Comprehensive evaluation of metal pollution and ecological risk in settling material from differently impacted sites in the Río de la Plata basin

Abstract

Metal speciation, geoaccumulation indices and sediment quality guidelines were used to evaluate the ecological risk of metals in settling material from the polluted metropolitan Buenos Aires coast in the Río de la Plata (RLP) compared to a more pristine northern site in the Uruguay River (UR). The content of organic carbon and trace metals (Zn, Cr, Cu, Pb and Ni) in RLP was 2-6 times higher reflecting the anthropogenic impact in the metropolitan coastal area. In contrast, Fe and Mn were 4-5 times lower, reflecting basically redox-driven evasion from polluted anoxic settling material. In addition, at RLP, trace metals were mainly found in the potentially bioavailable fractions (51-84%) compared to the prevailing, immobile residual fractionation at UR (63-86%), while Fe and Mn displayed a similar fractionation at both sites (Fe: 86-90% in the residual fraction; Mn: 71-82% in non-residual fractions). The geoaccumulation index relative to local background values (Igeo), the effects-based sediment quality guidelines (SQG) and the risk assessment code (RAC) from speciation data revealed some discrepancies in the risk evaluation. Although concentrations of Zn, Cr, Cu and Pb in RLP were enriched compared to background concentrations (1 ≤ Igeo ≤ 2) and exceed SQGs for the protection of aquatic life, according to RAC, only Zn, Mn and, to a lesser extent, Ni presented probable risks for the aquatic environment.