Nota de investigación

 

Zinc concentrations in the water column influenced by the oil spill in the vicinity of the Prestige shipwreck

 

Concentraciones de zinc en la columna de agua influida por el derrame de fuel en los alrededores a la zona de naufragio del Prestige

 

Ricardo Prego^1* and Antonio Cobelo-García¹

 

¹ Marine Biogeochemistry Reseach Group Instituto de Investigaciones Marinas (CSIC) C/ Eduardo Cabello 6 36208 Vigo, España. *E-mail: prego@iim.csic.es

 

Recibida en enero de 2003;
aceptada en febrero de 2003.

 

Abstract

Sixteen days after the Prestige tanker sank in the Northeast Atlantic (42°11' N, 12°02' W), a sampling campaign in the neighbouring water column was carried out in order to evaluate the posibility of zinc contamination due to the oil spill. Samples were taken in the water column (0-190 m) and in bottom waters -in the vicinity of the bow and stern- where the tanker lies (3520-3820 m). Total zinc concentrations found ranged from 14 to 571 nM, which are from one to two orders of magnitude higher than typical concentrations in these waters (0.2-3.2 nM). Therefore, a contamination by this metal, contained in the oil spilt from the tanks of the sunken ship, was detected. Zinc was also found to be a good tracer to detect the presence of fuel in those areas free of patches.

Key words: zinc, oil spill, seawater, Northeast Atlantic Ocean.

 

Resumen

Dieciséis días después del hundimiento del petrolero Prestige en el Atlántico Noreste (zona 42°11' N, 12°02' W) se llevó a cabo una campaña de muestreo en la columna de agua adyacente para evaluar la posible contaminación de zinc debido al fuel derramado al agua. Para ello se tomaron muestras en la columna de agua (0-190 m) y en el fondo marino -en las proximidades de la proa y popa- donde yace el petrolero (3530-3820 m). Las concentraciones totales de zinc obtenidas oscilaron entre 14 y 571 nM, esto es, de uno a dos órdenes de magnitud superiores a las normales en esas aguas (0.2-3.2 nM). Se detecta, pues, una contaminación por este metal contenido en el fuel vertido procedente de los tanques del buque hundido. El zinc resulta, además, un buen trazador para detectar la presencia de fuel en zonas limpias de manchas.

Palabras clave: zinc, derrame de fuel, agua de mar, Océano Atlántico Noreste.

 

Introduction

Zinc is a relatively abundant element in the environment (70 µg g^-1 in the continental crust; Wedepohl, 1991) and is essential for living organisms. Concentrations of zinc in ocean waters range from 0.05 to 9 nM, with an average value of 6 nM (Millero and Sohn, 1992). Contamination by this metal normally occurs in the coastal systems impacted by urban and industrial effluents (Nriagu and Pacyna, 1988) that cause an increase of its concentration in the water column and sediments. This increase has been detected in several points along the Galician coast (Prego and Cobelo-García, 2003). However, zinc contamination in oceanic waters is atypical.

The occurrence of heavy metals in the oil is consubstantial to its origin (up to 0.03% of metals; Marañón, 2000). The presence of metals (including zinc) in the oil spilt off the coast of Kuwait during the Gulf War has been indicted as one of the major causes of the coastal contamination (Bu-Olayan et al., 1998). The unrefined oil transported by the Prestige tanker contains, based on emulsion analysis, approximately 1 ppm of zinc. The satellite photos taken of the ocean surface in the area of the Prestige shipwreck showed several oil patches; for this reason, SASEMAR (Sociedad Española de Salvamento y Seguridad Marítima) planned the PRESTINAUT campaign in order to evaluate, among other matters, the potential contamination of the water column.

 

Material and methods

In the area comprised by the two fragments of the sunken Prestige, 32 water column samples were taken and analyzed for total zinc concentrations. The samples were collected during 4-8 December 2002, both in the water column close to the ocean surface over the vertical of the tanker wreckage and in bottom waters close to where the bow and stern of the tanker lie. The first set of samples was collected from the R/V Atalante, which, as it was assigned to observe the wreckage, only had 200 m of wire suitable for the hydro-graphic bottles (1.7 L; General Oceanics). These bottles were used at four stations to collect water from 0, 5, 10, 20, 50, 100 and 190 m depth. The second set of samples was taken from the Nautile manned submersible, to which a hydrographic bottle was attached. The bottle was closed after sample collection with the articulated arm. Using this procedure, samples were taken at four different immersions, three of them less than 5 m from the tankers and one at 100 m above the bow (table 1).

