo les along he Odiel i e basin (see Fig. 1) du ing July 1990. Two samples we e collec ed om each sampling s a ion: one du ing high wa e and he o he du ing low wa e so as o s udy he in luence o idal oscilla ions on he esul s. The wa e was il e ed as soon as possible h ough p e iously weighed Nuclepo e il e s (0.4pm po e size) o sepa a e he suspended ma e . The il e was hen d ied and weighed o calcula e he mass o eco e ed ma e . Typical suspended ma e concen a ions in he Odiel i e anged om 24 o 50ppm, depending on he sampling poin and he idal s a e du ing sampling. The il e was hen co e ed wi h HCI and in oduced in o an ul asonic ba h o hal an hou , du ing which, he suspended ma e was sepa a ed om he il e and dissol ed. The il e was hen washed wi h Fig, 1. Map o he Odiel i e showing he sampling poin s. The ec angula box is he g id used in he model. 284 R. Pe ici iez, A. Ma inez-Agui e HNOs which was added o he HCI o o m aqua egia. This solu ion was slowly hea ed o dissol e all emaining pa icles. U anium-232 and 229Th spikes and Fe ca ie we e added o he il e ed wa e and o he solu ion ob ained a e he dissolu ion o suspended ma e pa icles. P ecipi a ion o Fe wi h oxyhyd oxides wi h concen a ed amonium hen ollowed. U anium- and Th-iso opes we e ex ac ed om he p ecipi a es by using anion exchange esins (DOWEX AGlX8, HCl o m) and elec opla ed on o s ainless s eel planche s. Ac i i ies we e measu ed wi h Si ion-implan ed de ec o alpha spec ome y. De ails o he adiochemical and measu e- men me hods can be ound in Ma inez-Agui e (1991). 3 EXPERIMENTAL RESULTS Wa e samples a e deno ed by ‘0’ and suspended ma e samples by ‘SO’. The numbe s gi en o each sample iden i y he sampling s a ions (see Fig. l), which a e dis ibu ed along he low eaches o he i e , close o he e ilize complex. S a ion 1 is, howe e , a ups eam o his a ea. As i is no belie ed o be a ec ed by idal oscilla ions, only one sample was collec ed he e. The esul s o low and high wa e suspended ma e samples a e p esen ed in Tables 1 and 2, espec i ely. In he case o he low wa e samples, he e is a high concen a ion peak a s a ion S04, o bo h U and Th, e ealing he p esence o a local sou ce o ac i i y. Up o 86 and 116 pg g- o U and Th, espec i ely, ha e been measu ed in his sample, alues which a e much highe han hose p e iously ound o suspended ma e samples collec ed om o he i e s. Indeed, U concen a ions anging om 1.8 o 2.4 pg gg’ ha e been de ec ed in he Amazon and Mississippi i e s (Moo e, 1967). On he o he hand, Th concen a ions in hese i e s ange om some 8 o lOpgg_’ (Moo e, 1967). In some Japa- nese i e s, Th concen a ions ange om some 2.2 o 6.7 pg g-’ (Miyake e al., 1973). Thus, i seems clea ha samples SO4 and S05, collec ed close o he e ilize complex, a e a ec ed by he was e disposal om he complex. In he case o he high wa e suspended ma e samples, he dis ibu ion o Th and U is di e en . The maximum Th concen a ion is 0.7pgg-‘, and he e is a gene al homogeniza ion o concen a ion le els. This could be due o idal mixing, inpu o non-con amina ed suspended ma e om he sea and a non-cons an inpu o con aminan s om he sou ce ( he e ilize complex). On he o he hand, U concen a ions a