Modelling the spatio-temporal evolution of H-3 in the waters of the River Tagus

Modelling he spa io- empo al e olu ion o 3 H in he wa e s o he Ri e Tagus A. Baeza a, *, E. Ga cı ´a b , C. Mi o ´ a , R. Pe ia ´n ˜ez c a Uni e sidad de Ex emadu a, Facul ad de Ve e ina ia, Dp o. de Fı´sica, A da. Uni e sidad sn, 10071 Ca´ce es, Spain b Uni e sidad de Ex emadu a, Ingenie ı´a Te´cnica Fo es al, Dp o. de Fı´sica, 10600 Plasencia (Ca´ce es), Spain c Uni e sidad de Se illa, E.U. de Ingenie ı´a Te´cnica Ag ı´cola, Dp o. de Fı´sica Aplicada, 1, 41013 Se illa, Spain Recei ed 7 Ap il 2005; ecei ed in e ised o m 2 Oc obe 2005; accep ed 23 Oc obe 2005 A ailable online 15 Decembe 2005 Abs ac Measu emen s o i ium speci ic ac i i y le els and o empe a u es in wa e s o he To ejo ´neTagus ese oi (Spain) showed ha hei adioac i e cha ac e is ics we e basically in luenced by he adioac i e liquid e luen om he Alma az Nuclea Powe Plan . This en e s he To ejo ´neTagus ese oi ia he A ocampo cooling ese oi , which exchanges wa e wi h he i s . We s udied he empo al and spa ial (in wo dimensions) e olu ion o he men ioned pa ame e s o yea s 1997 and 1998. The i ium le els we e ound o be signi ican ly co ela ed wi h empe a u e. Two nume ical models we e cons uc ed o a quan i a i e s udy o he i ium le els along To ejo ´n ese oi : a 1D model was used o he dispe sion o i ium along he whole leng h o he ese oi , and a 2D dep h-a e aged model was used o a de ailed s udy o he a ea whe e i ium is eleased in o he ese oi . Bo h models sol e he hyd odynamic equa- ions, o ob ain he cu en s induced by he exchanges o wa e be ween he ese oi s in he Ri e Tagus and A ocampo, and he ad ec ion/di usion equa ion o calcula e he dispe sion o i ium. In gene al, he model esul s we e in ag eemen wi h he expe imen al obse a ions. Ó2005 Else ie L d. All igh s ese ed. Keywo ds: Rese oi wa e ; T i ium; Tempe a u e; Nume ical modelling; Dispe sion * Co esponding au ho . Tel./ ax: þ34 927 25 71 53. E-mail add ess: [email p o ec ed]s (A. Baeza). 0265-931X/$ - see on ma e Ó2005 Else ie L d. All igh s ese ed. doi:10.1016/j.jen ad.2005.10.004 Jou nal o En i onmen al Radioac i i y 86 (2006) 367e383 www.else ie .com/loca e/jen ad 1. In oduc ion The quali y o he plane wa e s no ably in luence hei use, wi h he p esence o adionu- clides being one o he limi ing ac o s. In his sense, he s udy o he i ium con en o di e en wa e s has a h ee old in e es . Fi s ly, i s uni e sali y, since i ium le els due o na u al o igins (Kau man and Libby, 1954) may be aised by se e al o de s o magni ude by an an h opogenic componen (UNSCEAR, 1988; IAEA, 1990; Okada and Momoshima, 1993). Secondly, i s adio- oxici y, which, while ela i ely low in compa ison wi h ha o o he adionuclides, is 25 000 imes g ea e when he 3 H is in he o m o HTO ins ead o HT (ICRP, 1978). Thi dly, he p ac- ically indis inguishabili y o hyd aulic sys ems o i ia ed wa e om un i ia ed wa e (Thi- bodeaux, 1979), so ha his adionuclide may be a good pa ame e o s udying he dynamics o wa e masses (Fon es, 1983a,b; Baeza e al., 2002), and in pa icula o s udying he spa ial and empo al dis ibu ion o o he subs ances con ained in hem, whe he conse a i e o no . One o he Ibe ian Peninsula