Immediately after arrival of the hydrographic bottles to the deck of the R/V Atalante, the water samples were taken for the analysis of zinc using new low-density polyethylene bottles (0.5 L). These bottles were previously acid-washed (10% nitric acid) for a week, rinsed (five times) with Milli-Q⁵⁰ (Millipore), filled with Milli-Q⁵⁰ acidified to pH 2 using HNO₃ 65% (Merck Suprapur) and stored in zip-lock plastic bags. Just before taking the samples, the bottles were emptied and rinsed with the sample; once the sample was taken, the bottles were again packed in zip-lock plastic bags and frozen. At the onshore clean laboratory, samples were thawed and acidified, using HNO₃ 65% Merck Suprapur, to pH ~3 and digested for 1 h using a 705 UV Digestor (Metrohm).

The analysis of zinc was carried out by means of differential pulse anodic stripping voltammetry (DPASV), using a 745 VA Trace Analyzer (Metrohm) attached to a 695 Autosampler (Metrohm). All the analytical procedure was undertaken inside a clean laboratory using clean techniques in order to avoid the contamination of samples. Blanks of the analytical procedure were run -one blank every four samples- and results are blank corrected.

The accuracy of the analytical procedure was checked by analyzing two different certified reference materials, CRM-403 (North Sea Water; BCR, Community Bureau of Reference) and CASS-4 (Coastal Seawater; NRC, Canada), obtaining a good agreement with the certified concentrations. Thus, a concentration of [Zn] = 23.4 ± 2.9 nM, n = 4 (certified value [Zn] = 26.4 ± 3.0 nM) for CRM-403 and of [Zn] = 5.35 ± 0.14, n = 3 (certified value [Zn] = 5.83 ± 0.87 nM) for CASS-4 were obtained.

 

Results and discussion

Results of the zinc analysis in the water column at the four stations (A1-A4) are plotted in figure 1. The values obtained are much higher than the zinc concentrations reported for the Northeast Atlantic Ocean; thus, values of total zinc ranging from 0.1 to 3.0 nM were reported (Landing et al., 1995; Ellwood and Van den Berg, 2000) for the water column (0-4000 m depth) with a typical nutrient-type behaviour, that is, a minimum at surface and maximum values at deep waters, always below 190 m.

The results indicate a contamination by zinc (table 2) in the water column. This contamination is maximum at the surface (0-5 m) even at those sampling stations where no oil patches were visible. Typical zinc concentrations in surface waters -dissolved and total- generally range from 0.28 to 1.53 nM (Ellwood and Van den Berg, 2000), whereas the concentrations found in this study range from 160 to 570 nM, that is, two orders of magnitude higher. Below 10 m depth, the zinc concentrations found ranged from 20 to 140 nM, always decreasing with depth, but much higher than the typical oceanic zinc concentrations (0.2-1.6 nM; Landing et al., 1995).

The rubble of the tanker lies at 3530 and 3819 m depth. This depth corresponds to the North-East Atlantic Deep Water (NEADW), which is of homogeneous characteristics both in space and time. The temperature at the wreckage depth is around 2.52-2.58°C and the salinity, 34.89-34.92 (Arhan et al., 1991). Typical zinc concentrations in the NEADW, taking as reference the 3000-4000 m depth in the North Atlantic given by Landing et al. (1995), range from 1.5 to 2.5 nM for dissolved zinc and from 1.5 to 3.2 for total zinc. Therefore, the results obtained in this deep water also show high zinc concentrations; concentrations are lower close to the bow (14 and 59 nM at stations N3 and N4) than close to the stern (66 nM at N2) and also lower than 100 m above (125 nM at N1).

The occurrence of fuel in the samples was analyzed by means of fluorescence (Álvarez-Salgado, pers. comm.), expressed as relative units. The good correlation shown between the fluorescence and the total zinc concentrations can be seen in figure 2. Therefore, compared to other trace metals measured such as copper and lead, zinc is a good tracer for the contamination from the fuel even in those areas where no oil patches are observed. The relationship between fuel and zinc can be expressed by the following equation:

where the fuel concentration in the water is expressed as fluorescence relative units (RU) and the total zinc concentration in nM.

Zinc is a common element in the environment and is essential for living organisms, but it may be toxic to the marine biota at high concentrations (DelValls and Chapman, 1998). In the oceanic surface waters free of oil patches, the zinc concentrations found were much higher than those reported as the toxic threshold for several phytoplankton species, such as the coastal diatom Thalassiosira weissflogii, for which toxic effects have been observed at zinc concentrations higher than 10 nM (Kozelka and Bruland, 1998). However, superior species are more resistant to zinc contamination; in this sense, no adverse effects were observed below concentrations of 10,700 nM (Martín-Díaz, 2002) in living organisms such as Carcinus maenas, Procambarus clarkii or Scrobicularia plana. The concentrations in those areas affected by masses of fuel may be higher than those reported here, and it is necessary to quantify the occurrence of this metal. Nevertheless, it has been shown that considerable concentrations of zinc can be adsorbed by aquatic plants, being passively transported only in the intracellular spaces (Inglett, 1983; Ohnesorge and Wilhelm, 1991); moreover, zinc concentrations in several bivalves and crustaceans can be up to three orders of magnitude higher than the typical (unpolluted) concentrations in seawater (Bockris, 1977).

