U- and Th-isotopes in an estuarine system in southwest Spain: Tidal and seasonal variations

~) Pe gamon Appl. Radia . Iso . Vol. 47, No. 9/10, pp. 1121-1125, 1996 Copy igh © 1996 Else ie Science L d P in ed in G ea B i ain. All igh s ese ed PII: S0969-8043(96)00117-0 0969-8043/96 $15.00 + 0.00 Um and Th-iso opes in an Es ua ine Sys em in Sou hwes Spain: Tidal and Seasonal Va ia ions R. PERIAlqEZ l, A. MARTINEZ-AGUIRRE 1 and M. GARCiA-LEON 2 ~Dp o Fisica Aplicada, E.U. Ingenie ia T cnica Ag icola, Uni e sidad de Se illa, C a U e a Km 1, 41071, Se illa, Spain and 2Dp o Fisica A 6mica Molecula y Nuclea , Uni e sidad de Se illa, Apdo 1065, 41080, Se illa, Spain The p esence o U- and Th-iso opes in suspended ma e om an es ua ine sys em which su ounds a phospha e e ilize p ocessing complex has been in es iga ed. A clea adioac i e impac om he complex has been con i med since up o 86 and 116/ g/g o U and Th espec i ely ha e been de ec ed. The e ec o ides and seasonal condi ions, empe a u e and salini y, on he esul s has also been in es iga ed. Some dis ibu ion coe icien s a e gi en. Copy igh © 1996 Else ie Science L d In oduc ion The Odiel i e is loca ed in he sou hwes o Spain and discha ges i s wa e s o he A lan ic Ocean. In he lowe eaches, i o ms an es ua ine idal sys em which su ounds a la ge indus ial a ea whe e a phospha e e ilize complex is loca ed. This complex eleases pa o i s was es di ec ly in o he Odiel i e . I is well known ha such was es con ain signi ican amoun s o na u al adionuclides: Ra, U-, Th-iso opes and hei daugh e s. The p esence o Ra-iso opes in wa e and suspended ma e samples collec ed om he Odiel i e has al eady been s udied (Pe i i iez and Ga cia-Le6n, 1993; Pe i~i iez e al., 1994). These s udies ha e e ealed ha a clea adioac i e impac is being p oduced by he e ilize complex since, o ins ance, up o 670 mBq/L o 226Ra ha e been de ec ed in a wa e sample collec ed close o he complex. Al hough he e a e some da a on he U- and Th-iso opes con en in he Odiel i e suspended ma e (Ma inez-Agui e e al., 1994), a mo e de ailed s udy is ca ied ou . The objec i e o his wo k is o achie e a comple e unde s anding o he dis ibu ion o U and Th in suspended ma e in he Odiel i e , including he e ec o idal and seasonal condi ions on he adionuclide concen a ions. This in o ma ion, oge he wi h some Kd dis ibu ion coe icien s which ha e also been measu ed, will be necessa y o comple e some modelling wo ks on he dispe sion o adionuclides in aqua ic idal sys ems (including he ans e s be ween solid and liquid phases) which a e being ca ied ou by ou g oup. The nex sec ion desc ibes he sampling wo k as well as he iso ope de e mina ion me hods. In ollowing sec ions he esul s a e p esen ed and discussed. Expe imen al Samples o i e wa e we e collec ed along he Odiel i e basin (see Fig. 1) du ing July 1990 (d y season) and Ma ch 1991 (we season) o s udy he in luence o seasonal condi ions on he U and Th concen a ions. Wa e samples we e collec ed in plas ic bo les. Two samples we e aken om each sampling s a ion: one du ing high wa e and he o he du ing low wa e o s udy he in luence o idal oscilla ions. The wa e was il e ed as soon as possible h ough p e iously weighed Nuclepo e il e s, 0.4 pm po e size, so as 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 . A e his, he il e was co e ed wi h HC1 and placed in o an ul asonic ba h o some hal an hou du ing which ime he suspended ma e is sepa a ed om he il e and dissol ed. The il e was hen washed wi h HNO3 which is added o he HC1 o o m aqua egia. This solu ion is hea ed slowly o dissol e all emaining pa icles. A e cooling, 232U and 229Th spikes and some Fe ca ie we e added o he solu ion. P ecipi a ion o Fe wi h oxyhyd oxides wi h concen- a ed amonium hyd oxide ollows. U- and Th-iso opes we e ex ac ed om he p ecipi a e using anion exchange esins (DOWEX AG 1X8, HC1 o m) and elec opla ed on o s ainless s eel planche s. Ac i i ies we e measu ed by Si ion-implan ed de ec o alpha spec ome y. Mo e 1121 1122 R. Pe i~ iez e al. / / FORET ]A FOSFORICO I -u ¢ -g ~x ~x ~ g- ,V / 1 km I i Fig. 1. Map o he Odiel i e showing he sampling s a ions. de ails o he adiochemical and measu emen me hods can be ound in Ma inez-Agui e (1991). Resul s and Discussion In wha ollows, he samples a e deno ed by '0' (Odiel). The numbe s gi en o each sample iden i y he sampling s a ion (see Fig. 1), which a e dis ibu ed along he lowe 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 nea he i e sou ce. As i is no supposed o be a ec ed by idal oscilla ions, only one sample was collec ed he e. Tables 1 and 2 p esen he esul s ob ained o he samples collec ed in 1990, o low and high wa e espec i ely. These gi e he U and Th concen a ions in suspended ma e in #g/g and some in e es ing ac i i y a ios. In he case o he low wa e samples, he e is a high concen a ion peak in s a ion 04, 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 de ec ed in his sample, U- and Th-iso opes in an es ua ine sys em Table 1. Concen a ions o U and Th in #g/g o he suspended ma e samples collec ed du ing low wa e in 1990. Some ac i i y a ios a e also gi en 1123 Sample U Th 234U ¢23gU 23°Th/232Th Th/U Ol 1.10 + 0.24 1.2 +_ 0.3 02 1.08 +_ 0.13 0.07 _.+ 0.04 1.27 +_ 0.14 7 ___ 4 0.06 -1- 0.04 03 2.03 + 0.24 0.11 + 0.05 1.39 + 0.17 9 + 4 0.05 _~ 0.03 04 86 + 10 116 4- 6 0.98 + 0.05 5.28 - - 0.13 1.35 ___ 0.17 05 25.3 _ 1.3 31 _+ 3 1.00 + 0.07 6.1 __. 0.7 1.22 _ 0.13 06 13.4 -£- 1.5 0.052 + 0.024 1.07 -£- 0.17 2.4 + 1.4 (3.9 __. 1.8) x 10 -3 07 19.1 _+ 1.7 1.06 + 0.06 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, Moo e (1967) has measu ed U concen a ions in Amazon and Mississippi i e s anging om 1.8 o 2.4 #g/g. On he o he hand, Th concen a ions in hese i e s ange om some 8 o 10 #g/g (Moo e, 1967). In some japanese i e s Th concen a ions ange om some 2.2 o 6.7/ g/g (Miyake e al., 1973). Thus, i seems clea ha s a ions 04 and 05, close o he e ilize complex, a e a ec ed by he was e disposal om i . In he case o he high wa e samples, he dis ibu ion o U and Th is di e en . The maximum Th concen a ion is 0.7 #g/g and he e is a gene al homogeniza ion o concen a ion le els, which may be due o he idal mixing and o he inpu o non-con amina ed suspended ma e om he sea. 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 s a ion 05, which is close o he e ilize complex. This e ec was al eady obse ed o 226Ra (Pe i i iez e al., 1994). I is in e es ing o no e ha he con amina ion is e y localized a ound he e ilize complex. Howe e , some U con amina ion seems o a ec all he s udied a ea downs eam he complex du ing low wa e . In gene al, 234U/E38U ac i i y a ios a e compa ible wi h he exis ence o secula equilib ium in hose 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., 1994). The secula equilib ium ound in he Odiel i e sugges s an ex e nal o igin o he suspended ma e pa icles, p obably om he e ilize complex. The 23°Th/232Th ac i i y a ios a e > 1 all along he i e o bo h se s o samples. This e eals he high con amina ion o he i e 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 by he Th/U mass a io. As U is la gely mo e soluble han Th, i is o en ound in de icien 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 ha e Th/U mass a ios abo e uni y. This is no he case wi h he Odiel i e suspended pa icles. I can be seen in Tables 1 and 2 ha , wi h he excep ion o samples 04 and 05 a low wa e , he Th/U mass a ios a e below uni y. This e eals he exis ence o a ex e nal sou ce o U con amina ed pa icles o he i e . The excep ions a e samples 04 and 05 which show Th/U mass a ios abo e uni y, ypical o unpe u bed i e s suspended pa icles. Howe e , hese samples being he mos con amina ed, he a ios