hyd ological sys ems ha is mos a ec ed by he p esence o i ium is he Ri e Tagus. The p incipal eason is ha h ee nuclea powe plan s use i s wa e o cooling (CSN, 1998). I s cou se is highly egula ed by an almos comple ely coupled se ies o ese oi s, which a e especially impo an in he i e passage h ough he egions o Ex em- adu a (Spain) and he Alen ejo (Po ugal), wi h i e ese oi s in 200 km. The au ho s s udied p e iously, o he yea s 1994e1996, he basic aspec s o he i ium’s spa ial and empo al e o- lu ion (Baeza e al., 1997), including a ious simple models ha allowed us o unde s and ce - ain ace s o i s dynamics, such as i s mean ansi speed (Baeza e al., 2001, 2004). One ac deduced was he impo ance o To ejo ´n ese oi in unde s anding he spa ial and empo al e olu ion o i ium in his pa o he Ri e Tagus, since he wa e is used no only o hyd oelec ic powe p oduc ion and i iga ion, bu also o cool he Alma az Nuclea Powe Plan (hence o h, ANPP) h ough wa e exchanges wi h A ocampo ese oi . In he p esen wo k, we s udy in de ail he in luence o his wa e exchange on he dis ibu ion o i ium in To ejo ´n ese oi , using da a o yea s 1997 and 1998. To his end, we collec ed mon hly wa e samples, in which we de e mined hei empe a u es simul aneously wi h he collec ion. In his way we we e able o analyze bo h he spa ial dis ibu ion o i ium le els in wo dimen- sions in To ejo ´n ese oi , ans e sally and longi udinally, and hei empo al e olu ion. The cu en s caused by he exchange o wa e be ween he di e en ese oi s in his pa o he Ri e Tagus make i di icul o gi e a de ailed quan i a i e in e p e a ion o he measu ed i ium le els, in a o m allowing p edic ions o hei u u e e olu ion. A p e equisi e is o con- s uc app op ia e models o To ejo ´n ese oi . We made a i s app oxima ion by using simple box models o simula e he empo al e olu ion o i ium le els in he A ocampo (Baeza e al., 1997) and To ejo ´n(Baeza e al., 2001) ese oi s, in pa icula s udying he a ea whe e he exchange o wa e wi h he A ocampo ese oi akes place. The p esen wo k desc ibes he de elopmen o wo new nume ical models o he To ejo ´n ese oi . The i s is a one-dimen- sional model cons uc ed o ep oduce he la ge scale dispe sion o i ium, co e ing he o al leng h o he ci ed ese oi . This model canno , howe e , desc ibe he dispe sion o i ium a a small scale, in pa icula in he a ea o he discha ges om A ocampo ese oi . A highe esolu ion wo-dimensional dep h-a e aged model was he e o e cons uc ed o his a ea. The ad ance ep esen ed by hese models is ha hey a e physical models in which he ins an- aneous cu en s a e ob ained o each poin o he To ejo ´n ese oi and o each ins an o ime. These cu en s a e hen used o calcula e he i ium dispe sion. Hence, he i ium con- cen a ions may be calcula ed o any posi ion and ime in his ese oi . 