Abstract

The sediments carried by the superficial streaming derived of urban areas represent an important source of diffuse pollution for the water bodies especially when treating to the presence of heavy metals. This study aims to evaluate the concentration and to identify the geochemical compartments where these metals are found in order to infer on the conditions that will enable to release these metals. The sediments studied in this work had been originated in a small watershed (0.83 km.) located in the metropolitan region of Porto Alegre city, Southern Brazil. The fluvial sediments in suspension had been collected during the flood. To establish the background, composed samples of the soil in an area of a native forest next to the beginning of the flowing had been collected, inside of the studied watershed. With sediment samples collected in the streets and stream, it was possible to understand that the paved streets are the main source of contaminants in this urban area. To determine the geochemical compartments where the heavy metals are found it had been used a sequential extraction method by Tessier et al. (1979). The results had shown that while Cr occurs predominantly in the residual phase (69%) and it is connected to organics (20%), Ni occurs in crystalline (54%), oxides (18%), organics (12%) and CaCO3 (11%), and Cu occurs on the oxides (63%). Therefore, it is possible to infer that the alterations in the pH and the Eh could release part of these metals to the soluble phase.

Keywords: Urban sediments, metals, sequential extraction