Toxic trace elements transport in stream sediments from the world-class Monte Amiata Hg mining district: Potential impact to the Mediterranean Sea

The spatial and temporal variability of Hg, As, and Sb contents were assessed in stream sediments across the Fiora River catchment (Italy), which drains the dismissed Monte Amiata Mining District, the 3rd largest Hg producer worldwide. Mercury, As, and Sb concentrations of samples collected in 2022 along Fiora River tributaries were compared to data collected in 1985 after mine decommissioning. In 2022, the Fiora River showed downstream magnification of Hg pollution, close to the outflow into the Mediterranean Sea. At several sampling sites, concentrations are above the Italian safety limit of 1 mg/kg (up to 3300 mg/kg). Arsenic and Sb concentrations are high in the mid-catchment tributaries (up to 84 and 79 mg/kg, respectively). The river contributes to the pollutant budget of the Mediterranean Sea, and showed low resilience to Hg pollution in the period 1985–2022, whereas for As and Sb some attenuation occurred. The mass loads of Hg, As, and Sb discharged into the Mediterranean Sea from the Fiora River are at least 0.9, 2.2, and 1.8 t/y respectively. The estimated potential sediment-bound Hg flux from the whole Monte Amiata district to the Mediterranean Sea (1.2–6.6 t/y) is in the same range of that calculated for the Idrija district, highlighting the pivotal role of the Mt. Amiata district on the Mediterranean Sea pollution.