368 A. Baeza e al. / J. En i on. Radioac i i y 86 (2006) 367e383 2. Expe imen al 2.1. S udy a ea The Ri e Tagus has i s sou ce in he Sie a de Alba acı ´n (Spain), and i s mou h in Lisbon (Po ugal). I s o al leng h is 1107 km (Fig. 1). The a ea o he Tagus basin is abou 8.1 10 4 km 2 , o which 69% is in Spain. The ANPP, wi h wo 930 MWe PWR- ype uni s is loca ed on he i e igh bank, in he egion o Ex emadu a and 180 km wes esou hwes o Mad id. The Ri e Tagus ecei es discha ges om he ANPP (Baeza e al., 1991) and om o he wo nuclea powe plan s loca ed below 300 km ups eam eJose ´Cab e a and T illo eas well as se e al smalle con ibu ions om a signi ican numbe o labo a o ies and hospi als (CSN, 1998). The low egime o he Ri e Tagus is con olled h ough a chain o ese oi s. One o hem, in he egion o Ex emadu a (Fig. 1), is, in cubic hec ome es (1 hm 3 ¼10 6 m 3 ), he 166 hm 3 capaci y To ejo ´n ese oi , which is he objec o he de ailed s udy o he p esen wo k. I is 42 km long, wi h a mean wid h o 130 m and a mean dep h o 20 m. Immedia ely ups eam, he 500 hm 3 capaci y Valdecan ˜as ese oi is loca ed. The Alca ´n a a ese oi , wi h a no mal ol- ume o 1600 hm 3 , is si ua ed downs eam om To ejo ´n. The wa e o To ejo ´n ese oi is pe- iodically pa ially exchanged wi h ha o A ocampo, since he la e is used as he ANPP’s Fig. 1. Map o he Ri e Tagus wi h he loca ions o he sampling poin s. The loca ion o he dam o each ese oi is indica ed by <. 369A. Baeza e al. / J. En i on. Radioac i i y 86 (2006) 367e383 seconda y cooling ci cui . The pu pose o he exchange is o main ain A ocampo’s olume p ac ically cons an a 35 hm 3 (ANPP, 1998) and i s wa e empe a u e su icien ly low o ul il i s cooling unc ion. The lowe pa o Fig. 2 shows he empo al e olu ion o he lows be ween he A ocampo and To ejo ´n ese oi s. The s ong 12-mon h pe iod seasonali y o hese exchanges, as has been desc ibed in p e ious s udies (Baeza e al., 2001), is obse ed. The uppe pa o he Fig. 2 shows he mon hly discha ges o wa e be ween he Tagus ese oi s p esen in Fig. 1, i.e., om Valdecan ˜as o To ejo ´n, and om he la e o Alca ´n a a. I can be seen ha he man- agemen o he discha ges be ween hese h ee ese oi s is pe ec ly coupled, which, as was indica ed abo e, is a common cha ac e is ic o all he Ri e Tagus ese oi s. On he o he hand, his managemen is adically di e en om ha be ween To ejo ´n and A ocampo, since he obse ed seasonali y in his case is condi ioned by he a ailabili y o wa e in he Tagus Ri e , which in u n is ela ed o he in ensi y and equency o ain alls in he a ea. 2.2. Sampling Two sampling p og ams eone mon hly and he o he in ensi e ewe e ca ied ou in 1997 and 1998 in he abo e desc ibed s udy a ea. The in ensi e p og am o sampling was designed o de e mine he spa ial dis ibu ion o i ium le els in wo dimensions in To ejo ´n ese oi : lon- gi udinally, o 17 sampling poin s dis ibu ed along he leng h o he ese oi , and ans e - sally, in he cen e and on he wo banks o he ese oi o each o hose 17 poin s. The mon hly p og am was o allow us o analyze he empo al e olu ion on he basis o mon hly DISCHARGES 0 500 1000 1500 1 3 5 7 9 11 13 15 17 19 21 23 1997-1998 1357 9 11 13 15 17 19 21 23 1997-1998 (hm 3 ) (hm 3 ) Valdecañas To ejón 0 25 50 75 100 A ocampo o To ejón To ejón o A ocampo Fig. 2. Uppe pa : mon hly discha ges o wa e om Valdecan ˜as o To ejo ´n, con inuous line, and om To ejo ´n o Alca ´n a a, do ed line. Lowe pa : mon hly exchange o wa e om A ocampo ese oi o To ejo ´n ese oi , con in- uous line, and om To ejo ´n o A ocampo ese oi , do ed line. 370 A. Baeza e al. / J. En i on. Radioac i i y 86 (2006) 367e383 eco ds a h ee o he