Heavy metal distribution in recent sediments of the Tietê-Pinheiros river system in São Paulo state, Brazil

Applied Geochemistry 17 (2002) 105–116 www.elsevier.com/locate/apgeochem

Heavy metal distribution in recent sediments of the Tieteˆ-Pinheiros river system in Sa˜o Paulo state, Brazil Ivone S. da Silva, Gilberto Abate, Jaim Lichtig, Jorge C. Masini* Instituto de Quı´mica, Universidade de Sa˜o Paulo, Caixa Postal 26077, Cep 05513-970, Sa˜o Paulo, SP, Brazil Received 30 March 2000; accepted 18 June 2001 Editorial handling by O. Selinus

Abstract The concentrations and possible chemical associations of Al, Fe, Mn, Ca, Cu, Pb, Cd, Zn, Ni and Cr in sediments of the Tieteˆ-Pinheiros river system in Sa˜o Paulo state, Brazil, were studied using a 3-step sequential extraction protocol recommended by the Standards, Measurements and Testing Programme (SM&T, formerly BCR). A single extraction with 0.1 mol l
1 HCl was applied in parallel to anoxic and air-dried samples. The river system crosses the metropolitan area of Sa˜o Paulo (MASP), which houses a population of nearly 17 million people, and receives a large load of industrial and domestic wastes. Samples were collected from reservoirs in the surroundings of MASP, named Billings, Pirapora and Rasga˜o, and from the Barra Bonita reservoir, that is located in the Tieteˆ river, 270 km downstream from Sa˜o Paulo city. The distribution of metals indicates the recent pollution characteristics for samples from the Billings, Pirapora and Rasga˜o reservoirs. In these sediments the metals are associated to a large degree with reactive forms such as sulphides and carbonates, or adsorbed to amorphous oxyhydroxides of Fe and Mn. In samples from Barra Bonita, heavy metals are mainly associated with the residual fraction, suggesting that their concentrations are controlled significantly by transport processes with fine particles as carriers from diffuse pollution sources. # 2001 Elsevier Science Ltd. All rights reserved.

1. Introduction Heavy metals are considered serious inorganic pollutants because of their toxic effects on biota (Alloway and Ayres, 1997), having a high enrichment factor and slow removal rate (McEldowney et al., 1993). Riverine or lacustrine sediments function as a sink for heavy metals from diverse sources, reflecting the natural soil composition of the surrounding areas, as well human activities. In an aquatic environment, heavy metal ions are subject to precipitation, complexation or adsorption and solubilization reactions, depending on the physical and chemical characteristics of the water body (Morgan and Stumm, 1991). According to Quevauviller et al. (1993), the determination of specific chemical forms, or the nature of

* Corresponding author. Fax: +55-11-38155579. E-mail address: [email protected] (J.C. Masini).

binding, is much more valuable than determination of the total metal content, since the toxic effects and the geochemical pathways of heavy metals are determined mainly by their mobile species. The knowledge of such chemical forms enables prediction and prevention of the adverse effects of heavy metals to affected communities (Benjamin and Honeyman, 1992). To evaluate the fraction of heavy metals that could be available for biota, several extraction procedures have been proposed (Kersten and Fo¨rstner, 1995). The idea is to extract the metals sequentially by increasing the reactivity of the extractant solution, so that in the first extraction step only adsorbed and exchangeable heavy metal cations would be solubilized, while in the last step the attack would extract cations associated with the sedimentary matrix (residual fraction). Pempkowiak et al. (1999) demonstrated that mussels living in two environments with similar total contents of heavy metals contained different concentrations of these elements in their soft tissues. Greater concentrations were observed in the

