- Email: [email protected]
127I ratios and 129I concentrations in a recent sea sediment core and in rainwater from Sevilla (Spain) by AMS
Nuclear Instruments and Methods in Physics Research B 172 (2000) 574±578
129
www.elsevier.nl/locate/nimb
I/127 I ratios and 129 I concentrations in a recent sea sediment core and in rainwater from Sevilla (Spain) by AMS
J.M. L opez-Gutierrez
a,*
, M. Garcõa-Le on a, R. Garcõa-Tenorio e, Ch. Schnabel M. Suter d, H.-A. Synal c, S. Szidat b
b,1
,
a
b
Dpto. de Fõsica At omica, Molecular y Nuclear, Universidad de Sevilla, Apdo. 1065, 41080 Sevilla, Spain Zentrum fur Strahlenschutz und Radio okologie (ZSR), Universit at Hannover, Am Kleinen Felde 30, D-30167 Hannover, Germany c Paul Scherrer Institut, ETH H onggerberg, CH-8093 Z urich, Switzerland d Institute of Particle Physics, ETH H onggerberg, CH-8093 Z urich, Switzerland e Dpto. de Fõsica Aplicada, ETS Arquitectura, Avda. Reina Mercedes, s/n, 41013 Sevilla, Spain
Abstract In spite of the environmental relevance of 129 I, there is still a scarcity of data about its presence in the dierent natural compartments. In this work, results are presented on the concentration of 129 I in rainwater samples taken in Sevilla (southwestern Spain) and in a sediment core taken near the Ringhals coast (Sweden). Typical concentrations of 108 and 109 129 I at/l are found in rainwater samples, similar to other values in literature. In the case of the sediment core, our results clearly show the impact of anthropogenic sources, with concentrations in the order of 1013 129 I at./kg and isotopic ratios 129 I/127 I in the order of 10ÿ8 in the higher layers. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 29.30.)h; 06.60.Ei; 89.60.+x; 92.60.Jq Keywords: Accelerator mass spectrometry; Iodine-129; Sample preparation; Sediment; Rainwater
1. Introduction 129 I is a long-lived T1=2 15:7 106 yr cosmogenic radionuclide for which the natural abundances have been altered in a signi®cant way by human action. Pre-nuclear 129 I/127 I ratios (in the
*
Corresponding author. Tel.: +34-954-55-2891; fax: +34954-55-2890. E-mail address: [email protected] (J.M. LoÂpez-GutieÂrrez). 1 Present address: Institute of Particle Physics, ETH H onggerberg, CH-8093 Z urich, Switzerland.
order of 10ÿ13 and 10ÿ12 [1]) have been raised to values between 10ÿ10 and 10ÿ5 [2±5] by the arti®cial nuclear activities (bomb tests, power plants and especially nuclear fuel reprocessing plants). Due to its physical and chemical properties, 129 I is very important from the environmental point of view [1±3]. In spite of that, data on the presence of 129 I in nature are scarce, especially in the background zones. In this paper, we try to contribute to the increasing of the 129 I data archive by its measurement in rainwater from Sevilla (Spain) and in a sea sediment core from Sweden. The results will give information about the large scale and long
0168-583X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 0 ) 0 0 1 0 4 - X
J.M. L opez-Gutierrez et al. / Nucl. Instr. and Meth. in Phys. Res. B 172 (2000) 574±578
term impact of the dierent anthropogenic sources in areas located far away from them.