e also lowe han du ing low wa e , al hough he e is s ill an impo an peak in sample U anium and ho ium in an es ua y 285 TABLE 1 Concen a ions o U and Th (pgg-‘) o he Suspended Ma e Samples Collec ed Du ing Low Wa e , and Ac i i y Ra ios Sample U1 [Th/ 234 Ui23R U 230Th/232Th ThjU so1 l.lOz O.24 1.2 I 0.3 so2 1.08 0.13 0.07 I 0.04 1.27 0.14 7*4 0.06 i 0.04 so3 2.03 i 0.24 0.11 0.05 1.39z o.17 9 4 0.05 x 0.03 so4 86 I 10 116&6 0.98 0.05 5.28 z!z 0.13 1.35 0.17 so5 25.3 z 1.3 31 3 1 .OO 0.07 6.1 0.7 1.22z o.13 SO6 13.4 1.5 0.052 z 0.024 1.07 0.17 2.4 & 1.4 (3.9 + 1.8) x 1O-3 so7 19.1 I 1.7 1.06 z 0.06 E o s a e la. TABLE 2 Concen a ions o U and Th (pg g-‘) o he Suspended Ma e Samples Collec ed Du ing High Wa e , and Ac i i y Ra ios Sample so1 so2 so3 so4 so5 SO6 so7 l.lOz O.24 3.9 * 0.5 2.8 0.3 9.oz 1.0 69 i 3 1.6 IIZ 0.4 2.4 zk 0.4 F”hl 0.12 0.07 0.19 z 0.07 0.27 0.13 0.06 0.04 0.7 * 0.2 234u1238u 230Th/232Th ThjU 1.2zlco.3 1.12 0.16 13 8 0.031 0.018 1.09 0.10 3.2 1.4 0.07 * 0.03 1.24 0.10 1719 0.030 z 0.015 1.04 * 0.07 0.9 k 0.3 8 5 0.04 0.03 0.91 0.17 5.1 * 1.9 0.29 k 0.10 E o s a e la. S05, which is close o he e ilize complex. This e ec has al eady been obse ed o 226Ra (Pe ia iez e al., 1994a). In gene al, 234U/238U ac i i y a ios a e compa ible wi h he exis ence o secula equilib ium in samples in which high U concen a ions ha e been de ec ed. This is no a ypical ea u e o i e s, bu i is ypical o he mine als used o e ilize p oduc ion (Ma inez-Agui e e al., 1994a). The secula equilib ium ound sugges s an ex e nal o igin o he suspen- ded ma e pa icles, which mus be he e ilize complex. The 230Th/232Th ac i i y a ios a e highe han 1 all along he i e o bo h high and low wa e suspended ma e samples. This e eals he high con amina ion by membe s o he 238U adioac i e chain. The exis ence o an ex e nal sou ce o ac i i y is con i med om he Th/ U mass a ios. Since U is conside ably mo e soluble han Th, i is o en ound in de ici wi h espec o Th in he solid su ace en i onmen . Thus, suspended ma e in unpe u bed i e s usually has ThjU mass a ios 286 R. Pe i&ez, A. Ma inez-Agui e abo e 1. This is no he case wi h he Odiel i e . I can be seen, in Tables 1 and 2, ha , wi h he excep ion o samples SO4 and SO5 in low wa e , he Th/U mass a ios a e below 1. This con i ms he exis ence o an ex e nal sou ce o U con amina ed pa icles o he i e . The excep ions men ioned abo e show a ios abo e 1, alues which a e ypical o unpe - u bed i e s’ suspended pa icles (al hough hey a e he mos con ami- na ed samples). These esul s will be discussed u he , wi h he help o he ma hema ical model. The esul s o he wa e samples a e p esen ed in Table 3. Un o u- na ely, only he low wa e samples could be measu ed (due o echnical p oblems in he labo a o y). Sample 01 shows an anomalously high U concen a ion wi h espec o he es o he da a. The e y low pH o he wa e , 2.86 (Pe iaiiez & Ga cia-Leon, 1993) due o local geology condi- ions (Ma inez-Agui e e al., 1994b), accoun s o he enhanced U concen a ion. The acid wa e s a ound s a ion 1 p o oke a dissolu ion o U om he solid phase (suspended ma e and bo om sedimen s) o he liquid phase. This e ec has also been obse ed o Ra-iso