Artículos

 

Presence, distribution and contamination levels of lead in the surface sediments of the Ria of Ferrol (NW Spain)

 

Presencia, distribución y niveles de contaminación de plomo en el sedimentos superficiales de la Ría de Ferrol (NO de España)

 

Ricardo Prego¹*, Antonio Cobelo-García¹, Carmen Tubío¹ and M^a del Carmen Barciela²

 

¹ Marine Biogeochemistry Research Group, Instituto de Investigaciones Marinas (CSIC), Spain. *E-mail: prego@iim.csic.es

² Departamento de Química Analítica, Facultad de Química, Universidad de Santiago de Compostela, Spain.

 

Recibido en octubre de 2001;
aceptado en octubre de 2002.

 

Abstract

This article discuss, using sediment samples from the Ria of Ferrol, the need for a better understanding of the biogeochemical cycle of lead in the Galician rias which, until now, was based on differing methodologies mainly applied to the biota in only few areas of the most important rias from a socio-economic point of view. Thirty-five samples of superficial sediment were analysed for lead (in the < 63 µm and total fractions) and opal percentage and organic carbon in the total. An analytical method is proposed employing ETAAS, whereby the sediment is introduced directly as a slurry together with the optimised amount of chemical modifiers. The accuracy of the method was checked by means of a certified reference material, obtaining a sensitivity of 2.72 µgg^-1 Pb with an RSD of 5.9%.

Values of lead in the superficial sediment of the Ria of Ferrol ranged from 13.5 to 159.1 µg g^-1 for the fraction < 63 µm, and from 12.1 to 142.5 µg g^-1 for the total fraction. The distribution does not follow a decreasing trend from the innermost part of the ria, as may have been expected, but rather the maximum values were found in the vicinity of the harbour area in the middle ria, indicating that rias behave as a lead deposition barrier avoiding the export of the metal to the open sea. Lead dispersion is mainly controlled by the tidal currents, given its lowest concentrations in the zones of strongest current, and also by the plankton, following the relationship obtained [Pb] = 48 [opal] + 16. A baseline lead concentration of 18.2 µg g^-1 was obtained for the fraction < 63 µm using core samples dated at 3745 ± 70 years before present. Applying this background values, the lead enrichment in the Ria of Ferrol is classified from moderate to considerable.

Key words: lead, contamination, sediment, ria, Galicia.

 

Resumen

El artículo aborda, a partir del sedimento de la Ría de Ferrol, la necesidad de un mayor conocimiento del ciclo biogeoquímico del plomo en las rías gallegas, basado hasta este momento en metodologías dispares aplicadas primordialmente a la biota en zonas puntuales de las rías más importantes socioeconómicamente. Se partió de 35 muestras de sedimento superficial, en cuya fracción total se analizaron la concentración de plomo, los porcentajes de ópalo y el carbono orgánico, mientras que en la fracción < 63 µm sólo se midió el plomo. Para estas muestras se propuso un procedimiento analítico basado en la espectrometría de absorción atómica con atomización en cámara de grafito, donde el sedimento en forma de suspensión se introdujo directamente junto con la cantidad optimizada de modificadores químicos. La exactitud del método se verificó mediante el uso de un material de referencia, obteniéndose una sensibilidad de 2.72 µg g^-1 de Pb y una precisión de RSD 5.9 %. Los valores medidos para el plomo en el sedimento superficial de la Ría de Ferrol se encontraron entre 13.5 y 159.1 µg g^-1 para la fracción menor de 63 µm y de 12.1 a 142.5 µg g^-1 para la total. La distribución no sigue en patrón decreciente desde el fondo de la ría, como era de esperar, sino que tiene su máximo en el área portuaria situada en la parte media de la ría, indicando que las rías deben actuar como una barrera de deposición de plomo. En la dispersión de este metal intervienen principalmente la marea, dada su menor concentración en zonas de mayor corriente, y el plancton, por la relación: [Pb] = 48 [ópalo] + 16. Se ha cuantificado un valor de fondo de 18.2 µg g^-1 de Pb para la fracción sedimentaria < 63 µm con muestras fechadas con 3745 ± 70 años de antigüedad. Aplicando éste, los factores de enriqueciminento para la Ría de Ferrol permiten clasificar su contaminación por plomo entre moderada y considerable

Palabras clave: plomo, contaminación, sedimento, ría, Galicia.

