Guanabara Bay

As part of the‘‘TAGUBAR’’(TAngential GUanabara Bay Aeration Recovery) project, surface and long core sediment sof the Guanabara Bay (Rio de Janeiro, Brazil) were investigated for mercury (Hg). The main,but not the only, input of Hg into the Bay’s waters is known to be a Chlor-Alkali Plant (CAP) located in the Acarì-São João de MeritÍ River system, on the northwestern side of the Bay. Mercury distribution in surface sediments (< 0.1– 3.22 mg kg-1, average 0.87 ± 0.80, n=40) seems to be controlled by the organic component, along with sulfur rather than grain-size, where Hg concentrations are less than 1 mg kg-1. Conversely, where the metal contents are higher than 1 mg kg-1, accumulation in surface sediments is mostly related to the presence of nearby contamination sources, such as industrial and urban settlements in the western sector of the Bay. Although total Hg contents in surface sediments exceed the values suggested by the effects-based standard quality guidelines as potentially toxic for the benthic community, results from a sequential extraction procedure showed that the contribution of the more soluble, easily exchange able and eventually bioavailable Hg phases was found almost negligible (< 0.1%). Most of the metal is strongly bound to the mineral lattice of the sedimentary matrix and should therefore be considered almost immobilized. The reduction in Hg accumulation in bottom sediments, expected as a consequence of the adoption of contamination control policies (i.e. Hg-free technologies in the CAP and sewage treating facilities), has not been clearly observed in the core profiles. Current estimates of Hg accumulation rates at the core top range from approximately 1 to 18 mg m-2 yr-1. Pre-industrial bottom core samples indicate that the central and northeastern sectors of the Bay are strongly affected by Hg enrichment: concentrations exceed the estimated baseline concentration by up to 20 factors. A cumulative Hg inventory suggests that the metal content has increased with the same order of magnitude in the vicinity of potential contamination sources on the western side of the Bay, but at a different rate; this is apparently determined by local conditions. A natural attenuation of Hg concentrations to background levels is not predictable in the near future.

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.).