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organisms living in the environment in which heavy metals were extracted in the earlier steps of sequential extraction. The present study was performed in water reservoirs of the Pinheiros and Tieteˆ rivers in Sa˜o Paulo state, Brazil. Two regions were studied: one located in the surroundings of the Metropolitan Area of Sa˜o Paulo (MASP) city, that has a population of nearly 17 million people, at the Billings, Pirapora and Rasga˜o reservoirs, while another region was in the countryside of Sa˜o Paulo state, at the Barra Bonita reservoir, located 270 km downriver from the MASP. These reservoirs are connected by the so called Tieteˆ-Pinheiros river system. The Tieteˆ river flows through the countryside of Sa˜o Paulo state, starting in the coastal mountain chain, and receives the waters of the Pinheiros river in Sa˜o Paulo city. Until 1992, the natural flow of the Pinheiros river could be reversed into Billings reservoir (Fig. 1) for electricity generation. After 1992, this has not been permitted, apart from flood control in Sa˜o Paulo city, to prevent the high levels of water pollution in the Billings reservoir, which in principle could serve as a huge water resource for the MASP. The aim of this work is to assess the mobility and to establish the possible forms or phases in which heavy metals are associated with the sediments of the TieteˆPinheiros system. For this purpose, the sediments were submitted to a single extraction with 0.1 mol l
1 HCl and to the sequential extraction procedure proposed by Ure et al. (1993a), recommended by the Standards, Measurements and Testing Programme (SM&T) of the European Union. In parallel, chemical and physical parameters such as acid volatile sulphides (AVS), organic and inorganic C, grain size and mineralogy were determined in order to identify the mechanisms involved in the distribution of heavy metals in the sediments.

2. Materials and methods Fig. 1 shows the area of study and the sampling stations in the Billings (B1 and B2), Pirapora (P1), Rasga˜o (P2) and Barra Bonita (BB1 and BB2) reservoirs. Sediments were collected in the rainy season (February and March 1998) using a gravity sampler. The samples were kept isolated in the sampler acrylic tubes to avoid contact with air and consequent oxidation. For this isolation, water in contact (immediately above) the sediment layer was also sampled. The tubes containing approximately 30 cm of sediments were capped and brought to the laboratory. Chemical and physical variables such as pH, EH and dissolved O2 (DO) of the bottom water were measured in the field, as well as the depth of the water column. Inside a glove box under a N2 atmosphere, a layer of approximately 10 cm of the sediment closest to the

water column was transferred to polyethylene flasks. These flasks were kept in desiccators at approximately 4  C until the time of the analysis. To keep the anoxic conditions during the sequential extraction experiment, all transferences were performed inside the glove box. Extractions were performed in tightly closed 50 ml polypropylene centrifuge tubes from Corning1. The sequential extraction protocol proposed by Ure et al. (1993a) consists of 3 steps. The reagents for sequential extractions are: reagent 1: 0.11 mol l-1 acetic acid (step 1 of the extraction); reagent 2: 0.1 mol l-1 hydroxylamine hydrochloride acidified to pH 2 with HNO3 (step 2); reagent 3: 8.8 mol l-1 H2O2 acidified to pH 2–3 with HNO3; and reagent 4: 1 mol l-1 ammonium acetate acidified to pH 2 with HNO3 (step 3). All materials and flasks used were immersed in 50% (v:v) analytical grade HNO3 overnight and then extensively washed with deionized water before use. The sequential extraction procedure was performed in duplicate. Step 1 consists of shaking 1.0 g of sediment with 40 ml of reagent 1 for 16 h. In step 2, the residue obtained from step 1 is shaken for 16 h with 40 ml of reagent 2. All solutions used in the setps 1 and 2 of the extration were prepared in deionized water free of dissolved O2. Finally, the residue of step 2 is treated with 10 ml of reagent 3 (twice, at 85  C) followed by extraction with 40 ml of reagent 4, after eliminating the excess of H2O2. Extracts were separated from the sediment by centrifugation at 3000 rpm. Detailed description of the protocol can be found elsewhere (Ure et al., 1993a; Fiedler, 1995). A fourth step was applied in order to obtain the residual fraction, following the procedure proposed by Kersten and Fo¨rstner (1986). This fraction was obtained by adding concentrated HNO3 to the residue from the last step of the sequential method. The same samples were also extracted with 0.1 mol l
1 HCl for 2 h under two different conditions: (i) keeping the sample wet and anoxic in a glove box with a N2 atmosphere and (ii), after air drying the sample at room temperature. A ratio of about 1:25 (mass:volume) was used for this procedure. Heavy metals (Zn, Pb, Cr, Cd, Cu, Ni and Mn), as well as major element constituents of the mineralogical matrix (Fe, Al and Ca) were determined by Flame Atomic Absorption Spectrometry (FAAS) on a CG AA 7000 BC spectrometer with background correction, using direct calibration with reagent-matched standard solutions. Extracts were filtered through 0.45 mm membranes before analysis. All results of the analysis of the wet sediments (sequential extraction and treatment with HCl) are quoted as dry weight of sediment. For the determination of the total metal content, samples were put into an oven at 300  C for 1 h, transferred to TeflonTM beakers and then attacked with H2O2 to eliminate organic matter. Finally, a mixture of HNO3 and HF (3:7) was added to complete the sample

I.S. da Silva et al. / Applied Geochemistry 17 (2002) 105–116 107

Fig. 1. Location of sampling sites: Billings reservoir (B-1 and B-2), Pirapora reservoir (P-1), Rasga˜o reservoir (P-2) located in the Metropolitan Area of Sa˜o Paulo city are shown in A. The Barra Bonita reservoir (BB-1 and BB-2) area is shown in B.

