Artículos

 

Relationship between PCBs in suspended and settled sediments from a coastal lagoon

 

Relación entre PCBs en sedimentos suspendidos y depositados de una laguna costera

 

Luísa A. Barreira¹, Maria J. Bebianno¹*, Stephen M. Mudge², Ana M. Ferreira³, Clarisse I. Albino¹, Luís M. Veriato¹

 

¹ CIMA Universidade do Algarve Campus de Gambelas 8000 Faro, Portugal. * E-mail: mbebian@ualg.pt

² School of Ocean Sciences, University of Wales Bangor, Menai Bridge, UK.

³ IPIMAR, Instituto Português de Investigação Marítima Av. Brasilia, 1400 Lisboa, Portugal.

 

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

 

Abstract

Polychlorinated biphenyl (PCB) congener concentrations and organic matter content were determined in 84 samples of suspended and settled sediments collected from six different locations in the Ria Formosa lagoon (Portugal). Total PCB (tPCB) concentrations were higher in the suspended matter (1.00-39.80 ng g^-1 dw) than in the sediments (0.10-2.10 ng g^-1 dw) and, in general, the same was true for the organic matter. Partial least squares (PLS) analysis using the suspended sediments as signatures indicated that they better explained the settled sediment pattern compared to the reverse. This suggests a suspended sediment source outside the lagoon as source of these PCBs. PLS analysis also indicated that Aroclor 1242 best explained the congener pattern in the sediments, but lower fits were obtained for the suspended matter. The tri- and tetra-chlorinated biphenyls were the most abundant congeners (about 60% of tPCB), followed by the hexa- (20%), penta- (11%) and hepta + octa-chlorobiphenyls (9%). The principal congener differences between the suspended and settled sediments were for the smaller tri- and tetra-chlorinated biphenyls, which were preferentially accumulated in the settled sediments. In contrast, congener 49 was significantly more abundant in the suspended phase and may be formed through the anaerobic UV degradation pathway either in suspension or on the surface of strongly anoxic sediments at low tide.

Key words: PCB, congeners, sediments, coastal lagoons, Ría Formosa.

 

Resumen

Se determinaron las concentraciones de congéneres de bifenilos policlorados (PCBs) y el contenido de materia orgánica en 84 muestras de sedimentos suspendidos y depositados, recolectadas de seis localidades diferentes de una laguna costera (Ría Formosa, Portugal). Las concentraciones totales de PCB (tPCB) fueron mayores en el material suspendido (1.00-39.80 ng g^-1 ps) que en los sedimentos depositados (0.10-2.10 ng g^-1 ps) y, en general, lo mismo fue cierto para la materia orgánica. El análisis de cuadrados mínimos parciales usando los sedimentos suspendidos como huellas indicó que éstos explican mejor el patrón de los sedimentos depositados que al revés. Esto sugiere una fuente de sedimento suspendido afuera de la laguna como fuente de estos PCBs. El análisis de cuadrados mínimos parciales también mostró que el Aroclor 1242 explicó mejor el patrón de congéneres en los sedimentos, pero se obtuvieron ajustes menores para el material suspendido. Los bifenilos triclorados y tetraclorados fueron los congéneres más abundantes (alrededor de 60% del tPCB), seguidos por los bifenilos hexaclorados (20%), pentaclorados (11%) y hepta + octaclorados (9%). Las principales diferencias de congéneres entre los sedimentos suspendidos y los depositados correspondieron a bifenilos triclorados y tetraclorados, que fueron acumulados preferencialmente en los sedimentos depositados. En contraste, el congénere 49 resultó significativamente más abundante en la fase suspendida y puede formarse por degradación anaerobia por irradiación UV, ya sea en suspensión o en la superficie de sedimentos altamente anóxicos durante la marea baja.

Palabras clave: PCBs, congéneres, sedimentos, lagunas costeras, Ría Formosa.