a o emen ioned sampling poin s in To ejo ´n ese oi and a ano he poin in A ocampo ese oi . Wa e empe a u e was simul aneously eco ded wi h each sam- ple collec ion. 2.2.1. Mon hly sampling E e y mon h, su ace wa e samples we e collec ed a he h ee sampling poin s o he To - ejo ´n ese oi iden i ied as 1, 7, and 8 in Fig. 1, and a one sampling poin o he A ocampo ese oi iden i ied as A in he igu e. As was no ed abo e, A ocampo is he cooling ese oi o he ANPP. The e is a cons an o ced ci cula ion o wa e a a a e o 88 m 3 /s a ound he sepa a ion dyke cons uc ed in he cen e o he ese oi . As a esul , i s wa e has pe ec ly homogeneous physicochemical p ope ies (Baeza e al., 1991), so ha poin A is en i ely ep- esen a i e o he cha ac e is ics o wa e in he ese oi . The loca ions o poin s 1, 7, and 8 in To ejo ´n ese oi a e he ollowing. Poin 1 is some 10 m om he To ejo ´n dam, on he le bank o he Tagus. Poin 7 is downs eam he A ocampo dam and close o he ou le om ha ese oi in o To ejo ´n, on he igh bank o he Tagus. Finally, poin 8 is some 400 m ups eam o poin 7, on he le bank o he i e and clea ly in he opposi e di ec ion o he na u al low o he wa e . These las wo poin s a e logically hose ha will be mos in ensely a ec ed by he mixing o wa e s be ween he To ejo ´n and A ocampo ese oi s, which, as we shall see below, ha e e y di e en adiological and he mal cha ac e is ics. 2.2.2. In ensi e sampling In ensi e sampling was ca ied ou on wo speci ic da es: 1997-08-01 and 1998-02-12. As can be seen in he lowe pa o Fig. 2, he o me da e co esponds o he ime o yea when he exchange o wa e be ween To ejo ´n and A ocampo ese oi s is maximum, while he la e is ep esen a i e o he ime o yea when he olumes o exchanged wa e a e ela- i ely low. On bo h da es, su ace wa e samples we e collec ed in a numbe o longi udinal and ans e sal ansec s o he To ejo ´n ese oi . Fig. 1 shows he loca ion o he poin s in he h ee longi udinal ansec s, co e ing he 42 km leng h o he ese oi . The sepa a ion o hese poin s was kep as homogeneous as possible, excep o a zone o he ese oi ha is especially p o ec ed because o i s wildli e. The numbe o sampling poin s on he 1998-02-12 da e was inc eased in ligh o he esul s o he analysis o he 1997-08-01 samples. 2.3. Analy ical me hods A each sampling poin , 1.5 L o wa e was collec ed in polye hylene bo les, and empe a- u e was measu ed wi h a p ecision o 0.1 C. A e anspo o he labo a o y, he samples we e il e ed h ough 0.45 mm po e il e s p io o hei p epa a ion o 3 H assay. Fo he i ium assay, a 50 mL aliquo was aken and subjec ed o a p ocess o simple dis- illa ion o elimina e colou a ion, o ganic ma e , and sal s ha migh in e e e wi h he analysis. The dis illed sample was hen mixed wi h a scin illa ion solu ion o Op iphase Hisa e 3 om LKB-Wallac a a sample/scin illa o a io o 8/12 (Scho ¨nho e and Hen ich, 1987)in20mL e lon-coa ed polye hylene ials. T i ium con en was de e mined using a Quan ulus-1220, LKB-Wallac, liquid-scin illa ion alphaebe a spec ome e . The quenching calib a ion was pe - o med wi h i ium s anda ds quenched o inc easing deg ees wi h CCl 4 . The backg ound in he egion o he i ium b-emission was o he o de o 1.5 cpm. The sys ema ically used coun - ing ime was 1000 min (200 min 5 measu emen cycles). The esul ing Lowe De ec ion 371A. Baeza e al. / J. En i on. Radioac i i y 86 (2006) 367e383 Limi a a 95% con idence o i ium was 0.5 Bq/L and he igu e o me i , (e iciency) 2 /back- g ound, was 320 (Baeza e al., 1999). 