129
I
2. AMS measurement AMS measurements have been carried out at the PSI/ETH AMS facility in Zurich. Ions are extracted from a cesium sputter ion source and an initial mass and energy analysis is performed by an electrostatic (90°) and a magnetic (90°) de¯ector before the ion beam is injected into the accelerator. The selected terminal voltage for 129 I is 4.7 MV in a 6 MV EN tandem. After acceleration, charge state 5+ is selected in a 15° electrostatic de¯ector. Final mass analysis is carried out in a 90° magnetic de¯ector. The 127 I5 current is measured in a Faraday Cup behind this de¯ector. The detection system consists of a time-of-¯ight (TOF) spectrometer and a DE±E ionization chamber. This chamber is necessary to avoid interferences from 128 Te5 ions entering the accelerator as 128 TeHÿ and from other molecular fragments with the same TOF as 129 I. The TOF spectrometer has been recently developed especially for 129 I and consists on two TOF detectors separated by 1.1 m [6]. The typical time resolution is about 400 ps, which is enough to discriminate 129 I from 128 Te, even if the number of 128 Te is some orders of magnitude higher than 129 I counts. The ionization chamber anode is divided into two parts in order to give information about both energy and energy loss and separate 129 I from the lighter molecular fragments with the same TOF. 3. Sampling and radiochemical methods 3.1. Rainwater Samples were collected regularly at the roof of the Faculty of Physics in Sevilla (37.4°N, 6°W) with a circular funnel, 1 m2 surface, and stored in 25 l plastic bottles. After collection, water is acidi®ed in order to avoid the formation of microorganisms and the hydrolysis cations which could modify the original concentration in the sample.
575
Approximately 4 mg Woodward iodine carrier are added to 1 l of rainwater. The solution is turned to alkaline pH by the addition of NaOH and iodine is reduced by NaHSO3 . The sample is then stored in the dark for 1 day. After that, the solution is acidi®ed with 7 M HNO3 and solid NaNO2 is added. Iodine is then extracted into chloroform in a separatory funnel and back-extracted into aqueous solution afterwards by the addition of NaHSO3 and H2 SO4 . These puri®cation steps are repeated twice. Finally, AgI is precipitated by the addition of AgNO3 , mixed with Ag powder and pressed in a pure tantalum target for AMS measurement. 3.2. Sediment: isotopic ratios The sediment core was taken at the coasts of Ringhals (Sweden), in the Kattegat area (57°400 3500 N, 11°240 0400 E). This core was sampled in 1984, i.e., before the Chernobyl accident, at about 96 m depth and 20 km from the coast. Its total length was 17 cm and it was divided into 1cm sections. 137 Cs concentration was determined in each section by high-resolution c-spectrometry [7]. A combustion method was used for the determination of the 129 I/127 I isotopic ratio. Some 15±40 g sediment are introduced into two quartz bottles connected by a small quartz tube. The bottles are placed into a small oven. Their entrances are connected with the outside of the oven by other two quartz tubes. One of them is used to inject O2 and N2 into the bottles. N2 is necessary to avoid violent reactions inside the bottles by diluting O2 . The gases resulting from the combustion of the sample escape through the second tube, which ®nishes into 80 ml of a 0.3 M NaHSO3 solution at laboratory temperature. This solution traps iodine by reducing it to iodide, which is a non-volatile chemical form. After pre-combustion (1 h at 300°C) to burn the organic matter, the O2 current is maintained for 3 h at 1100°C. After combustion, an aliquot from the trapping solution is measured by ion chromatography in order to know the total iodine concentration. Approximately 3 mg of carrier is added to the rest of the solution. The subsequent chemical process is identical to that described in Section 3.1:
576
J.M. L opez-Gutierrez et al. / Nucl. Instr. and Meth. in Phys. Res. B 172 (2000) 574±578
extraction in chloroform, back-extraction in aqueous solution and precipitation. The AMS measurements gives the amount of 129 I in the solution. 3.3. Sediments: concentrations In this case, an alkaline leaching method is used [8]. 6 g of NaOH, 10 mg of iodide carrier (Woodward) in 1 ml water and 0.3 mg of iodate carrier prepared from sodium iodate (Fluka, puriss., 129 I/127 I 2 10ÿ13 ) in 200 ll water are added to 2 g of sediment in a nickel crucible. Then, alkaline leaching is carried out for 1 h at 150°C, then for 2 h at 200°C followed by 3 h at 275°C. The melt is extracted by water. NaHSO3 and H2 SO4 are added to it and the solution is centrifuged. After that, the previously described puri®cation and precipitation schemes are followed. 4. Results and discussion 4.1. Rainwater Fig. 1 shows the results obtained for the 129 I concentration in rainwater from Sevilla during the studied periods: January±July 1996 and January± December 1997. Results range from 4:7 107 129 I at./l (19.2%) in January 1996 to 4:97 109 129 I at./l
Fig. 1.
