AN OVERALL VIEW OF CADMIUM AND ZINC CONTAMINATION IN SEPETIBA BAY, RIO DE JANEIRO, BRAZIL

J.E.L. Maddock*, L.D. Lacerda, Dept. Geoquímica, U.F.F., Niteroi,RJ, Brazil, geojelm, geodrud @ vm.uff.br, J.G. Rees, N. Breward, B.G.S.,Keyworth, Nottingham, U.K., jgre, nbr @ wpo.nerc.ac.uk

Abstract

A survey of contamination of water and sediments of Sepetiba Bay, Rio de Janeiro State, Brazil, with zinc and cadmium indicated that most of the estimated 49,000 tons Zn and 910 tons Cd emitted during the last 35 years by a hydrometalurgy plant on its north shore has been retain in the sediments of the bay. However, resuspension and sediment transport have dispersed the metals over a wide area in the bay.

History of Sepetiba Bay and its contamination with metals

Sepetiba Bay is a semi enclosed saline coastal lagoon, about 450km² in area, separated from the Atlantic Ocean to the south by a sand spit, and has some characteristics of an estuary. During the last 35 years, it has received metal contamination from an industrial park on its north shore, especially from a zinc/cadmium hydrometalurgy plant, closed since 1999, whose effluent and tailings pile runoff drained, and in the case of the tailings pile, still drains directly into the bay. The bay also receives sediment load from the Canal de São Francisco which carries about 170m³/s of water diverted, since 1953, from the Paraiba do Sul River, through a hydroelectric scheme, into a once small (~10 m³/s) water course (Figure 1). Effluent from potable water treatment for Rio de Janeiro flows into Sepetiba Bay, together with domestic sewage from several neighbouring towns. Until recently, the natural catchment area had a relatively low population density, so that only the sediments near the shore, east of the industrial area, contain sewage material. The bay has bulk carrier port facilities and is important for shrimp and other fishing, and bathing beaches.

Metal Contamination Survey

We undertook a comprehensive survey of the contamination of the sediments and water of Sepetiba Bay in July/August of 1997 (Rees et al., 1998). Filtered water samples and suspended material (s.s), taken from 43 points spread over the area of the bay (Figure 1), were analysed for contents of 8 trace metals. Sediment cores, of up to 1.4m. in length, were taken at 29 of these points and grab samples at the remaining points. Cores were sectioned to study vertical metal distributions. Metal concentrations were determined in the resulting sub-samples, as well as in the grab samples. Acidified water samples were analysed by ASV and material in suspension by ICP-MS after total digestion of the 0.45mm pore membrane filters. Sediment samples were dried at 60°C and the sieved -150mm fraction (practically the whole sample in most cases) digested in hot conc. HNO₃ for subsequent analysis by ICP or ICP-MS.

Results

Water and Suspended Particles

Dissolved cadmium and zinc were both found in greatest concentrations near the source and to the east of this, along the north part of the bay, where they were carried by the prevailing clockwise current. However, the median [Zn] = 4,5mg/l, was not much higher than “average coastal water” value quoted by Delft Hydraulics Laboratory (1985) of 2mg/l and median [Cd] was <0,02mg/l. Despite the introduction of the metals into the bay in dissolved form, their removal by absorbtion and deposition must be rapid enough for water from most of the bay not to contain abnormally high dissolved concentrations. Maximum dissolved concentrations were 0.341mgCd/l and 64.5mgZn/l, which did not exceeded national water quality limits. At the mouth of the bay, dissolved [Cd] was below detection limit (<0.02mg/l) at 3 of 5 points, [Cd]=0.02mg/l at the other 2, and the mean [Zn] was 3,2mg/l , with little variation between points.

[Cd] in s.s. was below the effective detection limit of 10ng/l and maximum [Zn] in s.s. was 31.9mg/l. At the mouth of the bay, mean [Zn] in s.s. was 1.7mg/l (at 5 points).

