Artículos

 

PCB composition in flood material and sediments from the Guadiana River estuary

 

Composición de PCB en el material y los sedimentos de inundación del estuario del Río Guadiana

 

Marta Martins, Ana Maria Ferreira* and Carlos Vale

 

^* Instituto de Investigação das Pescas e do Mar Av. Brasilia, 1449-006 Lisboa Portugal * E-mail: amfer@ipimar.pt

 

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

 

Abstract

Coarse material and freshly deposited particles transported by the exceptional flood of January 2001 were sampled at eight sites along the Guadiana River and estuarine shorelines. Two months after moderate river flows, we collected 27 surface sediments from the river-estuarine system and one sediment core from the coastal area. The PCB levels in sediments were lower than 1.1 ng g^-1, which is in conformity with the absence of industrial activities in the Guadiana drainage basin. Material transported by the flood contained higher PCB concentrations (0.8-1.8 ng g^-1) than surface sediments (0.1-1.1 ng g^-1), indicating the importance of these events in the transport of PCB to this low-contaminated estuarine system. These differences were not related to organic carbon content. Furthermore, this material is enriched in less chlorinated compounds as usually occurs far away from point sources. The diffuse transport influences the CB composition of sediment deposits on the adjacent coast, as revealed by variations in the depth profiles of the tri-/hexa-chlorinated ratio, organic carbon content and Si/Al ratio.

Key words: PCB, congeners, sediments, flood material, Guadiana River.

 

Resumen

Se analizaron el material grueso y las partículas recién depositadas transportadas por la inundación excepcional de enero de 2001 en ocho puntos del Río Guadiana y los márgenes del estuario. Después de dos meses de caudal moderado, se recolectaron 27 muestras de sedimento superficial en el sistema fluvial-estuarino y un core de sedimento en la zona costera. Los niveles de PCB en los sedimentos estuvieron por debajo de 1.1 ng g^-1, lo cual concuerda con la ausencia de actividades industriales en la cuenca hidrográfica del Guadiana. Las concentraciones de PCB fueron mayores en el material transportado por la inundación (0.8-1.8 ng g^-1) que en los sedimentos superficiales (0.1-1.1 ng g^-1), lo que demuestra la importancia de estos eventos en el transporte de PCB a este sistema estuarino poco contaminado. Estas diferencias no se relacionaron con el contenido de carbono orgánico. Además, este material se encuentra enriquecido de compuestos menos clorados como es usual en lugares alejados de las fuentes puntuales. El transporte difuso afecta la composición de bifenilos clorados de los sedimentos depositados en la costa adyacente, según muestran las variaciones de la relación triclorados/hexaclorados, el contenido de carbono orgánico y la relación Si/Al, en los perfiles de profundidad.

Palabras clave: PCB, congéneres, sedimentos, material de inundación, Río Guadiana.

 

Introduction

Polychlorinated biphenyls (PCBs) are among the most widespread and persistent contaminants in the coastal environment, being sorbed onto suspended particulate matter and bottom sediments (Dannenberger, 1996; Guruge and Tanabe, 2001; Lee et al., 2001). These compounds may be supplied to the coastal zones through river inputs (Van Zoest and Van Eck, 1990), atmospheric deposition (Sanders et al., 1996), spills (Ferreira and Vale, 2000) and dumping of dredging material (Tang and Myers, 2002). Estuaries in particular tend to accumulate large amounts of polluted riverine particulate matter, contributing to the environmental stress on the estuarine ecosystems (Van Zoest and Van Eck, 1990). As a result of international restrictions on manufacture and use of PCBs, their direct discharges by industry became less common (Connell et al., 1998). Trace quantities of these pollutants are also introduced into the aquatic environment during storm water runoff with diffuse sources progressively replacing the active point sources (Makepeace et al., 1995).

Before reaching the estuarine zone, the Guadiana River crosses approximately 130 km of a rural area where there are no point sources of PCBs. The fluvial regime is characterized by low flows in summer (79 m³ s^-1) and episodic runoff periods in winter (thousands m³ s^-1), carrying eroded soil particles from the drainage basin. This fluvial-estuarine system thus appears to be a suitable environment to assess the importance of diffuse sources associated with river runoff in the transport of PCB to the coastal zone.