 

Introduction

Lead is present in nature at low concentrations except for localized areas generally under anthropogenic influence. The important increase in lead consumption since 1945 (Metallstatistik, 1988) has affected the coastal systems, where this metal is dumped from industries settled along the coastline and transported by continental waters (Gordeev, 1986) and the atmosphere (Chow, 1978). This has lead to an increase in the flux of this element to the environment around 20 times higher than two centuries ago (Murozumi et al., 1969) and to an important change in the biogeochemical cycle of lead in an important number of coastal systems (Nriagu, 1978; Branica and Konrad, 1980). The sources of estuarine contamination come from both organometallic compounds in leaded gasolines and from inorganic lead species, mainly as a result of petroleum and coal combustion, batteries and chemical pigments (Robinson, 1978). The most important inorganic lead species is the Pb(II) cation that forms insoluble oxides and sulphates, and precipitates as sulfur compounds in organic-rich areas, such as the Galician rias (Prego, 1993). The importance of lead in the marine environment is due to its high toxicity (Taylor, 1981) because small amounts of this metal may cause important alterations in marine organisms, which can become chronic (USEPA, 1980). For several decades this potential danger has driven important scientific work regarding lead and the ways to control it in the environment (GESAMP, 1985).

The studies on lead in the Galician coast began 40 years ago; however, following the review by Prego and Cobelo-García (2003), around 30 papers are only available up till now about this metal. The presence of lead in the biota has focused special interest due to the exceptional conditions of the rias for aquaculture (Prego, 1995). The research carried out has been mainly restricted to the biggest and most important rias from the socioeconomic point of view (Prego and Cobelo-García, 2003). Therefore a substantial better knowledge of lead distribution in the rias is required, namely normalization of sampling and the fractions studied, unification or intercalibration of the methods so the results can be comparable and, finally, focus on the sedimentary biogeochemical compartment to asses its contamination state.

The Ria of Ferrol is a good example for applying the aforementioned because this ria was previously studied, mainly regarding its biological characteristics (Lueiro and Prego, 1999) and, although an environmental awareness already existed before 1988, little work had been carried out (table 1). It was after 1993 and, more recently, during the 1997-1999 period when the state of contamination of the Ria of Ferrol was considered, mainly due to the catastrophe originated by the sinking of the Aegean Sea tanker in the neighboring Ártabro Gulf (Ros, 1996).

This paper aims, in the one hand, to contribute to the general knowledge of the biogeochemical cycle of lead in estuaries, particularly in rias for which there is an important lack of information and, on the other hand, to show a procedure for the treatment of sediment samples in the Galician coast and to evaluate, for the first time in the Ria of Ferrol, the presence of lead.

 

Study area

The Ria of Ferrol (fig. 1) shows the typical extension for the Rias Altas, that is, those rias located north of the Finisterre Cape: 15 km long and a 21-km² area. This ria was classified by Torre-Enciso (1958) among the "funnel type" rias, since it corresponds to the low catchment area of a river flooded by the sea, in which fluvial erosion plays a major role whereas the lithological and tectonic factors are less important. The sediment contained in the ria basin is of continental and marine origin, the latter derived from the abundant biological activity in its waters (Lueiro and Prego, 1999). The granulometric plots show a predominance of fine sands in the mouth and the channel of the ria, poor in organic matter (1.7-2.5%), whereas in the rest of the ria there is fine mud with organic content ranging from 3.7 to 13.2% (López-Jamar et al., 1996).

The main fluvial input to the Ria of Ferrol is the Grande de Xubia River, flowing into the ria head with a mean annual flow of 5.5 m³ s^-1 (Río and Rodríguez, 1992) with the typical fluctuations of an oceanic rain pattern (Soto and Díaz-Fierros, 1996). This small continental input is the main drive of the positive estuarine residual circulation that is generally observed in this ria (Varela et al., 1996), which is controlled by the tidal currents.

The anthropogenic activities (IGE, 1995) and most of the industrial and urban influence is located in the middle-inner part of the ria, where not only most of the population is concentrated (143,000 inhabitants) but also the important industrial activity including shipyards, iron and steel industries and a commercial port with a mean annual vessel traffic around 1,500,000 gross register tons (IDASA, 1987).