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3. Results and discussion

column) measured during the field work. These parameters show the differences among the environments, especially in terms of DO. At the Billings reservoir, the sampling site B-1 was located close to the plant that connects water from the Pinheiros river, while B-2 is located in the main channel of the reservoir. Although the depth of the water column is very distinct in B-1 and B-2, the other chemical parameters, pH, EH and DO were very similar for the bottom water (overlaying layer to the sediments), denoting a strongly reducing environment. The Tieteˆ river receives water from the Pinheiros river, increasing the pollution load and resulting in the poor quality of the Pirapora reservoir (P-1), that represents one of the dams on the Tieteˆ river. The poor quality of this environment is indicated by the dark colour of the water, formation of foam along the dams and the intense smell of gases (probably CH4 and H2S). The Rasga˜o reservoir, another dam on Tieteˆ river, is very similar to Pirapora. From the parameters presented in Table 1, it is noticeable that there is a complete absence of DO and there are similar values of EH for both environments (also comparable to the sampling sites from Billings). The absence of DO could be explained by the microbial activity that metabolises the high levels of organic matter from domestic and industrial wastes (Fifield, 1995). On the other hand, the sampling sites from the Barra Bonita reservoir (BB-1 and BB-2) are distinctly characteristic in relation to the other sites mentioned. These sampling points had DO in the bottom waters and higher values of EH in comparison to samples from Billings, Pirapora and Rasga˜o. The negative EH still denotes a reducing character for the Barra Bonita bottom waters (Appelo and Postma, 1993). Barra Bonita is exploited for tourism, boating, fishing and electricity generation.

3.1. General characteristics of the Tieteˆ-Pinheiros system

3.2. Grain size distribution and mineralogy of the sediments

Table 1 presents the chemical and physical parameters (pH, EH, dissolved O2 (DO) and depth of water

Table 2 illustrates the grain size distribution for the sediments collected from the Tieteˆ-Pinheiros river system. The fractions were separated according to Ackermann et al. (1983), who suggested that the <20 mm

digestion. This procedure was carried out on a hot plate. The fine dark residue of the digestion was analysed by X-ray fluorescence. For analysis of acid volatile sulphide (AVS), extractions were performed in a closed system similar to the Johnson- Nishita apparatus (Johnson and Nishita, 1952), using 1 mol l-1 HCl. The system was flushed for 30 min. with N2, collecting the liberated H2S in a 50 or 100 ml volumetric flask containing 25 mmol l-1 deaerated NaOH solution, immersed in an ice bath. The determination of AVS was performed by the automated methylene blue method adapted from the American Public Health Association (1997), according to Silva et al. (2001). Carbon was determined using a Perkin-Elmer 2400 CHN analyser. The total C content was determined directly in the dried samples. For organic C determination, another aliquot was shaken with 2 mol l
1 HCl for 12 h. The residue was separated by centrifugation and, after extensive washing with deionized water for elimination of HCl, it was dried at 60  C and submitted to analysis. The inorganic C was obtained by difference between the total and organic contents. All samples were submitted to mineralogical analysis by X-ray diffraction (XRD) on a Siemens D5000 instrument. The main mineralogical phases were determined directly in samples dried at 60  C and ground. The grain size analysis was carried out on a Master Size Malvern with dried samples dispersed in water. The grain size curves were plotted in a logarithmic scale where the particle diameter varied from 0.1 to 1000.0 mm.

Table 1 Values of pH, EH, depth of water column and dissolved O2 (DO) measured during the field work in Tiete-Pinheiros river system Sampling site

Depth of water column (m)

pH

B-1 B-2 P-1 P-2 BB-1 BB-2

9 18 8 12 20 12

6.84 6.79 6.81 6.74 6.77 6.91

a

Not detectable.