 

Sampling and methods

Coarse material and freshly deposited particles transported by the Guadiana River after an exceptional episode of river flooding in January 2001 (max. 1200 m³ s^-1) were collected along 70 km of the river and estuarine shorelines (fig. 1). At the flood peak, the river transported coarser material that was sampled from a flat area of the right margin (station R1). During the declining flood phase, the river transported finer particles that were deposited on the margins. These brownish deposits were sampled one week after the flood at seven stations (R2 to R8) along the river and estuarine zone (fig. 1). In March 2001, after the river had reached moderate flows, 27 surface sediments were collected with a grab at several cross-sections of the same river-estuarine zone (stations A to J). At each cross-section, one to three samples were collected. Sediments consisted of a mixture of sand and fine particles. At each station, the first 2-cm layer of the sampled sediments was chosen for chemical analysis. In the same period, one sediment core (station M1) was sampled with a multicorer Midicorer Mark II-400 equipped with transparent tubes in the coastal area adjacent to the Guadiana estuary (fig. 1). The visual observation of the cores evidences mud-sand interlayers, with a thicker sand layer near the surface. One of the four cores collected was selected and sliced on board in layers of 2-cm thickness until 36-cm depth. All samples were stored frozen until analysis in the laboratory.

Aluminium, silicon, organic carbon (C_org) and PCB congeners were analyzed in all the material; C_org was analyzed in a Carlo Erba Elemental Analyzer, before and after heating the samples at 450°C. Aluminium and silicon were determined by atomic absorption spectrophotometry after acid digestion described by Rantala and Loring (1977). For PCB analysis, dry sediment samples were Soxhlet extracted with n-hexane for 16 h. The extracts were cleaned up with Florisil and sulphuric acid following the procedure described in Ferreira and Vale (2001). The elemental sulphur was removed with activated copper. The PCB congeners were analyzed using a Hewlett-Packard 6890 gas chromatograph with an electron capture detector and a capillary column (DB5, J&W, 60 m). In this study, PCB means the sum of congeners (CB) with IUPAC numbers: tri- (18, 26, 31), tetra- (44, 49, 52), penta- (101, 105, 118), hexa- (128, 138, 149, 151, 153) and hepta-CB (170, 180, 183). All concentrations are expressed on a dry weight basis.

 

Results

Salinity gradients along the river-estuarine zone

During the flood period in January 2001, fresh water filled almost the entire estuarine zone, salinity reaching 5 only at the outlet channel of the estuary (cross-section J). During the campaign of March 2001, the river flows were back to moderate values and salinity ranged from 1 to 35 in the estuarine zone between cross-sections D and G.

Organic carbon content

Coarser material and freshly deposited particles transported by the flood presented a broad range of C_org (0.4-2.8%), as well as surface sediments collected along the river-estuarine system (0.01-3.2%). All the samples were separated into two groups containing statistically different levels (P < 0.05) of C_org: low C_org (0.01-0.57%) and moderate C_org samples (0.97-3.20%). The longitudinal distributions of C_org along the Guadiana River estuary are shown in figure 2. The freshly deposited particles (represented by black bars) are included in the second group, while the coarser material collected at the flood peak (dashed bar) has low C_org. Those freshly deposited particles were found along the estuarine system.

PCB concentrations

The longitudinal distributions of PCB concentrations in the two sets of samples are presented in figure 3. Clearly, the coarse material and freshly deposited particles transported by the flood contained higher PCB levels (0.8-1.8 ng g^-1) than the surface sediments (0.1-1.1 ng g^-1). Whereas sediments impoverished in C_org contained low PCB concentrations, the coarse material presented low C_org but PCB concentrations similar to the fine material. No correlation between PCB and C_org was obtained and the representation of the PCB/C_org ratio presented a similar longitudinal distribution.

PCB composition

The mean proportions of tri-, tetra-, penta-, hexa- and hepta-chlorinated congeners (percentage of total) in surface sediments and in coarse and freshly deposited particles are compared in figure 4. On average, tri-CB accounted for around 60% of PCB in these materials, while in surface sediments it was less than 35%. The proportion of hexa- and hepta-CB was slightly higher in the surface sediments.