 

Material and methods

The sediment samples were taken from the R/V Mitylus and its auxiliary boat Zoea at the sampling stations shown in figure 1, using two Van Veen grabs of different size. From the collected sediment only the 1-cm-thick surface layer was taken, using a polyethylene spatula, and stored in polyethylene vials hermetically closed (previously acid-washed using 10% nitric acid for 24 h and rinsed with Milli-Q ultrapure water) and were left at 4°C in a fridge pending drying. The sediment, dried-up using an oven at a temperature lower than 50°C, was fractioned by means of plastic sieves with 2000 and 63-µm nylon meshes. The different fractions were stored in plastic tubes (pre-cleaned using the same procedure as for the vials) until analysis. Analysis was carried out in the < 63-µm fraction and the sediment size fraction between 63 and 2000 µm. The metal concentration in the total fraction was calculated using the percent of every size fraction obtained by weighing the sieved sediment.

The determination of lead in the samples was carried out by means of atomic absorption spectroscopy with electrothermal atomization (ET-AAS) using a Varian 220 device equipped with Zeeman background correction; the source of radiation was a hollow cathode lamp operating at 10 mA; the wavelength used was 283.3 nm and a slit of 0.5 nm. Pirolitic graphite tubes with L'vov platform were used for the sample atomization. The absorbance measurements were done using the peak area with a 3-s integration time.

In order to avoid sample contamination and analyte loss due to volatilization or to retention in insoluble materials associated with the digestion procedures, sediment slurries were prepared as sample pre-treatment. This procedure avoids the sample digestion and considerably reduces the time required for the analysis. The slurries were prepared using the method proposed by Bermejo Barrera et al. (1994): a 0.1-g aliquot of of sediment < 63-µm grain size is weighed and put in a polyethylene vial together with 5 g of zirconium pearls and 3 mL Milli-Q ultrapure water. The content of the vial is homogenized using a Vibromatic shaker for 60 min; then the zirconium pearls are removed by means of a porcelain sieving funnel. Finally, the slurry obtained is made up to 10 mL and stored at 4°C pending analysis. For the sediment fraction > 63 µm, after ground in an agate mortar, the same procedure was applied.

For every sample two slurries were prepared and two analyses were carried out for each slurry, i.e., four analyses were done per sample. If the results differed more than 5%, a third slurry was prepared and two more analyses were carried out.

Just before the analysis, Titron X-100 (0.1%) was added to a sample slurry aliquot as an stabilizing agent, together with the magnesium nitrate and palladium required as chemical modifiers. The optimum measurement conditions and the modifiers' concentrations required, 2 µg mL^-1 Pd and 15 µg mL^-1 Mg(NO₃)₂, were previously optimized and adapted to the device used and the type of sediment analyzed. The optimized temperature program is indicated in table 2. The sample volume spiked into the graphite tube was 20 µL.

The accuracy of the analytical procedure was checked using a certified reference material (PACS-1 of the National Research Council of Canada; [Pb] = 404 ± 20 µg g^-1). The results obtained, 409 ± 24 µg g^-1, agree well with the certified values, showing the accuracy of the measurements carried out. Sensitivity of the analytical procedure was checked as well, calculating the detection limit for 10 determinations of a slurry blank sample, being this 2.72 µg g^-1 of Pb. The precision of the method (RSD 5.9%) was also checked, by means of calibration with aqueous standards.

The determination of the amount of opal contained in the surface sediments was carried out following the method of alkaline digestion by Mortlock and Froelich (1989) and the following analysis of dissolved silicate in the obtained extract using the method reported by Hansen and Grasshoff (1983).

The organic carbon content in the surface sediments was determined by the difference between total carbon -measured in a Perkin Elmer elemental CNH analyzer- and inorganic carbon -analyzed by calcination loss between 550 and 975-. The Unidad de Servicios Generales de Apoyo a la Investigación at the Universidad de La Coruña carried out these organic carbon analyses.