 

Conclusions

The oil spill from the sunken tanker Prestige has originated a zinc contamination in the surrounding water column. Zinc has increased its concentrations from one to two orders of magnitude with respect to the natural values, reaching the highest concentrations at the ocean surface (190-570 nM). This metal was found to be a good tracer to study the occurrence of fuel in the water column.

 

Acknowledgements

The authors would like to thank the crew of the R/V Atalante and manned submersible Nautile for their cooperation and, particularly, Waldo Redondo for the sample collection; Fiz. F. Pérez for his helpful comments on the PRESTINAUT campaign; X.A. Álvarez-Salgado for the fluorescence data; Daniel Caride for technical assistance; and SASEMAR for letting us work on the campaign. The second author would like to thank the Spanish Ministry of Science and Technology for the financial support (FPI grant).

 

References

Arhan, M., Billand, A., Colin de Verdiere, A., Daniault, N. and Prego, R. (1991). Hydrography and velocity measurements offshore from the Iberian Peninsula. BORD-EST, Data Report. IFREMER, Campagnes Océanographiques Françaises, No. 2 (v. 2), 233 pp.         [ Links ]

Bockris, J.O'M. (1977). Environmental Chemistry. Plenum Press, pp. 461-467.         [ Links ]

Bu-Olayan, A.H., Subrahmanyam, M.N.V., Al-Sarawi, M. and Thomas B.V. (1998). Effects of the Gulf War oil spill in relation to trace metals in water, particulate matter, and PAHs from the Kuwait Coast. Environ. Intern., 24: 789-797.         [ Links ]

DelValls, A. and Chapman (1998). Site-specific sediment quality values for the Gulf of Cádiz (Spain) and San Francisco Bay (USA), using the sediment quality Triad and Multivariate Analysis. Ciencias Marinas, 24: 313-336.         [ Links ]

Ellwood, M.J. and Van den Berg, C.M.G. (2000). Zinc speciation in the Northeastern Atlantic Ocean. Mar. Chem., 68: 295-306.         [ Links ]

Inglett, G.E. (1983). Nutritional Bioavailability of Zinc. American Chemical Soc., Symp. Ser. 210pp.         [ Links ]

Kozelka, P.B. and Bruland, K.W. (1998). Chemical speciation of dissolved Cu, Zn, Cd, Pb in Narragansett Bay, Rhode Island. Mar. Chem., 60: 267-282.         [ Links ]

Landing, W.M., Cutter, G.A., Dalziel, J.A., Flegal, A.R., Powell, R.T., Schmidt, D., Shiller, E.A., Statham, P., Westerlund, S. and Resing, J. (1995). Analytical intercomparison results from the 1990 Intergovernmental Oceanographic Commision open-ocean baseline survey for trace metals: Atlantic Ocean. Mar. Chem., 49: 253-265.         [ Links ]

Marañón, E. (2000). La polución de los mares. En: J.L. Ruiz (ed.), Temas de Química Oceanográfica. Univ. Cádiz-UNED, pp. 189-250.         [ Links ]

Martín-Díaz, M.L. (2002). Utilización de biomarcadores como indicadores de efectos tóxicos producidos por Zn, Cd y Cu disueltos a concentraciones ambientales sobre Carcinus maenas. Tesis de licenciatura, Universidad de Cádiz.         [ Links ]

Millero, F.J. and Sohn, M.L. (1992). Minor elements of seawater. In: Chemical Oceanography. CRC, pp. 115-156.         [ Links ]

Nriagu, J.O. and Pacyna, J.M. (1988). Quantitative assessment of worldwide contamination of air, water and soils by trace metals. Nature, 333: 134-139.         [ Links ]

Ohnesorge, F.K. and Wilhelm, M. (1991). Zinc. In: E. Merian (ed.), Metals and their Compounds in the Environment (Part II). VCH, Weinheim, pp. 1309-1342.         [ Links ]

Prego, R. and Cobelo-García, A. (2003). Twentieth Century overview of heavy metals in the Galician Rias (NW Iberian Peninsula). Environ. Pollut., 121: 425-452.         [ Links ]

Wedepohl, K.H. (1991). The composition of the upper Earth's crust and the natural cycles of selected metals. Metals in raw materials. Natural resources. In: E. Merian (ed.), Metals and their Compounds in the Environment (Part I). VCH, Weinheim, pp. 3-17.         [ Links ]