The Urban Pressure Over the Sediment Contamination in a Southern Brazil Metropolis: the Case of Diluvio Stream

The Diluvio Stream flows in an area with the highest population density in Southern Brazil, where there are indications that urbanization has been negatively affecting the quality of the sediments. In the final section with the highest population density, the stream is channeled, and this is the most waterproofed part of the watershed, without any riparian forest, and surrounded by one of the biggest avenues of the metropolitan city of Porto Alegre. Thus, the aim of this study was to evaluate the spatial variability of the sediment pollution, by potentially toxic metals and macronutrients, from the source to the mouth of Diluvio Stream. Sediment samples were collected in 15 sites, along the entire course of the Diluvio Stream—from its springs to its mouth (18 km). We evaluated the pH, bulk density, grain size, electrical conductivity, total organic carbon (TOC), total phosphorus (P), inorganic P and organic P, total nitrogen (N), and pseudototal elements (Fe, Al, Ca, Mg, Na, K, Mn, Ba, Zn, Pb, Cu, Cr, Ni, Cd). The sediments of Diluvio Stream are predominantly sandy, with higher pollution in the sites with smaller particles and more organic carbon, increasing levels of metals and phosphorus to the Lake Guaíba. The pollution increases from its sources to its mouth, with an anthropogenic origin linked to the releases of wastewater and sewage, the large waterproofed area, the lack of riparian forest, and the large flow of vehicles near to the watercourse. Values of carbon, nitrogen, and phosphorus indicate that most of this derive from anthropogenic activities (as sewage), carrying other pollutants together—such as potentially toxic metals (especially Zn, Cu, and Pb).