The complex handling of historical contaminated sites: the case of the world-class Mt. Amiata district (Italy)

Global population growth and industrialization have driven the demand for material resources with a staggering growth in the decades after the World War II and at the beginning of 21st century. However, mining activities inevitably cause environmental degradation, including contamination of watersheds by mercury (Hg). Mercury affects human health as well as the terrestrial and marine biodiversity. Close to historical mining sites, river self-restoration (i.e., without any management) is incompatible to human timescales, because Hg remains stored within floodplains for decades to millennia. In contrast, complete remediation of watersheds is often economically unstainable because of the extent of contamination that crosses regional and national borders (e.g., Grygar et al., 2022). This paper presents results of recent studies on the Paglia-Tiber River (PTR, Italy) watershed, affected by a long and intense mining exploitation, as an example of environmental degradation due to lack of management strategies. The nature and the extent of contamination suggest that no clean-up strategies are feasible from an economic and social point of view. The PTR receives part of the drainage from the world-class Mt. Amiata Hg district, where production ended in 1980s. Longitudinally, sediments of PTR are contaminated (Hg >1 mg/kg, as defined by the Italian law) for 200 km up to the city of Rome (Rimondi et al., 2019). Transversally, the Hg contamination area affects all the Paglia River floodplain, and extends up to the pre-anthropic Pleistocene fluvial terraces. Such distribution resulted from the interplay of Hg mining, that fed the floodplain with large amounts of Hg-contaminated sediments during the braided stage of the river (end of 1800-mid-1950s), and the subsequent morphological changes of the river after 1960, induced by anthropogenic activities like gravel mining, that led to the present-day single-channel morphology (Fornasaro et al., 2022). Most of Hg is now stacked in overbank sediments at a higher level than the present-day watercourse. Conservative estimates indicate that at least 60 tons of Hg are contained in the sediments of the first 40 km of the Paglia River course (Colica et al., 2019). Under high flow conditions, and especially in coincidence with intense rain events, large amounts of Hg stored in the overbank sediments are physically mobilized and redistributed along the PTR and eventually to the Mediterranean Sea. Extreme weather events, expected to intensify for climate change, will further exacerbate these processes. In similar settings, given the impossibility of total remediation, mitigation strategies must be found to guarantee that the coexistence between humans and Hg contamination occurs at minimum risk. With respect to the PRT, ongoing studies are evaluating the potential role of poplar trees plantation along the riverbanks to favour the physical retention of Hgrich particles from the overbanks. A side benefit would be biomass recovery for energy production. Colica A., Benvenuti M., Chiarantini L., Costagliola P., Lattanzi P., Rimondi V. & Rinaldi M. (2019) - From point source to diffuse source of contaminants: the example of mercury dispersion in the Paglia River (Central Italy). Catena, 172, 488-500. https://doi.org/10.1016/j.catena.2018.08.043. Fornasaro S., Morelli G., Rimondi V., Fagotti C., Friani R., Lattanzi P. & Costagliola P. (2022) -The extensive mercury contamination in soil and legacy sediments of the Paglia River basin (Tuscany, Italy): interplay between Hg-mining waste discharge along rivers, 1960s economic boom,and ongoing climate change. J. Soil. Sediment., 22(2), 656-671. https://doi.org/10.1007/s11368-021-03129-0. Grygar T.M., Hošek M., Navrátil T., Bednárek J., Hönig J., Elznicová J., Pacina J., Rohovec J., Sedláček J. & Sass O. (2022) - Lessons learnt from the revitalisation of chemical factory in Marktredwitz and riverbanks downstream: when ‘renaturation’ can be harmful. Water-SUI, 14, 3481. https://doi.org/10.3390/w14213481. Rimondi V., Costagliola P., Lattanzi P., Morelli G., Cara G., Cencetti C., Fagotti C., Fredduzzi A., Marchetti G., Sconocchia A. & Torricelli S. (2019) - A 200 km-long mercury contamination of the Paglia and Tiber floodplain: Monitoring results and implications for environmental management. Environ. Pollut., 255(1), 113191. https://doi.org/10.1016/j. envpol.2019.113191.