Geochemical Background and Baselines values of PTEs (Potentially Toxic Elements) in soils and stream sediments from decommissioned mining areas: the Hg-district of Mt. Amiata (central Italy)

This PhD thesis was aimed at evaluating mercury (Hg) contamination and other potentially toxic elements (PTEs) in soils and stream sediments from the eastern sector of the Mt Amiata district (central Italy), with a specific focus on the former mining area of Abbadia San Salvatore (ASS). In particular, the study area covers a rectangle of about 120 km2, where eight of the most important former Hg-mines in Italy and in the world are present. The region, historically a major global producer of Hg, faces significant environmental challenges due to legacy contamination from cinnabar (α-HgS) extraction. The primary objectives of this research are to define speciation and distribution of Hg in various environmental matrices (soils, sediments, waters), to determine its mobility and bioavailability, and to establish geochemical background/baseline values for Hg and selected PTEs (i.e. As, Sb, Cu, Co, Cr, V and Ni) in soils and sediments. Additionally, this study seeks to evaluate remediation strategies for the safe management and reclamation operation of contaminated areas. A crucial component of this research involved the systematic sampling of the environmental matrices. Soil samples were collected from both top- (10-50 cm) and sub- (50-150cm) soils following a twofold strategy: a) one sample each 2 km2 and b) 5 to 6 samples each 1 km2 when approaching the former Hg-mines. The sampling strategy for stream sediments involved the collection of the stream sediments from major watercourses present in the area. Additionally, every 4 months a water survey (major and minor solutes and trace elements, including Hg, As and Sb) inside and outside the ASS mine area was carried out with the aim to study ten years of temporal series and present hydrogeochemical modeling (a flow and transport model developed using ModFlow 6.4.0) assess at which extent the high concentrations of dissolved Hg (up to 695 µg/L) is occurring in order to facilitate water remediation actions for the removal of Hg. Differently, As and Sb only occasionally showed concentration about the Italian limit imposed by the law, i.e. 10 and 5 g/L, indicating that by an environmental point of view they have no notable impact on the local surface and ground waters. In this PhD thesis, a new technique to determine Hg0 in solid matrices, utilizing thermal desorption (TD), is proposed. The related results were compared with those obtained by leaching tests to verify its speciation and bioavailability. It was found that the main Hg-species in soils and stream sediments are α-HgS and metacinnabar (β-Hg) and, subordinately, Hg bounded to Organic Matter (OM). Only few samples showed the presence of HgCl2 and HgSO4, suggesting a very low Hg-bioavailability to the biota and thus, a limited availability for the biogeochemical cycle. In addition, Hg-bioaccumulation by the main plant species growing into the mining area of ASS was investigated. It was evidenced that the preferred pathway of plant accumulation is the foliage. This should reduce bioaccumulation by plants and the negative effect that Hg may have. Only Sambucs nigra turns out to be the plant that better adsorbs Hg via the root system. Moreover, in the same area, dry and wet deposition by mosses was investigated to understand the dispersion and transport of Hg in the air. It was noted that once adsorbed, elemental gaseous mercury (GEM) strongly binds to mosses and is no longer released. In the ex-mining artificial Lake Muraglione, different remediation tests were carried out to reduce Hg since contents up to 900 mg/kg were determined. Physicochemical tests, including TD, indicated that most Hg is in an insoluble form. Therefore, this notably reduces the potential risk to the environment. The main achievement resulting by the geochemical characterization carried out in this thesis at the Lake Muraglione is that the Le Lame mining dump has been indicated as the main site to host the Hg-rich sediments from the lake. One of the main goals of this study was the determination of geochemical background values for Hg and other PTEs in soils and stream sediments. The geochemical background/baseline values of the soils/fluvial sediments were calculated using a geochemical and geostatistical approach, also using compositional data (CoDa) to better understand the relationships between the various elements and following the SNPA (National System of Environmental Protection) guidelines. For the determination of these values, it has been essential to recognize the element source(s) in the soils/fluvial sediments. It was indeed decided to provide geochemical baseline values according to the different lithology onto which soils and sediments were associated. Additionally, it was also proposed to identify a geochemical baseline range of values that would take into account the analytical uncertainties (10 %). The present PhD thesis tried to couple the scientific investigation with an applicative part since all the results were finalized to offer the basis for practical solutions to the remediation operations that in some sites (e.g. ASS) are already going on and safeguard both the environment and public health in those areas where the mining activities strongly affected the environmental matrices. I hope that the content of this work can be of some help for the decision-makers at both local and regional levels, ensuring that the regional mining legacy can be managed sustainably.