Sediments

Provisional reference values for assessing sediment contamination were promoted via a draft criteria document at an international sediment quality forum in 1992 (Van Veen and Strotelder 1988). “Target”, “standard” and “limit” metal concentrations were proposed. Mean [Cd] in surface sediments, of 3.04mg/kg, was near the “standard” value, 4mg/kg, and locally “limit” values (30 mg/kg) were approached. [Zn] at most sites, and the mean concentration, exceeded the standard value, 750mg/kg, and at site 1, [Zn], = 5100mg/kg, greatly exceeded the “limit” value of 2500mg/kg.

Many of the cores showed distinct step decreases in Cd and Zn concentrations, at depths where sediment ages corresponded to the startup of the hydrometalurgy plant, in 1965, p.ex.: Figure 2. Thus background concentrations in the sediments could be estimated from the concentrations in the lower parts of cores. This “step” may have corresponded to the lower limit of bioturbation but background concentrations would still be indicated in the same way.

Metal concentrations in suspended material sampled at the mouth of the bay, [Cd] < 0.1mg/kg (not detected) and range [Zn] = 5.0 – 15.5mg/kg, were lower than the background concentrations found in cores [Cd] ~ 0 (n.d.) and [Zn] = 156mg/kg. Concentrations in bottom sediments from this area were also extemely low [Cd] < 1mg/kg (n.d.) and range [Zn] = 26 –30mg/kg, except near the north shore, where higher values were found: [Cd] = 2mg/kg and [Zn] =529mg/kg. These figures, together with the values for dissolved metals, indicate that little export occurs of these metals from the bay.

Quantities of metals in the upper parts of the cores, in excess of those calculated from background concentrations, i.e. due to contamination from the hydrometalurgy plant, were estimated. The area mass density distribution of Zn is shown in figure 3 and that for Cd was similar but slightly less dispersed. The total tonnages of excess metals deposited in the bay’s sediments, 110,000 tons Zn and 670 tons Cd, were calculated by krigging these data.

Discussion and Conclusions

Maximum total emissions of Zn and Cd were calculated by Barcellos et al (1991), from the differences between total contents of the ore processed and tonnages of metals produced, as 49,000 tons Zn and 910 tons Cd. Extrapolation of these figures, assuming 1990 loss rates up to 1994, when pollution control measures were implanted, resulted in estimated total losses of 64,000 tons Zn and 1330 tons Cd.

The calculated quantities of Zn and Cd found in the sediments in this survey correspond to 170% and 50%, respectively, of these estimates of total losses from the refinery. It is probable that part of the cadmium losses is still be stockpiled in the tailings. If this is the case, part of the zinc must also be, and the excess zinc in the sediment must be attributed to other recent sources. Concentrations of Zn in the water and suspended sediments of the Canal de São Francisco indicate this as a source. On the basis of data published in other countries for typical source strengths and an audit of human activities present in the area, Barcellos & Lacerda (1994) attributed 30% of the Zn entering the Bay to sewage and urban drainage, solid waste (rubbish tip) leachate, port activities and paper manufacture effluent.

We conclude that not all the zinc and cadmium lost from the ore processing has reached the bay but virtually all that has was rapidly taken up by suspended matter and has been spread over a large part of the bay where it is retained in the sediments.

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Barcellos C. & Lacerda L.D. (1994) “Cadmium and zinc source assessment in the sepetiba Bay and region basin.” Env. Monit. & Assess. 29: 183-199.

Delft Hydraulics Laboratory (1985) “Coastal zone information system and water quality modelling for management of coastal waters”. BTA-60, Rep. R1182, Delft, Holand

Rees J.G., Breward N., Maddock J.E.L., Ferguson A.J. & Williams T.M. (1998) “Assessment of contamination by metals and selected organic compounds in coastal sediments and waters of Sepetiba Bay, Rio de Janeiro State, Brazil.” Tech. Rep. WC/98/30 BGS, Keyworth, Notts., UK. 93p.

Van Veen R.J. & Strotelder P.B.M. (1988) “Research on contaminated sediments in the Netherlands.” In: Wolf K., Van de Brink W.J. & Colon F.J. (Eds.). Contaminated Soil. p 1263-1275. Acad. Publ.

Contours from 0 to 2kg Zn.m^-2 at intervals of 400g Zn.m^-2