Vertical distribution of radiocaesium, plutonium and americium in the Catalan Sea (northwestern...

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J. Emiron. Radioucrivi!y 26 (1995) 205 2 I6 1 1995 Elsevier Science Limited

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0265-931X(94)00033-6

Vertical Distribution of Radiocaesium, Plutonium and Americium in the Catalan Sea (Northwestern

Mediterranean)

J. Molero,” J. A. Sanchez-Cabeza,” J. Merino,’ LL. Pujol,” P. I. Mitchellb & A. Vidal-Quadras”

“Grup de Fisica de les Radiations, Departament de Fisica, Facultat de Cikcies. Universitat Aut6noma de Barcelona, 08 193 Bellaterra, Spain

‘Laboratory of Radiation Physics, University College, Dublin, Ireland

(Received 27 September 1993; accepted 9 July 1994)

ABSTRACT

Caesium-137, 239, 240Pu and 24’Am concentration profiles (O-l 000 m) have

been determined in unfiltered large volume water samples collected from

the Catalan Sea (northwestern Mediterranean). Results showed that

radiocaesium concentration decreases quickly through the water column

while the transuranic concentration increases with depth, showing a faster

migration to the bottom layers. Comparing our results with those reported

by% other authors (1975-1980), radiocaesium input,from Chernobyl releases

has been ident$ed through the profile. In addition, transuranic concentra-

tions have decreased considerably in the different layers of the profile.

Integrated activities through the water column were 2.6 & 0.2 kBq m-2 ,for

‘j7Cs, 23 * 3 Bq rnp2 ,for 239~240Pu and 1.5 & 0.5 Bqm-2 .for 24’Am.

According to the estimated inventory and the integrated ,fall-out in the

Mediterranean area, about 56% of ‘37Cs, 27% of 23y,240Pu and 4% of

24’Am which was deposited over the Mediterranean Sea by generalized,fall-

out remains at present in the ,first 1000 m of the w,ater column. Vertical

distribution of 239, 240Pu/137Cs and 24’Am/‘37Cs activity ratios showed a

clear separation between transuranics and radiocaesium through the water

projl’le. 241Am123Y, 240 Pu activity ratio revealed,jaster sinking of americium

relative to plutonium.

206 J. Molero et al.

INTRODUCTION

Since 1945, nuclear weapons tests have spread artificial radionuclides all around the world. Moreover, the nuclear industry has introduced a large inventory of man-made radionuclides into the environment. Natural processes occurring in the marine environment define an interesting scenario in which to study the behaviour and fate of these radionuclides. Amongst other natural and artificial long-lived radionuclides, radiocaesium and the transuranics, plutonium and americium, are present, although usually in very low concentrations, in most physical and biological compartments of the oceans. These radionuclides incorporate into the biogeochemical cycles that take place in the marine environment. Soluble radionuclides tend to be dispersed following the water masses flow, while the more insoluble radionuclides may undergo fixation on

diverse particulate types such as clay minerals, plankton, faecal pellets, etc. A study of their vertical distribution assists our understanding of the processes governing the transport of materials within the water mass. Therefore, radiocaesium, plutonium and americium may be used as tracers in the study of marine processes.

A water column profile from the northwestern Mediterranean Sea was sampled and radiocaesium, plutonium and americium concentrations at different depths were determined. The results obtained from these deter- minations are presented in this paper. The observed vertical distribution of activities makes it possible to differentiate the behaviour of radiocaesium, plutonium and americium in the water column.

SAMPLING AND ANALYSIS

Large volume (200-300 litre) water samples were collected in June 1991, from the Catalan Sea, located in the northwestern Mediterranean between the Balearic Islands and the Spanish coast (see Fig. 1). Samples were taken from surface (335 m) and different depths (100 m, 500 m and 1000 m) on board the R.V. Garcia de1 Cid of the Consejo Superior de Investigaciones Cientificas. Surface samples were collected with submersible pumps, while deeper samples were collected using Niskin bottles arranged over an oceanographic ‘Rossette’ with a CTD probe. A combination of samples from different locations was necessary in order to achieve the large volume required for the radionuclide detection. As shown in Fig. 1, samples are representative of open waters in the Catalan Sea. Thus, the contribution of Ebro river inputs and the influence of coastal circulation patterns can be ignored.