 

Introduction

Polychlorinated biphenyls (PCBs) were produced and commercialized as mixtures of several congeners (Aroclors) worldwide. The contamination of the environment by these compounds is a global problem due to their wide use in industry as dielectric fluids in capacitors and transformers (Sawhney, 1986). A series of toxicological studies (Hutzinger et al., 1980) highlighted the threat of these compounds to the environment and human health, and as a result their production was prohibited. Although production has stopped, PCBs are still in use in closed systems. In Portugal, an estimated 500 tons of PCBs are still in use in transformers and capacitors (DGA, 1995). Therefore, contamination of the marine environment may still occur from leakage of capacitors and transformers or from waste disposal (Schwarzenbach et al., 1993).

PCBs reach the marine environment by different sources: waste water and industrial effluents, run-off from land or deposition from the atmosphere (Abdullah et al., 1982; Dickhut and Gustafson, 1995; Ferreira and Vale, 1995; Larsson and Sodergrem, 1987). Once released into the marine environment, they tend to immediately adsorb onto suspended particles due to their low solubility in water and high K_ow values and, therefore, end up trapped in the sediments (Borglin et al., 1996). In coastal lagoon systems like Ria Formosa, the distribution of these compounds depends not only on their properties but also on the characteristics of the system (Pereira et al., 1988; Philips, 1995). More recently, it was discovered that PCBs might be reductively dechlorinated. These processes are very important since they may reduce the toxicity of PCB mixtures. Higher proportions of the less chlorinated congeners and with chlorines in ortho positions may be evidence of the dechlorina-tion processes (Bedard and Quensen, 1995). Dechlorination of PCBs is generally accomplished by anaerobic microorganisms, although aerobic dechlorination has also been reported (Flanagan and May, 1993). Different distributions of dechlori-nated biphenyls were found as a consequence of several processes of individual PCB dechlorination (Bedard and Quensen, 1995). Environmental dechlorination has been reported in sediments of estuaries (Lake et al., 1992), ponds (Bedard and May, 1996), lakes (Natarajan et al., 1998) and rivers (Abramowicz et al., 1993; Sokol et al., 1994).

In southern Portugal PCBs may enter the marine environment from the rivers draining the coastal zone. Rainfall is infrequent but locally heavy. This leads to periodic washout from the terrestrial environment, and sediments and their associated contaminants may be trapped at the interface between the land and the sea. In this region, the Ria Formosa lagoon could be an important sink. Although the concentrations of PCBs previously detected in this coastal lagoon were relatively low when compared with other European coastal systems (Ferreira et al., 1989; Castro and Vale, 1995; Ferreira and Vale, 1996), PCBs have been detected in the whole soft tissue of oysters (Ferreira and Vale, 1995) and clams (Ferreira and Vale, 1998) collected from different areas of the lagoon. The seasonal and spatial variation of PCB concentrations in the suspended matter and the sediments in the Ria Formosa lagoon has not been reported. This paper describes the variations of PCB concentrations in the suspended and settled sediments and the congener speciation in the Ria Formosa lagoon.

Description of the area

Ria Formosa is a coastal lagoon on the south coast of Portugal, permanently connected with the sea through several narrow inlets. The lagoon is about 55 km long and 6 km in its widest part, and covers an area of approximately 160 km², one third being intertidal (Bebianno, 1995). The average depth is approximately 3.5 m, and tidal amplitude varies from a maximum of 4 m at spring tide to 2 m at neap tide, which results in important fluctuations (either twice a day or every fortnight) of the water volume inside the lagoon (Bebianno, 1995). In each tidal cycle, 50-75% of the water mass of the lagoon is exchanged with the ocean (Sprung, 1994). The current speed decreases from the outside to the inside of the lagoon, being almost zero in the intertidal areas (Bebianno, 1995). Grain sizes of bottom sediments vary from mud in inner areas to coarse sand in the inlet channels. There are few fresh-water inflows into this lagoon system and their influence is seasonal. The only relevant input is from the Gilao River, which is the only river that does not dry up during summer (Mudge et al., 1999) . Therefore, salinity remains around 36 psu all year round, except for short periods when run-off becomes important (Falcao and Vale, 1990).

