Artículos

 

Two opposite cases of metal accumulation in ria sediments: Ferrol and Corme-Laxe (Galicia, NW Iberian Peninsula)

 

Dos casos opuestos en la acumulación de metales en el sedimento de una ría: Ferrol y Corme-Laxe (Galicia, NO Península Ibérica)

 

Antonio Cobelo-García*, Ana Labandeira and Ricardo Prego

 

Grupo de Biogeoquímica Marina Instituto de Investigaciones Marinas (CSIC) Eduardo Cabello 6 E-36208 Vigo, España. * E-mail: acobelo@iim.csic.es

 

Recibido en junio de 2003;
aceptado en febrero de 2005.

 

Abstract

The accumulation of heavy metals (Cd, Cu, Pb and Zn) in surface sediments (<63 µm) of the Ferrol and Corme-Laxe rias was studied. These two rias have opposite characteristics from the point of view of both the morphology and the anthropogenic impact. Low metal concentrations were found in the Corme-Laxe Ria, with the highest values observed in the Anllóns River estuary, probably due to a higher lithogenic composition of the sediments. Concentrations in the Corme-Laxe Ria (Cd, 0.10 ± 0.03 µg g⁴; Cu, 5 ± 2 µg g⁴; Pb, 12 ± 4 µg g⁴; and Zn, 49 ± 17 µg g⁴) can be used as a good reference for metal background values for sediments from the 18 Galician rias. Although these background values are lower than those previously reported for the Galician coast, they are similar to those found in other uncontaminated coastal and marine systems. The highest concentrations for Ferrol were generally found in the middle part of the ria, which receives the most important urban and industrial impact. The values obtained show evident metal contamination in the middle and inner parts of the Ferrol Ria, with enrichment factors of 4-10 for Cd, 5 for Cu, 2-3 for Pb and 4-5 for Zn.

Key words: heavy metals, contamination, sediments, background values, Ferrol, Corme-Laxe, Galicia.

 

Resumen

Se estudió la acumulación de metales pesados (Cd, Cu, Pb y Zn) en el sedimento (<63 µm) superficial en las rías de Ferrol y Corme-Laxe, las cuales poseen características contrapuestas tanto desde el punto de vista de su morfología como de su influencia urbana e industrial. La ría de Corme-Laxe presentó bajos niveles de metales en los sedimentos, siendo mayores en el estuario del Río Anllóns probablemente debido a un mayor grado litogénico de los sedimentos. Las concentraciones obtenidas en la ría de Laxe (0.10 ± 0.03 µg g⁴ de Cd, 5 ± 2 µg g⁴ de Cu, 12 ± 4 µg g⁴ de Pb y 49 ± 17 µg g⁴ de Zn) resultan una buena referencia de valores naturales de fondo para metales en las 18 rías gallegas. Aunque éstos son menores a otros considerados para la costa gallega, son similares a los medidos a nivel mundial en sedimentos marinos no contaminados. En Ferrol, las máximas concentraciones de metales se presentan generalmente en la parte media de la ría, donde se localiza el mayor núcleo de población y buena parte de la actividad industrial. Los datos allí obtenidos señalan la existencia de una evidente contaminación metálica en la parte media e interna de la ría de Ferrol con factores de enriquecimiento comprendidos entre 4-10 para Cd, 5 para Cu, 2-3 para Pb y 4-5 para Zn.

Palabras clave: metales pesados, contaminación, sedimentos, valores de fondo, Ferrol, Corme-Laxe, Galicia.

 

Introduction

Sediments play a key role in the biogeochemical cycles of metals, providing useful information on their inputs to coastal and marine systems (Zwolsman et al., 1996). Accordingly, sediments can help us to localize the contamination sources in the estuarine-coastal zone, where industrial and urban effluents are usually dumped. The sediments of estuaries and rias act as a sink for the metals released into the coastal zone (Millward and Glegg, 1997; Balci, 1999; Helland, 2001).

About 40% of the Galician population lives close to the rias and an important fraction of the total industrial activity is located along their margins, leading to an evident risk of contamination in these environments (Cobelo-García et al., 2004). Hence, information on metal concentrations in a pristine ria that can be used as reference for those anthropogenically impacted is needed. The morphology must also be taken into account because of the critical role it may have in the accumulation of metals and other contaminants within the ria. No such studies, however, have been carried out to date (Prego and Cobelo-García, 2003).

The Ferrol Ria supports a population of 143,000 inhabitants distributed in the towns of Ferrol, Fene, Mugardos, Neda and Narón. Fene and Farrol are important industrial areas, including brick manufacturing, wood and milk products, polypropylene, iron and steel, and shipyard activities, and have a busy commercial port with a vessel trade of around 1,500,000 tons per year, transporting goods such as dunite, coal, scrap iron, wood, and iron and steel products (Lueiro and Prego, 1999). On the other hand, the Corme-Laxe Ria is completely different from the Ferrol Ria in regard to anthropogenic impact. A population of only 18,100 inhabitants is settled along its marginótowns of Laxe, Cabana and Pontecesoóand the industrial activity is very scarce, fishing and farming being the main activities.

