The Idrija Mine is the second largest Hg mine in the world which operated for 500 years. Mercury (Hg)-laden tailings still line the banks, and the system is a threat to the Idrija River and water bodies downstream including the Soca/Isonzo River and the Gulf of Trieste in the northern Adriatic Sea. A multidisciplinary study was conducted in June 1998 on water samples collected 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 the mine from <3 to >60 ng liter-1 with methyl mercury (MeHg) accounting for ~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), such genes were detected in bacteria collected downstream. Benthic macroinvertebrate diversity decreased downstream of the mine. Gulf waters near the river mouth contained up to 65 ng liter-1 total Hg with ~0.05 ng liter-1 MeHg. Gulf sediments near the river mouth contained 40 ??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 with highest activities near the surface. MeHg was degraded by an oxidative pathway with >97% C released from MeHg as CO2. Hg methylation depth profiles resembled profiles of dissolved MeHg. Hg-laden waters still strongly impact the riverine, estuarine, and marine systems. Macroinvertebrates and bacteria 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 the 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.