Artículos

 

Influence of Aveiro Lagoon heavy metal contents in the adjacent continental shelf (Portugal)

 

Influencia de la Laguna de Aveiro en los contenidos de metales pesados en la plataforma continental adyacente (Portugal)

 

I. Abrantes^1, F. Rocha²*, J. Vidinha² and J.A. Dias³

 

¹ E.S.E./Instituto Superior Politécnico de Viseu R. Dr Maximiano Aragao 3500 Viseu, Portugal.

² Centro de Minerais Industriais e Argilas Universidade do Aveiro Campus de Santiago 3810 Aveiro, Portugal * E-mail: frocha@geo.ua.pt

³ Universidade do Algarve UCTRA, Campus de Gambelas. 8000 Faro, Portugal.

 

Recibido en junio de 2003;
aceptado en mayo de 2004.

 

Abstract

A chemical analysis of Al, Ca, Fe, Mg, Cu, Pb, Zn, Cr, Co, Ni and Cd was carried out in 22 surface sediment samples of the continental shelf and upper slope between Espinho and Aveiro (Portugal). To reduce the disturbing effects of grain size, all samples were size-normalized by wet sieving (<63 µm). The fine fraction was digested with concentrated acids and analyzed by flame atomic absorption spectrometry. The Al and heavy metal concentrations displayed a similar distribution pattern, decreasing seaward. Exceptions to this general pattern are Ca, Mg and Pb distributions. The Ca contents are related to carbonated biogenic particles that are very abundant in the outer shelf. Multivariate statistical analysis was applied to all data to assess the behaviour and sources of each element analyzed. The results from factor analysis showed that terrigenous supply (from Aveiro Lagoon flow, coastal erosion and littoral drift), grain size (clay) sorting and anthropogenic influences from Aveiro Lagoon are the major factors controlling spatial variations of chemical elements in the continental shelf between Espinho and Aveiro. Unusually high contents of Pb were found in the middle shelf off Furadouro, but we could not determine the possible source of this contamination.

Key words: heavy metals, sediments, continental shelf, Aveiro Lagoon, multivariate statistical analysis.

 

Resumen

Se realizaron análisis químicos de Al, Ca, Fe, Mg, Pb, Zn, Cr, Co, Ni y Cd en 22 muestras de sedimento superficial de la plataforma y vertiente continental entre Espinho y Aveiro (Portugal). Para reducir el efecto de la granulometría del sedimento, se normalizó el tamaño de las muestras mediante tamizado (<63 µm). Se procedió a la digestión de la fracción fina con ácidos concentrados y se analizó mediante espectrometría de absorción atómica con llama. Las concentraciones de Al y metales pesados muestran un patrón de distribución similar, disminuyendo hacia la vertiente. Las excepciones a este patrón general son las distribuciones de Ca, Mg y Pb. El contenido de Ca está relacionado con las partículas biogénicas carbonatadas que son muy abundantes en la plataforma externa. Con el objetivo de determinar el comportamiento y el origen de cada uno de los elementos analizados, se aplicaron técnicas de análisis estadístico multivariante. Los resultados obtenidos muestran que los principales factores que controlan las variaciones espaciales de los elementos químicos en la plataforma continental entre Espinho y Aveiro son los sedimentos terrígenos procedentes de la Laguna de Aveiro, de la erosión costera y de la deriva litoral; el tamaño de grano (tamaño arcillas); y la influencia antropogénica en la Laguna de Aveiro. En la plataforma media, a la altura de Furadouro, se encontraron valores elevados de Pb, aunque no hemos podido determinar la posible fuente de esta contaminación.

Palabras clave: metales pesados, sedimentos, plataforma continental, Laguna de Aveiro, análisis estadístico multivariante.

 

Introduction

Several studies regarding mineralogical distribution in the fine fractions of the sediments from the Portuguese Atlantic coast and continental platform in relation to circulation/ paleocirculation have been carried out during the last decade (Drago et al., 2002; Machado et al., 1999a, 1999b, 2000, 2001a, 2001b, 2002; Martins et al., 2001a, 2001b, 2002; Oliveira et al., 1998, 2000, 2001, 2002; Vidinha et al., 1998, 2000, 2002). The main topics were and still are:

1. Analyses of the mineralogical and chemical composition of littoral beach and dune sands aiming to determine the source rocks of sediments, their pathways into the present-day deposits, the reworking processes, the determination of shoreline stability and the dispersion of pollution.