Ratio between tri- and hexa-chlorinated congeners

To better illustrate the differences in PCB composition between materials transported by the flood and surface sediments, the ratio between the levels of tri-CB (CB18 + CB26 + CB31) and hexa-CB (CB128 + CB138 + CB149 + CB151 + CB153) was calculated. The results obtained for samples with low and moderate C_org contents are presented in two plots (fig. 5). In general, higher ratios were obtained for coarse material and freshly deposited particles transported by the river flood (max. 9.4). Most of the surface sediments containing lower C_org showed tri-CB/hexa-CB ratios greater than one (max. 3.6), and sediments with moderate levels of C_org ratios less than one. A few exceptions were registered. Although the longitudinal trend was not well defined, all samples from the lower estuary (sections I and J) presented ratios less than one.

PCB, Si/Al ratio and organic carbon in coastal sediments

Depth variations of the Si/Al ratio, C_org content, PCB concentrations and tri-CB/hexa-CB ratio in the sediment core M1 from the coastal area adjacent to the estuary are presented in figure 6. The core is characterized by pronounced depth variations of the Si/Al ratio and C_org, reflecting interlayers of coarse and fine particles as well as material with a broad range of C_org. The low C_org in the sub-surface layer (0.20%) coincides with the increase of the Si/Al ratio (4.1). This layer (5-7 cm depth) showed higher values of PCB (0.75 ng g^-1) and of the tri-CB/ hexa-CB ratio (4.0) than other sediment layers containing lower values of both C_org and the Si/Al ratio. The second peak of the Si/Al ratio recorded between 10 and 14 cm depth was also accomplished by increases of PCB and of the CB ratio, although the variation of C_org content was minor. At deeper layers (15-20 cm depth), the sediment presented higher C_org without substantial alterations in the Si/Al ratio and PCB.

 

Discussion

The low PCB concentrations in sediments from the Guadiana estuary (<2 ng g^-1) indicate that this fluvial-estuarine system has a low degree of contamination in comparison to other estuarine systems (Frignani et al., 2001; Jeong et al., 2001). The occurrence of a slight increase of PCB in sediments from the estuarine mouth (G3, H2, I1, I2, I3, J1, fig. 3) suggests, however, the existence of slight point sources in the urbanized areas of the margins. In addition to this increment, the tri-CB/hexa-CB ratio decreased in sediments from this part of the estuary. The predominance of hexa-CB in this area corroborates the hypothesis of local sources, since proportions of higher chlorinated congeners are usually higher near point sources because of lower chlorinated congeners being more volatile, degradable and lost more readily (Lee et al., 2001).

Particles transported by the flood and deposited on the shoreline exhibited higher concentrations of PCB than sediments; however, the proportions of higher chlorinated compounds are lower in comparison to surface sediments. The alterations of CB composition in conjunction with the absence of industries in the drainage river basin, suggest that the increase of PCB in material transported by river flood resulted from diffuse transport of these compounds rather than from the influence of active point sources. Several works have shown the importance of diffuse sources on the transport and distribution of PCB in coastal environments, associated with storm water runoff (Ferreira and Vale, 1995; Makepeace et al., 1995). The predominance of less chlorinated compounds observed in the coarse and fine soil particles transported by the storm water runoff also reflects the influence of diffuse sources due to the higher mobility of these congeners both in the atmosphere and aquatic milieu (Iwata et al., 1995; Dannenberger, 1996; Lee et al., 2001). The absence of active point sources in the Guadiana drainage basin suggests that lower CB proportions in soil particles are derived from atmospheric transport. The assumption of PCB mixture enriched in less chlorinated congeners from a potential source in remote areas is less realistic, as shown by the enrichment of higher chlorinated congeners in sediments at the mouth of the Guadiana estuary and after an accidental spill in the Tagus estuary (Ferreira and Vale, 2000).