opes (Pe iaiiez & Ga cia-Leon, 1993). The pH o wa e s downs eam om s a ion 01 anges om 6.3 o 7.6; hus, his e ec does no ake place (Pe iaiiez & Ga cia-Leon, 1993). High U concen a ions a e also obse ed all along he s udied a ea wi h a peak a ound sample 04, close o he complex. Indeed, U concen a ions in i e wa e ange om some 0.02 pgll’ in he Amazon i e (Be ine e al., 1970) o some 3.5 pgll’ in he Nahe i e (Mangini e al., 1979). Simila alues we e ound in some Indian i e s (Bha & K ishnaswamy, 1969). Thus, i seems ha an inpu o U- con amina ed wa e is aking place a ound he sampling s a ion 04. This inpu is again p obably ela ed o he ope a ion o he e ilize complex. In he case o Th, concen a ions a leas one o de o magni ude highe TABLE 3 Concen a ions o U and Th in Wa e Samples (pgl-‘) Collec ed Du ing Low Wa e , and Ac i i y Ra ios Sample [VI [Th/ 234 38 u 230 Th/232 Th Thl U 01 7.5 0.3 1.89 0.03 02 2.85 0.16 0.102 0.018 1.09 & 0.05 7.9 1.4 0.014 0.003 03 3.09 0.18 0.147 0.020 1.02 0.04 7.5 1 .o 0.048 & 0.007 04 6.9 & 0.4 8.4 . 1.0 1 .oo 0.05 5.4 * 0.02 1.22 0.16 05 5.3 0.3 0.58 0.19 1.06 0.09 11 z 4 0.11 0.04 06 4.5 0.3 0.29 0.05 1.09 &O.ll 5.8 1.1 0.064 0.012 07 4.98 z 0.24 0.14 0.02 1.08 z 0.04 7.4 1.2 0.028 0.004 E o s a e la. U anium and ho ium in an es ua y 287 han in o he non-pe u bed wo ld i e s (Moo e, 1967; Miyake e al., 1973) ha e been de ec ed. A high concen a ion peak (8.4pgl-‘) has been ound in sample 04, p obably ela ed o he ope a ion o he complex. The 234U/238U ac i i y a ios a e, in gene al, compa ible wi h he exis- ence o secula equilib ium. This is no he ypical disequilib ium obse ed in i e wa e s (Sco , 1982) bu is ypical o he mine als used o e ilize p oduc ion. The high a io obse ed in sample 01 mus be due o he highe end o 234U o be dissol ed. The 230Th/232Th ac i i y a io is abo e uni y all along he i e , indica ing g ea e con amina ion by adionuclides o he 238U adioac i e chain. Finally, he ThjU mass a io is, wi h he excep ion o sample 04, below uni y. This is consis en wi h he end o Th being associa ed wi h he solid phases in i e s. The excep ion mus be, as in he case o suspended ma e , ela ed o he con aminan s. This will be discussed la e , wi h he help o modelling. kd dis ibu ion coe icien s be ween suspended ma e and wa e o 238~ 232Th and 230Th a e p esen ed in Table 4. In he case o 238U, ou esul s a e in ag eemen wi h he alues ound in li e a u e. Indeed, kd o U in coas al wa e can ange om 0.2 o 5 lg-‘, he mean alue being 1 lg-’ (IAEA, 1985). The kds a e e y simila o bo h Th-iso opes, indi- ca ing ha hey a e dis ibu ed be ween wa e and suspended ma e in he same way. Howe e , alues below he ange o a ia ion which can be ound in he li e a u e (IAEA, 1985) ha e been ob ained. Ne e heless, a clea conclusion o hese esul s is he la ge kd a ia ion be ween di e en sampling poin s (up o wo o de s o magni ude). This a ia ion comes om he kd de ini ion i sel : i assumes an equilib ium si ua ion o he exchanges be ween he solid and liquid phases. This si ua ion is no always achie ed when an in-si u measu emen o kd is pe o med, especially in si es whe e he inpu in o he sys em is changing wi h ime (man-made pollu