Sedimentation rates and contaminant chronology in a tropical estuary, Sepetiba Bay, Rio de Janeiro, Brazil

With regard to industrial activity and urban development, the state of Rio de Janeiro, at the southeastern Brazilian coast, is the second most important in Brazil. About 15 million inhabitants are living in the region of the city of Rio de Janeiro. In the Sepetiba Bay area, about 60 km south of the city, anthropogenic pressure is high. The area has experienced fast growth of urbanization and industrialization in the last four decades. The construction of a large harbor in the early 1970s made the area attractive for industrial development. For the last three decades the population of the region increased from 600,000 to 1.2 million inhabitants. The Sepetiba Bay is a semi-enclosed body water separated from the ocean by a beach barrier located approximately 60 km west of Rio de Janeiro City (latitude 23 deg. S and longitude 44 deg. W). The area has a hot and humid tropical climate with annual precipitation of 1,500 mm - 2500 mm. Geology is characterized by Pre-Cambrian granitic and metamorphic rocks. Drainage basin of the bay, with 447 km2, is formed by nine rivers draining a quaternary plain in its northeastern shore. Mean depth is about 6 m and annual freshwater input by rivers to the bay is 7.6 million m3. From 1950, the Sao Francisco channel has allowed water of the Paraiba do Sul River (at the north of the Rio de Janeiro State) to be deviated to the bay of Sepetiba , and now contributes to 87% of the fresh water supply. There are around 400 industrial facilities located in Sepetiba that can be grouped into pyrometallurgic (including two large steel plants), aluminum plants and thermal power plants. The metallurgic industry contributes with 24 t Cd y-1 and 3,360 t Zn y-1. This industrial park is responsible for the input of large amounts of heavy metals into the surrounding environments, reaching the coast via rivers, by industrial waste erosion and leaching, and by atmospheric deposition. In particular, cadmium and zinc inputs increased by three orders of magnitude relative to pre-industrial fluxes, and these metals were directly discharged into the bay during a long period. One of the most important Cd and Zn sources is the Ilha da Madeira plant, located near the studied site. This plant started in 1960 and first produced only Zn. In the early 70s Cd extraction begun. The treated ore is constituted by Camina (Zn2SiO3(OH)2) and Willemita (SiO4Zn2). Since 1980, after a dam was built before the metallurgic plant, less wastes have been introduced into the bay. A sediment core was collected in the Bay of Sepetiba on the shore at low tide, close to the Ilha da Madeira metallurgic plant. The core samples were analyzed for their Cd, Cu, Zn, Pb contents and lead isotopes. On this basis, three successive events were observed : (i) at the depth of 50-52 cm, a marked change in the stable lead isotopic ratios, (ii) at 38-42 cm depth, the beginning of the Zn concentration upwards increase, and (iii) above 30 cm, relatively high Cd concentrations. Such records can easily be explained by the local metallurgic development history. However, sediment accumulation rates evaluated using these reference levels are higher than those calculated from the 210Pb profile using the Constant Initial Concentration (CIC) or Constant Rate of Supply (CRS) models. As the core was collected directly close to the beach strand, it can be assumed that the influence of the flat zone before the mangrove on the sedimentation increased when the water depth decreased at the site where the core was collected; therefore particles relatively rich in 210Pb, eroded from this zone, concentrated more and more at the surface. A model, taking into account an exponential increase of the initial 210Pb activity allows the calculated rates to fit well with those evaluated from the 'historic' events