2. Investigations of the mineralogical composition of sea-bottom sediments, of suspended particulate materials and on cores, together with paleontological and geochemical data, aiming at paleoclimatic and paleoenvironmental reconstructions, sea-level changes analysis, identification of stratigraphic correlations, past and present sources of the sediments and means of terrigenous supply, dispersion of pollution and, finally, to describe the present oceanic water masses.

The northern Portuguese shelf is characterized by a highly energetic hydrodynamic regime of waves and tides. In the summer, northerly winds drive a coastal upwelling (Wooster et al., 1976; Fiúza et al., 1982; Fiúza, 1983), with an associated equatorward flow on the shelf, whereas during the winter, westerly winds with a southerly component promote down-welling with poleward flow (Drago et al., 1998; Vitorino et al., 2000, 2002).

The surface sediments of this sector of the Portuguese continental shelf came, essentially, from the Douro watershed, located at the northern extremity of this coastal sector, and also from the Vouga watershed, that drains to a wide lagoon (Aveiro Lagoon), which in turn is connected to the Atlantic Ocean through an artificial inlet. Mainly granites, schists and greywackes outcrop in the Douro watershed, while gneisses, migmatites, granitoids, micaschists, schists, sandstones and shales outcrop in the Vouga watershed and in the surrounding region of Aveiro Lagoon.

Geological studies of the sediments of the continental shelf and upper slope between Espinho and Aveiro have been carried out (Abrantes, 1994; Abrantes and Magalhães, 2000; Abrantes et al., 1994, 2001, 2002; Cascalho, 2000; Cascalho and Magalhães, 2000) using sedimentological, paleoecological, geochemical and mineralogical analysis of sediment samples collected systematically in this sector of the Portuguese Atlantic coast. The surface sediments of the continental shelf and upper slope between Espinho and Aveiro have been characterizeded by the authors, based on both sediment texture and coarse-fraction granulometric and compositional analysis.

The study area is located in the western Portuguese continental margin, between 40°31' N and 40°58' N (fig. 1). The shelf is narrow, ranging from 38 km, near Aveiro Canyon, to 50 km, off Furadouro.

Mesozoic carbonate rocks outcrop at the middle and outer shelf. The bottom sedimentary cover is dominated by sand particles, in the inner shelf (>30 m) by fine sand, in the mid-shelf (30-80 m) by coarse sand and gravel, and in the outer shelf by carbonate-rich sand, poorly sorted. Gravel particles occur essentially along two bands correlated with paleo-littorals. The fine fraction content is generally low (average 5%), increasing from the coast to the upper slope (fig. 1). The fine fraction content is probably related to the effectiveness of Aveiro Lagoon as a natural sediment trap, the sedimentary dynamics of this region and the bathymetric control.

This shallow coastal lagoon (also known as Ría de Aveiro) separated from the sea by a sand bar discharges into the shelf through an artificial channel. The flows from the Vouga and Antua rivers are the most important input of fresh water into this lagoon. During a tidal cycle the total mean river discharge is about 1.8 x 10⁶ m³ (Moreira et al., 1993), although the tidal prism at the mouth during a spring tide, with a tidal range of 2.48 m, is about 70 x 10⁶ m³ (Vicente, 1985). At low tide, water covers an area of 66 km², while at high tide, this area increases to 83 km² (Dias et al., 1999). The lagoon provides conditions for harbour, navigation and naval industry and is still a place of industrial and domestic waste discharges.

Waters flowing from the north, mainly from the Douro River, extend their influence to the study area through littoral drift currents. The continental area drained to this shelf is almost dominated by granitic and schist-graywake formations.