EH (mV)
584.5
589.0
583.0
568.0
442.0
254.8

DO (mg l
1) 0.45 0.04 n.d.a n.d.a 2.94 1.67

Table 2 Grain size distribution expressed in % for sediments from Tieteˆ-Pinheiros river system Sampling site

<20 mm

20–60 mm

60–200 mm

200–600 mm

B-1 B-2 P-1 P-2 BB-1 BB-2

70.5 65.2 79.6 65.8 42.6 25.1

26.9 33.4 19.5 30.1 17.4 20.0

2.4 1.4 0.9 3.7 28.4 32.3

0.2 – – 0.4 11.6 22.6

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fraction includes the organic matter, while the <60 mm fraction is the break between silt and sand. The material carried in suspension is mainly in the < 60 mm fraction (WHO, 1982). Sediments from B-1, B-2, P-1 and P-2 sampling sites were enriched in the <20 mm fraction and also in the fraction in the interval 20–60 mm. The contribution of particles over 60 mm to these sediments was small. This distribution was completely different in relation to the samples from the Barra Bonita reservoir (BB-1 and BB2) where there was an appreciable contribution of particles in all ranges assessed. The main mineralogical phases determined in the sediments were quartz, gibbsite, goethite, kaolinite and feldspar. This group of minerals were determined in all samples. It was not possible to assess the quantity of each mineralogical phase or their association with each interval of the grain size distribution. 3.3. Carbon and acid volatile sulphides The results for C and AVS are shown in Table 3. AVS concentrations were very high in samples from Billings, Pirapora and Rasga˜o reservoirs while in Barra Bonita, these values were very low. The high contents of AVS in the samples close to MASP can be assigned to the microbial mediated reduction of SO24 , which is present

in high concentrations in these environments (Silva and Masini, 1999). In the Billings reservoir, large amounts of CuSO4 are added as algaecide. The C was mainly found in the organic fraction. The total C content was higher in the Billings reservoir, owing to the important contribution of aquatic plant debris and municipal wastes. In the Pirapora and Rasga˜o reservoirs the organic C is mainly due to municipal and industrial wastes. Samples from the Barra Bonita have the lowest organic C content. In Barra Bonita the organic C may be associated with humic matter (Abate and Masini, 1999), owing to the less important direct contribution of anthropogenic sources in this environment. 3.4. Total metal contents Table 4 shows the total concentration of metals in the sediments. This determination was made after extraction of the whole sediment, but the results were expressed in terms of g or mg of metal cation per kg of the sediment fraction < 60 mm. This is considered the sandfree fraction of the sediment (WHO, 1982), which is the major adsorbent of heavy metal cations. Results presented in Table 4 show that the total concentrations of heavy metals were significantly higher for Pb and Zn in samples from sites B-1, B-2, P-1 and P-2 (Billings,

Table 3 Organic, inorganic and total C and acid volatile sulphides (AVS) in the sediments of the Tieteˆ-Pinheiros river system Sampling site

Organic C (%)

Inorganic C (%)

Total C (%)

AVSb (g kg
1)

B-1 B-2 P-1 P-2 BB-1 BB-2

5.03 7.09 2.18 3.64 2.70 1.02

0.08 0.16 0.28 0.26 n.d.a 0.17

5.11 7.25 2.46 3.90 2.71 1.19

2.20.2 2.90. 2 2.370.03 1.260.04 0.0230.002 0.0200.001

a b

Not detectable. AVS concentrations expressed in terms of the mass of the fraction < 60 mm.

Table 4 Total metal contents in sediments from Tieteˆ-Pinheiros river system. Concentrations were expressed in terms of the mass of the fraction < 60 mm (sand free basis) Sampling site

Ala

Caa

Cdb

Crb

Cub

Fea

Mna

Nib

Pbb

Zna

B-1 B-2 P-1 P-2 BB-1 BB-2

166 3 151 5 157 7 164 5 73 2 73 2

3.60.4 2.20.3 2.00.3 3.30.6 3.00.2 0.990.02

5.60.4 6.70.4 3.40.1 4.00.4 51 6.90.4

206 5 235 14 115 4 150 16 168 48 75 2

354 2 260 4 964 140 19 255 2 166 2

763 852 533 685 15310 1721

0.438 0.003 1.09 0.03 0.226 0.004 0.41 0.04 4.8 0.5 3.63 0.02

913 1246 1164 10716 1075 982

1472 1431 954 11917 438 7113

0.68 0.01 0.6560.006 0.42 0.06 0.6 0.1 0.23 0.01 0.2240.004

a b

Concentrations in g kg
1. Concentrations in mg kg
1.