129
(5.9%) in April 1996, with the exception of one sample that presented a specially high concentration of 6:02 1010 129 I at./l (1.4%) also in January 1996. It is dicult to compare these results to others due to the scarcity of data on 129 I, especially in places far away from the direct impact of nuclear facilities or accidents like Sevilla. Typical 129 I concentrations in rainwater in these zones have traditionally been found to be in the order of 107 and 108 at./l [9±12]. However, concentrations found in recent measurements in Europe range from 108 to 1010 129 I at./kg [13,14], being typically over 109 at./kg. As commented before, the concentrations measured in Seville are typically in the order of 108 and 109 129 I at./kg, in good agreement with these results, that correspond to zones far away from the direct impact of nuclear facilities. Our results also agree with the generalized idea that an increase in the 129 I concentration in rainwater is taking place during the last years, probably caused by the large reprocessing plants emissions. 4.2. Sediment core 127
The results for the 129 I concentration and 129 I/ I ratio in the sediment core are depicted in Fig. 2.
I depositions (at./m2 d) and concentrations (at./kg) in rainwater from 1996 to 1997 taken at Sevilla (Spain).
J.M. L opez-Gutierrez et al. / Nucl. Instr. and Meth. in Phys. Res. B 172 (2000) 574±578
Both the values increase from the bottom layers to the top ones, showing the evident impact of anthropogenic sources. In Fig. 2, the 129 I concentration pro®le is also compared to the results for 137 Cs, measured by cspectrometry in 1986 [7]. The general pattern of the two curves is the same, re¯ecting that very likely the presence of 129 I and 137 Cs in the sediment is caused by the same source. The only measurements of 129 I in sediments we have found in literature were carried out at Cape Hatteras [15] and some points at the American Paci®c coast [16]. In all cases, highest ratios are found in the top layers, reaching values in the order of 10ÿ11 and 10ÿ12 , respectively. They are several orders of magnitude lower than those presented in this work, what reveals the strong impact of several anthropogenic sources in our
577
sediment. In fact, the high 129 I/127 I ratios and 129 I concentrations in this sediment compared to others show that 129 I coming from reprocessing plants must be present in an important proportion in this sediment. This is a very likely fact if the geographical situation of the sampling point is considered. Low concentrations and isotopic ratios under 13±14 cm could reveal a pre-anthropogenic origin of these bottom layers, although some migration of the 129 I must have taken place (ratios are in the order of 10ÿ10 ). An increase of these magnitudes can be appreciated in 13±14 cm, possibly corresponding with the beginning of nuclear weapons tests in 1954, followed by a stable concentrations and ratios zone. The values corresponding to layer 1±2 cm is very high compared to those is the second zone. This increase cannot be explained by the eect of nuclear weapons tests exclusively, and it must be caused by the arrival of 129 I from the nuclear fuel reprocessing plants of Sella®eld and La Hague to this area. Indeed, Aarkrog et al. [17] calculated that the transit time of the seawater from these reprocessing plants to the place this sediment was taken is approximately 3 yr. These results con®rm the fact that 129 I can be used as a tracer of marine water movements, which has already been shown by other authors [2]. It is clear that a more precise interpretation of the results must include migration processes of the radionuclides along the sediment length. 5. Conclusions
Fig. 2. Depth pro®le distribution of the 137 Cs concentration, 129 I concentration and 129 I/127 I ratio in a sea sediment core from the coasts of Ringhals (Sweden).
The capacity of 129 I for tracing the movement of marine waters has been con®rmed by the measurement of 129 I concentrations and 129 I/127 I ratios in a sediment core from the Kattegat area (Sweden). The results have shown an important impact of the residues from the nuclear fuel reprocessing plants of Sella®eld and La Hague that are transported to the sampling place. The comparison between results obtained recently in Europe and those measured some years ago in rainwater probably demonstrates the impact of the 129 I released by the nuclear fuel reprocessing plants, which is transported to places far from these facilities by geological transport
578
J.M. L opez-Gutierrez et al. / Nucl. Instr. and Meth. in Phys. Res. B 172 (2000) 574±578
processes. This transport is probably leading to a quite fast homogenization of the 129 I releases from reprocessing plants in the Northern Hemisphere. Acknowledgements J.M. L opez-Gutierrez is deeply indebted to the Spanish Ministerio de Educaci on y Cultura, the Andalousian Consejerõa de Educaci on y Ciencia and to the Institute of Particle Physics of the ETHZurich and the Paul Scherrer Institut for the ®nancial support of this work and his stays at ETH. This work has been partially ®nanced by the project PB 98-1114 of the Spanish Government.
