Water Quality and Sediment Behaviour of the Future: Predictions for the
21st Century (Proceedings of Symposium HS2005 at IUGG2007, Perugia, July
2007). IAHS Publ. 314, 2007, 163-172
The distribution of metals (Co, As and Zn) in a
residential urban watershed, typical of the Brazilian suburbs
CRISTIANO
POLETO & GUSTAVO HENRIQUE MERTEN
Hydraulic
Research Institute - IPH, Federal University of Rio Grande do Sul - UFRGS, Avenida Bento Gonçalves 9500, Caixa Postal
150.29, Porto Alegre, Brazil
cristiano_poleto@hotmail.com
Abstract Environmental contamination by heavy metals has been a constant concern in areas with industrial activity. However, little research has been devoted to the presence of metals in residential areas, especially in the urban periphery. In Brazil, these areas are characterized by disordered urban development and mixed land use, including homes, businesses and services. There is normally no control over point source pollution (domestic sewage, motor oils) in these areas, much less non-point source pollution (sediment from paved and unpaved roads). This study seeks to investigate the concentrations of Co, As and Zn in soil and aquatic environments in a 0.83 km2 urban watershed in the metropolitan region of Porto Alegre, in southern Brazil. Forty-seven samples per km2 were collected from paved and unpaved roads, vegetated areas, the riverbed and its banks. The presence of metals in suspended fluvial sediment was also studied, and sequential extraction was performed to evaluate the geochemical fractions occupied by these metals. Analysis indicates that the metals in question are widely distributed throughout the study area, with higher concentrations in streets with more intense automobile traffic. The metals were found in concentrations above the background values in fluvial sediment; however, sequential extractions showed that both Co and As are found mainly (>50%) in the crystalline phase, offering little risk to the aquatic environment. On the other hand, Zn was distributed among the easily reducible (12%), oxides (41%) and carbonates (20%), representing greater mobility in the aquatic environment in the event of changes in the pH and/or Eh of the water. This non-industrial study area presented high concentrations of the three metals studied, but only Zn poses significant risk of contaminating the aquatic environment.
Key
words urban sediments; metals; sequential extraction