Natural content and antrophogenic input of heavy metals in sediments from Bahía Blanca Estuary (Argentina)

GRECCO, L.E.; MARCOS, A.O.; GÓMEZ, E.A.; BOTTÉ, S.E.; MARCOVECCHIO, J.E.

Itajaí, Santa Catalina, Brazil

Simposio; 8th INTERNATIONAL COASTAL SYMPOSIUM (ICS'04); 2004

Centro de Ciéncias Tecnologicas da Terra e do Mar (CTTMar)/ Universidade do Vale do Itajaí (UNIVALI)

Estuaries usually behave as a “geochemical trap” for dissolved materials, firstly increasing their concentration and eventually transferring them to cohesive sediments through different processes. Many anthropogenic pollutants are associated to clay minerals, or have at least a similar dynamical behavior. Such reasons sustain the fact that potentially dangerous substances (including heavy metals) spread out to be concentrated in those places where low environmental energy allows fine sediment to settle: tidal flats and artificial mooring sites. Bahía Blanca estuary presents good chances for man-activity introduction of pollutants into the environment, as it includes the most important artificial deep harbor system within Argentina, as well as industrial nucleous and large cities. Once these substances are deposited, no negative consequences on the environment are produced because immobilization. However, the Bahía Blanca Estuary mooring sites are periodically dredged by water injection (ca. 400,000 m3/yr), potentially promoting the transference of those originally immobilized substances to bioavailable compounds, and so its dissemination through the environment. To determine the concentration of potentially dangerous elements which naturally occur on sediments within the environment is necessary in order to be able to assess the man-originated chemical pollution degree on materials which are being dredged. In the present study these values were determined by analyses of sediment samples from a functional tidal flat from a southern island and from non-functional tidal flats deposited between 3500 and 3300 years ago. At the same time, samples close to the potential pollutant input (mooring sites) were obtained. On all the samples, grain size and mineralogical analysis were performed, while geochemical fractioning was carried out following the methodology proposed by Megalatti et al. (1983) and modified by Lacerda et al. (1988) and Maddock & Lopes (1988), in order to determine the bioavailability of studied heavy metals. All samples show a great proportion of silt and clay (< 63 m). Tidal flats have a greater proportion of sand, while the percentage of clay is incremented at the mooring sites. The fraction minor to 63 m is composed mainly by plagioclase and quartz and abundant volcanic glass (around 50%), while clay minerals are composed mainly by smectites (beidellite) and scarce illite. The total concentration of heavy metals (mg/g, dry weight) ranges as follows: Cd (1.5-0.5), Pb(23-1), Cu(23-5), Zn (87-23), Cr(13-4), Ni(16-8), Mn(522-149) and Fe (26300-12000). These values represent percentages varying from 99.8 to 45.5 of metals linked with the residual fraction (F5), from 13.8 to 0.1 in reducible compounds (F4), from 19.3 to 0.1 linked to carbonates (F3), from 27.5 to 0.2 in oxidizable complexes (F2), and from 15.2 to 0.1 in easily exchangeable compounds (F1). In nearly all the samples, geochemical partitioning gives low percentages of bioavailable compounds (F1+F2). However, in samples from free-contaminated areas a little increase is observed in some metals as Cd and Pb in F1 and F2. It is possible to conclude that natural oscillations of the analyzed heavy metals content and its partitioning within the environment are greater, or at least similar, to the anthropogenic input of potential harmful compounds. So, at least for the elements analyzed, dredging tasks would not being negatively affecting the environment. Naturally occurring heavy metal compounds within the environment would be related to the devitrification and alteration of basaltic volcanic glass which naturally conforms an important proportion of the estuarine sediments