Fig. 1. Sampling area in the Catalan Sea (northwestern Mediterranean) at which the water profile (O-l 000 m) was collected.

After addition of caesium and iron carriers, as well as 242Pu and 243Am tracers, the samples were treated on board by radiocaesium, plutonium and americium simultaneous concentration procedure as has been described by Vidal-Quadras et al. (1989). Briefly, plutonium and americium were coprecipitated with iron hydroxides whilst caesium was retained in the supernate. Once concentrated, plutonium and americium were sepa- rated by sorption on consecutive ion-exchange columns and differential elutions with changing acid eluents. To solve interferences, intermediate purification steps were required, involving ion-exchange and organic solvent extraction (Wong, 1971; Murray & Statham, 1976). After puriti- cation, plutonium and americium alpha sources respectively were prepared by electrolytic plating onto stainless steel discs using a procedure based on a method developed by Puphal and Olsen (1972).

Caesium was concentrated from the remaining supernate by scavenging with ammonium molybdophosphate (AMP) as described by Miyake et al.

(1961) and used by us in previous works (Vidal-Quadras et al., 1988, 1989; Sanchez-Cabeza et al., 1990). Once the supernate was discarded, the AMP powder was dried and placed in a geometry suitable for gamma counting. Our studies have established that the scavenging procedure is quantitative (Molero et al., 1993a).

Plutonium-239, 240, 238Pu and 24’ Am determination was carried out by counting for a minimum of lo6 s in alpha spectrometers equipped with low background SSB detectors. Chemical recoveries ranged from 50 to 90% and energy resolution from 20 to 35 keV. In order to determine ‘37Cs, AMP was measured for a minimum of 3 x lo5 s using a HPGe detector surrounded by a 1Ocm copper-cadmium-lead shield and linked to an 8k MCA with spectrum stabilizer. Spectra were analysed by means of a modified version of the code SAMP080 (Koskelo et al., 1981). Detector backgrounds and reagent blanks were monitored regularly. Results obtained in national and international intercomparison exercises showed the reliability of our measurements.

RESULTS AND DISCUSSION

Vertical distribution

The activities and isotopic ratios of 137Cs, 239,240Pu, 238Pu and 24’Am in samples taken from surface to 1000 m depth are given in Table 1. The data showed that radiocaesium concentration decreased quickly through the water column while the concentration of the transuranic elements appeared to increase toward the bottom. Caesium-137 concentration at 1 OOOm depth, 1.80 * 0.15 Bq m-j, represented 40% of the surface levels, 4.4 f 0.2 Bq rnp3, whereas 239,240Pu concentrations below 100m depth, 21 to 26 mBq md3, were about twice those representative of the top layer, 13.9 f 1 .O mBq rnp3. For the 241Am, despite its scarce presence in the Mediterranean waters (about 1 mBq me3), concentrations showed a slight trend to increase with depth, although the uncertainties of measurements at these levels may mask any variation.

Figure 2 shows the vertical distribution of radiocaesium, plutonium and americium concentrations observed in the present work, together with those obtained in 1976 by Fukai et al. (1979) for a location close to the northeastern border of the Gulf of Lyons. Both profiles showed a notable radiocaesium impoverishment through the water column, although concentrations given in the present study appeared to be higher than those reported earlier. Such an increase may be explained as a result of radiocaesium deposition over the northwestern Mediterranean Sea from Cher- nobyl releases in 1986, as has been reported by the Marine Environmental

Verticul distribution of’Cs, Pu and Am in the Cat&n Sea 209

TABLE 1 Radiocaesium, Plutonium and Americium Concentrations in Large Volume Water Samples Collected from Different Depths in June 1991 in the Catalan Sea (Northwestern

Mediterranean)

Depth (WI) Concentration

Surface 100 500

137Cs 239. 240pu 23xPU 24’ Am

(Bqm ‘) (mBqm-“) (mBqmi’) (mBqm_‘)

