Distribution and heavy metal pollution of the suspended particulate matter on the barcelona continental shelf (North-Western Mediterranean)

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Environmental Pollution 85 (1994) 205-215

DISTRIBUTION A N D HEAVY METAL POLLUTION OF THE S U S P E N D E D PARTICULATE MATTER ON THE B A R C E L O N A CONTINENTAL SHELF (NORTH-WESTERN M E D I T E R R A N E A N ) Albert Palanques Institut de Cidncies del Mar ( CSIC), Passeig Joan de Borb6 s/n, Barcelona 08039, Spain (Received 15 June 1992; accepted 4 May 1993)

are also associated with the transport and deposition of the suspended particulate matter (SPM) discharged into the sea. The suspended waste and contaminants are dispersed by mixing and advective processes. The ultimate sink is influenced by the prevailing short-term currents (Csanady, 1981). After sedimentation, contaminants and waste can be resuspended either by bioturbation or by physical erosion. The impact of the waste at sea is highly controlled by physical, chemical and biological factors. The final situation is the effect of a dynamic interaction between anthropogenic input and natural processes. This paper focuses on the study of the heavy metals associated with suspended sediment discharged onto the Barcelona continental shelf (Fig. 1). Barcelona is one of the main industrial cities on the Mediterranean coast. The major sources of metals on this continental shelf are the Bes6s River, the littoral sewers, and the pipeline of the Barcelona-Bes6s wastewater treatment plant. The distribution of particulate heavy metals and SPM is analysed along with hydrographic parameters and currents in order to determine the main processes that control this distribution.

Abstract The distribution of heavy metal pollution associated with suspended particulate matter on the Barcelona continental shelf has been studied to evaluate the environmental impact o f anthropogenic metals in this Mediterranean area. The main sources o f heavy metal pollution on this continental shelf are the Bes6s River and the sewage sludge produced in the Barcelona-Bes6s wastewater treatment plant. The levels of Pb, Cr, Cu, Cd and Ni are very high around the mouths of the river and the pipeline of the wastewater treatment plant. The highest suspended sediment and heavy metal concentrations are along the inner and mid-shelf due to aggregation processes and the low energy of the dominant currents flowing in the study area. However, a significant amount of polluted suspended sediment is transferred to the slope by advective processes. This amount may be increased significantly by the action of wave-induced currents during strong storms. Trawl fishing may also contribute to the shelf-slope transfer o f contaminated particles. INTRODUCTION

Sedimentological setting On the Barcelona continental shelf sediment distribution is highly influenced by the regime of currents. In this area currents circulate in directions parallel to the coast, for 80-90% of the time. The most persistent current direction is towards the south-west. During the remaining time, currents are influenced by the action of the wind. Wind generated currents are important in surface waters and in the shallow part of the shelf. The average current velocity is between 5 and 10 cm s ~ and the maximum recorded current velocity was approximately 40 cm s ~ (Ballester & Julia, 1979; Amengual et al., 1988). On this continental shelf there are three main types of bottom sediment (Medialdea et al., 1989; Palanques & Diaz, 1994): (1) the littoral sediment which covers the area shallower than 15-20 m and is lithogenic, wellsorted sand; (2) the prodeltaic sediment which is mainly mud (silt+clay content >80%) that has accumulated along the inner and mid-shelf between the 20 and the 60 m isobaths, southward from the river mouth; and

Large coastal cities are one of the most important sources of pollution in the north-western Mediterranean. The rivers that discharge near these cities are highly polluted, and most of the waste discharged by them accumulates in the nearshore area. This situation is aggravated by the sewers discharging along the city littoral. Domestic wastewater treatment plants contribute to improving the quality of the littoral waters. However, the sewage sludge produced in the digestion tanks of these plants is in some cases discharged through pipelines into deeper areas where most of it is often not dispersed by currents, but accumulates near the pipeline mouth. Many of the contaminants, such as heavy metals discharged by rivers, sewers and pipelines, are associated with sludge and river particles (FOrstner & Wittmann, 1981; Salomons & FOrstner, 1984; Capuzzo et al., 1985). Therefore, the transport and fate of the contaminants Environ. Pollut. 0269-7491/94/$07.00 © 1994 Elsevier Science Limited, England. Printed in Great Britain 205

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was measured by two Aanderaa RCM-8 current meters deployed at the 22 m isobath (C, Fig. 1), one of them 8 m deep and the other 15 m deep. Water samples were vacuum-filtered on board through pre-weighed polycarbonate Nucleopore membranes (0.4 /zm pore diameter), which had been previously washed, dried and weighed, in order to determine suspended particulate matter concentrations and perform the heavy metal analyses. Particulate matter samples plus the Nucleopore membranes were digested in hot nitric acid (95°C) by the method described in Palanques et al. (1989). Cd, Co, Cr and Cu were analysed by induction coupled plasma; Pb and Ni by graphite furnace and Fe by flame atomic absorption. A smaller filter of each sample was studied by optical microscopy.