 

Materials and methods

Sample collection and storage

Samples of suspended matter and surface sediments (top 3 cm) were collected from six areas across the Ria Formosa lagoon, Portugal (fig. 1). Sample collection took place every two months, from January 1996 to February 1997.

Samples of suspended matter were collected using sediment traps, consisting of six PVC tubes of 4 cm internal diameter and 30 cm high. The sediment traps were placed in the intertidal zone at each site; the samples were uncovered at low tide and remained under water the rest of the time. Samples were collected every two months at low tide and transferred to solvent-cleaned glass jars with aluminium caps. Samples were filtered and the solid material was wrapped in pre-cleaned aluminium foil and stored at -20°C until further treatment.

The first 3 cm of the surface sediments were collected from the same locations using a clean stainless steel spoon, and stored at -20°C in solvent-cleaned glass jars with aluminium caps until subsequent analysis.

Determination of organic matter content

An aliquot of 10 g of each suspended matter or sediment sample was weighed and air-dried to determine the wet:dry ratio. These sub-samples were then burned in a furnace at 500°C for 3 h; the organic matter (OM) content was determined by weight loss.

PCB extractions and gas chromatographic analysis

About 40 g of wet samples of suspended matter and sediments were refluxed in a solution of 6% NaOH in methanol for 3 h. After cooling, the extracts were centrifuged at 1000 g for 5 min, to coalesce suspended solids, and the liquor was decanted into a separating funnel (Mudge et al., 1999). The PCBs were then extracted with n-hexane. Sulphur was eliminated with mercury and the extracts were purified on a Florisil column and with a sulphuric acid treatment (1 mL lipids to 4 mL acid) to remove interfering lipids.

The PCB congeners were determined using a Hewlett-Packard 5890A gas chromatograph with capillary column (DB5, J&W, 60 m) and automatic injection, with the following temperature program: 60°C - 20°C/min - 210°C (8 min) - 2°C/ min - 250°C (17 min) - 4°C/min - 260°C (5 min). Helium was used as a carrier gas at a flow of 1.6 mL min^-1 and a mixture of argon and methane (9:1) was used as a makeup gas, at a flow of 30 mL min^-1. Detection was made with an electron capture detector and the assignment of peak identification was made by comparison with an external standard (PCB-1, IOC-Kiel), acquired from QUASIMEME and maintained frozen, containing known concentrations of all the PCB congeners presented in this work, IUPAC numbers 18, 26, 31, 44, 49, 52, 101, 105, 118, 128, 138, 149, 151, 153, 170, 183, 180, 187 and 194 (Ballschmitter and Zell, 1980). Concentrations were determined using the peak heights. Total PCB (tPCB) is designated as the sum of the quantified congeners, and concentrations were expressed as ng g^-1 of dry weight (dw) and as ng g^-1 of OM. Recovery rates were determined by submitting three samples to two successive extractions and these indicate that the percent recoveries for all samples were in the 65%-67% range. For quality assurance, blank samples were extracted using the same procedure. PCBs were not detectable in these samples.

Statistical analysis

Statistical analyses were performed using the Kruskal-Wallis ANOVA by ranks and confirmed by the Friedmen ANOVA by ranks. Differences between PCB concentrations in suspended matter and sediments were determined using the Mann-Whitney U-test. Multivariate statistical methods were also used in the identification of sources and co-variant behaviour. These methods included principal component analysis (PCA), partial least squares (PLS) and cluster analyses, performed assuming single linkage and Euclidean distances.

PCA has been widely used to assist in the interpretation of large chemical datasets but as a multivariate statistical technique. PLS is relatively recent. This technique was developed by Wold and has evolved into a powerful analytical tool. A new use is environmental forensics, as demonstrated by Yunker et al. (1995) to determine the origin of particulates deposited in Arctic waters, and by Mudge and Seguel (1999) to trace the dispersion of known pollutant sources in the environment. In an attempt to resolve issues of source identification and partitioning, sets of data are used to generate signatures, and the extent to which each signature explains the variance seen in the environmental data is a quantitative measure of their similarity.

