Assessment of heavy metal contamination in sub-tropical riverine sediments using geoaccumulation index

Heavy metal determination in surface sediment allows pollutant assessment when the detection in the water column is not possible. Geoaccumulation Index (Igeo) and the Sediment Quality Guidelines (SQGs) were used to quantitatively evaluate the degree of heavy metal contamination in fluvial sediments. Both of these methods had already been used by several authors and the generated indexes allowed a good comparison parameter. In the present study, heavy metal in sediments (Mn, Zn, Cd, Cr, Cu, Pb and Ni) were analyzed in 14 sampling points, distributed in the upper watercourse of Subaé river, Feira de Santana – BA, in two field campaigns in order to compare dry and wet seasons. The quantification of heavy metal contents was measured by flame atomic absorption spectrometer. The heavy metal levels in the sampling points were low when compared to the global reference values for world sediments. Cadmium concentrations were always below the detection limit of the used spectrometer, and the relative abundance of the studied metals was the following: Mn>Zn>Pb>Cu>Cr>Ni. The variation range of each element concentrations was: Mn 69.3 and 266 mg Kg-1; Zn 30.7 and 426 mg Kg-1; Pb 6.00 and 63.6 mg Kg-1; Cu 4.26 and 66.2 mg Kg-1; Cr 3.24 and 52.9 mg Kg-1; Ni 1.96 and 24.7 mg Kg-1. These results are indicating low contamination risk to biota, despite of the fact that some points presented moderate risk of toxicity. The Igeo ranged from unpolluted (Zn, Mn, Cr, and Ni) to slightly polluted (Pb and Cu) in the assessed sediments. The different approaches to geochemical information treatment allow a wide and coherent debate about the environmental risk of heavy metals from anthropogenic diffuse sources.