Both rias (fig. 1), situated 50 km apart, are suitable for a morphological comparison because they have similar dimensions and the same mean depth (table 1). The Ferrol Ria, however, is a semi-enclosed embayment connected to the ocean by a narrow channel (0.35 km wide and 2 km long), whereas the Corme-Laxe Ria has a wide mouth through which potential contaminants are supposedly washed out by the sea. The fluvial discharges are also different (table 1), favouring water renewal (Prego and Fraga, 1992) in the Corme-Laxe Ria as opposed to the Ferrol Ria.

From a lithological standpoint they are quite similar since both watersheds are formed by igneous rocks. The Ferrol Ria and its adjacent watersheds are dominated by schists and gneisses, while the Corme-Laxe Ria and the Anllóns River are dominated by schists, gneisses and granites (Carballeira et al., 2000).

These two rias are therefore useful for a comparative study on metal accumulation in the sediments. The main objectives of the present study are (i) to ascertain the state of metal contamination in the Ferrol Ria in relation to the Corme-Laxe Ria and (ii) to establish the values found in the Corme-Laxe Ria as backgound reference for the rest of the rias.

 

Material and methods

Sediment samples were taken at 16 sampling sites in the Ferrol Ria (September 1998) and at 13 sites in the Corme-Laxe Ria (July 2000), from the R/V CSIC Mytilus and its auxiliary boat Zoea. Stations are indicated in figure 1.

Two Van Veen grab samplers of different size were used. Only the surface layer of the sediment was collected (<1 cm), using polyethylene (PE) spatulas, and stored in hermetically-closed PE vials (previously acid-washed with HNO3 10%) and kept in a cooler at 4°C; samples were later dried in an oven (≤50°C). Once dried, sediments were sieved in order to get the fine fraction (<63 µm).

The determination of Cd, Pb and Cu was carried out by direct injection of sediment slurries (Bermejo-Barrera et al., 1994), using electro-thermal atomic absorption spectrometry (ET-AAS) with a Zeeman background correction Varian 220 apparatus; Zn was determined by means of flame (air-acetylene) atomic absorption spectrometry (FAAS), using a Perkin-Elmer 2380 after digestion of the samples. Samples were microwave (Milestone 1200 Mega) digested in Teflon bombs using a HNO3 65% and HF 48% mixture following the EPA 3052 guideline (EPA, 1996) for siliceous-type sediments.

The accuracy of the analytical procedure was checked using two certified reference materials: PACS-l and PACS-2. A good agreement was found between the values obtained and the certified concentrations: 409 ± 24 µg g⁴ (Pb) and 424 ± 37 µg g⁴ (Cu) vs the PACS-l certified values of 404 ± 20 µg g^-(Pb) and 452 ± 16 µg g^-l (Cu); and 2.14 ± 0.09 µg g^- (Cd) and 360 ± 2 µg g^-l (Zn) vs the PACS-2 certified concentrations of 2.11 ± 0.15 µg g^-l (Cd) and 364 ± 23 µg g^-l (Zn).

 

Results

Average results for the different areasóinner (estuarine), middle and outeróof the Ferrol and Corme-Laxe rias are indicated in table 2. From five to six sampling sites were considered for the inner and middle areas, and from three to four for the outer ria.

The Cd concentrations in the Ferrol Ria decreased seawards, ranging from 0.2 µg g^-l in the outer area to 1.8 µg g^- in the inner part. The Corme-Laxe Ria showed a similar trend but with lower Cd concentrations, ranging from 0.3 µg g⁴ in the estuary to around 0.1 µg g^-l in the rest of the ria.

The highest Cu concentrations in the Corme-Laxe Ria were found at its head, reaching values of up to 49 µg g⁴, with a sixfold decrease towards the middle and outer areas (table 2). In Ferrol, the highest values were obtained in the inner and middle parts, with an average of 58 and 66 µg g⁴, respectively, but with a high dispersion of data in the latter case (table 2); the Cu values in the outer ria were around three times lower.

The Pb concentrations in Corme-Laxe decreased from the estuarine area towards the middle and outer parts, from mean values of around 30 to 12 g⁴. Significantly higher values were obtained in Ferrol, with maximum concentrations in the middle ria (75 ± 21 µg g⁴) that are three times higher than the values found at the mouth (table 2).

The distribution of Zn in Ferrol was similar to that of Cu and Pb: the highest concentrations (267 ± 43 µg g⁴) were found in the middle ria and were more than two times higher than at the mouth (table 2), where the lowest values were found (115 ± 5 µg gr⁴). The Zn concentrations in the outer Ferrol Ria, like those of Cu, were similar to those found in the estuarine area of the Corme-Laxe Ria. In the middle and outer parts of this ria, the Zn concentrations were considerably lower, 49 µg g^-l (table 2).