ion). Thus, when a kd measu emen is ca ied ou , an ex ensi e TABLE 4 Dis ibu ion Coe icien s (1 g-‘) o he Low-wa e Samples Sample 238 I/ 232Th 230Th 01 0.15 z 0.3 02 0.38 z 0.05 0.7 I 0.4 0.59 IO.17 03 0.91 * 0.12 0.7 4 0.3 0.91 0.17 04 12.5 z 1.6 13.8 1.8 13.3 S 1.6 0.5 4.8 IIZ 0.4 531 18 30 4 06 3.0 0.4 0.2 * 0.1 0.07 x 0.02 07 3.8 z 0.4 E o s a e lo 288 R. Pe i i iez, A. Ma inez-Agui e desc ip ion o he sampling condi ions (pH, empe a u e and salini y) should be made in o de o in e p e he esul . In open dynamic sys ems, such as in an es ua y, equilib ium condi ions will p obably ne e be eached. Thus, ma hema ical models designed o non-equlib ium condi- ions a e impo an ools o s udy he dispe sion o adionuclides in dynamic aqua ic sys ems. 4 THE MODELLING STUDY 4.1 Model desc ip ion The i s models which we e de eloped o s udy he dispe sion o non- conse a i e adionuclides in aqua ic sys ems we e a e aged box models in which he exchanges o adionuclides be ween he liquid and solid phases we e desc ibed in e ms o kd, assuming comple e equilib ium in he sys em (Howo h and Eggle on, 1988; Ab il & Ga cia-Leon, 1993). Howe e , in open sys ems whe e he inpu is changing wi h ime (as in he case o man-made pollu ion) and when s udying he dispe sion o adio- nuclides in small coas al egions (such as es ua ies), he equilib ium app oach will no hold. This is due o he ac ha , because o he high spa ial and empo al esolu ions ha a e imposed by he nume ical scheme, he equilib ium will no be eached in each ime s ep (Pe ia iez e al., 19966). In hese cases, a kine ic app oach is mo e app op ia e. Thus, he equilib ium will be he dynamic balance be ween wo opposi e chemi- cal eac ions (Ny ele e al., 1984). The model de eloped o s udy he dspe sion o adionuclides in he Odiel i e es ua y (which, o cou se, could be applied o any o he si e) makes use o his kine ic app oach. The model is p esen ed in de ail in Pe iaiiez e al. (1996b); hus, only a b ie desc ip ion o he p ocesses ha a e included will be gi en he e. The sys em unde s udy is di ided in o a numbe o g id cells o ‘compa men s’. Fou phases o subcompa men s a e p esen in each g id cell, which a e wa e , suspended ma e and wo sedimen g ain size ac ions. In Fig. 2, a g id cell is shown. Radionuclides can be dissol ed o associa ed wi h suspended ma e . As ides p oduce a con inuous mo emen o wa e , adionuclides in bo h phases will be anspo ed om one g id cell o ano he by ad ec i e and di usi e p ocesses. Only pa icles wi h a diame e 4 < 62.5 pm a e conside ed o be p esen in he wa e column as suspended ma e (Gu bu e al., 1987): la ge pa icles will apidly sink o he bo om. In sedimen s, we will conside wo g ain size ac ions: pa icles wi h diame e 4 < 62.5 ,um (small g ain size ac ion) and pa icles wi h 4 > 62.5 pm U anium and ho ium in an es ua y 289 ad ec ion and di usion 0 0 Fig. 2. G id .