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Fig. 1. Map showing the Barcelona continental shelf and the location of the hydrographic stations sampled for this study. C: currentmeter location. (3) the outer shelf sediment, which is muddy biogenic sand covering the shelf between 60 m deep and the shelf break. Modern sediment accumulates mainly on the inner part of the shelf. The biogenic sand of the outer shelf is a relict sediment. Anthropogenic heavy metal pollution in the fine fraction of the bottom sediment is clearly detected in the prodeltaic sediment shallower than 50 m, between the Bests River mouth and about 10 km southward (Fig. 2) (Palanques & Diaz, 1994). On this part of the shelf the higher heavy metal levels are located around the mouths of the Bes6s River and the Bogatell sewer. The heavy metal pollution decreases seaward from these locations. In the mid-shelf, heavy metal pollution is relatively low, except in the area where sewage sludge from the Barcelona-Bests wastewater treatment plant accumulates. This waste has been discharged through a pipeline about 4 km long since 1979. The mouth of the pipeline is located near the 56 m isobath, in the transition area between the prodeltaic sediment and the outer shelf sand. The waste deposit in front of the pipeline covers an area of 1 km 2 and its maximum thickness is about 2.5 m. This waste is finer than the natural sediment in the Bes6s prodelta and is also highly polluted by heavy metals (Palanques et al., 1991). METHODS Three hydrographic cruises were carried out, one in November 1987, one in February 1988 and one in May 1988 with the R/V Garcia del Cid. The location of the hydrographic stations is shown in Fig. 1. Water samples, temperature and salinity data were taken at several depths by means of Niskin bottles. Water sampling was made in less than 12 h in order to get a reasonable synopticity for the distribution of different parameters. In addition, water from the Bes6s River was sampled nine times during 1 year. Current during the cruises

Suspended particulate matter distribution

The main source of SPM in the study area is the Bes6s River. The SPM concentrations in the lower part of this river are variable and relatively high (Table 1). These values are usual for a short river with a mountainous drainage basin affected seasonally by heavy rains (Milliman, 1980; Berner & Berner, 1987), but anthropogenie activities (deforestation, industrial and domestic dumping) must also contribute to an increase in the SPM concentration to a certain extent. At the river mouth, the fluvial SPM is quickly dispersed and the SPM concentration decreases very sharply from the freshwater-saltwater interface. The range of SPM concentrations on the continental shelf is shown in Table 1. The maximum concentrations were detected around the mouths of the Bes6s River and the pipeline (Fig. 3). On the continental shelf the suspended particles were not homogeneously distributed in the water column, but were more concentrated in surface and near-bottom waters, forming a surface nepheloid layer (SNL) and a near-bottom nepheloid layer (BNL) separated by relatively clear intermediate water (Fig. 4). Seaward, the SPM concentration tended to decrease, although this trend was locally disrupted by the input of particles discharged from the pipeline in the mid-shelf (Figs 3 and 4). The concentration of SPM around and over the pipeline mouth ranged from 1 to some tens of milligrams per litre. Table 1. Mean, maximum (Max.), minimum (Min.) and standard deviation (SD) values of suspended particulate matter concentration in surface water (SF) and in near-bottom water (NB) of the Barcelona continental shelf during the hydrographic cruises carried out in this study. S P M concentration in the lower Besbs River are also shown.

