Vertical distribution and contamination assessment of heavy metals in the vadose zone of a 50-year-old smelter dumpsite, San Antonio Bay, Argentina

This research aimed to assess the contamination levels and mineralogical association of heavy metals in the vadose zone beneath smelter residues that were freely deposited over natural sediments from the 1960 s to the 1980 s. The results revealed elevated concentrations of heavy metals, particularly Fe, Pb, Zn, Cu, and Cd. The highest concentrations were found within the superficial layer, corresponding to the smelter slag (203,703 – 219,998 mg kg−1 Fe; 73,708 – 136,390 mg kg−1 Pb; 18,922 – 29,484 mg kg−1 Zn; 6473 – 7887 mg kg−1 Cu; 20 – 48 mg kg−1 Cd). This residue is an acidic material mainly constituted by secondary sulfates and oxyhydroxides. Although concentrations decreased with depth, the levels of these metals remained higher than the background even at depths of 300 cm. Precipitation of secondary minerals (plumbojarosite, anglesite, Zn-sulfates, and Fe oxydroxides) from the percolating leachates was detected in the vadose zone based on SEM and EDS analysis. According to their vertical distribution, Cd and Zn showed a higher mobility than Cu, Fe and Pb, which were primarily concentrated at the surface layer. The geo-accumulation index (Igeo) was Pb > Cu > Zn > Cd > Ni. The vadose zone is severely polluted and represents a very high risk to the environment based on the ecological risk factor (Eri) and risk index (RI). The metals that contribute most to this condition are Pb > Cd ≈ Cu > Zn. Given the severity of the situation, remediation of the study area is recommended. [Display omitted] •Smelter residues contained extremely high concentrations of Fe, Pb, Cu, Cd and Zn.•Metal concentrations were related to the mineralogical assembly of the sediments.•Secondary sulfates and oxyhydroxides provide evidence of leaching of metals.•The assessed environmental risk proved to be extremely high.