Mobility of metals in river sediments from a watershed in the Iron Quadrangle, Brazil

The dynamics of the behavior of metals in freshwater environments has been one of the main focuses of environmental monitoring in the last years. The present study investigated the distribution of heavy metals in the river sediments of the Santa Bárbara River Watershed (Minas Gerais, Brazil). The study area suffered from problems related to mining and several high impact activities, such as farming, forestry, and reforestation for charcoal production. Surface sediments were collected at four points in the Conceição, Caraça, and Santa Bárbara rivers. These sediments were analyzed for exchangeable, reducible, oxidizible, and residual fractions through the sequential extraction procedure that was proposed by the European Community Bureau of Reference (BCR). The results indicate that the Cd and Pb are mostly present in the reducible fraction, while Fe, Ni, Cr, and Zn were predominantly found in the residual fraction, Mn and Cu are the elements that proportionally present more exchangeable fractions and greater mobility. Factors such as pH and redox potential influence several processes which alter the mobility of metals, such as adsorption and complexation. The values of these parameters have shown that the sampled points have reducing and acidic characteristics. The identification of the geochemical stages of metals in sediments allowed for the comprehension of the contamination in the medium and the identification of possible sources. This enables the assessment of the quality of the sediments in the studied watershed and serves as a reference for the monitoring of the anthropic effect to the environment.