 

Discussion

Low Cd, Cu, Pb and Zn concentrations were observed in the surface sediments of the Corme-Laxe Ria, following a decreasing trend from the head towards the mouth. The relatively higher concentrations obtained in the Anllóns River estuary are largely due to the higher lithogenic composition of the sediment, deposited by the river, which is enriched in metals compared to the ocean-borne particulate matter (Labandeira, 2003). Other factors, such as contamination inputs throughout the estuary, may also play a role in the concentrations found. Outside the estuarine area (beyond the sandy bar, fig. 1), Cu and Zn concentrations were similar in both the middle and outer ria (table 2). The exception was Pb, whose concentration in the middle ria was similarówith a high dispersion of dataóto that found in the estuary.

If the average Cd, Cu, Zn and Pb concentrations found in the middle and outer parts of the Corme-Laxe Ria are compared with the background values reported for the Galician rias, with typical values for the lithosphere or with uncontami-nated coastal and marine sediments (table 3), it is evident that the values recorded in Corme-Laxe are lower than those previously reported for the Galician rias (Carral et al., 1995; Carballeira et al., 2000; Cobelo-García and Prego, 2003). On the other hand, these values are similar to those reported for other uncontaminated coastal systems or the lithosphere (table 3). Therefore, Corme-Laxe can be considered a non-contaminated ria. Accordingly, the metal concentrations presented herein can be used as reference or background values to study metal contamination in other Galician rias. Moreover, the Cd concentrations reported here constitute the first background values given for the Galician coastal system.

With the exception of Cd, the highest concentrations in the Ferrol Ria were found in the middle part (table 2), where most of the population and the industrial activities are settled. Using the background values reported for this ria (Cobelo-García and Prego, 2003; table 3), the metal contamination can be estimated using the enrichment factors (EF; Hakanson, 1980), defined as: EF = CV/BV, where CV represents the current value observed and BV the background value. Accordingly, Cu contamination is considerable (3 < EF < 6) in the inner and middle ria (EF_Cu = 4.8 ± 3.3 and 5.5 ± 1.4, respectively) and moderate (1 < EF < 3) in the outer part (EF_Cu = 1.7 ± 0.6), while Pb contamination is moderate in the inner (EF_Pb = 1.8 ± 0.5) and the middle ria (EF_Pb = 2.8 ± 0.8), and low or negligible in the outer ria (EF_Pb = 1.5 ± 1.0). On the other hand, Zn contamination is considerable in the inner (EF_Zn = 4.3 ± 1.0) and middle ria (EF_Zn = 4.9 ± 0.8) and moderate in the outer part (EF_Zn = 2.1 ± 0.2). With respect to Cd in Ferrol, a decrease in concentrations was found from the inner to the outer ria. Background values estimated for the Corme-Laxe Ria helped us to calculate the EF, resulting in a very high Cd contamination in the inner Ferrol Ria (EF > 6), considerable in the middle ria and moderate in the outer ria.

The presence of metal contamination in sediments from the Ferrol Ria compared with those from the Corme-Laxe Ria is not only due to the higher anthropogenic impact, which is significant in Ferrol (Cobelo-García et al., 2004), but also to the morphological characteristics of the ria. The higher fresh-water discharge and the wide, open mouth to the ocean favours the dispersion and flushing of contaminants. The semi-enclosed shape of the populated and industrial Ferrol Ria, connected to the ocean by a narrow channel, enhances the entrapment of contaminants within the bay. Moreover, the Corme-Laxe Ria is close to the summer upwelling events that occur off Cape Finisterre (Prego and Bao, 1997), favouring the renewal of the ria water, whereas this is of lesser importance in the Ferrol Ria since it is distant from the upwelling detected at Cape Prior (Prego and Varela, 1998).

The results obtained show that the Corme-Laxe Ria can be used as a good reference to study metal concentrations and contamination of the 18 Galician rias. The null or scarce anthropogenic impact in the sediments of this ria can be attributed to the scarce anthropogenic activities and its inherent morphological characteristics that favour the flushing of contaminants and water renewal. The Ferrol Ria represents the opposite case: a semi-enclosed embayment with an important urban and industrial impact, trapping contaminants within the bay. From the metal concentrations found in the surface sediments, this ria can be defined as contaminated in Cd, Cu, Pb and Zn in the middle and inner parts.

 

Acknowledgements

The authors thank the crew of the R/V CSIC Mytilus for their kind cooperation, Monserrat Martínez and Daniel Caride for their assistance during the sampling and handling, and Carmen Tubio for the chemical analysis. The first author thanks the Spanish Ministry of Science and Technology for financial support (FPI grant). This study is a contribution to the FEDER-CICYT projects "Procesos biogeoquímicos en la ría de Ferrol: Origen de su fertilización por sales nutrientes y variación espacial e histórica de metales en el sedimento" (ref. IFD97-0479-CO3-02), and "Hidrodinámica e hidro-química del sistema integrado cuenca del río Anllóns-ría de Laxe: Situaciones estacionales y respuesta a eventos de avenidas extraordinarias" (ref. HID99-0699).

 

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