3. Resul s and discussion 3.1. Tempo al e olu ion (1997e1998) Fig. 3 shows he empo al e olu ion o 3 H le els and he empe a u e a he su ace wa e o he sampling poin s iden i ied in Fig. 1 as 1, 7, and 8, belonging o To ejo ´n ese oi , and poin A in A ocampo ese oi . Table 1 lis s he mean alues and anges eco ded a hese sampling poin s o he ci ed a iables e i ium and empe a u e. The ollowing esul s s and ou . The highes ac i i y le els we e eco ded a poin A, as was o be expec ed since his is he cooling ese oi o he ANPP. Ac i i y le els and hei empo al e olu ion a e consis en wi h he indings o p e ious s udies (Baeza e al., 2001). The obse ed empo al e olu ion is a con- sequence o he managemen o hese wa e s, whose egime o exchange wi h hose o he To - ejo ´n ese oi (see lowe pa o Fig. 2) is condi ioned by he maximum empe a u e ha he wa e can each o be used o cool he ANPP. Thus, poin A no only had he highes empe - a u es o all he s udy a ea, bu i also had he na owes ange o empe a u es as well. This is because in he cold mon hs o he yea , when i is unnecessa y o exchange wa e in o de o sa is y he he mal equi emen s, i s empe a u e alls o less han ha o he To ejo ´n ese oi wa e s. The ei e a ed use o he A ocampo wa e as coolan du ing win e is also he eason why 3 H le els a poin A we e highe in hese mon hs. In pa icula , he e was a signi ican neg- a i e co ela ion be ween i ium le els and empe a u e a poin A (see he las ow o Table 1). Fo he h ee poin s sampled mon hly in he To ejo ´n ese oi epoin s 1, 7, and 8 ebo h i ium le els and empe a u e dec eased om poin 7 o 8, and om he las o poin 1. This is eadily unde s andable gi en he spa ial loca ion o hese h ee poin s (see Fig. 1) ela i e o he 0 50 100 150 200 250 300 0 10 20 30 40 Sampling poin A Sampling poin 7 0 100 200 300 400 500 1357911131517192123 1997-1998 1357911131517192123 1997-1998 1 3 5 7 9 11 13 15 17 19 21 23 1997-1998 1 3 5 7 9 11 13 15 17 19 21 23 1997-1998 3H (Bq·L-1) 3H (Bq·L-1) 3H (Bq·L-1) 3H (Bq·L-1) 0 10 20 30 T (ºC) T (ºC) T (ºC) T (ºC) H-3 T H-3 T H-3 T H-3 T 600 40 Sampling poin 8 0 50 100 150 200 250 300 Sampling poin 1 0 20 40 60 80 0 10 20 30 40 0 10 20 30 40 Fig. 3. Speci ic ac i i y o i ium and empe a u e measu ed du ing 1997e1998 a sampling poin s A, 1, 7, and 8 o he s udy a ea. 372 A. Baeza e al. / J. En i on. Radioac i i y 86 (2006) 367e383 local sou ce o i ium and hea in o To ejo ´n ese oi , i.e., he discha ge om he A ocampo ese oi . Local empo a y inc eases in ac i i y we e di ec ly co ela ed wi h inc eases in he exchange o wa e be ween he A ocampo and To ejo ´n ese oi s (see lowe pa o Fig. 2), and he e o e had a s a is ically signi ican posi i e co ela ion wi h ises in To ejo ´n ese oi wa e empe a u e (see he las ow o Table 1). These empe a u e inc eases o cou se ha e a na u al componen du ing he ho mon hs, ein o ced by he ci ed exchanges wi h he ho e wa e in A ocampo ese oi . The coupled egula ed ansi o wa e in his zone o he Ri e