Presence of uranium and plutonium in marine ... - Springer Link

J Radioanal Nucl Chem (2013) 298:981–986 DOI 10.1007/s10967-013-2496-9

Presence of uranium and plutonium in marine sediments from gulf of tehuantepec, mexico E. Ordoń˜ez-Regil • M. G. Almazań-Torres • J. A. Sanchez-Cabeza • A. C. Ruiz-Fernańdez

Received: 21 January 2013 / Published online: 13 April 2013 Ó Akade´miai Kiado´, Budapest, Hungary 2013

Abstract Uranium and plutonium were determined in the Tehua II-21 sediment core collected from the Gulf of Tehuantepec, Mexico. The analyses were performed using radiochemical separation and alpha spectroscopy. Activity concentrations of alpha emitters in the sediment samples were from 2.56 to 43.1 Bq/kg for 238U, from 3.15 to 43.1 Bq/kg for 234U and from 0.69 to 2.95 Bq/Kg for 239?240 Pu. Uranium activity concentration in marine sediment studied is generally high compared with those found in sediments from other marine coastal areas in the world. The presence of relatively high concentrations of anthropogenic plutonium in the sediments from the Gulf of Tehuantepec suggests that anthropogenic radionuclides have been incorporated and dispersed into the global marine environment. Keywords Uranium Plutonium Alpha spectrometry Marine sediments Gulf of tehuantepec Upwelling

E. Ordoń˜ez-Regil M. G. Almazań-Torres (&) Instituto Nacional de Investigaciones Nucleares, Carretera Me´xico Toluca s/n, La Marquesa, c.p. 52750 Ocoyoacac, Mexico, Mexico e-mail: [email protected] J. A. Sanchez-Cabeza Departamento de Fı´sica, Facultat de Ciencies, Universitat Auto`noma de Barcelona, 08193 Bellaterra, Spain A. C. Ruiz-Fernańdez Instituto de Ciencias del Mar y Limnologıá, Universidad Nacional Autońoma de Me´xico, U. Acade´mica Mazatlań, Calz. J. Montes Camarena s/n, Playa Sur, 82040 Mazatlań, Sinaloa, Mexico

Introduction The presence of uranium and plutonium at relatively high concentrations in the natural environment has motivated several studies in order to assess the potential environmental impacts. Uranium is a naturally occurring element that can be found in low levels within igneous rock, soil, and water. Uranium in soil and rocks is distributed throughout the environment by wind, rain and geologic processes. The uranium is leached from rocks, soils and sediments, after which the waters containing the element enter lakes and rivers. The concentration of uranium in each natural material is not homogeneous: in minerals of igneous rocks, uranium is found in concentrations of parts per million whereas the infiltration waters of these materials hardly attain concentrations of parts per billion [1, 2]. Uranium is frequently found in marine sediments, especially those rich in organic matter, phosphate, or both [3]. Some studies suggest that most of the uranium in marine sediments has been derived from sea water [4, 5]. Naturally occurring uranium consists of three isotopes: 238U, 235U and 234U. In a closed system, 234U and 238U are in secular equilibrium (234U/238U = 1.0). Due to weathering, the content of 234U may vary slightly in the nature; therefore the isotopes 234U and 238U are found frequently in radioactive disequilibrium. This disequilibrium is greater in seawater than in terrestrial materials. 234U/238U activity ratio in seawater is *1.15 whereas in rocks, soils and sediments the ratio is B1.0. Plutonium isotopes are produced artificially by bombardment of uranium with neutrons through a series of neutron capture and radioactive decay reactions. Plutonium-239 is naturally occurring in extremely small amounts by neutron capture in uranium mineral [6]. The major concentrations of plutonium in the environment are