Value

Mean Max. Min. SD

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May 88

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NB

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NB

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NB

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1-5 4.6 0.5 1-2

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SPM from the BNL consists dominantly of terrigenic particles. Particles from the SNL are dominantly terrigenic around the river mouth (where SPM concentration >1 mg litre ]), but seaward they become dominantly biogenic. SPM distribution changed on each cruise according to the river discharge, the hydrographic situation and hydrodynamic processes. The structure of the water column during each cruise is shown in Fig. 5, and the water discharge and currents in Table 2. On the November cruise and on the February cruise the water column was vertically mixed and there was a

Fig. 2. Maps showing the distribution of heavy metals in bottom sediment. Concentrations are expressed in ppm except Fe which is in per cent.

salinity front lying landward around the river mouth (Fig. 5). This front was caused by the river freshwater discharge and limited a low salinity surface water layer in the inner shelf. SPM concentration in this low salinity water ranged between 2 and 4 mg litre t. This contributed to the presence of higher SPM concentrations in the SNL than in the BNL around the river mouth. However, the BNL was thicker and extended further seaward than the SNL. During both cruises the SPM concentration was higher than 1 mg litre ~ along the littoral area and along most of the SNL and BNL of

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Suspended particulate matter on the Barcelona Continental Shelf Table 2. River (R) discharge, surface (S) current and nearbottom (NB) current during the cruises

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the inner and mid-shelf (Fig. 4). The SPM concentration increased in front and over the pipeline mouth to several milligrams per litre. On the N o v e m b e r cruise a turbid water patch of several hundred square metres was found in the surface water over the pipeline mouth (Figs 3 and 4). On the May cruise there was also a salinity front around the river mouth, but the water column was in the initial stages of the seasonal water stratification. During this cruise the BNL was less developed, although the m a x i m u m SPM concentration was near the b o t t o m in the inner shelf around the Bes6s and Riera del Bogatell mouths. The S N L showed a wide along-shelf continuity [Figs 3 and 4(c)] instead of a m a x i m u m SPM concentration near the river mouth, as in previous cruises. The spring bloom of biological productivity produced an increase of chlorophyll A content in the upper 5 m of the water column, where this content ranged from 24 /xg litre ] around the river mouth to about 2 - 5 / z g litre t in the deeper stations (Amengual et al., 1988). During the previous cruises the m a x i m u m surface values of chlorophyll A hardly exceeded 2 /zg litre ]. The intermediate water was clearer than on the

Table 3. Natural levels of heavy metals in the fine fraction of bottom sediment of the study area (from Palanques & Diaz, 1994)

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previous cruises in most of the study area, even over the pipeline m o u t h where the SPM concentration was no higher than 2 /zg litre ' near the bottom and between 2 and 3mg litre ' in the surface water. Distribution of heavy metal pollution in suspended sediment The pollutants associated with suspended matter can be expected to have a very mixed history. The heavy metal-SPM interaction depends largely upon the conditions in the immediate vicinity of the particles, as well as upon the nature of the particles and the pollutants involved (Eisma, 1981). Natural levels of heavy metals in the SPM of this shelf are difficult to estimate because of the present contamination. However, natural levels of heavy metals in the fine fraction of the b o t t o m sediment were determined (Palanques & Diaz, 1994) and are shown in Table 3. In this study area the SPM shows a wide range of

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Table 4. Mean, maximum (Max.), minimum (Min.) and standard deviation (SD) values of heavy metals in surface suspended particulate matter (SF) an in near-bottom suspended particulate matter (NB) of the Barcelona continental shelf during the hydrographic cruises carried out in this study. Heavy metal concentrations in the SPM of the lower Bes6s River are also shown. Fe concentration expressed as a percentage. All other elements expressed in ppm.

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February 88

NB

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374 1 192 20 369

550 1079 118 331

613 1 576 97 469

446 854 97 222

551 1290 173 374

459 054 183 268

737 1378 372 433

Mean Max. Min. SD

976 2 572 250 754

709 2 236 160 564

849 3 357 41 957

343 889 60 271

516 1 712 184 408

340 248 76 335

1163 2402 323 675

Mean Max. Min. SD

864 1295 200 346

794 1250 270 339

500 888 173 242

347 666 138 153

568 942 128 266

465 866 172 191

428 818 136 218

Mean Max. Min. SD

10 40 <2 12

12 45 <2 17

17 44 <2 14

9 39 <2 11

9 34 <2 11

9 32 <2 11

Mean Max. Min. SD

103 272 27 80

101 240 25 77

105 258 56 56

85 150 32 37

88 237 34 57

80 213 32 54

151 227 105 37

Mean Max. Min. SD

15 25 <7 7

19 43 <7 12

12 38 <7 9

14 39 <7 10

12 34 <7 8

11 30 <7 7

21 35 8 9

Mean Max. Min. SD

3-4 4.5 1-4 0.9

3-5 4-6 2-6 0.6

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The higher heavy metal concentrations in marine SPM, which were similar or even higher than those of the river SPM, were located around the mouths of the river and the pipeline. Heavy metal concentrations decreased sharply around these two areas. The most remarkable examples of such a distribution were Pb and Cr (Figs 6-8). Cu and Cd showed a more dispersed pattern along the inner shelf, with a more gradual decrease seaward from the river mouth. Pb, Cr, Cu and Cd remained relatively high along the inner and mid-shelf beyond the river and the pipeline mouths. Heavy metals decreased to low concentrations on the outer shelf. Metal concentrations usually showed a sharper decreasing trend across the