References [1] J. Fabryka-Martin, S.N. Davis, Nucl. Instr. and Meth. B 29 (1987) 361. [2] F. Yiou, G.M. Raisbeck, Z.Q. Zhou, L.R. Kilius, Nucl. Instr. and Meth. B 92 (1994) 436. [3] J. Handl, E. Oliver, D. Jakob, K.J. Johanson, P. Schuller, Health Phys. 65 (1993) 265. [4] L.R. Kilius, J.C. Rucklidge, C. Soto, Nucl. Instr. and Meth. B 92 (1994) 393.
[5] E. Robens, D.C. Aumann, J. Environ. Radioact. 7 (1988) 159. [6] J.M. L opez-Gutierrez, M. Garcõa-Le on, Ch. Schnabel, A. Schmidt, R. Michel, H.-A. Synal, M. Suter, Appl. Radiat. Isotopes 51 (1999) 315. [7] C.I. S anchez, R. Garcõa-Tenorio, M. Garcõa-Le on, J.M. Abril, F. El-Dahoushy, Nucl. Geophys. 6 (3) (1992) 395. [8] A. Schmidt, Ch. Schnabel, J. Handl, D. Jakob, R. Michel, H.-A. Synal, J.M. Lopez, M. Suter, The Science of the Total Environment 223 (1998) 131. [9] M. Paul, D. Fink, G. Hollos, A. Kaufman, W. Kutschera, M. Margaritz, Nucl. Instr. and Meth. B 29 (1987) 341. [10] M.J.M. Wagner, B. Dittrich-Hannen, H.-A. Synal, M. Suter, U. Schotterer, Nucl. Instr. and Meth. B 113 (1996) 490. [11] Y. Muramatsu, Y. Ohmomo, The Science of the Total Environment 48 (1986) 33. [12] Y. Muramatsu, Y. Ohmomo, and M. Sumiya, International Conference on Methods and Applications of Radioanalytical Chemistry, 5±10 April 1987, Kona, Hawaii. [13] H.-A. Synal, Personal communication. [14] G. Krupp, D.C. Aumann, J. Environ. Radioact. 46 (1999) 287. [15] U. Fehn, G.R. Holdren, D. Elmore, T. Brunelle, R. Teng, P.W. Kubik, Geophys. Res. Lett. 13 (1) (1986) 137. [16] J.E. Moran, U. Fehn, R.T.D. Teng, Chem. Geo. 152 (1998) 193. [17] A. Aarkrog, L. Botter-Jensen, H. Dahlgaard, H. Hansen, J. Lippert, S.P. Nielsen, K. Nilsson, Riso National Laboratory Report, Riso-R-421-1980.