In essence, PLS initially performs PCA on a set of data that is defined as the signature (Geladi and Kowalski, 1986; Naftz, 1996). In this situation, the suspended sediment data from each of the six sites were used to explain the variance in all the remaining data. The settled or suspended sediment PCB data were used to generate the first two principal components (PC1 and PC2) that explain most of the variance in the data. PC2 is fitted orthogonally to the first component so there is no aspect of PC1 in PC2. These projections in n-dimensional space, where n is the number of PCBs analyzed, can be described by a series of loading factors on each compound; those compounds having a major impact on each PC will have high loadings (either positive or negative), whereas those PCBs that are relatively unimportant and, therefore, do not have a major influence on the data, will have values close to zero. These projections are applied to the remaining data, and the amount of variance explained by the signatures quantified.

 

Results and discussion

Total organic carbon content in suspended matter and sediments

Figure 2 shows the spatial and seasonal variation of the OM content (mean ± standard deviation) in the suspended matter and the sediments. The OM content was higher in the suspended matter samples than in the sediments in every sampling site at each sampling time (P < 0.01). The OM content ranged from 3.9% to 21.1% in the suspended matter and from 0.4% to 18% in the sediments. These sediment values are slightly higher than the values found in a previous study of the same area (Yunker et al., 1995). The highest values were observed in muddy areas with fine-grained sediment accumulation, near sewage outfalls or fresh-water input. Statistical analyses revealed no significant seasonal variation (P > 0.05) in the OM content for both the suspended matter and the sediments; however, the OM content was highest at site 5 for the sediments (P < 0.05) and at sites 4 and 5 for the suspended matter.

Spatial and seasonal variation of total PCB concentrations

Figure 3 shows the tPCB concentrations in the suspended matter and the sediments collected from the six sites of the Ria Formosa lagoon. The tPCB concentrations were much higher in the suspended matter (1.00-39.80 ng g^-1 dw) than in the sediments (0.10-2.10 ng g^-1 dw). Such a difference in the contamination levels (at least by one order of magnitude) (P < 0.01) suggests that particles may be the major transport and redistribution mechanism of PCBs in the Ria Formosa lagoon.

Similar results were observed in the estuaries of the Sado (Ferreira and Vale, 1996) and Humber (Tyler and Millward, 1996) rivers. It should be pointed out, however, that the distribution and transport of tPCB in the suspended phase is highly dependent upon the hydrodynamics of the system. The PCB concentrations measured in the suspended matter samples are of the same order of magnitude as others previously reported for the Ria Formosa lagoon and the Sado Estuary (Ferreira et al., 1989; Ferreira and Vale, 1995, 1996). The tPCB levels measured in the sediments are, in general, lower than those found in the Tagus Estuary and in other places worldwide (Abdullah et al., 1982; Baldi et al., 1983; Camacho-Ibar and McEvoy, 1996).

Since the OM content in the suspended particles and in the sediments may play a role in the distribution of PCB concentrations in the lagoon, the relationship between the tPCB concentrations and the OM content was studied. Significant positive linear relationships were obtained (P < 0.01) between tPCB concentrations and the OM content in both the suspended matter (tPCB = 0.887 OM - 3.786; r = 0.442, n = 41) and the sediments (tPCB = 0.042 OM + 0.564; r = 0.459, n = 38). The OM, therefore, plays an important role in the partitioning of the PCB congeners between the suspended and settled sediment phases and in the distribution of these compounds within the lagoon. This influence seems to be greater for the suspended sediments than for the settled sediments, since the slope of the linear regression is greater for the former. So, to better understand the behaviour of these compounds in the Ria Formosa lagoon and to identify possible sources of PCBs for this system, it is necessary to normalize the data in terms of the OM content of the samples, expressing tPCB concentrations in terms of ng g^-1 of OM.

