TRACE METAL DISTRIBUTION IN SURFACE SEDIMENTS FROM THE EASTERN BRAZILIAN COAST

Luciano Guedes; Cristina M.M. Souza; Carlos E.V. de Carvalho; Alvaro R.C. Ovalle; Carlos E. Rezende &, Marcelo G. Almeida (Laboratório de Ciências Ambientais, CBB, UENF. Av. Alberto Lamego 2000, Horto, Campos dos Goytacazes, R.J. CEP 28.015-620. Brazil. Tel +55 24 726-3709. Fax +55 24 726-3720. E-mail: cristal@cbb.uenf.br )

 

Abstract

The present work studied the heavy metals distribution on surface sediments in the Brazilian Continental Shelf. Total and potential available extractions were performed in the total bulk of sediments in order to evaluate the contamination and bioavailability of these elements for marine biota. Granulometric analysis were also performed in order to characterize the sediments and help the trace metal distribution interpretation. The sediment granulometry showed a predominance of the Silt-Clay fraction (up to 80%). The total metals concentrations (Fe, Ni, Mn, Cu and Cr) were negatively correlated to particle size, except Zn that presented no correlation. Positive correlation was also observed among the studied metals, probably indicating an association between these elements and the litological source. The potentially available fraction presented higher concentrations in shallow areas near the influence of the small and medium size rivers. Heavy metals concentrations showed decrease values seaward possibly due the transport rivers capacity and the mixture with biogenic sediment.

 

Introduction

The present study is part of the Brazilian and German Bilateral Scientific Cooperation known as Joint Oceanografic Projects (JOP’S II). The sediment sampling was performed in the beginning of 1995 on board of  the  Research Vessel Victor Hensen along in Brazilian waters.          

 

            The East Brazilian  continental shelf  include a very interesting geomorphological feature the Abrolhos Bank, which form a natural barrier to the Brazil Current (Ekau, et al. 1999). This barrier promotes upwelling areas in the south of the Abrolhos Bank  (Vitória Edge), increasing nutrient input, biodiversity and local primary  productivity (Gaeta et al. 1999). The large number of small and medium sized rivers in this region and the lack of information on pollutants input, increase the importance of the study of heavy metal from natural and  antropogenic sources. These elements reach the coastal zone mainly by rivers, generally being adsorbed in solid phase and subsequently deposited in sediments by chemical and mechanical processes. In sediments metals could be associated with specific geochemical phases that are susceptible to changes depending on  environmental conditions. In sediments metal associations will define the availability and incorporation rate to the bentonic organism (Salomons & Förstner, 1984).

  

Material and Methodology  

In this work were analyzed samples collected on 22 stations, comprising 5 sampled areas during the period of 28.12.94 to 11.01.95, from Doce River at Espírito Santo to estuarine zone of  São Francisco River at Sergipe.        

 

            The sediment granulometry was analyzed following the methodology described by ABNT/NBR 7181(1984). The metal (Fe, Mn, Zn, Cr, Ni and Cu) extractions were based on methodology described by Krause et al. (1995)  modified by Almeida (1998). Metal determination was performed using an atomic absorption spectrophotometry, with flame (Varian, model AAS 120). All the samples were digested in duplicate and blanks were analyzed in order to avoid sample contamination. The precision and accuracy of the analytic method was determined by analyzing certified estuarine sediment (Standard Reference Material 1646a) provided by NIST/USA. The available heavy metal fraction (Exchangeable + Oxydizible + Reducible) was determined using the methodology described by Carvalho (1997), where 2g of sediments were added to 15 ml of HNO3 (0,5N) at room temperature and kept shaking for 24h. The extracts were than filtered and analyzed  for heavy metal concentration in an ICP-AES (Varian - Liberty Series II).

 

 Results 

            Table 1 presents heavy metals concentration intervals in superficial sediments (mg.g -1) in both of the studied fractions (total and available). Heavy metals distribution along different stations in the total and  available fractions  are shown in the Figure 1. The results showed the following order of abundance for total concentration  Fe> Mn> Cr> Zn> Ni> Cu.  