# ce lame ac ions cell in which he adionuclide ans e p ocesses be ween he ou phases a e ep esen ed. (la ge g ain size ac ion). Only he small g ain size ac ion can be esuspended in he wa e column and inco po a ed in o suspended ma e . On he o he hand, when suspended ma e is desposi ed on he es ua y bed, i will be inco po a ed in o he small g ain size ac ion o he sedimen s. Thus, he deposi ion and esuspension p ocesses p oduce an exchange o adionuclides be ween he suspended ma e and he small g ain size ac ion o he sedimen . The wa e is in con ac wi h he o he h ee phases; hus, adso p ion and deso p ion eac ions ake place. These eac ions a e desc ibed in e ms o kine ic ans e coe i- cien s. Finally, ex e nal sou ces o adionuclides (in dissol ed and suspended phases) may exis in each g id cell. We will conside ha he adso p ion p ocess ( ans e om wa e o he solid phases) is go e ned by a coe icien kl and he in e se p ocess (deso p ion owa ds he liquid phase) by a coe icien k2. The adso p ion p ocess is a su ace phenomenon and will depend on he su ace o pa icles pe wa e olume uni in o he g id cell. This quan i y will be deno ed he exchange su ace. I we now conside only suspended ma e as he solid phase (neglec ing sedimen s), we ha e: kl = xl &> (1) 296 R. Pe ihiez, A. Ma inez-Agui e 12 - ,' 1 : : I' 10 - ,' : ,jl : 8 2 ,' 8- s /' ,' ,/ 6- / ,/:: : IL 4- '! I' I ,I : ,' ' ,' '-_ 2- --.__.________,,.---_ ,I' L_ ,' NC __________________- m 0 I 1 I I I I I 0 0 5 10 15 20 25 30 35 40 Fig. 5, Compu ed (line) and measu ed (poin s) dis ibu ion coe kien (I g-‘) o U du ing low wa e . gene al beha iou o he Th/U mass a ios is well ep oduced by he model in all cases. I was men ioned in Sec ion 3 ha he U excess wi h espec o Th in suspended ma e indica es he exis ence o an ex e nal sou ce o U con amina ed pa icles o he i e . The e was, howe e , an E , I I I I 1 1 I 1.4 - exp. 8"s. - - A model susp. ---- model disc -- 1.2 - ,I' l- , ,' : /' : : . ' 0.8 - I' : I : : : : 0.6 - : (1 : : I' 0.4 - : '._ I' : __--__________ 1' 0.2 - ' ' ' L__/ __.-. I I I 0 5 IO 15 20 25 30 35 40 Fig. 6. Model esul s o Th dispe sion. (A) Wa e and suspended ma e du ing high wa e . (B) Suspended ma e du ing low wa e . (C) Wa e du ing low wa e . Concen a- ions a e gi en in pg 1-l and pg g-’ o wa e and suspended ma e samples, espec i ely. U anium and ho ium in an es ua y 297 F 140 1 I 1 I , I b =P. w B model ---- 120 - 100 I : - I : / / : 80 - : ) : : : , /j ,' : 60 - ,' ,' ,I' ,' 40 - ;p ,/' I 20 - ,' I' : ,' I' I ,/ : . . 0 5 10 15 20 25 30 35 40 I 9 c 8 exp. - model ---- 10 15 20 25 30 35 40 Fig. 6. Con inued. excep ion (samples SO4 and SO5 which we e he mos con amina ed). Indeed, mo e Th han U is discha ged in o he i e , in bo h dissol ed and suspended o ms. F om he second inpu a es ( he i s a e no indica i e because hey a e used jus o c ea e he backg ound), i can be shown ha , in solu ion, ThjU = 2.4, and in suspended ma e , Th/U = 1.5. Thus, mo e Th han U is discha ged and, as a consequence, he e is an excess o Th in wa e and suspended ma e nea he sou ce. Howe e , he e is an 298 R. Pe ibiez, A. Ma inez-Agui e 100 80 20 1 I I I I I I exp. - model ..~_ ,,-- >’ I ‘. ,/- Y_ I’ -._ _______.-._____-I --._________.----.-_ ok ” I I I I I I IA & 1 0 5 10 15 20 25 30 35 40 Fig. 7. Compu ed (line) and measu ed (poin s) dis ibu ion coe icien (1 g-‘) o Th du ing low wa e . excess o W in he es o he i e in bo h liquid and suspended phases. I seems clea ha he dissol ed Th ha is eleased o he i e is quickly ixed o solid pa icles, and hese pa icles ( oge he wi h he pa icles ha a e eleased om he sou ce and a e ich in Th) a e deposi ed on he i e 0.25 0.2 3 E 0.15 0.1 5.000000e-02 0 I - _‘.. .