Mercury and arsenic in stream sediments and surface waters of the Orcia River basin, southern Tuscany, Italy.

The Orcia river basin lies north of the Mt. Amiata mining district, and may receive toxic elements such as mercury and arsenic therefrom. The Orcia River eventually flows to the Ombrone River, which in turn flows to the Tyrrhenian Sea. The analysis of stream sediments collected in the Orcia River and its main tributaries, as well as in the Ombrone River, indicates moderate contents of both Hg and As (median values: Hg 118 μg/kg, As 5.25 mg/kg), rarely exceeding the Italian limits of environmental quality standards. Only in the close proximity of the former Hg mine of Pietrineri exceptionally high values for both elements are observed (Hg 195 mg/kg, As 35 mg/kg). Travertine and unconsolidated deposits associated with thermal springs of the area also show generally low Hg contents (4 - 320 μg/kg), with a significant exception of 23 mg/kg. Arsenic contents in the same deposits are more variable, and may reach values up to 358 mg/kg. Surface waters collected at the same sites as stream sediments show Hg and As contents below the Italian mandatory limits for drinking waters (1 μg/L for Hg, and 10 μg/L for As). Likewise, in thermal springs Hg contents are low, whereas As contents (in agreement with previous studies) may reach 23.4 μg/L. At present, the input of toxic elements from the mining district into the Orcia and Ombrone watersheds is apparently much lower than documented in the Paglia and Tiber catchments south of Mt. Amiata, and does not pose an immediate environmental threat. However, the possible remobilization of Hg-contaminated sediments during flash flood events cannot be dismissed.