Assessment of heavy metals contamination in coastal sediments of Guanabara Bay (Rio de Janeiro, Brazil) through regional geochemical baselines

Assessment of the contamination degree in coastal sediments depends on the choice, sometimes underestimated, of appropriate natural geochemical baselines. Average values (i.e. shale composition, crustal abundance data, etc.) are often used as background but they do not take into account the natural geochemical variability existing among different marine environments. For this purpose, data from pre-industrial deep levels of local sediment cores may be considered the most suitable baselines. However, considering the textural variability of the sediments, more than single baseline levels, regional normalization functions (trace element vs. grain-size proxy), are the best choice. On the basis of these functions, sediment quality can be estimated through easy understandable enrichment factors (EFs) and quantified respect to natural backgrounds. Contaminated sea-bottom sectors, anthropogenic sources and diffusion vectors can be consequently recognized (discovered). The Guanabara Bay, located in the center of the metropolitan region of Rio de Janeiro (Brazil), is subjected to a high anthropogenic impact due to the intense and relatively recent urbanization and industrialization of the territory. High levels of contaminants have been detected in all compartments of the estuarine coastal waters of the bay. The industrial discharges of sewage and process wastes represent the principal source of contamination accounting for 25 % and 90% of the organic and chemical contamination, respectively. Additional contribution is due to the domestic sewage, to the two oil refineries and to the intensive port activities, although large amounts of metals are supplied into the bay by the rivers, accumulating in the bottom sediments. The geological-environmental approach applied to Guanabara Bay addresses to investigate physical, chemical and mineralogical characteristics of the bottom sediments and to determine regional pre-industrial geochemical functions. Evaluation of the enrichment factor distributions allows the identification of single metal enriched sectors that are related to all the potential contamination sources in the coastal area (rivers, waste waters, inert material waste disposals, industrial and urban sites, etc.).