129
www.elsevier.nl/locate/nimb
I/127 I ratios and 129 I concentrations in a recent sea sediment core and in rainwater from Sevilla (Spain) by AMS
J.M. L opez-Gutierrez
a,*
, M. Garcõa-Le on a, R. Garcõa-Tenorio e, Ch. Schnabel M. Suter d, H.-A. Synal c, S. Szidat b
b,1
,
a
b
Dpto. de Fõsica At omica, Molecular y Nuclear, Universidad de Sevilla, Apdo. 1065, 41080 Sevilla, Spain Zentrum fur Strahlenschutz und Radio okologie (ZSR), Universit at Hannover, Am Kleinen Felde 30, D-30167 Hannover, Germany c Paul Scherrer Institut, ETH H onggerberg, CH-8093 Z urich, Switzerland d Institute of Particle Physics, ETH H onggerberg, CH-8093 Z urich, Switzerland e Dpto. de Fõsica Aplicada, ETS Arquitectura, Avda. Reina Mercedes, s/n, 41013 Sevilla, Spain
Abstract In spite of the environmental relevance of 129 I, there is still a scarcity of data about its presence in the dierent natural compartments. In this work, results are presented on the concentration of 129 I in rainwater samples taken in Sevilla (southwestern Spain) and in a sediment core taken near the Ringhals coast (Sweden). Typical concentrations of 108 and 109 129 I at/l are found in rainwater samples, similar to other values in literature. In the case of the sediment core, our results clearly show the impact of anthropogenic sources, with concentrations in the order of 1013 129 I at./kg and isotopic ratios 129 I/127 I in the order of 10ÿ8 in the higher layers. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 29.30.)h; 06.60.Ei; 89.60.+x; 92.60.Jq Keywords: Accelerator mass spectrometry; Iodine-129; Sample preparation; Sediment; Rainwater
1. Introduction 129 I is a long-lived T1=2 15:7 106 yr cosmogenic radionuclide for which the natural abundances have been altered in a signi®cant way by human action. Pre-nuclear 129 I/127 I ratios (in the
*
Corresponding author. Tel.: +34-954-55-2891; fax: +34954-55-2890. E-mail address: [email protected] (J.M. LoÂpez-GutieÂrrez). 1 Present address: Institute of Particle Physics, ETH H onggerberg, CH-8093 Z urich, Switzerland.
order of 10ÿ13 and 10ÿ12 [1]) have been raised to values between 10ÿ10 and 10ÿ5 [2±5] by the arti®cial nuclear activities (bomb tests, power plants and especially nuclear fuel reprocessing plants). Due to its physical and chemical properties, 129 I is very important from the environmental point of view [1±3]. In spite of that, data on the presence of 129 I in nature are scarce, especially in the background zones. In this paper, we try to contribute to the increasing of the 129 I data archive by its measurement in rainwater from Sevilla (Spain) and in a sea sediment core from Sweden. The results will give information about the large scale and long
0168-583X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 0 ) 0 0 1 0 4 - X
J.M. L opez-Gutierrez et al. / Nucl. Instr. and Meth. in Phys. Res. B 172 (2000) 574±578
term impact of the dierent anthropogenic sources in areas located far away from them.
129
I
2. AMS measurement AMS measurements have been carried out at the PSI/ETH AMS facility in Zurich. Ions are extracted from a cesium sputter ion source and an initial mass and energy analysis is performed by an electrostatic (90°) and a magnetic (90°) de¯ector before the ion beam is injected into the accelerator. The selected terminal voltage for 129 I is 4.7 MV in a 6 MV EN tandem. After acceleration, charge state 5+ is selected in a 15° electrostatic de¯ector. Final mass analysis is carried out in a 90° magnetic de¯ector. The 127 I5 current is measured in a Faraday Cup behind this de¯ector. The detection system consists of a time-of-¯ight (TOF) spectrometer and a DE±E ionization chamber. This chamber is necessary to avoid interferences from 128 Te5 ions entering the accelerator as 128 TeHÿ and from other molecular fragments with the same TOF as 129 I. The TOF spectrometer has been recently developed especially for 129 I and consists on two TOF detectors separated by 1.1 m [6]. The typical time resolution is about 400 ps, which is enough to discriminate 129 I from 128 Te, even if the number of 128 Te is some orders of magnitude higher than 129 I counts. The ionization chamber anode is divided into two parts in order to give information about both energy and energy loss and separate 129 I from the lighter molecular fragments with the same TOF. 3. Sampling and radiochemical methods 3.1. Rainwater Samples were collected regularly at the roof of the Faculty of Physics in Sevilla (37.4°N, 6°W) with a circular funnel, 1 m2 surface, and stored in 25 l plastic bottles. After collection, water is acidi®ed in order to avoid the formation of microorganisms and the hydrolysis cations which could modify the original concentration in the sample.
