Distribution of Thallium in Sediments of the Fiora River Catchment, Central Italy: Implications for Its Sources

Previous studies documented the contribution of toxic elements (Hg, As, and Sb) from the dismissed Monte Amiata Mining District (Italy), the third largest Hg producer worldwide, to the Mediterranean Sea. Another highly toxic element, thallium (Tl), received less attention. Here we report a reconnaissance study of the spatial variability of Tl content in stream sediments across the Fiora River catchment, which drains part of the Hg and Sb mining districts. Thallium contents are comparatively low (≤0.4 mg/kg) in sediments of creeks directly draining the mining areas, whereas they increase up to 2 mg/kg in the catchment section that drains the Vulsini ultrapotassic volcanic province, where no known mineral deposits occur. Results suggest that Hg and Sb deposits cannot be the only Tl source in the catchment. The most likely alternative/additional candidate are the high-K volcanic rocks. Although no specific data for the Vulsini district exist, a distinct Tl geochemical anomaly linked to the Latium volcanic province is present. The total Tl mass contained in sediments discharged yearly into the Mediterranean Sea from the Fiora River is estimated in the order of 0.1 t. This reconnaissance study indicates a non-negligible potential release of Tl from the mining districts and volcanic catchments in Central Italy and suggests the opportunity of further investigation on Tl distribution and speciation in the area.