123

982

due to human activities. The main sources of anthropogenic plutonium are the atmospheric testing of nuclear weapons, nuclear accidents, and releases from fuel reprocessing plants [7–10]. The North Pacific and Atlantic oceans have been the major repository for these releases [11, 12]. Knowledge of the behavior and distribution of radionuclides in the marine environment has been important for a variety of issues including study of oceanic process and environmental impacts [13]. The presence of anthropogenic radionuclides into the seawater has generally had a more local character; however, soluble radionuclides have been transported over long distances by prevailing currents. The distribution of radionuclides in the marine environment has been examined elsewhere e.g. Sanchez–Cabeza and Molero [14], Lee et al. [12], Mahmood et al. (2010) [15]. However, the majority of these studies have been carried out at the sites close to the contamination sources (fallout from nuclear weapons test, nuclear accidents and radioactive discharges). Hence, the Gulf of Tehuantepec is an interesting site to assess natural and anthropogenic radionuclide dispersion in the marine environment due to its location away from sources of radioactive contamination and nuclear facilities. The Gulf of Tehuantepec is located in a region where the Intertropical Convergence Zone is displaced longitudinally every year, resulting in large seasonal meteorological and oceanographic fluctuations. The oceanographic fluctuation promotes water mass mixing and upwelling processes, which in turn support a high biogenic productivity [16]. The anaerobic activity in this region is also important due to its influence on precipitation of Fig. 1 Sampling area (Gulf of Tehuantepec, Mexico at 15°59.9870 N and 94°48.4690 W, at 66.7 m depth)

123

J Radioanal Nucl Chem (2013) 298:981–986

dissolved minerals like those of uranium. These minerals, which are heterogeneously distributed in the coast through fluvial sediments and soils and dispersed by streams, accumulate at the base of the continental slope. Thus, radionuclide content in marine sediments is enriched by continuous deposition of radionuclide from seawater [5]. The aim of this investigation was to obtain information on the presence and concentration levels of uranium and plutonium in the Tehua II-21 sediment core collected in the continental shelf of the Gulf of Tehuantepec, Mexico.

Materials and methods Study site and sample collection The Gulf of Tehuantepec is located in the Mexican Tropical Pacific, approximately between 14°300 –16°120 N and 92°000 –96°000 W (Fig. 1). It extends 500 km from Puerto Angel, in southern Oaxaca State, southeastward to Barra del Suchiate, in southeastern Chiapas State, and has a mouth approximately 160 km wide. A strong, gale-force wind called the Tehuano, periodically blows out over the waters of the Gulf of Tehuantepec, inducing strong upwelling of nutrient-rich waters which support abundant sea life [16, 17]. The sediment core Tehua II-21 (18 cm long) was extracted from the Gulf of Tehuantepec, Mexico (15°59.9870 N and 94°48.4690 W, water depth *66.7 m) in October 2004. Small portions at 0.3 and 1 cm intervals were taken from this core and analyzed. Details of


983

Fig. 2 Alpha spectrum of a uranium separated from leachable fraction and b uranium and plutonium separated from residual fraction

extraction and preparation of sediment samples are described by Ruiz-Fernandez et al. [18]. Analytical methods Radiochemical separation and electroplating Six samples extracted from different sections along the sediment core sample were analyzed. Analytical method consisted of radiochemical separation coupled with electrodeposition. This method has been considered one of the most effective ways to separate and purify several actinides and lanthanides from natural samples [4, 15, 19, 20]. Radiochemical separation was executed by partition chromatographic method using di-(2-Ethyl-Hexil) phosphoric acid (D2EHPA) as stationary phase [21, 22]. All sediment samples were digested in 50 mL of 6 M HCl and 0.1 M NH4Cl under continues agitation during 48 h. Residual mineral was separated by filtration and dissolved in a mixture of concentrated acids (nitric, hydrochloric and hydrofluoric acid). Both leachable and residual fractions were concentrated using a water bath until evaporation was nearly completed. The dry residue was recovered with 10 mL of concentrated hydrochloric acid and diluted until HCl concentrations were between 0.1 and 0.6 M. This solution was passed through a chromatography column containing Teflon granules impregnated with D2EHPA. At this stage, uranium, iron and plutonium were retained on the stationary phase. Iron and uranium fractions were sequentially eluted with diluted and concentrated hydrochloric acid from the leachable fraction [21]. Plutonium and uranium were eluted with hydrofluoric acid from residual fraction. The respective eluate was then evaporated until incipient dryness. The extract was dissolved in 50 mL of 0.1 M NaF solution and

pH adjusted to 8 and then placed in the electrolytic cell. The thin sources for alpha spectrometry were prepared by electrodeposition of alpha emitters on 304 mirror polished stainless steel discs for 24 h at 15 mA. Finally, the discs were removed and rinsed with deionized water and allowed to dry air. The yield of the extraction process for alpha emitters was 70 ± 5 %. The alpha spectra obtained from some thin sources are presented in Fig. 2. Alpha spectrometry The activities of the alpha emitters were measured using a dual alpha spectrometer 576A, EG&G ORTEC, with rough detectors of 300 mm2 and residual pressure of 10-3 Torr. The spectra were collected in a multichannel analyzer spectrum master 9191 EG&G ORTEC. The thin sources were counted for about a week. Spectra were analyzed using Maestro-32 for windows MCA emulation software. Detectors were calibrated with a triple source of 239Pu (Ea = 5.16 MeV, 241Am (Ea = 5.49 MeV) and 244Cm (Ea = 5.81 MeV). The full width at half-maximum (FWHM) resolution for 239Pu peak was 25 keV and detector counting efficiency was 25 %.