575
Approximately 4 mg Woodward iodine carrier are added to 1 l of rainwater. The solution is turned to alkaline pH by the addition of NaOH and iodine is reduced by NaHSO3 . The sample is then stored in the dark for 1 day. After that, the solution is acidi®ed with 7 M HNO3 and solid NaNO2 is added. Iodine is then extracted into chloroform in a separatory funnel and back-extracted into aqueous solution afterwards by the addition of NaHSO3 and H2 SO4 . These puri®cation steps are repeated twice. Finally, AgI is precipitated by the addition of AgNO3 , mixed with Ag powder and pressed in a pure tantalum target for AMS measurement. 3.2. Sediment: isotopic ratios The sediment core was taken at the coasts of Ringhals (Sweden), in the Kattegat area (57°400 3500 N, 11°240 0400 E). This core was sampled in 1984, i.e., before the Chernobyl accident, at about 96 m depth and 20 km from the coast. Its total length was 17 cm and it was divided into 1cm sections. 137 Cs concentration was determined in each section by high-resolution c-spectrometry [7]. A combustion method was used for the determination of the 129 I/127 I isotopic ratio. Some 15±40 g sediment are introduced into two quartz bottles connected by a small quartz tube. The bottles are placed into a small oven. Their entrances are connected with the outside of the oven by other two quartz tubes. One of them is used to inject O2 and N2 into the bottles. N2 is necessary to avoid violent reactions inside the bottles by diluting O2 . The gases resulting from the combustion of the sample escape through the second tube, which ®nishes into 80 ml of a 0.3 M NaHSO3 solution at laboratory temperature. This solution traps iodine by reducing it to iodide, which is a non-volatile chemical form. After pre-combustion (1 h at 300°C) to burn the organic matter, the O2 current is maintained for 3 h at 1100°C. After combustion, an aliquot from the trapping solution is measured by ion chromatography in order to know the total iodine concentration. Approximately 3 mg of carrier is added to the rest of the solution. The subsequent chemical process is identical to that described in Section 3.1:
576
J.M. L opez-Gutierrez et al. / Nucl. Instr. and Meth. in Phys. Res. B 172 (2000) 574±578
extraction in chloroform, back-extraction in aqueous solution and precipitation. The AMS measurements gives the amount of 129 I in the solution. 3.3. Sediments: concentrations In this case, an alkaline leaching method is used [8]. 6 g of NaOH, 10 mg of iodide carrier (Woodward) in 1 ml water and 0.3 mg of iodate carrier prepared from sodium iodate (Fluka, puriss., 129 I/127 I 2 10ÿ13 ) in 200 ll water are added to 2 g of sediment in a nickel crucible. Then, alkaline leaching is carried out for 1 h at 150°C, then for 2 h at 200°C followed by 3 h at 275°C. The melt is extracted by water. NaHSO3 and H2 SO4 are added to it and the solution is centrifuged. After that, the previously described puri®cation and precipitation schemes are followed. 4. Results and discussion 4.1. Rainwater Fig. 1 shows the results obtained for the 129 I concentration in rainwater from Sevilla during the studied periods: January±July 1996 and January± December 1997. Results range from 4:7 107 129 I at./l (19.2%) in January 1996 to 4:97 109 129 I at./l
Fig. 1.
129
(5.9%) in April 1996, with the exception of one sample that presented a specially high concentration of 6:02 1010 129 I at./l (1.4%) also in January 1996. It is dicult to compare these results to others due to the scarcity of data on 129 I, especially in places far away from the direct impact of nuclear facilities or accidents like Sevilla. Typical 129 I concentrations in rainwater in these zones have traditionally been found to be in the order of 107 and 108 at./l [9±12]. However, concentrations found in recent measurements in Europe range from 108 to 1010 129 I at./kg [13,14], being typically over 109 at./kg. As commented before, the concentrations measured in Seville are typically in the order of 108 and 109 129 I at./kg, in good agreement with these results, that correspond to zones far away from the direct impact of nuclear facilities. Our results also agree with the generalized idea that an increase in the 129 I concentration in rainwater is taking place during the last years, probably caused by the large reprocessing plants emissions. 4.2. Sediment core 127
The results for the 129 I concentration and 129 I/ I ratio in the sediment core are depicted in Fig. 2.