should be ela ed o he con aminan s. Tables 3 and 4 p esen he esul s o he low and high wa e samples, espec i ely, collec ed in 1991. The beha iou o Th is simila o ha o he 1990 sampling campaign. The e is a concen a ion peak in s a ion 05, close o he e ilize complex, du ing low wa e and a e y homogeneous concen a ion p o ile du ing high wa e . Ne e heless, he e is a signi ican di e ence. The concen a ion le els, including ha o he peak in 05, a e simila o hose ound in unpe u bed i e s (Moo e, 1967; Miyake e al., 1973). Ne e heless, he p esence o Th con amina ed pa icles is clea since he Th concen a ion a s a ion 05, du ing low wa e , is some ou imes la ge han in he su oundings. The case o U is di e en , he e a e high concen a ion le els du ing bo h high and low wa e all along he i e . Now he con amina ion is no educed in he a ea close o he e ilize complex, bu ex ends om 02 o 07. This could be a seasonal e ec . Du ing he 1990 sampling campaign (summe , d y season) he wa e salini y was abou ou imes highe han du ing he 1991 sampling campaign (win e , we season), see Pe i~i iez (1995). On he o he hand, he wa e empe a u es we e 27.5 _ 1.2 and 15.0 4- 0.8°C du ing he sampling campaigns o Table 2. Concen a ions o U and Th in #g/g o he suspended ma e samples collec ed du ing high wa e in 1990. Some ac i i y a ios a e also gi en Sample U Th 234U/238U 2~Fh/232Th Th/U O1 1.I0 + 0.24 1.2 + 0.3 02 3.9 ___+_ 0.5 0.12 + 0.07 1.12 + 0.16 13 + 8 0.031 + 0.018 03 2.8 + 0.3 0.19 + 0.07 1.09 + 1.10 3.2 + 1.4 0.07 __+ 0.03 04 9.0 + 1.0 0.27 + 0.13 1.24 __+ 0.10 17 + 9 0.030 + 0.015 05 69 4-__ 3 1.04 + 0.07 06 1.6 4- 0.4 0.06 __ 0.04 0.9 __ 0.3 8 __+ 5 0.04 + 0.03 07 2.4 + 0.4 0.7 __ 0.2 0.91 __+ 0.17 5,1 + 1.9 0.29 __+ 0.10 ARI 47/9-I0--K 1124 R. Pe i~ iez e al. Table 3. Concen a ions o U and Th in #g/g o he suspended ma e samples collec ed du ing low wa e in 1991. Some ac i i y a ios a e also gi en Sample U Th 2uU/2~sU 23°Th/232Th Th/U Ol 1.55 __. 0.19 0.58 __ 0.17 1.67 ___ 0.18 2.1 ___ 0.7 0.37 __. 0.12 02 11.2 __. 1.1 0.09 4- 0.06 1.24 ___ 0.09 27 __. 19 (8 _ 5) x 10 -3 03 8.7 _+ 1.5 0.07 +_ 0.04 1.06 ___ 0.19 8 -i- 5 (8 __ 5) x 10 -3 04 27.9 _ 2.7 0.34 + 0.15 1.05 _ 0.07 9 _ 4 (2.8 _ 1.3) x 10 -3 05 30 + 3 11.3 + 0.9 1.12 _+ 0.09 5.8 + 0.4 0.38 4- 0.05 06 48 + 6 2.6 + 0.5 1.13 + 0.09 4.6 + 1.0 0.054 + 0.012 07 31 + 4 2.1 + 0.6 1.17 + 0.11 2.3 _+ 0.8 0.070 + 0.021 1990 and 1991 espec i ely. I is well known ha an inc ease o salini y leads o dissolu ion o ca ions om suspended ma e . This e ec has been ound o 226Ra in, o ins ance, he Pee Dee i e (Elsinge and Moo e, 1980) and he Odiel i e (Pe i/ iez e al., 1994) and o 23sU in some labo a o y expe imen s ca ied ou by Bi d and E enden (1994). On he o he hand, an inc ease o wa e empe a u e leads o a as e dissolu ion o 23sU om suspended ma e (Ames e al., 1983). Thus, du ing he 1990 sampling campaign he U may ha e dissol ed apidly while du ing he 1991 campaign i emained ixed o solid pa icles o a longe ime. Again, he 234U/23sU a ios a e in gene al compa - ible wi h secula equilib ium in he pa o he i e close o he complex. The 23°Th/232Th a ios a e again la ge han uni y, which shows a high con amina ion by membe s o he 23sU chain. Finally, he Th/U mass a ios a e much below uni y, which con i ms he exis ence o an ex e nal sou ce o ac i i y. Some dis ibu ion coe icien s o 23su, 232Th and 23°Th a e p esen ed in Table 5. The ac i i ies o U and Th iso opes in he liquid phase ha e been ob ained om Pe i~i iez (1995). 