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heavy metal concentrations (Table 4). In the lower Bes6s River, metal levels of the SPM are high and variable. This river is highly polluted by Pb and Cr and is also affected by Cu, Ni and Cd pollution. On the continental shell metal levels and distributions of the SPM changed on each cruise, but some c o m m o n features and trends were observed (Figs 6-8): (1)

May 88

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shelf than along the shelf. This trend was locally disrupted by the strong heavy metal anomaly of the suspended waste discharged from the pipeline. On the outer shelf, concentrations of Co, Ni, Cr and Cd were similar to the natural levels estimated for the fine fraction of bottom sediment. Occasionally, high concentrations of some heavy metals were detected along the midshelf (Fig. 6), probably as a consequence of the diffusion of contaminated particles from the effluent discharged by the pipeline. The distribution of particulate metals is mainly controlled by SPM behaviour and dynamics. The maximum heavy metal concentrations around the mouths of the river and the pipeline were caused by the scavenging of metals that takes place when particles and colloids flocculate in the freshwater-saltwater interface. When there is a transition from reducing to oxidising environments, as in sewage sludge discharge, organic matter and iron hydrous oxides also play an important role in ftoc formation and scavenging of heavy metals. Such processes have been widely described by several authors (Beck et al., 1974; Rashid, 1974; Monaco, 1977; Krone,

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1978; Hawley, 1982; Gibbs, 1986). Simultaneously with flocculation, diluted trace metals may be removed by co-precipitation, leading to an increase in the metal content of the SPM (Sholkovitz, 1976, 1978). Gibbs (1986) observed that heavy metal concentrations in flocs was significantly higher than in discrete single particles with a lower settling velocity. These mechanisms generate maximum heavy metal concentrations where flocculation occurs, and decreasing heavy metal gradients around these maximum areas. In the study area, sharp decreasing gradients of Pb and Cr were produced when their concentrations were higher than 1000 ppm, suggesting that high pollution levels of these metals were strongly associated with the fiocs that settled near the river and the pipeline mouths. Seaward from the mouths of the river and the pipeline, dispersion of SPM and associated metals takes place mainly in the nepheloid layers. Suspended particles from the BNL have a different nature than those from the SNL, and are differently affected by currents and biological activity. Marine currents constitute one of the main factors controlling SPM dispersion and distribution. In this study area the surface currents were relatively weak (Table 2), and had a different orientation on each cruise. During the November cruise the surface current was towards the south-west, and dispersion was dominantly along the shelf. The higher mean values and maximum values of most of the metals were detected during the November cruise (Table 4), when the shelf currents were lower (Table 2). During the February cruise, the orientation of the surface current was landward and the contaminated surface suspended particles discharged from the pipeline were dominantly dispersed towards the shore, as is indicated by the surface Pb, Cr and SPM distributions (Fig. 7). This contributed to increasing SPM pollution near the coast. During the May cruise the surface current orientation was seaward, and so the surface SPM was dominantly dispersed offshore and showed a wider dispersion than in the other cruises (Figs 3 and 4). This situation was probably favoured by the initial stage of surface water stratification and by the spring bloom of biological productivity. During these cruises the near-bottom current was towards the north-east. This produced a north-eastward displacement of the near-bottom polluted particles. However, north-eastward currents alternate with the stronger and more common south-westward currents. Consequently, near-bottom SPM can be displaced north-eastward and south-westward before settling, producing an along-shelf metal anomaly. Thus, on the Barcelona continental shelf the SPM distributions are dominantly affected by the action of lowenergy along-shelf currents. This favours the deposition of flocs and associated heavy metals on the inner and mid-shelf. On the sea-bed, the maximum heavy metal concentrations are located in front of the river mouth. in the area more persistently located under the low salinity surface water, and around the pipeline mouth. Fine sediment and heavy metal distributions showed a