Results and discussion The results of measurements of uranium and plutonium concentration in Tehua II-21 sediment samples and the calculated 234 238 U/ U and 239?240Pu/238U activity ratios are summarized in Table 1. The results of activity concentrations are given with standard deviation (SD) calculated for 95 % confidence intervals. Due to the complex composition of marine sediment samples, the resulting uranium concentration is an average

123

984


Table 1 Activity concentrations of uranium and plutonium in Tehua II-21 sediment core Depth (cm)

238

234

U (Bq/kg)

U (Bq/kg)

239?240

Pu (Bq/kg)

234

U/238U ratio

239?240

Pu/238U ratio

0.6

2.56 ± 0.13

3.15 ± 0.16

2.95 ± 0.15

1.23 ± 0.06

1.15 ± 0.06

0.9

2.98 ± 0.15

3.07 ± 0.15

2.13 ± 0.11

1.03 ± 0.05

0.71 ± 0.04

1.2

0.82 ± 0.04

0.81 ± 0.04

0.92 ± 0.05

0.99 ± 0.05

1.12 ± 0.06

2.1

1.62 ± 0.08

1.44 ± 0.07

0.19 ± 0.01

0.89 ± 0.04

0.11 ± 0.01

6.6

6.25 ± 0.31

6.01 ± 0.30

0.23 ± 0.01

1.03 ± 0.05

0.03 ± 0.001

15.2

43.1 ± 2.16

43.24 ± 2.16

0.69 ± 0.03

1.00 ± 0.05

0.016 ± 0.001

over the uranium of both leaching and residual fractions. The total 234U and 238U activity concentrations ranged from 0.81 to 43.2 Bq/kg and 0.82 to 43.1 Bq/kg, respectively and 239?240Pu activity concentrations ranged from 0.19 to 2.95 Bq/kg. Table 2 shows the ranges of uranium and plutonium activity concentrations in marine sediments from other marine sites of the world. The maximum value of 238U activity concentration in marine sediments from nonupwelling areas varies only a few Bq/kg [4, 23]. However, the difference between the values found in upwelling regions is 30 and 50 Bq/Kg [24, 25]. This can be explained by the fact that within upwelling areas, a relatively high flux of particulate organic matter can easily make the regions suboxic or anoxic environments; thus, uranium shows distinctly non-conservative behavior [24]. The maximum value of 239?240Pu activity concentration in the marine sediments we studied is high compared to that found in sediments from regions relatively close to sources of radioactive pollution [23, 26, 27], and similar to that found in sediments from Dabob Bay in the eastern subarctic Pacific Ocean [28]. Our value is a little low

compared to that found in slightly polluted sediments from Palomares [29]. It is considerably lower than found in more highly polluted sediments from the same region [30]. Uranium and plutonium concentration profiles depend on depth. See Fig. 3a, b, respectively. The lowest concentrations of uranium were found in the top layers of the sediment core (0–2 cm). The concentration of uranium increases with depth. That is because uranium enters

Table 2 Comparison of uranium and plutonium activity concentration in sediments from other coastal marine sites of the world Region

238

U (Bq/Kg)

Reference

North sea

5.1–10.6

[23]

Red sea

2.7–13.4

[4]

Benguela current

5.3–10.7

[24]

Concepcion bay

26–101

[25]

0.8–43.1

Present work

Gulf of Tehuantepec Region

239?240

Reference

Palomares, Spain

3.3–5.3

[29]

Palomares, Spain

0.26–50.4

[30]

Mediterranean sea

0.05–0.6

[26]

North sea

0.14–0.41

[23]

Pu (Bq/Kg)

Dabob Bay, Washington

1.75–2.86

[28]

Gulf of Mexico

0.0003–0.18

[27]

