Heavy metal enrichment and potential ecological risks from solid mine waste: The case study of the Lame Hg mining dump (Abbadia San Salvatore, Mt. Amiata, Southern Tuscany)

Abstract Mining activity has generated substantial amounts of solid byproducts, despite its notable role in advancing the socio-economic progress worldwide. Mining leftovers, such as excess earth and debris, as well as waste materials from mineral processing operations, often contain hazardous substances such as heavy metals (HMs) that can have severe consequences for both human well-being and the environment. Solid mine wastes consist of multiple elements including sulfides, Fe-Mn oxides, carbonates, silicates, and HMs. Each of these elements possesses distinctive chemical, biological, and ecotoxicological properties. As a result, the effective management of mine wastes requires the use of diverse analytical techniques to accurately assess the elemental constituents, their concentrations, and their individual and combined effects. Researchers frequently apply various indexes such as enrichment factor (EF), contamination factor/degree, and ecological risk (ER) to evaluate the quality of the surrounding environment or to determine the potential ecological hazards associated with distinct solid wastes. Located on the eastern side of Mt. Amiata, a few km north of the Abbadia San Salvatore (ASS) mine, the Le Lame mining dump covers an area of roughly 120,000 m2. The transportation of calcines, which are post-processed products, to the mining dump commenced during the 1930s. This was done in order to increase the production of liquid Hg, and new automated conveyor belt systems were established to transfer the Hg-rich material from the extraction shafts to the highly efficient Gould Furnaces. One of the objectives of the remediation of the former Hg mine in ASS is to secure and morphologically reshape the mining dump at Le Lame. In 2022, a new soil sampling was carried out at two depths (0.8-1.2 m and 1.2-2.5 m) to obtain detailed information on the distribution of HMs (e.g. Hg, Sb, As, Be, Cr, Co, Cd, V, Zn, Cu, Fe, Mn, Ni, Pb, Tl) and the respective EF and ER index in the Lame soil. Only Hg, Sb and As were found to have concentrations higher than the D.Lgs 152/06, which regulate the threshold concentrations of contamination, being characterized by the following ranges 6.6-890 mg/kg, 1-1980 mg/kg and 5-88 mg/kg, respectively. In order to understand the relationship between metals, in top and sub-soil, a Compositional Data Analysis (CoDA) approach was adopted on the two datasets. The EF of Co, Cr, Ni, V, Zn and Cu in the top soils is found to be minimal to significantly enriched. Arsenic appears to have an EF up to very high enrichment, while Sb up to extremely high enrichment. Mercury is the only element to have a consistently very high enrichment in the top- and sub-soils. In the sub-soil Co, Ni and Zn are found to have minimal to moderate enrichment, while Cu has significant enrichment. Arsenic, Sb and Hg in the sub-soil reflect the same result as in the top soils. The ER, calculated only on Hg and Sb in both top- and sub-soils, appears to be low to severe.