 

Results and discussion

The lead concentration found in the surface sediment of the Ria of Ferrol ranged from 13.5 to 159.1 µg g^-1 in the fine (< 63 µm) fraction (fig. 2) and from 12.1 to 142.5 µg g^-1 in the total fraction (fig. 3). The scarce data previously reported (table 1) lie within this range, excep for those belonging to intertidal areas, which are slightly higher. In comparison with the values found in other Galician rias, reaching Pb concentrations sometimes higher than 400 µg g^-1 (Prego and Cobelo-García, 2003), lead concentrations excessively high are not observed in the Ria of Ferrol. The concentrations are more similar to the values found in oceanic basins (8-60 µg g^-1; Nriagu, 1978) than to those reported for estuaries under a strong anthropogenic influence where lead concentrations up to 1450 µg g^-1 were found in harbor sediments (Standford et al., 1981).

In order to define the importance of the metallic contaminants it has to be considered that the high degree of water exchange in the rias (Prego and Fraga, 1992) reduces the interest of using data of dissolved Pb in the water column. Sediments, due to their inherent temporal stability and higher contamination levels -because of the metal accumulation-, are more appropriate to evaluate the state of heavy metals in the rias (Carballeira et al., 1997). The marked adsorbing and transport capacities of the the fine fraction (< 63 µm) related to metallic traces allow a good comparison of the values obtained for the sediments of the Galician coast (Guerrero et al., 1988; Carral et al., 1995); moreover, the data available for the fraction smaller than 2 µm or 63 µm in Galician estuaries do not show any perceptible differences in the lead concentrations (Belzunce et al., 1997a). As a consequence, the use of the fraction < 63 µm is advisable as a start point to discuss the lead concentration levels in the Ria of Ferrol, and also to contrast their situation with other estuaries (Fõrsner, 1980).

Therefore, in the present paper the fraction < 63 µm was chosen as a reference. Nevertheless, when comparing the results obtained for this fine fraction with the total fraction (fig. 3), a straight line with a slope value near 1 (0.99) with a good correlation (r = 0.91) is obtained. This indicates that the use of both sedimentary fractions to study the lead concentration in the Ria of Ferrol would be correct and, furthermore, a different accumulation behavior of this metal is not likely with respect to the grain size in this ria.

Lead distribution in the sediment

The isoline map of lead concentration for the surface sediments in the Ria of Ferrol clearly indicates a maximum in the area surrounded by the harbor (150-µg-g^-1 isoline, fig. 2) and two secondary maxima in the inner part of the ria (90 µg g^-1, fig. 2). On the contrary, lead concentrations are lower in those areas where tidal currents are stronger: the inner part in the vicinity of a channel (40 µg g^-1, fig. 2) and in the mouth of the ria (30-40 µg gT¹, fig. 2). These values are close to those measured in the same sedimentary fraction for the neighboring continental shelf (21-33 µg gT¹; Guerrero et al., 1988).

Although an inner fluvial pattern is assumed for the lead concentrations in the Galician rias (see Herbello and Prego, 1998), this can not be taken as a general rule. Despite the previously reported data for intertidal sediments also pointed to the same assumption for the Ria of Ferrol (Carballeira et al., 1997), following the horizontal lead distribution (fig. 2) there is an asymmetry towards its northern margin in the middle part of the ria, which is a consequence of the anthropogenic inputs. Something similar was also reported for the Ria of Vigo in the harbor area located at the middle-southern margin (> 150 µg g^-1; Besada et al., 1997; Belzunce et al., 2000), but in this ria lead values are even higher in its head (Belzunce et al., 1997b). Therefore, inside the rias as well as in other estuaries (Ewers and Schlipköter, 1991), most of the lead inputs generally occur directly from freshwater currents and this metal tends to be located in the surrounding area of discharge points, due to the low solubility of the lead compounds in seawater. Consequently rias, which are mixing areas of fresh and saline waters with high primary production and strong sedimentation, can be considered as sedimentation traps for lead, which accumulates in the vicinity of the discharge source points.

The physical factors participating on the dispersion of lead inside the Ria of Ferrol are controlled by the tidal circulation. The fact that lead concentration is not likely to depend on the sediment fraction (fig. 3) suggests that lower lead concentrations in several parts of this ria are caused by: (a) the "cleansing" of the sediment by the tidal current-transported water in the inner part of the ria; and (b) this along with a suspended particulate matter exchange with the shelf -which is poor in lead- in the mouth of the ria.