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Pirapora and Rasga˜o reservoirs) in relation to BB-1 and BB-2 (Barra Bonita reservoir). Nickel contents were very similar in all sampling sites, while Cr and Cu were higher in samples from the Billings reservoir, but quite high in the Barra Bonita reservoir as well. Cadmium levels were surprisingly elevated in the Barra Bonita reservoir, especially at site BB-2. Once all these results were corrected for a sand free basis, it is important to observe that the Barra Bonita reservoir is as enriched in heavy metals as the Billings, Pirapora or Rasga˜o reservoirs. Comparing the heavy metal sources for all of these environments, as discussed above, considerable higher concentrations for the Billings, Pirapora and Rasga˜o reservoirs would be expected. If the results of the total metal contents were expressed in terms of the whole mass of sediment used in the extraction (without correction for the < 60 mm fraction), the total concentrations of heavy metals (Cd, Cr, Ni, Pb, Zn and Cu) would be almost the same in samples from Billings reservoir, which contains > 97% of grains which are < 60 mm (Table 2). On the other hand, lower concentrations in the sediments from the Barra Bonita reservoir would be observed, since these samples contained only 60% (BB-1) and 45.1% (BB-2) of particles < 60 m m. hese results suggest that heavy metals in Barra Bonita are mainly due to transport of fine particles, indicating the influence of diffuse pollution sources from upriver areas (Esteves, 1986). As major elements, Al, Ca, Fe and Mn were determined to contrast the natural weathering processes from soils and rocks in the surrounding areas of each sampling site. Their total contents (Table 4) are quite representative of the differences in the geological formations in the surrounding areas that contribute these elements to the sediments. Aluminium was higher in the MASP (Billings, Pirapora and Rasga˜o reservoirs) where granites, phyllites, schists, quartzites, and meta-limestones are the prominent geological formations. In the Barra Bonita area the geology is predominantly sedimentary rocks of the Parana´ basin (especially red sandstones) and basalts that reflect the volcanism that affected this basin (IPT, 1981). As a result, sediments from this area contain higher concentrations of Fe and Mn and lower contents of Al in comparison to the reservoirs located in the MASP. On the other hand, Ca did not appear to be more concentrated in any reservoir. The fine and dark residues from the chemical digestion were submitted to X-ray fluorescence, indicating the presence of Fe, Al and Mn, as well as a small amount of C (determined by elemental analysis). It was not possible to perform quantitative determinations of these elements in the residue. The accuracy of the analytical procedure used for the total digestion of the samples was investigated using a standard reference sediment (SRM-NIST1646a), obtaining recoveries between 84 and 105% for all elements.

3.5. Single 0.1 mol l
1 HCl extraction. The role of sulphide Sulphide in sediments plays a major role in the transport and partition of heavy metals between the aqueous and solid phases. Owing to the low solubility of their sulphide compounds, heavy metals are immobilized in anoxic sediments. On the other hand, when anoxic contaminated sediments are dredged or submitted to forced oxygenation, suphides are oxidized to SO24 , liberating H+ ions to the aqueous phase according to reactions such as (Fo¨rstner, 1995): þ H2 S þ 2O2 ! SO2
4 þ 2H þ 4FeS þ 9O2 þ 6H2 O ! 4FeOOH þ 4SO2
4 þ 8H þ FeS2 þ 15=4 O2 þ 5=2H2 O ! FeOOH þ 2SO2
4 þ 4H

Fig. 2 shows the distribution of metals extracted with 0.1 mol l
1 HCl. The total metal contents are also given to evaluate the efficiency of this leaching procedure. Copper was not detected in any of the extracts obtained from the crude anoxic sediment (except for BB-2, at very low concentration), a fact that may be explained by the low solubility of CuS (Ksp=8.510-45). On the other hand, the air drying process led to extraction of 60–80% of the total Cu content in the samples with high AVS (B-1, B-2, P-1 and P-2), while in the BB-1 and BB2 samples, only 23–27% of Cu was extracted. This result is consistent with the findings of Simpson et al. (1998), who have assigned the increased solubility of Cu from anoxic sediments resuspended in aerated water to CuS oxidation by Fe(III). Lead and Cd showed similar behaviour, but as their sulphides are more soluble than CuS (Ksp=710
28 and 110
28, respectively), a significant fraction of them was extracted in 0.1 mol l
1 HCl even from the crude and anoxic sediments, in agreement with the results of Cooper and Morse (1998), who verified the solubility of greenockite (CdS) and galena (PbS) in HCl. After the drying process a significant increase in the extractability of Pb and Cd occurs, which may be due to sulphide oxidation, but also to an increase in the crystallinity of Fe and Mn oxides and consequent decrease of the adsorption capacity of these phases (Kersten and Fo¨rstner, 1989). In the sequential extraction, Cd showed an important association with the oxidizable phase (sulphide and organic matter) only in the P-1 sample, suggesting that adsorption to mineral phases such as clays, oxy hydroxides of Fe, Al and Mn, as well as pyritic phases, plays a major role in the control of Cd(II) availability. Zinc sulphide is fairly soluble in 0.1 mol l
1 HCl, so that only a small increase in the availability of this cation was observed after the drying of the sediments. The amount of Ni extracted from all samples was not significantly altered after air drying, suggesting that no