Tagus (see uppe pa o Fig. 2) is ul ima ely esponsible o he anspo o i ium discha ged in o he To ejo ´n ese oi o wa e s below To ejo ´n dam. This ansi occu s p e - e en ially du ing win e , so ha i ium is sp ead ou along he men ioned ese oi du ing he summe mon hs. 3.2. Spa ial e olu ion To check he e aci y o he o egoing s a emen , we pe o med an in ensi e s udy on he spa ial dis ibu ion o i ium and empe a u e on wo da es ha we e clea ly iden i iable wi h he p e iously ypi ied cha ac e is ics o he mon hs o win e and o summe . Resul s a e p esen ed in Table 2. The able lis s, i s ly, he dis ance o he 17 poin s selec ed along he leng h o he To ejo ´n ese oi (see Fig. 1) om he ou le o he discha ges om he A - ocampo ese oi (poin 7). Secondly, o each o he wo sampling da es and each sampling poin , i lis s he 3 H le el and, in pa en heses, wa e empe a u e in he cen e and nea he banks. I can be obse ed ha he esul s we e adically di e en in he condi ions iden i ied as summe (sampling da e 1997-08-01) and win e (sampling da e 1998-02-19). The summe sam- ples showed a good ans e sal mixing in bo h i ium le els and empe a u es a each sampling poin . Indeed, he maximum ela i e ans e sal a ia ion was 13.8% o 3 H and 5.2% o em- pe a u e, bo h co esponding o poin 17. Likewise, he o e all spa ial homogenei y was also e y signi ican . In pa icula , omi ing he ac i i ies and empe a u es measu ed a poin 17 which is 12.4 km away in he opposi e di ec ion o no mal wa e low in he ese oi , we can conside ha he es o he ese oi has a well de ined i ium le el and empe a u e in he condi ions ha we e iden i ied as summe . In pa icula , o ha sampling da e, he alues we e 56.6 3.2 (SD) Bq/L and 29.2 1.5 (SD) C, espec i ely. The wo pa ame e s we e closely co ela ed, wi h a co ela ion coe icien o 0.880 a a signi icance le el quan i ied by he alue o he pa ame e p¼510 4 . Table 1 Mean alues and anges o i ium and empe a u e eco ded du ing yea s 1997 and 1998 in wa e s o A ocampo es- e oi (poin A), and To ejo ´n ese oi (poin s 1, 7, and 8) Sampling poin A 1 7 8 3 HSD (Bq/L) 270 144 22 19 101 77 57 63 Range 3 H (Bq/L) 59e496 1.4e6.0 2.4e243 1.2e199 TSD (C) 28 5185218207 Range T(C) 19e36.2 9e26.5 10e30.5 10e28.5 Co ela ion 3 HeT(signi icance le el) 0.774 (<0.0001) 0.676 (0.0003) 0.484 (0.0165) 0.711 (<0.0001) De ails o he co ela ion be ween hese wo pa ame e s, a he ou sampling poin s ha we e selec ed o he s udy o he empo al e olu ion, a e also gi en. 373A. Baeza e al. / J. En i on. Radioac i i y 86 (2006) 367e383 Table 2 De ails o he wo in ensi e sampling campaigns, iden i ying he dis ance (m) o each sampling poin om he ou le o wa e discha ged om A ocampo ese oi in o To ejo ´n ese oi No a ion Dis ance (m) Sampling 1997-08-01 Sampling 1998-02-19 Le side Cen um Righ side Le side Cen um Righ side 1þ28 600 53 3 (26.0) 54 3 (26.5) 56 3 (26.0) 1.5 1.5 (11.5) 1.5 1.5 (11.5) 1.5 1.5 (11.5) 2þ13 100 58 3 (27.0) 58 3 (27.0) 61 4 (27.0) 17.5 2.0 (10.5) 17.7 2.0 (10.5) 17.8 2.0 (10.5) 3þ2600 56 3 (28.7) 55 3 (29.0) 56 3 (29.0) 34 3 (11.0) 16.6 1.9 (10.0) 10.7 1.7 (11.0) 4þ1600 57 3 (29.0) 58 3 (29.5) 57 3 (29.0) 44 3 (11.5) 48 3 (12.0) 7.6 1.7 (11.0) 5þ600 57 3 (29.5) 58 3 (29.5) 60 4 (30.0) 32 3 (12.0) 87 5 (13.0) 58 4 (12.0) 6þ300 53 3 (30.2) 57 3 (30.5) 56 3 (30.0) 88 5 (13.5) 111 6 (14.0) 81 5 (13.0) 70 594 (31.0) 58 3 (31.5) 55 3 (32.0) 3.7 1.6 (10.0) 229 11 (18.5) 371 18 (24.0) 8400 58 3 (30.5) 54 3 (30.5) 55 3 (30.5) 1.6 1.5 (10.0) 2.5 1.5 (10.0) 1.5 1.5 (9.5) 92400 54 3 (29.5) 56 3 (29.5) 53 3 (29.5) 3.9 1.6 (10.5) 5.0 1.7 (9.5) 4.4 1.6 (10.0) 10 2900 52 3 (28.0) 51 3 (29.0) 53 3 (29.0) 3.7 1.5 (11.0) 3.8 1.5 (10.0) 1.5 1.5 (9.5) 11 3900 No da a No da a No da a 3.3 1.6 (10.0) 3.0 1.5 (10.0) 2.0 1.5 (10.0) 12 4900 60 4 (30.3) 67 4 (30.2) 61 4 (29.0) 2.2 1.5 (9.5) 2.5 1.5 (9.5) 2.4 1.5 (9.5) 13 6400 No da a No da a No da a 2.9 1.5 (9.5) 3.8 1.5 (9.5) 2.8 1.6 (9.5) 14 7900 No da a No da a No da a 2.2 1.5 (9.5) 2.2 1.5 (9.5) 2.2 1.5 (9.5) 15 8900 No da a No da a No da a 2.2 2.0 (10.5) 1.5 1.5 (9.5) 2.5 1.5 (9.5) 16 11 400 No da a No da a No da a 1.5 1.5 (9.5) 1.5 1.5 (9.5) 2.6 1.6 (9.5) 17 12 400 2.2 1.3 (19.0) 2.0 1.3 (19.0) 2.3 1.3 (20.0) 1.5 1.5 (9.5) 1.5 1.5 (9.5) 2.8 1.6 (9.5) The sign þo indica es whe he i is in he same di ec ion o he opposed one, espec i ely, as he no mal ad ance o he wa e in he To ejo ´n ese oi . The able lis s, o each o he wo sampling da es, he 3 H le els (Bq/L) and, in pa en heses, he empe a u e (C) o he su ace wa e close o he wo banks and in he cen e o he ese oi . 374 A. Baeza e al. / J. En i on. Radioac i i y 86 (2006) 367e383 Wi h espec o he spa ial dis ibu ion in he To ejo ´n ese oi de e mined on he sampling da e ypical o win e condi ions, i.e., when he e was a small low o ela i ely wa m wa e om he A ocampo ese oi con aining ela i ely high le els o i ium, as well as a la ge wa- e low in he di ec ion o he na u al cu en in his ac o he Ri e Tagus, he ollowing ac s may be obse ed. Fi s ly, he adioac i e and he mal con ibu ions om he A ocampo dis- cha ges we e only no able in a limi ed po ion o he To ejo ´n ese oi . This zone was in he di ec ion o he na u al ad ance o he wa e , also a ou ed by he di ec ion o he A o- campo discha ge canal on he igh bank o he Ri e Tagus a poin 7. One obse es in Table 2 ha i was p ecisely a ha poin whe e he highes empe a u e and 3 H ac i i y we e de ec ed. Bo h he ans e se and he longi udinal homogenei ies o hese pa ame e s in he To ejo ´n es- e oi wa e ha e disappea ed unde he ope a ing condi ions ha we e iden i ied as being yp- ical o win e . Indeed, du ing hese win e mon hs, he impac o he A ocampo discha ges was only pa ially de ec able downs eam om he ou le poin in o he To ejo ´n ese oi . I could no e en be de ec ed a he head o he ese oi , as indeed was he case on he summe da e o ou in ensi e sampling. None heless, he wo pa ame e s e 3 H ac i i y and empe a u e ewe e again pe ec ly co ela ed, e lec ing hei common o igin in he discha ges om A ocampo in o he To ejo ´n ese oi . The co ela ion coe icien was 0.974, wi h a signi icance le el quan i ied by he alue o he pa ame e p¼10 4 . 4. The modelling s udy 4.1. 