Provenance and distribution of potentially toxic elements (PTEs) in stream sediments from the eastern Hg-district of Mt. Amiata (central Italy)

Geochemical analysis of Potentially Toxic Elements (PTEs) in stream sediments is essential for understanding environmental impacts in areas with complex geology and mining activities. This study focuses on stream sediments from the eastern sector of the Mt. Amiata Hg-district (central Italy) to define the background values of As, Sb, Cr, V, Co, Cu, Ni, and Hg, the latter being speciated to assess its bioavailability and mobility for potential environmental risks. The stream sediments are divided into four different groups: (1) Volcanic, (2) Volcanic-dominated, (3) Sedimentary-dominated, and (4) Sedimentary. This subdivision is confirmed by spatial distribution maps. While Cr, Co, V, Cu, and Ni are related to mafic and ultramafic rocks, Hg shows higher concentrations (up to 850 mg/kg) close to the former mining sites and organic-rich areas. In streams draining the Mt. Amiata volcanics and hydrothermal zones, As is up to 311 mg/kg while Sb is uniformly distributed (up to 84 mg/kg), though depleted in volcanic rocks. Consolidated sediment clusters (group 1 and group 2 and group 3 and group 4, respectively) are used to calculate the geochemical background values (crucial for post-extractive land reclamation), which result to overcome the concentrations imposed by the Italian law for Hg in both clusters, As in cluster 1 and Co in cluster 2 (5.2-6.3, 20-24.3, 39.7-48.5, and 26.3-32.2 mg/kg, respectively). In the stream sediments, Hg speciation by thermal desorption highlights the presence of stable Hg forms. Thus, low Hg bioavailability is expected, being also consistent with the soils from the nearby mining areas.