Though the sedimentology and mineralogy of the shelf sediments of this region are well known (Dias and Nittrouer, 1984; Dias, 1987; Magalhães, 1993, 1999; Abrantes, 1994; Abrantes et al., 1994, 2001; Abrantes and Magalhães, 2000; Cascalho, 2000; Cascalho and Magalhães, 2000), there is a lack of information on the geochemistry of the surface sediments.

This work aims to evaluate the distribution of several elements, in particular the heavy metals, in the fine fraction of the surface sediments and to correlate trace metal content with natural or anthropogenic inputs.

 

Materials and methods

The 22 shelf sediment samples analyzed were collected using a Smith-McIntyre grab, during several cruises conducted under the SEPLAT program, promoted by the Portuguese Hydrographic Institute, between 1988 and 1990 (fig. 2).

To reduce the effects of variable grain size on the chemical and mineralogical composition, only the fine fraction (<63 µm) was analyzed. This fraction was obtained by wet sieving and dried at 60°C. Chemical analysis to determine Al, Ca, Fe, Mg, Cu, Pb, Zn, Cr, Co, Ni and Cd was carried out by flame atomic absorption spectrometry on a GBC 600 spectrophotometer.

The chemical analysis of the sediment samples was carried out following the method proposed by Lecomte and Sondag (1980): 1 g of sediment was digested with a mixture of 3 mL of HCl (37%), 2 mL of HNO₃ (65%) and 1 mL of HF (40%). After drying at 120°C, the residue was mixed with 10 mL of HNO₃. The resultant solution was centrifuged, filtered and mixed with demineralized water.

The quality of the analysis was tested by simultaneously extracting and analyzing duplicate samples. To control the analytical process and check for possible batch errors, samples from one batch were included in other batches. The precision of the analytical results was estimated by replicate analysis (Garrett, 1973). Analytical precision, defined as the percent relative standard variation at the 95% confidence level, ranged from 4% to 12% for the elements analyzed, considering the P_0.05 significance level.

The data obtained were submitted to multivariate statistical analysis (principal components analysis) using Statistica (v. 5.0) software.

Multivariate data analysis (principal components analysis) of the chemical data was carried out to obtain a convenient characterization of the data, through the reduction of the complexity of the model and classification of the variables and samples into natural groups, according to Imbrie and Van Andel (1964), Jöreskob et al. (1976), Davis (1986), Mezzadri and Saccani (1989) and Reyment and Jöreskob (1993).

 

Results and discussion

To compare sedimentary metal concentrations, a granulo-metric normalization approach was used to compensate grain size effects, according to Loring (1991). The concentrations of the elements analyzed in the fine fraction of the surface sediments studied are listed in table 1. Figures 3 and 4 show the distribution maps of these elements.

The values of Al, Fe and Cu are within currently accepted background levels (Salomons and Fóstener, 1984), whereas Pb, Zn, Cr and Ni show some anomalous values, higher than the currently accepted background levels, which may be related to anthropogenic contamination. All samples have Cd concentrations below the detection limit (0.05 mg kg^-1).

The highest concentrations of Al were found at depths above 100 m. On the contrary, Ca contents increased westwards, with the highest values in the outer shelf, where the sediments are enriched in carbonate biogenic particles like debris of molluscs and foraminifers.

The distribution of Fe showed an enrichment in the midle and outer southern shelf zones. Mg contents presented a homogeneous distribution, although two samples located in the mid-shelf and in the outer southern shelf show concentration values much higher than the others.

The concentrations of Cu, Zn, Cr, Co and Ni displayed a similar distribution pattern, decreasing seaward. The highest values of Cu and Zn were found in the inner and middle shelf sediments in front of the Ría de Aveiro mouth. However, the higher Cr and Ni concentrations occurred in the inner shelf region, extending mainly from the Ría de Aveiro mouth to the northern end of the study area.

The results obtained by Rocha et al. (2005) show that the surface and near-surface sediments of Aveiro Lagoon trap relatively large quantities of trace metals, such as Cu, Pb, Co, Ni, Cd, Zn, Fe, Mn and Cr, considered to be partially adsorbed, particularly on silty and clayey particles, and also may be co-precipitated on Fe and Mn oxyhydroxide forms precipitated in the topmost layers of sediments.

