A geochemical characterization of sediments of the Mt. Amiata Hg district and a preliminary estimation of mercury background

The Mt. Amiata region (southern Tuscany, Italy) hosts one of the most important Hg districts in the world, determining a widespread Hg (and associated As) geochemical anomaly. Mercury ores have been exploited for Hg production up to the end of 1970s, resulting in the widespread occurrence of Hg-contaminated discarded materials next to the mine centres. As a result, extensive Hg contamination affects the Pagliola Creek ecosystem, which drains the most important Hg mine of Abbadia San Salvatore. Stream sediments of different age were sampled along the Pagliola Creek in order to reconstruct the history of Hg(As) diffusion in this region. Moreover, a paleo-hydrographic reconstruction of the Pagliola Creek was attempted, and four different typologies of sediment sampled. Multivariate analysis was applied to observe if the operational subdivision of sediments by stratigraphy corresponded to remarkable differences in main geochemistry. Analysis showed that sediments can be clustered in two main groups, i.e. pre- and post-mining sediments, on the base of the Al2O3/CaO ratio. Accordingly, Al2O3/CaO >1 and Al2O3/CaO ≈1 characterized pre- and post-mining sediments, respectively, suggesting a progressive enrichment in CaO as sediments become younger in age (post-mining). According to our study, Hg in the Mt. Amiata sediments is not randomly distributed: Hg, in fact, is inversely correlated to Al2O3/CaO ratios (R2 = 0.87) and directly to the CaO content, increasing up to two orders of magnitude between pre- and post-mining sediments. Common to most of Hg mines worldwide, calcination was carried out at the Abbadia San Salvatore mine, which consisted in the addition of lime to a first roasted material to recover residues of metallic Hg. Final discarded wastes are then termed calcines, and are enriched in CaO and Hg, due to the inefficiency of the extraction process. Natural weathering of these materials caused CaO and Hg enrichment of recent and present-day (post-mining) stream sediments of Pagliola, which are still directly impacted by runoff from the Abbadia San Salvatore mine site. Accordingly to the results of our study, CaO likely represents a geochemical marker of such a weathering of calcines, and can act as a proxy for Hg in the Pagliola Creek watershed. A preliminary Hg regional background of 2-6 μg/g was set up for the Mt. Amiata area, which largely exceeds the Hg established contamination level of Italy. As a result of this study, a new reliable regulation for Hg should be defined for this region, taking into account the natural Hg presence in the environment.