Fe, Mn and the Cu showed the same total concentration distribution, in some cases the difference among the stations of the same sampling area, were orders of magnitude higher. Those differences were observed for the first two elements in the Canavieiras area and for Cu, in the Caravelas area. Fe presented the largest differences in the measured concentrations in Canavieiras, Caravelas and Espírito Santo areas. The other elements presented smaller differences of concentrations among the stations of same area, with factors that varied between 1.1 to 7.7. 

 

            All the metals presented representative distribution in the potential available fraction of the sediment, being Zn the element that presented the most homogeneous and expressive distribution in this fraction, followed by manganese. Among the areas, the Espírito Santo area had the highest percentile of  available phase for all the metals (Fe = <DL - 8,7 %; Mn = 36 % - 96 %; Cr = 0,30 % - 8,0 %; Zn = 15% - 60 %; Cu = <DL - 20 % and Ni = 1,0 - 1,7 %). 

Some sampling stations presented concentrations of metals (Fe, Cr, Zn, Cu and Ni) higher than the world average for sediment (Salomons & Förstner, 1984), with enrichment factors varying from 1.1 (Cr and Zn) and 1.5 times (Fe, Cu and Ni). The Espírito Santo area presented the highest  enriched factors among all the studied areas for Fe, Cu, Cr, Zn and Ni.   

 

The granulometric analysis results showed a very heterogeneous distribution along all sampling sites. Because of this large variation even in the same sampling areas, the establishment of an average sediment granulometry was avoided.

 

Table 1. Total and available fraction heavy metal concentrations on superficial sediment (mg.g -1)

 

Heavy Metals  (mg.g -1)

Areas

Fe

Mn

Cr

Zn

Cu

Ni

SF

A

<DL

189

0.31

15.4

0.23

0.86

 

T

37779 – 43984

311 - 424

70 -79

59 - 71

7.9 - 43

50 - 63

CN

A

<DL - 3418

3.0 - 248

<DL - 6,2

<DL - 17

<DL - 3,4

<DL – 3.1

 

T

2733 - 40046

34 - 433

24 - 94

22 - 58

9.7 - 30

31 – 63

CB

A

8.0 - 723

<DL - 68

0.04 – 1.4

15 - 20

<DL - 0,05

<DL – 0.4

 

T

7259 - 21943

55 - 302

32 – 58

32 - 54

9.6 - 14

28 – 49

CV

A

<DL - 984

<DL - 173

0.01 – 3.7

15 - 23

0.01 – 3.7

0.04 – 0.68

 

T

1704 - 37718

56 - 335

27 – 54

25 - 48

0.85 - 12

28 – 43

ES

A

416 - 7813

177 - 564

0.32 – 6.7

16 - 17

<DL – 5.1

0.62 – 1.2

 

T

50623 - 78580

483 - 644

66 - 112

27 - 107

6.6 - 51

57 – 73

SF= São Francisco; CN= Canavieiras; CB= Cabrália; CV= Caravelas; ES= Espírito Santo; T= Total fraction; A= Available fraction; <DL= lower than detection limit.

           

 

Discussion       

            The heavy metal distribution in surface sediments along the sampling stations showed different patterns in each area. This trend could be related to the distinct granulomety and/or to different  sources of  these elements (riverine x marine) and the distance of each sampling site from the point source. It is already known that the these factors increase heavy metals concentration in the water column and consequently in surface sediments (Lacerda et al. ,1992). 

 

            Tropical coastal sediments generally present a natural enrichment in Fe concentrations (Almeida, 1998) mainly due to the high weathering and erosion of the soils. This trend was observed in the Espírito Santo area for both fractions when compared to the others areas, although the increase in the available fraction values probably suggests the influence of antropogenic activities, once the distribution of that element in areas without pollution is limited to the residual fraction of the sediment (Salomons & Föstner, 1984). The distributions of Cr and Cu in this station also presented the same trend, corroborating the hypothesis of  possible antropogenic influence.