Gulf of Tehuantepec

0.69-2.95

Present work

123

Fig. 3 Activity concentration profiles of a uranium and b plutonium in marine sediments and activity ratios profiles of c 238U/234U and d 239?240Pu/238U


seawater primarily from underwater geologic formations and it has been constantly deposited on the seafloor. In contrast, the highest concentrations of plutonium were found in the top layer of the sediment core (0–0.6 cm). The plutonium concentration decreases with depth. This behavior is also expected because plutonium has entered the oceans primarily from the atmospheric testing of nuclear weapons. 234 U/238U activity ratio profile is shown in Fig. 3c. A 234 high U/238U activity ratio of 1.23 was found in the upper layer of sediment sample. The isotopic disequilibrium observed between uranium-238 and its daughter uranium234 can be explained by the coastal ocean movement in the Gulf of Tehuantepec, which promotes the dispersion of soluble radionuclides. The 234U/238U activity ratio value in the lower layers was close or equal to 1.00 (Fig. 3c), which is characteristic of unchanged systems like rocks and minerals. The 239?240Pu/238U activity ratios ranged from 1.15 near the surface to 0.016 with increasing depth (Fig. 3d). The minimum value is several thousand orders of magnitude higher than the lowest value of 10-12 found in uranium minerals [6]. These results therefore suggest that the presence of plutonium in the sediment samples is certainly due to global radioactive contamination caused by nuclear weapon testing and nuclear accidents, particularly in the North Pacific Ocean. Although the Gulf of Tehuantepec is located relatively far away from radioactive pollution sources (i.e. nuclear power plants), this study shows that anthropogenic radionuclides have been globally dispersed into the marine environment through their transport by oceans currents [9–11].

Conclusions Uranium and plutonium in marine sediments from the Gulf of Tehuantepec have been separated and measured by using radiochemical separations and alpha spectrometry. Uranium concentration in the sediment core Tehua II-21 is generally high compared with those found in marine sediments from other regions in the world, although this appears to be influenced by upwelling events. The relatively high concentration of plutonium in the sediment samples studied suggests that anthropogenic radionuclides resulting from radioactive pollution have been globally dispersed in the marine environment by oceanic currents. Vertical profiles of concentrations and activity ratios indicate the presence of natural uranium and the recent incorporation of anthropogenic plutonium in marine sediments from the Gulf of Tehuantepec. To advance in the study of anthropogenic radionuclide dispersion in the environment, additional studies on the presence and concentration of radionuclides in sediment samples from other

985

coasts and lagoons of Mexico and their environmental impact are required.

References 1. Chadima SA (1989) Generalized potential for radon emission based on estimated uranium content in geologic rock units. Science Center, University of Dakota, Madison, p 6 2. Dostrovsky I (1988) Energy and the missing resource: a view from the Laboratory. Pres Syndicate of the University of Cambridge, New York 3. Kolodny Y, Kaplan R (1970) Uranium isotopes in sea-floor phosphorites. Geochim Cosmochim Acta 34:3–24 4. Sam AL, Ahamed MMO, El Khangi FA, El Nigumi YO (2000) Uranium and thorium isotopes in some red sea sediments. Radiochim Acta 883:307–312 5. Veeh H (1967) Deposition of uranium from the ocean. Earth Planet Sci Lett 3:145–150 6. Essien IO (1991) Contamination of the earth’s surface by plutonium and uranium fallout. J Radioanal Nucl Chem 147:269–275 7. Mitchell PI, Sanchez-Cabeza JA, Ryan TP, McGarry AT, VidalQuadras A (1990) Preliminary estimates of cumulative caesium and plutonium deposition in the Irish Terrestrial Environment. J Radioanal Nucl Chem 138:241–256 8. Mitchell PI, Leoń Vintro´ L, Dahlgaard H, Gasco´ C, Sanchez-Cabeza JA (1997) Perturbation in the 240Pu/239Pu global fallout ratio in local sediments following the nuclear accidents at Thule (Greenland) and Palomares (Spain). Sci Total Environ 202:147–153 9. Strok M, Smodis B, Petrinec B (2010) Natural radionuclides in sediments and rocks from Adriatic Sea. J Radioanal Nucl Chem 286:303–308 10. Zheng J, Liao H, Wu F, Yamada M, Fu P, Liu C, Wan G (2008) Vertical distributions of 239?240Pu activity and 240Pu/239Pu atom ratio in sediment core of Lake Chenghai SW China. J Radional Nucl Chem 275(1):37–42 11. IAEA (2005) Worldwide marine radioactivity studies (WOMARS) radionuclide in oceans and seas. IAEA-TECDOC-1429 12. Lee SH, Gastauda J, Povinec PP, Hong GH, Kim SH, Chung CS, Lee KW, Petterson HBL (2003) Distribution of plutonium and americium in the marginal seas of the northwest Pacific Ocean. Deep-Sea Res II 50:2727–2750 13. Ivanovich M (1994) Uranium series disequilibrium: concepts and applications. Radiochim Acta 64:81–94 14. Sanchez-Cabeza JA, Molero J (2000) Plutonium, americium, and radiocaesium in the marine environment close to the Vandello´s I nuclear power plant before decommissioning. J Environ Radioact 51:211–228 15. Mahmood ZUW, Shahar H, Ahmad Z, Wo YM, Bakar ASA (2010) Behavior and distribution of 239?240Pu and 241Am in the east coast of Peninsular Malaysia marine environment. J Radioanal Nucl Chem 286:265–272 16. Molina-Cruz A, Martıńez-Lo´pez M (1994) Oceanography of the Gulf of Tehuantepec, Mexico, indicated by Radiolarian remains. Palaeogeogr Palaeoclimateol Palaeoecol 110(3–4):179–195 17. Tapia-Garcia M, Garcia-Abad MC, Carranza-Edward A, Vazquez-Gutierrez F (2007) Environmental characterization of the continental shelf of the Gulf of Tehuantepec. Mexico Geofı´s Int 46(4):249–260 18. Ruiz-Fernandez AC, Hillaire-Marcel C, de Vernal A, MachainCastillo ML, Va´squez L, Ghaleb B, Aspiazu-Fabiań JA, PaézOsuna F (2009) Changes of coastal sedimentation in the Gulf of Tehuantepec, South Pacific Mexico, over the last 100 years from short-lived radionuclide measurements. Est Coast Shelf Sci 82:525–536