I depositions (at./m2 d) and concentrations (at./kg) in rainwater from 1996 to 1997 taken at Sevilla (Spain).
J.M. L opez-Gutierrez et al. / Nucl. Instr. and Meth. in Phys. Res. B 172 (2000) 574±578
Both the values increase from the bottom layers to the top ones, showing the evident impact of anthropogenic sources. In Fig. 2, the 129 I concentration pro®le is also compared to the results for 137 Cs, measured by cspectrometry in 1986 [7]. The general pattern of the two curves is the same, re¯ecting that very likely the presence of 129 I and 137 Cs in the sediment is caused by the same source. The only measurements of 129 I in sediments we have found in literature were carried out at Cape Hatteras [15] and some points at the American Paci®c coast [16]. In all cases, highest ratios are found in the top layers, reaching values in the order of 10ÿ11 and 10ÿ12 , respectively. They are several orders of magnitude lower than those presented in this work, what reveals the strong impact of several anthropogenic sources in our
577
sediment. In fact, the high 129 I/127 I ratios and 129 I concentrations in this sediment compared to others show that 129 I coming from reprocessing plants must be present in an important proportion in this sediment. This is a very likely fact if the geographical situation of the sampling point is considered. Low concentrations and isotopic ratios under 13±14 cm could reveal a pre-anthropogenic origin of these bottom layers, although some migration of the 129 I must have taken place (ratios are in the order of 10ÿ10 ). An increase of these magnitudes can be appreciated in 13±14 cm, possibly corresponding with the beginning of nuclear weapons tests in 1954, followed by a stable concentrations and ratios zone. The values corresponding to layer 1±2 cm is very high compared to those is the second zone. This increase cannot be explained by the eect of nuclear weapons tests exclusively, and it must be caused by the arrival of 129 I from the nuclear fuel reprocessing plants of Sella®eld and La Hague to this area. Indeed, Aarkrog et al. [17] calculated that the transit time of the seawater from these reprocessing plants to the place this sediment was taken is approximately 3 yr. These results con®rm the fact that 129 I can be used as a tracer of marine water movements, which has already been shown by other authors [2]. It is clear that a more precise interpretation of the results must include migration processes of the radionuclides along the sediment length. 5. Conclusions
Fig. 2. Depth pro®le distribution of the 137 Cs concentration, 129 I concentration and 129 I/127 I ratio in a sea sediment core from the coasts of Ringhals (Sweden).
The capacity of 129 I for tracing the movement of marine waters has been con®rmed by the measurement of 129 I concentrations and 129 I/127 I ratios in a sediment core from the Kattegat area (Sweden). The results have shown an important impact of the residues from the nuclear fuel reprocessing plants of Sella®eld and La Hague that are transported to the sampling place. The comparison between results obtained recently in Europe and those measured some years ago in rainwater probably demonstrates the impact of the 129 I released by the nuclear fuel reprocessing plants, which is transported to places far from these facilities by geological transport
578
J.M. L opez-Gutierrez et al. / Nucl. Instr. and Meth. in Phys. Res. B 172 (2000) 574±578
processes. This transport is probably leading to a quite fast homogenization of the 129 I releases from reprocessing plants in the Northern Hemisphere. Acknowledgements J.M. L opez-Gutierrez is deeply indebted to the Spanish Ministerio de Educaci on y Cultura, the Andalousian Consejerõa de Educaci on y Ciencia and to the Institute of Particle Physics of the ETHZurich and the Paul Scherrer Institut for the ®nancial support of this work and his stays at ETH. This work has been partially ®nanced by the project PB 98-1114 of the Spanish Government.
References [1] J. Fabryka-Martin, S.N. Davis, Nucl. Instr. and Meth. B 29 (1987) 361. [2] F. Yiou, G.M. Raisbeck, Z.Q. Zhou, L.R. Kilius, Nucl. Instr. and Meth. B 92 (1994) 436. [3] J. Handl, E. Oliver, D. Jakob, K.J. Johanson, P. Schuller, Health Phys. 65 (1993) 265. [4] L.R. Kilius, J.C. Rucklidge, C. Soto, Nucl. Instr. and Meth. B 92 (1994) 393.
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