dominant southward orientation (Palanques & Diaz, 1993), indicating that SPM dispersion is mainly controlled by the southward alongshore mean flow and that many heavy metals are deposited, associated with fine particles, along the prodelta. Concentrations of most heavy metals are lower in the prodeltaic fine sediment than in the overlying SPM. This is favoured by: (1) The organic matter, the content of which is higher in the over-prodelta SPM (10-25%) than in the prodeltaic bottom sediment (9-13°/0) (Palanques & Diaz, 1994). (2) The segregation of metals in different size ftocs produced by coagulation; Gibbs (1986) observed that flocs with a high settling velocity (>5.5 cm min '), which are quickly deposited in the prodelta, have lower metal concentrations than flocs with lower settling velocity (0.9-5-5 cm rain '), which remain as SPM for longer periods of time. (3) The diagenetic mobilization of trace metals; the surface sediment of the study area shows negative redox potential values which range from -350 to -400 mV around the river and pipeline mouth (Amengual et al., 1988), where the organic matter content is relatively high (10-13%). In oxic waters overlying an anoxic sediment, as around the river and pipeline mouths, metal remobilization occurs at the sediment - water interface (Presley & Tefry, 1980). (4) The resuspension action of currents, benthic organisms or anthropogenic activities. These processes may contribute to the delivery of metals from the bottom sediment. The segregation of metals by coagulation (2) is also responsible for the higher average concentrations of the Pb, Cr, Cu, Cd and Ni in surface SPM than in nearbottom SPM, especially in the low salinity surface water around the river mouth. The higher organic matter content of the surface SPM may also contribute to this trend. A very important metal anomaly is located in the Bes6s prodeltaic sediment. However, not all the fine particles and associated heavy metals are accumulated in the prodelta. SPM and metal distribution show that a significant amount of particulate matter remains in suspension and is transported to other areas. During low-energy current conditions, advective processes are more dominant in the near-bottom water and SPM can be transported obliquely to the shelfbreak and transferred to the slope. During stronger storms, however, diffusive processes may become significant and the shelf-slope transfer probably increases. On this continental shelf, resuspension of prodeltaic bottom sediment by currents is not frequent, and when it occurs it mainly affects the littoral and the inner edge of the prodelta. In the central part of the prodelta, where the sediment is finer and more cohesive, stormwave induced currents are mitigated and could cause

Suspended particulate matter on the Barcelona Continental Shelf

resuspension only during the strongest and least frequent events. Sonographs taken in the Besrs prodelta show that the most evident resuspension process affecting this area is a h u m a n activity: trawl fishing (Palanques et al., 1991). The effect of this activity has been evaluated as a significant resuspension process in other areas (Churchill, 1989). Resuspension by biological reworking of surface sediments could also be significant. The resuspension events m a y contribute to increase the shelf-slope transfer. The fate of the material escaping from the Barcelona continental shelf area is still unknown, but very high metal contents detected in the SPM of the Ebro continental slope (Palanques et al., 1990), about 200 km south of the study area, suggest that highly polluted particles could escape from the Barcelona continental shelf and be transported by the mean flow along the slope. CONCLUSIONS SPM discharged onto the Barcelona continental shelf by the Bes6s River and the pipeline of the wastewater treatment plant is dispersed forming a surface and a nearbottom nepheloid layer. This SPM is contaminated by Pb, Cr, Cu, Cd and Ni. The strongest anomalies of these metals were located near the river and pipeline mouths where coagulation and metal scavenging occurs. Low-energy along-shelf currents are the dominant factor controlling distribution and dispersion of SPM and associated metals on the Barcelona continental shelf. These currents allow the deposition of polluted fine sediment in the Bes6s River prodelta. However, significant amounts of contaminated suspended particles are transported towards the slope. ACKNOWLEDGEMENTS R a m 6 n par6s, Andr6s M a l d o n a d o and Ignacio Diaz are thanked for their help and support and Ferrfin Valldespinos, Jorge Guill~n, Gabriel Borrfis, Ricard N a v a r r o and Agusti Julia for their assistance at sea and technical support. We also express our gratitude to the officers and crew of the R/V Garcia del Cid. Montse Roura, Montse Baucells, Gloria Lacort, Pilar Garcia and Elisenda Verg6s are thanked for their technical assistance. This study received economic support from the project A M B 92-0251-CO2-01 funded by the 'Comisi6n interministerial de Ciencia y Tecnologia' and the project PL-920027 funded by the CEE.

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