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, IAEA (1985) showed ha Kd o U in coas al wa e s can ange om 0.2 x 103 o 5 x 103 kg/kg. The K~s a e e y simila o bo h Th-iso opes, sugges 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 seen in he li e a u e ha e been ob ained. IAEA (1985) indica e ha Kd o Th can ange om 500 x 103 o 10,000 x 103 kg/kg. Ne e heless, a clea conclusion om 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 solid and liquid phases. This si ua ion is no always achie ed when an in si u measu emen o K~ is made, 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 de e mi- na ion o he sampling condi ions (salini y, empe a- u e, pH...) should be made in o de o be able o in e p e he esul . I is p obable ha he equilib ium condi ions can no easily be eached in na u e. Thus, labo a o y expe imen s and ma h- ema ical simula ions a e impo an ools which can help o unde s and he mechanisms o dis ibu ion o adioac i i y in aqua ic en i onmen s. This wo k has been success ully ca ied ou in ou g oup in he case o ~ Ra (Pe i~i iez e al., 1996a,b). Conclusions The p esence o signi ican concen a ions o U and Th in suspended ma e samples collec ed om he Odiel i e can be a ibu ed o was e disposal Table 4. Concen a ions o U and Th in gg/g o he suspended ma e samples collec ed du ing high wa e in 1991. Some ac i i y a ios a e also gi en Sample U Th 234U/'UsU 2a°Th/232Th Th/U O1 1.55 _ 0.19 0.58 _ 0.17 1.67 _+ 0.18 2.1 _ 0.7 0.37 _ 0.12 02 63 +_ 5 0.8 + 0.3 1.196 + 0.024 4.0 _+ 1.6 0.013 4- 0.005 03 31 + 3 0.41 _ 0.19 1.32 _+ 0.09 4.7 + 2.3 0.13 + 0.006 04 119 + 10 0.46 + 0.17 1.071 _+.+ 0.013 2.2 + 0.9 (3.9 + 1.5) x 10 -3 05 94 + 8 0.30 + 0.11 1.115 +_ 0.022 5.7 + 2.2 (3.2 + 1.2) x 10 -3 06 0.13 + 0.05 1.5 _+ 0.7 07 33 + 3 0.33 + 0.17 1.27 + 0.06 3.6 + 2.0 0.010 + 0.005 Table 5. Dis ibu ion coe icien s (kg/kg) o he 1990 low wa e samples Sample 23sU 232Th 23e' h O1 (0.15 +_ 0.3) x 103 02 (0.38 + 0.05) x 103 (0.7 _+ 0.4) x 103 (0.59 + 0.17) x 103 03 (0.91 4- 0.12) x 103 (0.7 -4- 0.3) x 10 a (0.91 + 0.17) x 103 04 (12.5 + 1.6) x 103 (13.8 + 1.8) x 103 (13.3 4" 1.6) x 103 05 (4.8 +_ 0.4) x 103 (53 + 18) x 103 (30 + 4) x 103 06 (3.0 4- 0.4) x 103 (0.2 _ 0.1) x 103 (0.07 + 0.02) x 103 07 (3.8 + 0.4) x 103 U- and Th-iso opes in an es ua ine sys em 1125 om a phospha e e ilize complex loca ed close o he i e . Tides and seasons ( h ough salini y and empe a u e) a ec he dis ibu ion o U and Th along he es ua y. Some ac i i y a ios ha e also been in es iga ed, which ha e con i med he ex e nal o igin o he con amina ed suspended ma e pa icles. The Kd o U- and Th-iso opes has been calcula ed. The esul s show impo an a ia ions om one sampling s a ion o ano he , which a e due o he ac ha equilib ium condi ions which a e necessa y o a co ec Kd calcula ion a e no easily achie ed in Na u e. Acknowledgemen --This wo k was pa ially suppo ed by ENRESA. Re e ences Ames L. L., Mc-Ga aw J. E., Walke B. A. and Sal e P. F. (1983) U and Ra so p ion on amo phous e ic oxyhyd oxide. Chem. Geol. 40, 135-148. Bi d G. A. and E enden W. G. (1994) E ec o sedimen ype, empe a u e and colloids on he ans e o adionuclides om wa e o sedimen . J. En i on. Radioac . 22, 219-242. Elsinge R. J. and Moo e W. S. (1980) 226Ra beha iou in he Pee Dee i e , Winyah bay. Plane . Sci. Le . 48, 239-249. IAEA (1985) Sedimen K~ and concen a ion ac o s o adionuclides in he ma ine en i onmen . Technical Repo s Se ies 247. IAEA, Vienna. Ma inez-Agui e A. (1991) Radioac i idad na u al en di e sos compa imen os na u ales de Andalucia. Ph.D. Thesis, Uni e sidad de Se illa (in Spanish). 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(1994) Radium iso opes in suspended ma e in an es ua ine sys em in he sou hwes o Spain. J. Radioanal. Nucl. Chem. A ic. 183, 395-407. Pe i~ iez R., Ab il J. M. and Ga cia-Le6n M. (1996a) Modelling he dispe sion o non-conse a i e adio- nuclides in idal wa e s. Pa 1: concep ual and ma hema ical model. J. En i on. Radioac . 31, 127-142. Pe i~i iez R., Ab il J. M. and Ga cia-Le6n M. (1996a) Modelling he dispe sion o non-conse a i e adio- nuclides in idal wa e s. Pa 2: applica ion o Z26Ra dispe sion in an es ua ine sys em. J. En i on. Radioac . 31, 253-272.