The lower values of these metals observed in the middle shelf correspond to the higher concentrations of other metals (Al, Cu and Zn). Pb showed an unusually high concentration in the mid-shelf sediments.

Factor analysis was used to correlate elements and identify possible sediment sources. Table 2 lists the factor loadings and figure 5 presents the contoured maps of each sample's factor scores. The analysis showed an extraction of three factors, explaining 78.9% of the total variance:

• Factor 1 represents carbonated biogenic particles (Ca) in opposition to terrigenous non-carbonated particles (Al, Cu, Zn, Cr, Co and Ni).

• Factor 2 explains Fe distribution, showing the positive scores in the northern outer shelf and in the southern at depths below 30 m.

• Factor 3 represents Pb distribution with the higher values in the middle shelf.

The higher Al concentrations found at depths above 100 m are related to the input of sediments from the weathering of continental soils and rocks (composed mainly of aluminosili-cates, such as micas, chlorites and feldspars).

The westward increase in Ca is in accordance with factor 2 mapping (fig. 5); this factor explains the distribution of bio-genic particles, which have the highest values in the outer shelf near the head of Aveiro Canyon. The decrease in the supply of terrigenous particles associated with a hydrodinamically calmer environment and also with high organic productivity induced by upwelling currents over Aveiro Canyon is probably responsible for the high Ca values displayed in the outer southern shelf.

The distribution of heavy metals (Cu, Zn, Cr, Co and Ni) displayed a spatial variation, with high concentrations in the inner-middle shelf and low concentrations in the outer and upper slope sediments. This general decrease is also observed in other areas of the Portuguese continental shelf and indicates that heavy metals of the shelf sediments have been depleted by geochemical processes, during the long residence time of suspended material, by dilution effects and/or by mixing with allochthonous particles such as biogenic carbonate particles (Araujo et al., 2002).

The high values of Cu and Zn found in the inner and middle shelf sediments in front of the Ría de Aveiro mouth, as well as the distribution patterns of Cr, Co and Ni concentrations allowed us to consider that inputs from Aveiro Lagoon (natural and anthropogenic) and from northern sediments (Douro River and coastal erosion) by littoral drift or by advective currents are the main sources of heavy metals in this region. Studying the surface sediments of Aveiro Lagoon, Rocha et al. (2005) found a clear association between Ni and Cr variables, the peculiar cartographic distribution of the factor scores suggesting either a lithological/mineralogical control (related to vermiculite and smectite rich muds) or some influence of different chemical parks surrounding the lagoon (Ovar, Estar-reja and Ílhavo), and pointing out the influence of the naval activities (port and shipyards) linked with Aveiro harbour (located close to the lagoon mouth).

Factor 2 is associated with Fe distribution. As Fe is an element commonly present in the crystallochemical matrix of clay minerals, this factor can be related to changes in clay mineral associations. Therefore, Fe enrichment in the middle and outer southern shelf zones can be attributed to clay minerals such as smectite and chlorite. Actually, smectite has higher values in the southern outer shelf, which seems to be a low-energy area favourable for the deposition of this very fine-grained mineral (Abrantes et al., 2001).

As we have already stated, Pb showed an unusually high concentration in the mid-shelf sediments, but the distance from the coast and the localization of these highest Pb levels does not suggest anthropogenic enrichment from a point source. So, these values must be further investigated.

In summary, particulate metals from natural and anthropogenic sources accumulate together and tend to be concentrated in the fine-grained particles from the inner and middle shelf. The Al and heavy metal concentrations displayed a similar distribution pattern, decreasing seaward. Exceptions to this general pattern are Ca, Mg and Pb distributions. The Ca contents are related to carbonated biogenic particles that are very abundant in the outer shelf. The major factors controlling the spatial variations of the elements analyzed in the study area are terrigenous supply from Ría de Aveiro, coastal erosion and Douro River; grain size (clay) sorting; and anthropogenic influences from Aveiro Lagoon.

 

Acknowledgements

The authors sincerely thank the Hydrographic Institute for providing the sampling to carry out the chemical analysis.

 

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