44 It 0.2 13.9 l I.0 0.80 f 0.14 I.0 f 0.1 4.1 It 0.2 216 i 1.2 1.2 i 0.3 1.1 i 0.3 3.2 f 0.2 21 f2 <2 1.9 f 0.5

I .80 f 0.15 26* 3 <2 I.5 i 0.4

Depth (m) Isotopic ratios

23H PU/ 24’Aml 23y~240P~ (x 10m3)i 24’Am (x 10m4)/ 23Y, z40pu 23Y. 240pu

137cs 137CS

Surface 0.058 f 0.011 0.072 i 0.009 3.2 & 0.3 2.3 f 0.2 100 0,056 i 0.015 0.051 i 0.014 5.3 zt 0.4 3It-1 500 <O.lO 0.09 It 0.02 6.6 f 0.8 6&2

1000 <O,lO 0.06 f 0.02 14f2 8*2

Errors are quoted to * 10.

13’Cs (BQ m-)) 23g240Pu (mBq m-3)

241Am (mBq m-3) 2 4 6 I I

\ \ \ \ \ A+ I I I I I I

* i 1 :F$“,“i;l. (1979) 1

Fig. 2. 13’Cs, 239,240Pu and 24’Am vertical distribution through the water column in the Catalan Sea (northwestern Mediterranean).

Laboratory of the IAEA (MEL-IAEA, 1990) and by ourselves (Molero, in press). In the case of 239. 240Pu and 241Am, results showed that concentrations of both radionuclides have decreased considerably at all depths. In general, transuranic radionuclides showed significant sinking in this area in the period 1976-1991, americium being the one which has undergone the greatest change relative to plutonium. The vertical distribution pattern of plutonium observed in the present work was different from that observed in 1976 by Fukai et al. (1979) where 239,240Pu concentration at 1 OOOm depth was lower than at the surface and a clear subsurface maxi- mum was identified. Higher 239,240Pu concentrations in deep layers observed in the present work could be explained in terms of plutonium resuspension with sediments, as the sampling area included locations placed over the continental slope.

The observed vertical distribution of radiocaesium agreed well with the consideration that this radionuclide remains mainly in solution (Hether- ington & Harvey, 1978; Fukai et al., 1979). As Chernobyl signal was registered at different depths, vertical migration would seem to be observed for this radionuclide. In spite of that, although the water surface layer (O-100m) develops a relatively high mixing grade, radiocaesium penetration to deeper layers is limited by the thermocline inhibiting vertical water exchange. Moreover, other contributions to the radiocaesium vertical migration such as, for example, particle deposition or plankton activity, must be of minimal incidence. Livingston et al. (1979) quantify as less than 0.1% the fraction of ‘37Cs associated with the particulate matter which travels through the water column to the bottom sediments. Thus, the generalized enhancement of radiocaesium levels at different depths should be mainly attributed to the different water mass movements across the Mediterranean basin. As reported by the Marine Environmental Laboratory (MEL-IAEA, 1990) enhanced concentrations of 137Cs were attributed to the penetration into the northwestern Mediterranean of the Levantine Intermediate Water which originates in the eastern Mediterra- nean, where higher amounts of Chernobyl 13’Cs were deposited (Nikitin

et al., 1989). Unlike radiocaesium, transuranic elements have high affinity with

suspended materials and are efficiently scavenged to the bottom sediments (Noshkin & Bowen, 1973; Livingston & Bowen, 1977). The observed decrease in plutonium and americium levels at different depths may therefore be attributed mainly to their preferential association with sinking particulate matter. Studies conducted by the present authors on particulate distribution of plutonium and americium in water samples from the Catalan Sea have shown that the suspended matter was enriched in 24’Am relative to 239. 240Pu by a factor 11 i 2 over the soluble phase (Molero

Vertical distribution cf Cs, Pu and Am in the Catalan Sea 211

clt al., 1993b), in close agreement with results obtained by Holm et al.

(1980). Livingston et al. (1979) also showed that americium transfer rate to the sediments was 4 to 8 times greater than for plutonium in the Mediterranean Sea.

