Contaminated sediment loads from ancient mercury ore roasting sites, Idrija area, Slovenia

• Stream sediments are highly contaminated with Hg.
• Strong effect of ancient Hg ore roasting sites on the Idrijca River sediment contamination
• Delivery of Hg by storm waters from ancient roasting sites is an ongoing issue.

The distribution and transport of Hg and other elements were studied in aquatic systems draining contaminated ancient Hg ore roasting sites Pšenk and Frbejžene trate in Idrija surroundings. Hg concentrations in sediments at the studied roasting site areas were found extremely high, ranging from 18 to 1240 mg/kg (average 387 mg/kg) in the < 0.063 mm grain-size fraction, and from 48 to 1080 (average 456 mg/kg) in the < 0.125 mm fraction. Further downstream along the Padarjeva grapa Stream, which drains both contaminated areas, Hg concentrations were 330–2040 (average 721 mg/kg; for < 0.125 mm) and 380–840 (average 531 mg/kg; for < 0.04 mm). Increased Hg concentrations were found in the Idrijca River sediments downstream of the Padarjeva grapa Stream confluence. The results of our investigation have proved that Hg-loaded materials are not only still present at the roasting sites, but are intensively eroded and transported downstream during high waters. Consequently they have significant influence on the Idrijca River sediment contamination. Delivery of Hg contaminants from historically contaminated roasting sites via Padarjeva grapa Stream during heavy rainfalls was most intense during operation of the roasting sites, but remains an ongoing issue.