The recognition of alterations occurred in the PCB composition input with floods is of great importance since it is expected that in the future, as point sources decrease or cease, the PCB levels in aquatic systems will be mainly influenced by diffuse transport (Connell et al., 1998). Pulse mechanism, associated with peak river floods, supplies the estuary with material containing predominantly lower CB. These compounds tend to be distributed along the estuary and in certain cases may reach the adjacent coastal zone, as registered by the depth profile of the proportion of tri-CB with respect to hexa-CB in the Guadiana system.

 

References

Connell, D.W., Wu, R.S.S., Richardson, B.J., Leung, K., Lam, P.S.K. and Connell, P.A. (1998). Occurrence of persistent organic contaminants and related substances in Hong Kong marine areas: An overview. Mar. Pollut. Bull., 36: 376-384.         [ Links ]

Dannenberger, D. (1996). Chlorinated microcontaminants in surface sediments of the Baltic Sea: Investigations in the Belt Sea, the Arkona Sea and the Pomeranian Bight. Mar. Pollut. Bull., 32: 772-781.         [ Links ]

Ferreira, A.M. and Vale, C. (1995). The importance of runoff to DDT and PCB inputs to the Sado estuary and Ria Formosa. Neth. J. Aquat. Ecol., 29: 211-216.         [ Links ]

Ferreira, A.M. and Vale, C. (2000). Aumento dos níveis de PCB nas partículas em suspensao, peixes e moluscos do estuário do Tejo e zona costeira adjacente após um derrame acidental. In: A. Duarte, C. Vale and R. Prego (eds), Estudos de Biogeoquímica na Zona Costeira Ibérica, Universidade de Aveiro, Portugal, pp. 55-60.         [ Links ]

Ferreira, A.M. and Vale, C. (2001). Seasonal and inter-annual variations of PCB and DDT contents in the oyster Crassostrea angulata from the Sado estuary (Portugal). Cienc. Mar., 27: 255-268.         [ Links ]

Frignani, M., Bellucci, L.G., Carraro, C. and Raccanelli, S. (2001). Polychlorinated biphenyls in sediments of the Venice Lagoon. Chemosphere, 43: 567-575.         [ Links ]

Guruge, K.S. and Tanabe, S. (2001). Contamination by persistent organochlorines and butyltin compounds in the west coast of Sri Lanka. Mar. Pollut. Bull., 42: 179-186.         [ Links ]

Iwata, H., Tanabe, S., Ueda, K. and Tatsukawa, R. (1995). Persistent organochlorine residues in air, water, sediments, and soils from the Lake Baikal region, Russia. Environ. Sci. Technol., 29: 792-801.         [ Links ]

Jeong, G.H., Kim, H.J., Joo, Y.J., Kim, Y.B. and So, H.Y. (2001). Distribution characteristics of PCBs in the sediments of the lower Nakdong River, Korea. Chemosphere, 44: 1403-1411.         [ Links ]

Lee, K.T., Tanabe, S. and Koh, C.H. (2001). Contamination of polychlorinated biphenyls (PCBs) in sediments from Kyeonggi Bay and nearby areas, Korea. Mar. Pollut. Bull., 42: 273-279.         [ Links ]

Makepeace, D.K., Smith, D.W. and Stanley, S.J. (1995). Urban stormwater quality: Summary of contaminant data. Rev. Environ. Sci. Technol., 28: 1278-1284.         [ Links ]

Rantala, R. and Loring, D. (1977). A rapid determination of 10 elements in marine suspended matter by atomic absorption spectrophotometry. Atomic Absorption Newslett., 16: 51-52.         [ Links ]

Sanders, G., Hamilton-Taylor, J. and Jones, K.C. (1996). PCB and PAH dynamics in a small rural lake. Environ. Sci. Technol., 30: 2958-2966.         [ Links ]

Tang, N.H. and Myers, T.E. (2002). PCB removal from contaminated dredge material. Chemosphere, 46: 477-484.         [ Links ]

Van Zoest, R. and Van Eck, G.T.M. (1990). Behaviour of particulate polychlorinated biphenyls and polycyclic aromatic hydrocarbons in the Sheldt estuary. Neth. J. Sea Res., 26: 89-96.         [ Links ]