Severe contamination of waters and stream sediments in an abandoned mine land from Alta Versilia (Southern Apuan Alps, Italy)

Natural weathering and human activities may both be sources of toxic and potentially toxic elements that are released and transported through contaminated plumes by surface and subsurface waters. Remediation strategies, to be effective, require knowledge of the source zone of contamination and of the processes that control the migration of the different contaminants, including the role of physical and chemical processes such as advection, dispersion and diffusion, besides the redox, chemical speciation adsorption/desorption and dissolution/precipitation reactions. In this context, the acid mine drainages that may develop as effluents from abandoned mine sites likely represent one of the largest environmental problems facing the mining industry. In the present study, preliminary results concerning the fate of toxic and potentially toxic elements released from acid mine drainages into the surface waters and stream sediments of the Baccatoio river, crossing abandoned mine sites in southern Apuan Alps (Tuscany region) and the Valdicastello Carducci village, are reported. The mining activity, that exploited ore bodies of pyrite + baryte and iron oxides + baryte hosted at the contact between the phyllites, related to the Paleozoic basement of Apuan Alps, and Triassic dolostones, ended in 1989. The data on mine effluents indicate that sulfide oxidation was the primary source of contaminants, releasing acid waters with high Fe (up to 14 g/L), Mn (19 mg/L)), Al (270 mg/L), Cd (2 mg/L), Sb (19 mg/L), Pb (3 mg/L), Zn (270 mg/L), Ni (5 mg/L), Tl (9 mg/L) and As (30 mg/L) concentration. After the outflowing in the superficial waters of the Baccatoio stream, most metals and metalloids are scavenged by the formation of large amounts of iron precipitates when Fe(II) is oxidized to Fe(III), whose composition is controlled by sulfate ion concentration, water pH and aging, ranging from jarosite to schwertmannite, ferrihydrite and goethite. In addition, the rate of Fe(II) oxidation is highly pH dependent, and, at the pH values measured in the Baccatoio waters, Fe(II) might increase its residence time, allowing further precipitation downgradient. Iron-rich stream sediments are characterized by high concentrations of Sb, As, Ni, Pb, Zn and Hg (up to about 540, 3600, 9200, 2100 and 80 mg/kg, respectively), acting as effective scavengers for these contaminants. However, the data indicate that Tl behaves almost conservatively in the surface waters of the Baccatoio, and it is readily transported by the aqueous phase. Indeed this very highly toxic element is missing or present in very low concentrations in the Baccatoio stream sediments while it persists in relatively high concentrations in the aqueous matrix, becoming a cause for concern for the environment and human health.