123

986 19. Aoyama M, Hirose K (2008) Radiometric determination of anthropogenic radionuclide in seawater. Radioact Environ 11:137–162 20. Harrison JJ, Zawadzki A, Chisari R, Wong HKY (2011) Separation and measurement of thorium, plutonium, americium, uranium and strontium in environmental matrices. J Environ Radioact 102:896–900 21. Ordoń˜ez-Regil E (1986) Preparacioń fuentes finas de uranio para la Espectroscopıá alfa. Rev Soc Quim Mex 30(2):84–88 22. Rout A, Venkatesan KA, Srinivasan TG, Vasudeva Rao PR (2010) Unusual extraction of plutonium(IV) from uranium(VI) and americium(III) using phosphonate based task specific ionic liquid. Radiochim Acta 98:459–466 23. Nedwell D, Parkes S, Upton A, Assinder DJ (1993) Seasonal fluxes across the sediment-water interface, and processes within North Sea sediments. Philos Trans R Soc Lond A 343:519–529 24. Yu EF, Liang ChH, Chen MT (1999) Authigenic in marine sediments of the benguela current upwelling region during the last glacial period. TAO 10(1):201–214 25. Farıás L, Salamanca MA, Chuecas LA (2000) 238U removal and accumulation in Concepcioń bay sediments. Rev Geol Chile v 27:241–254

123

J Radioanal Nucl Chem (2013) 298:981–986 26. Sanchez-Cabeza JA, Masque´ P, Ani-Ragolta I, Merino J, Frignan M, Alvisi F, Palanques A, Puig P (1999) Sediment rates in the southern Barcelona continental margin (NW Mediterranean Sea) derived from210Pb and 137 Cs chronology. Prog Oceanogr 44:313–332 27. Schott MR, Salter PF, Halverson JE (1983) Transport and deposition of plutonium in the ocean: evidence from Gulf of Mexico sediments. Earth Planet Sci Lett 63:202–222 28. Carpenter R (1981) Plutonium and americium in anoxic marine sediments: evidence against remobilization. Geochim Cosmochim Acta 45:1917–1930 29. Antoń MP, Gasco C, Sanchez-Cabeza JA, Ll Pujol (1994) Geochemical association of plutonium in marine sediments from palomares (Spain). Radiochim Acta 66(67):443–446 30. Chamizo E, Jimeńez-Ramos MC, Enamorado SM, Garcıá-Leoń M, Garcıá-Tenorio R, Mas JL, Masque´ P, Merino J, SańchezCabeza JA (2010) Characterisation of the plutonium isotopic composition of a sediment core from Palomares, Spain, by lowenergy AMS and alpha spectrometry. Nucl Instrum Methods Phys Res Sect B 268:1273–1276