The spatial and seasonal variations of the tPCB concentrations in the suspended and settled sediments were evaluated using the Kruskal-Wallis ANOVA by ranks. Differences in PCB concentrations among the different sites were still statistically significant (P < 0.05) after normalization. Factors such as particle size, for example, seem to influence the PCB distribution: small colloidal particles like those found in fine sediments have increased PCB sorption capabilities compared to larger particles like sand (Sawhney et al., 1981; Thompson et al., 1996). The dependence of PCB concentration on the grain size has been reported in several studies, in which PCBs were determined in bulk sediments (Larsen et al., 1985; Camacho-Ibar and McEvoy, 1996; Dannenberger, 1996; Piérard et al., 1996; Ferreira and Vale, 1996). Another factor that will influence the contaminant concentration is the source term (Ferreira and Vale, 1995). In the lagoon, both factors may be operating to determine the overall distribution seen in figure 3.

Regarding the particulate matter, site 5 exhibits the highest tPCB concentrations (7.30-39.80 ng g^-1 dw), while the other sites show relatively low concentrations (1.00-9.70 ng g^-1 dw) (fig. 3a). The normalization of these data to OM does not decrease this difference (fig. 3c), which indicates that the high value reported here is not a consequence of a high OM content. Site 5 is directly influenced by the fresh-water input of the Gilao River, receiving the contribution of drainage from the river basin (Ferreira et al., 1989). This might account for the high levels of PCBs detected in this area and may be the most important source of these compounds for the lagoon. This was also confirmed by other studies carried out in the lagoon (Ferreira and Vale, 1995). Site 2, although situated near the major sewage outfall, shows one of the lowest PCB concentrations. This indicates that sewage is not one of the primary sources of PCBs to this lagoon system. No significant seasonal variations of tPCB concentrations (expressed as ng g^-1 dw or OM) were observed (P > 0.05) for all sites, in the suspended matter (fig. 3b, d).

In the sediments, the spatial variation of tPCB concentrations was not statistically different (P > 0.05) when the data were expressed in terms of dry weight (fig. 3 a); however, when the data were expressed in terms of OM content, site 3 presents the highest concentrations of PCBs (fig. 3 c). This site is situated near one of the principal inlets of Ria Formosa, where major water exchanges with the Atlantic Ocean occur. Other studies have reported high values of contaminants that are generally associated with OM in this region (Bebianno, 1995; Yunker et al., 1995; Mudge and Bebianno, 1997).

There are statistically significant (P < 0.05) seasonal variations in tPCB levels in the sediments when expressed on a dry weight basis (fig. 3b), although these differences are no longer observed when the data are normalized to OM content (fig. 3d). Therefore, the tPCB concentrations are dependent upon the OM content that may fluctuate in response to local environmental conditions. The PCB concentrations found in the suspended matter and sediments at the other sites are relatively low and may be the result of atmospheric deposition or land run-off (Dickhut and Gustafson, 1995; Ferreira and Vale, 1995, 1996). The hydrodynamics of the system may also influence the distribution of PCBs associated with suspended particles.

Differences in the tPCB concentrations between the suspended matter and the sediments (on a dry weight basis) were not apparent after normalization of data (with the exception of site 5).

The suspended sediments may be a source of PCBs to the sediments and, conversely, the sediments may be the source of resuspended material. The similarity between the suspended and settled sediments was investigated with PLS. Each suspended sediment was used as a potential source, and the amount of variance explained in the settled sediments was quantified and averaged over the sampling period (~1 year). In a similar manner, the settled sediments were used as sources to see how much variance they explained in the suspended material. The results are presented in table 1.

These results indicate several points: (1) the suspended sediments from site 1 (ss1) have the greatest similarity with all the other sites, but especially with site 1, and ss4 was similar in this respect, although the other suspended sediments were not as similar to the settled sediments in the immediate vicinity; (2) ss3 explained a very small amount of the variance in the sediment data (up to 6%); (3) there is considerable overlap between the suspended sediment signatures since the total explained variance (sum of the rows) is much greater than 1.0; (4) when the sediments are used as potential sources of PCBs for the suspended matter (lower section of table 1), the fits are much poorer than the reverse scenario, i.e., the suspended sediments are a better reflection of the sediments than the other way around; and (5) due to the relatively homogenous nature of the sediments, there is little difference between sources (small range across the rows, table 1).