1D model The use o a longi udinal model o s udy he dispe sion o i ium in he whole To ejo ´n ese oi is jus i ied by i s na owness and shallowness ela i e o i s leng h (see he Sec ion 2.1). Also, discha ges om A ocampo a e designed o achie e a apid mixing o he pollu- an s bo h in dep h and ans e sally. Thus, we a e dealing, essen ially, wi h a one-dimensional dispe sion p oblem. Indeed, 1D models ha e been ound o be adequa e ools o s udying he dispe sion o pollu an s in channeled egions and i e s (Ab il and Abdel-Aal, 2000; Ab il e al., 2000; P andle, 1974). The hyd odynamic equa ions a e sol ed o ob ain he cu en s along he ese oi o he ime pe iod o in e es . In he p esen case, he ad ec ion/di usion dispe sion equa ion o i ium is sol ed simul aneously. The 1D hyd odynamic equa ions a e (P andle, 1974) dS z þ ðAuÞ xdxQ¼0 u þu u xþg z xþCd ujuj hþz¼0 whe e Sis he ho izon al su ace a ea, z he heigh o he wa e su ace abo e he mean le el, u he wa e eloci y along he xdi ec ion, A he c oss-sec ion a ea o he ese oi , g he accel- e a ion due o g a i y, Q he ex e nal low o wa e en e ing/lea ing he ese oi a each poin , and ha dep h pa ame e de ined as h¼A/bwhe e bis he wid h o he ese oi . The bed ic- ion coe icien is de ined as Cd¼g C2 h 375A. Baeza e al. / J. En i on. Radioac i i y 86 (2006) 367e383 e olu ion o he le els o i ium in he ese oi s o A ocampo and To ejo ´n hence depends s ongly on he managemen o hei wa e s. The empo al e olu ion o he wa e empe a u e is also condi ioned by he na u al e olu ion o he en i onmen al empe a u e. The wo magni- udes consequen ly p esen ed seasonal a ia ions wi h an annual pe iod, which we e six mon hs ou o phase in he A ocampo ese oi , and pe ec ly synch onized a he h ee sampling poin s ha we e sys ema ically analyzed in he To ejo ´n ese oi . Fo he s udy o he spa ial e olu ion o i ium in he To ejo ´n ese oi , we ca ied ou in- ensi e sampling a nume ous poin s o he ese oi on wo da es clea ly ypical o win e and o summe . In win e , due o he minimal exchange o wa e wi h he A ocampo ese oi and o he la ge wa e low in he na u al di ec ion o he Ri e Tagus wa e s, he To ejo ´n ese oi was ound o beha e in a o m simila o a i e wi h a quasi-na u al low egime. Consequen ly, he ela i ely insigni ican inpu s o i ium and hea om he A ocampo ese oi undamen- ally a ec ed he a ea close o and downs eam om he discha ge poin (poin 7 in ou s udy), wi h majo ans e se and longi udinal g adien s in bo h empe a u e and i ium ac i i y. On he con a y, in summe , he To ejo ´n ese oi was ound o beha e in a o m simila o a lake, since he e we e minimal lows o wa e om Valdecan ˜as ups eam, and in o Alca ´n a a downs eam. The exchange o wa e wi h he A ocampo ese oi , whe e he i ium le els and he empe a u e a e highe , was maximal a ha ime, howe e . In hese condi ions, bo h he 3 H and he hea di used p ac ically o e he whole To ejo ´n ese oi , wi h he consequen longi- udinal and ans e sal homogeniza ion o hese wo magni udes. A 1D model o he To ejo ´n ese oi was de eloped o s udy he beha iou o i ium. In gene al, he model yielded a empo al e olu ion o i ium speci ic ac i i ies along he ese oi , and i ium p o iles along he leng h o he ese oi o he wo in ensi e sampling campaigns, ha we e in ag eemen wi h he measu emen s. I was necessa y, howe e , o use a la ge alue o he di usion coe icien du ing he summe mon hs. A 2D model o he elease a ea was de eloped o s udy he beha iou o i ium nea he ANPP discha ge poin . 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