I.S. da Silva et al. / Applied Geochemistry 17 (2002) 105–116

oxidation of NiS (if present) occurred. These facts also suggest that surface adsorption processes are controlling the availability of these cations. Unlike Cu, Pb, Cd and Zn, the solubility of Fe in 0.1 mol l
1 HCl was diminished after drying in samples containing high AVS (Billings, Pirapora and Rasga˜o),

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which may be explained by the crystallisation of Fe oxides formed upon sediment oxidation (Cauwenberg and Maes, 1997). It is interesting to notice that in Barra Bonita sediments, where the AVS is very low, the drying process increases the extractability of Fe in 0.1 mol l-1 HCl.

Fig. 2. Distribution of metals extracted with 0.1 mol l-1 HCl and the total metal contents (all results are represented on a sand free basis) (continued).

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Fig. 2. (Continued).

As observed for Fe, the solubility of Mn and Cr decreased after drying, which may result from an increase in crystallisation of their oxides. Calcium and Al solubility was not significantly altered by drying and oxidation, since the availability of these cations is mainly governed by the solubility of their carbonates (Ca) and hydroxides (Al). 3.6. Sequential extraction The amount of metals extracted in each step of the sequential extraction procedure is given in Fig. 3. The sum of the 4 extracted fractions is in reasonable agreement with the total metal contents. For Fe and Al this comparison is not possible because most of these metals were not extracted in the treatment with HNO3 (residual fraction). Similarly, heavy metals such as Cd and Cr in samples from Billings, Pirapora and Rasga˜o reservoirs, and Cd, Cr, Ni, Pb, Zn and Cu in samples from Barra Bonita reservoir, contained lower values for the sum of the extracted fractions compared to the total contents. These differences may be explained by the residual fraction, which probably contained these metals. Some authors have used a stronger attack to extract residual metals using HNO3, HF and HClO4 (Tessier et al., 1979) or aqua regia (Davidson et al., 1994). Chromium, Ni, Zn and Cu in the Billings, Pirapora and Rasga˜o reservoirs are mainly associated with reactive forms (released in the first 3 steps of the sequential extraction procedure) while in Barra Bonita Cr, Cu, Cd and Pb were found, in greater proportion, bound to the sedimentary matrix (residual fraction). The highest total metal contents in all sampling sites clearly showed one dominant fraction (exception for Cd and Pb). For instance, Ni, Zn, Mn and Ca were extracted predominantly in the first step of the sequential extraction, which represents the metal bound to carbonates or sorbed/exchangeable phases (Ure et al., 1993a). Copper and Cr were determined mainly in the fraction extracted in the third step, as oxidizable phases (Fiedler, 1995). The major fraction of Fe and Al was retained in the residual fraction, which was confirmed by the iden-