  

            Significant positive correlation between Fe and Mn, and among those two elements and Cr, Zn and Ni in the total fraction was observed. According to Salomons & Förstner (1984), those correlation are probably indicating that this elements have the same source, possibly lithologic, characteristic of sediment non impacted, therefore, of natural origin. Although Espírito Santo area is close to urbanized areas, the other areas were characterized by processes related to the natural variations in the fluvial flows, according to the proximity of some stations the outlet of small rivers placed along the sampling stations.  Espírito Santo, São Francisco and Canavieiras areas presented total concentrations above the world average for sediments for Cu (1,3), Cr (1,3) and Ni (1,2). The observed concentrations in these sampling sites are probably related to the presence of  riverine material from the São Francisco and Belmonte rivers, that are responsible for a significant seasonal influence the  in the coastal zone (Danielson et. al. ,1983; Jennerjahn et al. ,1996).  

 

            Mn and Zn presented an important participation in the available fraction of the sediment. The manganese showed an accentuated remobilization capacity in the Espírito Santo area (up to 98 %), suggesting a strong association to substrate susceptible to redox reactions. The Zn presented values similar of concentration potentially in the available fraction along all the sampling stations, except for the station 8. That behavior is characteristic of these element in both natural and impacted reas (Almeida, 1998 ;Souza et. al., 1986).  

A significant positive correlation was observed between Fe and Mn and the other studied metals (Cu, Cr and Ni) in the available fraction, probably indicating the importance of the co-precipitation process controlled by Fe and Mn oxy-hydroxides. The oxides and hidroxides of Fe and Mn are considered as one of the main responsible geochemistry by the deposition and accumulation of metals in marine and fluvial sediments (Yucesoy & Ergin, 1992; Summerhayes & Thorpe, 1996). 

 

            The predominance of the sand size fraction in Canavieiras, Cabrália and Caravelas areas suggests a very dynamic area, mainly due to the absence of large rivers and the strong influence of the oceanic current. According to Summerhayes and Thorpe (1996) during the last regression there was a predominance of the silt-clay fraction in the sediment, although the coastal areas are characterized by heterogeneous  sediments granulometry. This variability generally depend on the proximity of sources and or dynamic transport processes characteristic of shallow coastal platform waters. Although one of the stations at this area presented  distinct granulometry distribution with the predomination of silt (39 %) and fine sands (36 %) fractions.

 

            In some areas as Canavieiras and Cabrália large variations in the metal concentrations, up to one order of magnitude, were observed for some of the studied metals (Fe, Mn and Cu). The stations that had high metal concentrations also presented the predominance of the sand fraction. This trend is probably suggesting the influence of the sand fraction in the metal transport, maybe promoting alterations in the relationship between the heavy metals and the finest fraction of the sediment (Goh & Chou, 1997), this could be related to the presence of small rivers in the area (Melo et. al. 1975).

 

In comparison to other studies in estuarine areas and of continental platform, the present study presented medium concentrations similar to these, having presented for Mn inferior values, but for Fe, Cr and Ni close values the found highest concentrations. Comparing with Souza, (1995), whose study was accomplished in a river considered non impacted, the concentrations presented similar variations for Fe, Ni, Cu and Cr, and inferior for Mn. 

 

Conclusion 

Espírito Santo and São Francisco regions were presented the highest metals concentrations probably due to the presence of the Doce and the São Francisco Rivers as the main  metal input.  In the Canavieiras area Belmonte river's influence is clearly shown although the effect of the coastal currents promote a rapid dillution and a large variability in the granulometric distribution.

 

All the studied metals presented positive correlation with iron and manganese, indicating association to the lithologic source of the material and the formation of Fe and Mn oxy-hydroxides in the continental shelf waters reinforcing this process as an important sedimentation process. In the general a decrease of heavy metals concentrations toward the ocean was observed, probably due to lost of the transport capacity and the dilution effect of caused by the mixture of riverine material (enriched in metal content) with marine material (generally poor in metal content). 

The granulometric distribution presented correlation with the distribution of the metals and with the distance of the coast, corroborating with observations done by other authors. The metals presented more homogeneous distribution near the coast, in areas of smaller energy. The available fraction presented larger concentrations near the coast, rivers outlet and areas with possible antropogenic influence. 

             


           

Figure 1 – Heavy  metal (total and available fraction) distribution in all sampling stations.  

 

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Acknowledgements

            The authors would like to thank the support by JOPS Project. and FENORTE