Mercury biogeochemistry in the Idrija River, Slovenia, from above the mineinto the Gulf of Trieste

The Idrija Mine is the second largest Hg mine in the world which operated f or 500 years. Mercury (Hg)-laden tailings still line the banks, and the sys tem is a threat to the Idrija River and water bodies downstream including t he Soca/Isonzo River and the Gulf of Trieste in the northern Adriatic Sea. A multidisciplinary study was conducted in June 1998 on water samples colle cted throughout the Idrija and Soca River systems and waters and sediments in the Gulf Total Hg in the Idrija River increased >20-fold downstream of t he mine from <3 to > 60 ng liter(-1) with methyl mercury (MeHg) accounting for similar to 0.5%, Concentrations increased again downstream and into the estuary with MeHg accounting for nearly 1.5% of the total. While bacteria upstream of the mine did not contain mercury detoxification genes (mer), su ch genes were detected in bacteria collected downstream. Benthic macroinver tebrate diversity decreased downstream of the mine. Gulf waters near the ri ver mouth contained up to 65 ng liter(-1) total Hg with similar to 0.05 ng liter(-1) MeHg. Gulf sediments near the river mouth contained 40 mu g g(-1) total Hg with MeHg concentrations of about 3 ng g(-1), Hg in sediment pore waters varied between 1 and 8 ng liter(-1), with MeHg accounting for up to 85%, Hg methylation and MeHg demethylation were active in Gulf sediments w ith highest activities near the surface. MeHg was degraded by an oxidative pathway with >97% C released from MeHg as CO2. Hg methylation depth profile s resembled profiles of dissolved MeHg. Hg-laden waters still strongly impa ct the riverine, estuarine, and marine systems, Macroinvertebrates and bact eria in the Idrija River responded to Hg stress, and high Hg levels persist into the Gulf. Increases in total Hg and MeHg in the estuary demonstrate t he remobilization of Hg, presumably as HgS dissolution and recycling, Gulf sediments actively produce MeHg, which enters bottom waters and presumably the marine food chain. (C) 2000 Academic Press.