tification of secondary minerals such as goethite and gibbsite. It should be noted, however, that 0.11 mol l-1 acetic acid extracted large amounts of Fe and Mn in their divalent oxidation states, suggesting that sulphides of Fe (II) and Mn (II) are dissolved in this step, which was confirmed by qualitative tests and liberation of H2S. The originally black sediment becomes brown after the acetic acid treatment, even in the N2 atmosphere in which the extraction is performed, indicating that most of the sulphide related to AVS is associated with Fe (II) in the crude sediment. Copper was mainly extracted after the treatment with H2O2, suggesting a strong association with sulphides and organic matter (Fiedler, 1995) in the Billings, Pirapora and Rasga˜o reservoirs. In Barra Bonita, despite of low AVS, a significant part of the Cu was released from oxidizable phases, suggesting a strong association with organic matter, in agreement with large conditional stability constants determined for the association between Cu and humic acid isolated from Barra Bonita sediment (Abate and Masini, 1999). In the first and second steps, Cu was present only in those samples where the total contents were higher, especially at the Billings reservoir, representing the greater reactivity of this environment. A small fraction of Pb was extracted by acetic acid in samples from the Billings, Pirapora and Rasga˜o reservoirs (similarly to Cu). Important fractions of Pb were extracted from reducible and oxidizable phases, but unlike Cu, the major association of Pb was with the residual phase, that accounted for about 50% of total Pb. In Barra Bonita samples, more than 90% of Pb was associated with the residual phase. In Billings (B-1 and B-2) and Rasga˜o (P-2) sediments, the reactive Cd (extracted in the first 3 steps of the sequential extraction) was mainly solubilized with acetic acid, which would not be expected, owing to the low solubility of CdS (Wallmann et al., 1993), suggesting that precipitation as sulphide is not governing the availability of this cation. In all samples a considerable amount of Cd was found in the residual fraction and, for Barra Bonita samples, it was the only fraction in

I.S. da Silva et al. / Applied Geochemistry 17 (2002) 105–116

which Cd was found. A possible explanation for extraction of Cd, Cu and Pb in the first and second steps from samples with high AVS would be a recent (and frequent) pollution load. Heavy metals would be adsorbed in upper sediments, and competitive equilibrium among the metal cations for the adsorbing phases (pyrite, clays, oxyhydroxides of Fe and Mn, organic matter)

113

are likely to occur, and the reaction kinetics would play a major role in determining the associations (Simpson et al., 1998). In Pirapora sediments (P-1), only very small amounts of Cu, Pb and Cd were extracted with acetic acid, suggesting that these metals are less reactive in comparison to other polluted samples from sites B-1, B-2 and P-2.

Fig. 3. Distribution of the metal fractions through the sequential extraction procedure (all results are represented on a sand free basis) (continued).

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Fig. 3. (Continued)

Pirapora sediment seems to be evolved in terms of the consolidation and intensity of the diagenetic processes in its sediments. The area of this reservoir could be compared to a swamp where reducing conditions dominate and dissolved O2 is scarce or absent (Table 1). The accumulation of fine sediments (Table 2) and the distance from the sampling site to the main channel of the Tieteˆ river (approximately 2 km) would contribute to isolation of this specific area within the Pirapora reservoir, resulting in a different stage of reactivity of heavy metals in relation to the Billings and Rasga˜o reservoirs. Nickel and Zn were similarly distributed through the several steps of the sequential extraction, with a major contribution of phases soluble in acetic acid (B-1, B-2, P-1 and P-2 samples). This agrees with Wallmann et al. (1993), who demonstrated that trace metal sulphide minerals are not extracted together in the same step of the sequential extraction, but in all fractions to an extent depending on their solubility. In addition, the distribution of Ni and Zn may be explained by their presence as carbonates or hydroxides, or adsorbed to the surface of the sediments. For both Ni and Zn, the residual fraction was unimportant in all sampling sites compared to the other fractions (except for Ni in Barra Bonita samples, which showed an approximately homogeneous distribution for all steps of the sequential extraction). Significant amounts of these metals were also extracted from reducible amorphous oxihydroxides of Fe and Mn, as well as from oxidizable organic matter or sulphidic phases. Thus, it seems that sulphide is not the only phase governing the availability of Ni(II) and Zn(II), as previously observed for Cu, Pb and Cd. Diagenetic processes and recent pollution sources in these aquatic environments would influence the reactivity of these metals. The geochemical behaviour of Ca in all sampling sites could be explained by the presence of inorganic C (IC) in the sediments (Table 3), so that this cation would be mainly precipitated as carbonate. Silva and Toledo (1997) pointed out the probable presence of gypsum in sediments of Pirapora reservoir which could be another possible explanation for the Ca distribution in that par-