The potential sources were further investigated with PLS using published Aroclor compositions (Frame et al., 1996). Each of five Aroclors (1016, 1242, 1248, 1254 and 1260) were used as signatures to explain the variance in the environmental data. The results for three Aroclor mixtures are presented in figure 4. The data show that Aroclors explained the variance in the sediments better than in the suspended matter and that the mixtures with lower chlorinations (e.g., 1242) were significantly better at explaining the sediment composition compared to the more highly chlorinated mixtures (e.g., 1254). Although the fit was worse in the suspended matter than in the settled sediments, the PCBs in the suspended sediments at sites 1 and 5 were best explained by Aroclors 1242 and 1248 (fig. 4). This may indicate that the major sources are at these sites and are derived initially from Aroclors, though their composition may have been altered before collection at the lagoon.

Concentrations and distribution of individual PCB congeners

The concentrations of the PCB congeners are presented in tables 2 and 3, and figure 5 presents the average distribution of these compounds, on a percentage basis, in the suspended matter and the sediments of Ria Formosa lagoon in 1996/1997. There were no significant seasonal and spatial variations of individual PCB congener concentrations, on a percentage basis, in the suspended matter and sediments (P > 0.05). The tri- and tetra-chlorobiphenyls accounted for approximately 60% of the tPCB concentrations; the penta-, hexa- and hepta + octa-chlorobiphenyls contributed 11%, 20% and 9%, respectively. The levels of the tri- and tetra-chlorinated PCB congeners were significantly different between the suspended and the sediment phase (P < 0.05). Congener 49 was clearly more abundant in the suspended sediments than in the settled sediments (21% of tPCB in suspended matter and 8% of tPCB in sediments).

In a previous study, the annual average percentage of PCB congeners was about 27% for tri- and tetra-chlorobiphenyls, 11% for penta-, 40% for hexa-, and 22% for hepta- and octa-chlorobiphenyls (specifically at site 5) (Ferreira and Vale, 1995). Comparing these values with those obtained in the present study, increases of about 50% are apparent in the tri-and tetra-chlorobiphenyl proportions, while there are decreases in the more chlorinated biphenyls (about 50% for hexa- and 60% for hepta + octa-chlorobiphenyls). Since the less chlorinated PCBs are the most volatile and biodegradable, such an enrichment may indicate the occurrence of microbial degradation with dechlorination of the more chlorinated hydrocarbons (Bedard and May, 1996; Dannenberger, 1996; Hope et al., 1997) or a change in source material (a switch from a more highly chlorinated Aroclor to one with a lower chlorination number).

The presence of higher proportions of congener 49, along with the predominance of the lesser chlorinated biphenyls in the suspended sediments of the lagoon, may indicate that there have been no recent spillages in this area and the Gilao River appears to be the major source of these compounds to this area.

The higher concentrations of congener 49 found in the suspended matter support the hypothesis of anaerobic biodeg-radation of the more chlorinated biphenyls, by a combination of both the N and P processes of dechlorination. These pathways lead to the preferential formation of congener 49 (Bedard and May, 1996). However, most probably no anaerobic biodeg-radation occurs in the suspended particles since these have, generally, more oxic conditions, and so biodegradation must occur only in the settled sediments. The sediments of the Ria Formosa lagoon are strongly anoxic and have high OM content, especially in the areas where fine grains may be found, conditions that strongly favour the anaerobic decomposition of PCBs. Although higher concentrations of congener 49 are not observed in the sediments, there is clearly a predominance of the tri- and tetra-chlorinated biphenyls, which may indicate anaerobic decomposition of PCBs through other processes.

