Geochemistry of Mine Stream Sediments and the Control on Potentially Toxic Element Migration: A Case Study from the Baccatoio Basin (Tuscany, Italy)

The concentration of 14 potentially toxic elements (PTE), including emerging contaminants, was determined in mine tunnel sludges, streambed sediments, and in the bank-full channel load of the Baccatoio stream, in Tuscany (Italy), in a catchment affected by AMD. The geochemical profile of streambed piston-cored sediments and the solute concentrations of the hyporheic pore water and surface water were also determined. Sediments are characterized by high concentrations of As, Cd, Tl, Sb, Ni, Cu, Zn, besides Al, Mn, and Fe. The As and Fe are closely coupled, indicating that iron oxyhydroxide precipitation plays a major role in controlling As sequestration. Mn oxides likely act as a sorbent for Zn, Ni, Sb, and Pb. Downstream geochemical trends indicate that precipitation and sorption represent the natural attenuation processes controlling the PTE loading released from the contaminated acidic effluents. The obtained results highlight that the Baccatoio stream sediments may act as secondary pollution sources via redox cycling, allowing contaminant remobilization to surface- and ground-water across the streambed, affecting the stream environment and aquatic biota.