ticular environment. However, XRD analysis did not confirm the presence of minerals such as calcite or dolomite in the sediments. Aluminium was predominantly bound to the residual fraction, but a significant amount was also released from oxidizable phases, which may be due to formation of strong complexes between organic matter and trivalent cations. Similarly, Cr was mainly found in association with residual or oxidizable phases, in agreement with the findings of Thomas et al. (1994) and Ure et al. (1993b). 3.7. Environmental implications Table 5 shows the individual and global contamination factors, Cif and Cf (Cif) respectively, (Fernandes, 1997; Barona et al.,1999), for the 6 sediment samples and trace heavy metals. In this work, Cif was calculated as the sum of concentrations of metals extracted in the first 3 steps of the sequential extraction divided by the concentration in the residual fraction (extracted with concentrated HNO3 from the residue of the sequential extraction). The sample from B-2, with a Cf of 48, represents the greatest contamination risk, followed by P-1 (Cf=36), B-1 (Cf=35), P-2 (Cf=29), BB-1 (Cf=5) and BB-2 (Cf=2.9). It is interesting to note that trace heavy metals in sediment from the Pirapora sample (P1) seemed to be less reactive in comparison to the other polluted points studied, since no significant extraction of Cu, Pb, Cd and Cr was observed during the acetic acid treatment. On the other hand, when the reducible and oxidizable fractions are considered, the contamination potential of this sample has a magnitude similar to the other samples. Billings sediments at point B-2 represent the major risk for water contamination. Despite the fact that since 1992 the waters from the Pinheiros river have not been systematically pumped to this reservoir, heavy metals remain reactive and readily available during changes of acidity and redox potential. Among the trace heavy metals, Ni and Zn gave the highest Cif values in all samples, followed by Cu, despite

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I.S. da Silva et al. / Applied Geochemistry 17 (2002) 105–116 Table 5 Individual (Cif) and global (Cf) contamination factors of Cu, Pb, Cd, Zn, Ni and Cr for the sediment samples Sampling site

B-1 B-2 P-1 P-2 BB-1 BB-2

Heavy metals (Cif)

Cf= Cif

Cu

Pb

Cd

Zn

Ni

Cr

8.80.1 102 4.10.1 4.10.1 0.950.05 0.930.01

1.10.1 1.40.4 1.30.2 0.950.50 0.20.1 0.20.1

1.10.3 1.50.5 1.60.2 1.00.1 0 0

15.60.5 204 121 13.10.7 1.40.2 0.80.3

6.40.3 111 152 8.00.6 2.20.2 0.80.2

2.40.1 3.70.1 1.90.5 2.00.1 0.40.2 0.20 0.05

the low solubility of its monosulphide and strong association with organic matter, it is not significantly retained by the sedimentary matrix in the contaminated sediments of Billings, Pirapora and Rasga˜o. The larger Cif in Billings (B-1 and B-2) sediments may be due to the frequent addition of CuSO4 (algaecide) to the waters of this reservoir. Comparatively, Cd and Pb gave the lowest Cif values. This fact, however, should be viewed with caution in samples from the Billings, Pirapora and Rasga˜o reservoirs, since the Cif values of between 0.95 and 1.6 mean that a fraction between 47 and 80% of these metals are extracted during the sequential extraction. Owing to the high toxicity of Cd and Pb, the Cif values obtained still represent a potential risk of contamination to the water phase, or to the biota, in samples from P-1, P-2, B-1, and B-2. Chromium Cif was intermediate between Cu and Cd or Pb in all samples.

4. Conclusion The distribution of heavy metals indicates recent and intense pollution loads in samples from the Billings, Pirapora and Rasga˜o reservoirs, located in the MASP. This was evidenced by the high concentration of heavy metals, as well as by the significant extraction of Cu, Pb, Cd and Ni from acid soluble and reducible phases, even under anoxic conditions, suggesting that surface adsorption, in addition to the formation of sulphide salts, are the major processes governing the availability of these cations to the aqueous phase. In samples from Barra Bonita, metals showed an association which was much closer to the sedimentary matrix (residual fraction). However, the total heavy metal contents were also high at this locality, which could be explained by the transport of fine particles that could carry the metals from upriver areas, or from diffuse pollution sources. Results obtained from the extraction of crude anoxic sediments with 0.1 mol l-1 HCl should be evaluated with caution, since their potential for contamination may be underestimated in such a treatment. This was evidenced

35 2 48 8 36 4 29 3 5.0 0.8 2.9 0.7

by the increased extractability observed for all trace metals (with the exception of Cr and Ni) after air drying of the sediment, which commonly occurs with dredged sediments.

Acknowledgements The authors are grateful to Fundac¸a˜o de Amparo a` Pesquisa do Estado de Sa˜o Paulo (FAPESP) and to Conselho Nacional de Desenvolvimento Cientı´fico e Tecnolo´gico (CNPq) for financial support, and fellowships.

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