It is possible to compare the relative importance of the individual congeners in the suspended and settled sediments using the principal components from the PLS analysis. If the scores for the settled sediments are subtracted from the suspended sediments, the behaviour of the individual congeners becomes evident (e.g., site 1 shown in fig. 6). For several congeners (white bars), there is little difference between the suspended and settled sediments. These tend to be the more chlorinated compounds. For three congeners (49, 128 and 170, grey bars), there are greater concentrations in the suspended material compared with the settled sediments. This is especially evident for congener 49. The remaining congeners (black bars), are preferentially accumulated in settled sediments; these are mainly tri- and tetra-chlorinated compounds. This trend is seen at all sites.

It does not seem intuitively likely that congener 49 is formed in the sediments together with other tri- and tetra-chlorinated compounds, but only 49 is mobilized into the suspended matter. Another mechanism for the formation of congener 49 may be through UV irradiation of more highly chlorinated PCBs in the suspended sediments. The lagoon is exposed to high levels of UV light, especially between noon and 3 p.m., and and these increase during the summer (M.J. Bebianno, pers. comm.). There is evidence of anaerobic PAH degradation in the same system since the surface sediments also receive high UV levels (Mudge and Bebianno, unpub. data). Other authors have found similar pathways in laboratory systems (Miao et al., 1999; Oida et al., 1999).

In mesotidal estuaries, like the Ria Formosa lagoon, resus-pension of surface sediments occurs during each tidal cycle (Vale et al., 1993). Therefore, the particles collected in the sediment traps are probably a mixture of the suspended particles circulating in the system and resuspended particles from the surface sediments (Ferreira and Vale, 1996), which could account for the same predominance of these less chlorinated PCBs in the suspended matter.

The non-appearance of the higher contributions of congener 49 in the sediment profiles may be explained by the large amount of sediment sample taken (top 3 cm), which is representative of a few years of deposition. This would tend to dilute any recently formed congeners at the surface where UV irradiation is greatest.

There is also the possibility of atmospheric deposition of PCBs. This process of environmental contamination would favour the less chlorinated biphenyls and would also give greater contributions in the summer, when volatilization of PCBs is more significant (Dickhut and Gustafson, 1995). A combination of all these processes is probably occurring in the Ria Formosa lagoon. This would explain why no seasonal differences were found in the PCB levels in the particulate matter: in summer, there may be a greater contribution from atmospheric deposition and in winter, land run-off would dominate.

 

Conclusions

Suspended sediments showed higher concentrations of PCBs compared with the settled sediments. The same result is obtained when the data are expressed either on a dry weight basis or per unit OM. There were no temporal differences in the concentrations, although spatial variations were apparent.

Due to the sandy nature of some sites, OM influenced the spatial distribution of the total PCBs in both phases.

The PLS analysis indicates that the suspended matter is the source of PCBs to the sediment and not the other way around. There is also considerable overlap between the signatures as the suspended sediments may have a similar origin. However, site 3 is different to the others since it is located near the major outlet of the lagoon and is on sandy sediments. Similar PLS analysis using published Aroclor compositions suggests that the low chlorinated mixtures (e.g., Aroclor 1242) are the best source for the sediments and that the more highly chlorinated mixtures (e.g., Aroclor 1254) poorly explain the variance in the data. For the suspended sediments, the overall fit from the Aroclors is much worse, though Aroclor 1242 and 1248 best explained the variance at sites 1 and 5. This may be due to the altered nature of the PCB mixture in the sediments because they undergo degradation and dechlorination.

When PCA is used to compare the congener distribution in both the suspended and settled sediments, no differences are found between the large PCBs. The tri- and tetra-chlorinated PCBs are present in the settled sediments in higher concentrations compared with the suspended sediments, where congener 49 (and to a lesser extent congeners 128 and 170) is relatively enriched. It is postulated that the tri- and tetra-chlorinated PCBs are formed in the sediments by anaerobic dechlorination, but congener 49 is formed in the suspended matter or at the surface of the anaerobic sediments through a UV degradation pathway.

 

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