Profile inventories

An estimation of radiocaesium, plutonium and americium water column inventories above 1000 m can be obtained by integrating concentrations at each depth. Assuming a linear behaviour for radiocaesium vertical distribution and a mean value for plutonium and americium concentrations throughout the water column, inventories in the profile result 2.6 f 0.2 kBq m-* for ‘37cs

$41 23 f 3 Bq me2 for 239, 240Pu and

1.5 f 0.5 Bq m-’ in the case of Am. Earlier studies carried out by the

Marine Environment Laboratory on the vertical distribution of these radionuclides in the northwestern Mediterranean have shown that 137Cs and 239,240Pu inventories between 0 and 1 OOOm in the water column are 2.33 & 0.05 kBq me2 and 25.0 f 0.7 Bq m-*, respectively (MEL-IAEA, 1990) in close agreement with our results.

From estimations of radiocaesium, plutonium and americium integrated world-wide fall-out deposition (Hardy et al., 1973; Krey et al.,

1976; UNSCEAR, 1982) around 5.1 kBq m-2 for ‘37Cs, 8 1 Bq me2 for 239. 240Pu, 730 Bq m-* for 241Pu and 25 Bq m-2 for 24’Am were introduced in the Mediterranean Sea until the seventies. In the case of ‘37Cs, radioactive decay correction gave us 3.2 kBq m-2 in the nineties. As 24’Am is also produced by 24’Pu decay, the estimated fall-out in the seventies includes americium deposition plus production by plutonium decay until the late seventies (UNSCEAR, 1982). Taking into account 241Pu decay from the late seventies to the present time, the integrated activity of “‘Am should be 40 Bq me2.

According to the estimated inventory in the profile, about 56% of ‘37Cs and 27% of 239, 240 Pu which were deposited over the Mediterranean Sea by global fall-out, are still present in the first 1 OOOm of the water column. In the case of radiocaesium, surface water measurements showed that about 30% of the estimated inventory in the northwestern Mediterranean corresponds to post-Chernobyl input (MEL-IAEA, 1990; Molero, 1992). Thus, this contribution has been subtracted from the total radiocaesium profile inventory. With regard to americium, results show that only about 4% of the integrated activity which has been estimated to be deposited by world-wide fall-out and produced by 24’Pu decay in Mediterranean waters to date is present in the first 1 000 m of the water column. Results reported by other authors (see Table 2) showed that a large fraction of the activity

TABLE 2 Estimated Fraction of ‘37Cs, 239.240Pu and 24’ Am Deposited over the Mediterranean Area by Radioactive Fall-Out from Weapon Atmospheric Tests which Remains at Present in the

Water Column

Rqference Year Depth (mj Fraction (%)

137cs 23Y. 240pu 241Am”

Livingston et al., 1979 1975 o-2 000 87 81 46 Fukai et al., 1979 1976 o-2 000 70 63 42 This work 1991 O-I 000 56’ 27 4

uFrom 24’Am deposition plus 24’Pu decay. ‘Post-Chernobyl contribution has been subtracted.

deposited over the Mediterranean remained within the water column in the middle seventies. However, our estimations revealed that plutonium and americium levels have been considerably reduced in the first 1000 m water layer.

Isotopic ratios

Vertical distributions of 239,240Pu/137Cs, 241Am/‘37Cs and 241Am/239,240P~ activity ratios are shown in Fig. 3. From this figure it is clear that the separation between transuranics and radiocaesium through the water column as plutonium and americium undergoes a faster migration, while caesium remains mainly solubilized.

Evolution of 241Am/239,240 Pu activity ratio is complex. As 24’Pu decays to 241Am, plutonium sinking rate must be considered to explain americium levels at different depths. Results revealed a significant loss of americium from the water column to the sediments and plutonium penetration to deeper layers. Fukai et al. (1979) reported a 241Am/239,240Pu ratio of 0.053 f 0.012 in surface waters, increasing to 0.3 f 0.05 at 1000 m depth. According to our results, this ratio appears to have held or even slightly increased in surface waters, namely 0.072 f 0.009, whilst the ratio observed at 1000 m depth, namely 0.006 f 0.02, shows a relevant decrease with respect to those reported earlier. The surface waters ratio can be explained as a result of the 241Am ingrowth by 241Pu decay from the seventies to the present time. As we have seen, 24’Pu deposition by generalized fall-out was 10 times greater than 239,240Pu (UNSCEAR, 1982). On the other hand, the decrease in the 241Am/239,240P~ ratio at 1 OOOm depth can be explained in terms of the preferential scavenging of americium relative to plutonium close to the sediments. Analysis of coastal sediments

VerticuI distribution of Cs, Pu and Am in thr Cutalan Seu 213

Fig. 3. Vertical distribution of activity ratios 239~240Pu/‘37Cs, 24’Am/“7Cs and 24’Am/2”,240Pu through the water column in the Catalan Sea (northwestern Mediterra-

nean).