__________-.. ,,. ,--’ Y ,’ ,,’ ,,-- ‘ _‘ I’ -*_- ._,_‘- $I ,I’ 1, , , , ‘, : I’ i : .-. ,’ ----.__________--- ** i i ( , 0 5 10 15 20 25 30 35 40 Fig. 8. Compu ed (lines) and measu ed (poin s) ThjU mass a ios o suspended ma e samples du ing high wa e (A), wa e samples du ing low wa e (B), and suspended ma e samples du ing low wa e (C). U anium and ho ium in an es ua y 299 1.4 I I ! exP. 1-41 model ---- l.2- 0 0.6 - I' ,,' /' I' 0.4 - I' I' /' I ,' 0.2 - I' ,' : : ,,= ,' I : * /__-' .I_ 0. ________- ____---- I I ---___ _-----__-- _io___ 0 5 10 15 20 25 30 35 40 1.4- c ,’ __-- 4 cup. w model ---- ____-- , ,/-- 1.2 - ,' I ' ,' ' l- ,' I I' 0.6 - : 0.6 - ,' , I ‘ ' I' : ), : >/ 0.4 - : : : II #' : 0.2 - : j _____----..___I '. '.. o- e 1 I I I I ---___________T---g I I 0 5 10 15 20 25 30 35 40 Fig. 8. Con inued. bed nea he sou ce. In his way, a U excess would exis in he es o he i e , as con i med by he expe imen al and he model esul s. This hypo hesis is also suppo ed by he ac ha an excess o Th has been measu ed in sedimen s collec ed close o he e ilize complex (Ma inez- Agui e e al., 19946). The sensi i i y o he model o he di e en pa ame e s, he way o c ea ing he backg ound and he magni ude o he sou ce e m ha e al eady been s udied (Pe ia iez e al., 19943, 1996a,c). 300 R. Pe icinez, A. Ma inez-Agui e 5 CONCLUSIONS U anium and Th concen a ions ha e been measu ed in wa e and suspended ma e samples in an es ua ine sys em a ec ed by was e dispo- sal om a e ilize p ocessing complex. Dis ibu ion coe icien s ha e also been calcula ed. The esul s ha e shown a adioac i e impac and a signi ican kd a iabili y. In o de o pe o m a quan i a i e s udy on he dis ibu ion o U and Th, as well as on he kd a iabili y, a modelling s udy has been ca ied ou . The model includes he pa i ion o adio ace s be ween ou phases, and his pa i ion is based on a kine ic app oach. The model sol es he hyd odynamic equa ions, he suspended ma e equa ions and he ou equa ions ha go e n he ime e olu ion o he adio ace concen a ions in each one o he ou phases. The model esul s a e in good ag eemen wi h he expe imen al da a o bo h U and Th, and o bo h wa e and suspended ma e samples. The kd a iabili y is ep oduced by he model, as well as he ThjU mass a ios. This shows he goodness o he desc ip ion o he ans e p ocesses, since good esul s a e ob ained o elemen s wi h a e y di e en chemical beha iou (Ra, U, Th) by simply changing he alue o a pa ame e : he exchange eloci y. ACKNOWLEDGEMENTS This wo k was pa ially suppo ed by ENRESA. The au ho is indeb ed o J. M. Ab il o help in he modelling wo k. REFERENCES Ab il, J. M. & Ga cia-Leon, M. (1993). A 2D-4 phases ma ine dispe sion model o adionuclides. Pa 1: concep ual and compu a ional model. J. En i on. Radioac ., 20, 71-88. Be ine, K. K., Chan, L. H. & Tu ekian, K. K. (1970). U anium de e mina ions in deep-sea sedimen s and na u al wa e s using ission acks. Geochim. Cosmochim. Ac a, 34, 641-8. Bha , S. G. & K ishnaswamy, S. (1969). Iso opes o u anium and adium in Indian i e s. P oc. Indian Acad. Sci., 70, 1-17. Fla he , R. A. (1994). A s o m su ge p edic ion model o he no he n bay o Bengal wi h applica ion o he cyclone disas e in Ap il 1991. J. Phys. Oceanog ., 24, 172-90. Fla he , R. 