There are also other biogeochemical processes that take part, within which plankton plays an important role. The good correlation found between lead concentrations and percent of opal in the surface sediments ([Pb] = 48 [ópalo] + 16; r = 0.88) suggests that, particularly, diatoms can be a substantial mechanism in scavenging lead to the sediment. In this sense, the scattering of data obtained in the plot of lead versus organic carbon (fig. 4) does not indicate any correlation of lead with the urban and industrial organic matter but with the organic matter with a C:N ratio similar to a phytoplankton-rich sediment (fig. 4).

Diatoms can concentrate up to 47 µg g^-1 of Pb (dry weight) in coastal areas (Martin and Knauer, 1973), and therefore can be good direct or indirect -by means of zooplankton faeces (Fowler, 1977; Reinfelder and Fisher, 1991)- pathways in scavenging lead to the sediments of the Ria of Ferrol.

Lead contamination in the ria

The lead concentrations in the fraction < 63 µm and the corresponding background value were taken to ascertain the point sources of contamination in the ria. To this aim the enrichment factors (EFs) were calculated as the ratio between the concentration of the current sample and the background value.

The lead background values (BV) available to date for the Galician coast were estimated by modal analysis from intertidal surface sediment samples (Carral et al., 1995). Using this approach, BVs of 78 µg g^-1, for rias of granitic lithology, and 50 µg g^-1 for rias of schist-gneiss lithology were reported (Carral et al., 1995).

In order to calculate the EF in the Ria of Ferrol, the determination of BV taking pre-industrial values obtained from dated sediment cores was chosen. For this, it is usual to use as background reference value the oldest layers (Establier et al., 1985) and therefore the samples between 130- and 170-cm depth were collected from a core taken using a "vibro-corer" in the central part (in the vicinity of station 9, fig. 1). A background value (BV) of 18.2 µg g^-1 was obtained, which corresponds to 3745 ± 70 years before present, calculated by ¹⁴C dating at 161 cm depth (Santos and Vidal-Romaní, 2000).

This newly BV determined is lower than the previously reported for sediments of rias, and is closer to the value of 32 µg g^-1 for granites and 16 µg g^-1 for schist-gneisses proposed by Wedepohl (1991) for world average sediments.

With this lead BV proposed for sediments of the Ria of Ferrol, the EFs shown in figure 5 were obtained. The contamination levels were determined according to the classification given by Hakanson (1980) for Swiss lakes: null < 1 > moderate < 3 > considerable < 6 > very high.

Generally speaking, the samples obtained in the Ria of Ferrol show EFs ranging from 2 to 6 (fig. 5), that is, a moderate to considerable contamination for lead in this ria. Out of this general findings are: (a) samples with concentrations similar to the natural background value, stations 42 and 3 at the mouth of the ria; stations 7, 22 (with the lowest EF = 0.8), 23 and 24 in the southern margin; stations 13, 14, 15, 16, 50 and 51 located in the inner part under strong tidal currents due to the funneling effect caused by land filling for the construction of two bridges; (b) samples with high contamination, which are located in the vicinity of the harbor area (stations 44b, 46, 47, 48 -with the highest EF = 9.3- and 49), and also in the northern margin of the innermost part of the ria (stations 52 and 53) in the vicinities of the iron and steel production facilities.

The existence of these different areas in the Ria of Ferrol with different contamination levels agrees with the previously reported results based on biomonitoring (Carballeira et al., 1997) using Scrobicularia plana and Enteromorpha sp.

 

Acknowledgements

Thanks to Jorge Alonso, captain of the R/V Mytilus and his crew for the help provided during the sampling campaign; to the Instituto de Geologia (ULC) for the samples of deep sediment; and to Montserrat Martínez and Juan R. González for their technical assistance. A. Cobelo-García thanks the Spanish Ministry of Science and Technology for its financial support (FPI grant). This paper is a contribution to the project "Biogeochemical processes in the Ria of Ferrol: origin of its fertilization by nutrients and spatial and historical variation of metals in sediment" (ref. 1FD97-0479-C03-02) financed by FEDER-CYCIT.

 

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