(Molero, in press) has revealed a 24’Am/239,240Pu ratio of 0.32 i 0.10, in agreement with the above consideration.

CONCLUSIONS

On the whole, as reported by several authors, transuranics are efficiently transferred to the sea bed in the Atlantic and Pacific oceans, as well as in more closed areas such as the Irish or the Mediterranean seas. Parti- culate association of transuranics must be considered to be the driving force causing this process. Results discussed above agree with the fact that americium has greater affinity than plutonium for suspended materials. Vertical distribution of radiocaesium and transuranics may reveal, beyond radiological considerations, the fate of soluble and particulate pollutants into the Sea. Diffusion or advection by water must be considered in the first case, while sedimentation and sinking to the bottom sediments must be considered in the second case. Clearly, research on the behaviour of radionuclides is useful in tracing general marine contaminants.

The study of radiocaesium, plutonium and americium concentrations profile (O-l OOOm) has enabled us to understand their behaviour in the marine environment. The main features of the present work may be summarized in the following conclusions.

J. Molero et al.

‘37Cs concentrations undergo a rapid decrease through the water column in such a way that observed surface levels, 4.4 III 0.2 Bq m-j, fall to 1.8 f 0.15 Bq me3 at 1 OOOm depth. Comparing ‘s7Cs levels obtained in this work with those reported by Fukai et al. (1979) in 1976, the input from Chernobyl releases has been detected at different depths. Transuranic concentrations increase with depth through the water column, rising from 13.9 X/Z 1.0 mBq m-3 at surface to 26 f 3 mBq m-’ at 1 000 m depth for 239.240Pu and from 1.0 f 0.1 mBq rnp3 to 1.5 f 0.4 mBq me3 for 24’Am. Comparing transuranic levels obtained in this work with those reported by Fukai et al. (1979) in 1976, the concentrations of plutonium and americium in the Mediterranean water masses have decreased significantly at all depths from the seventies to the present time. Integrated activity in the profile is 3.0 f 0.2 kBqm-* for ‘s7Cs, 23 f 3 Bq rnp2 for 239,240Pu and 1.5 f 0.5 Bq mm2 for 24’Am. According to this estimation, about 56% of ‘37Cs and 27% of 239,240Pu which was deposited over the Mediterranean Sea by generalized fall-out remains at present in the first 1 OOOm of the water column. With regard to americium, results show that only about 4% of the integrated activity which estimated to be deposited and produced in Mediterranean waters to date is present in the first 1 OOOm of the water column. The vertical distribution of 239,240Puj’37Cs and 24’Am/‘37Cs activity ratios at surface, (3.2 & 0.3) x 1O-3 and (2.3 f 0.2) x lop4 respectively, and at 1 OOOm depth, (14 & 2) x lop3 and (8 f 2) x lop4 respectively, showed a clear fractionation between radiocaesium and transuranics through the water column: transuranics have undergone a notable vertical sinking while caesium remains mainly in solution.

24’Am/239. 240 Pu activity ratio observed through the profile has been interpreted, after comparison with results reported previously by other authors, on the basis of the production in situ of 24’Am from 24’Pu decay and the scavenging of americium by particles close to the bottom.

ACKNOWLEDGEMENTS

The authors wish to acknowledge financial support received from the Empresa National de Residuos Radiactivos to develop this work. The

Vertical distribution of Cs, Pu and Am in the Catalan Sea 215

cooperation of the oceanographic research group of the Institut de Ciin- ties de1 Mar de Barcelona (CSIC) on board the R.V. Garcia de1 Cid, particularly that of Dr J. Salat and Dr J. Font, is also gratefully acknowledged.

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Vertical distribution of radiocaesium, plutonium and americium in the Catalan Sea (northwestern...

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