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Pugh, D. T. (1987). Tides, Su ges and Mean Sea Le el. John Wiley, Chiches e . Sco , M. R. (1982). The chemis y o U- and Th-se ies nuclides in i e s. In U anium Se ies Disequilib ium: Applica ions o En i onmen al P oblems, eds M. I ano ich & R. S. Ha mon. Cla endon P ess, Ox o d, pp. 181-201. Van de Heijde, H. B., Klijn, P. J. & Passchie , W. F. (1988). Radiological impac s o he disposal o phosphogypsum. Radia . P o . Dosim., 24, 419- 23. APPENDIX Dissol ed phase acL4 ---Z a -ki Cd j!$ + k2 C,m g + k2 (sed, + sedi) + (ad + di ), (Al) whe e Cd and C, a e he adionuclide concen a ions in wa e (mgmh3) and suspended ma e (mgg-‘), espec i ely, m is he suspended ma e concen a ion (mgl-‘) and H is he wa e column heigh . The ime de i- a i e mus be in e p e ed as in a o wa d ini e di e ences scheme. The * means ha he co esponding quan i y mus be e alua ed in he new ime s ep. The i s e m ep esen s he ans e o adionuclides om wa e o he h ee solid phases. Thus, kl is gi en by eqn (3). ad + di ep esen s ad ec i e plus di usi e anspo o dissol ed adionuclides om one g id cell o ano he . The hi d e m is he ans e o adionuclides om he wo ac ions o sedimen s o wa e : sed s H* sed I (A21 (A3) whe e a, and aI a e he adionuclide concen a ions in he small and he la ge g ain size ac ion o he sedimen s, espec i ely (mgg-*). The bulk densi y o he sedimen , pm, is gi en in kgme3. U anium and ho ium in an es ua y Suspended ma e 303 acs a = m*H* kl x - k2Cs s + ( es - dep) + (ad + di ). (-44) As he i s e m is he ans e o adionuclides om wa e o jus suspended ma e , kl is gi en by he i s e m o eqn (3). ad + di is he ad ec i e plus di usi e anspo o adionuclides in suspended ma e . The second e m is he ans e o adionuclides om suspended ma e o wa e , and es and dep ep esen he esuspension and deposi ion p oces- ses: d C,m dep = - m*H* iws 9 es = ___ m*H’ ( > -- I 103, VC, (A% whe e d and , a e he mean deposi ion and esuspension eloci ies, Cd and ,, a e he c i ical deposi ion and esuspension eloci ies and q is he wa e lux eloci y. Bo h in he dissol ed phase and in he suspended ma e equa ions, he ex e nal sou ces o adionuclides mus be included in he g id cells in which hey exis . Sedimen s The equa ion o he small g ain size ac ion is: &I, CdH a = kl - P& lo3 - kza, ,b + (dep - es). (A7) The i s e m is he ans e om wa e o he small ac ion o he sedi- men . Thus, kl is gi en by he second e m o eqn (3). The second e m is he ans e o adionuclides om he sedimen o wa e and dep - es ep esen s he ans e s be ween he sedimen and suspended ma e ( h ough deposi ion and esuspension): dep = (A@ a, ( > z-1 es = 7 ,, . (A9) 304 R. Pe ici ?ez, A. Ma inez-Agui e The equa ion o he la ge g ain size ac ion o he sedimen is: 8 al a = (AlO) whe e he i s e m is he ans e o adionuclides om wa e o he sedimen [kl is gi en by he hi d e m o eqn (3)], and he second e m ep esen s he in e se p ocess.