Mercury Transformations in the Idrija River System, Slovenia

Collaborators: Tamar Barkay (Rutgers University), Jadran Faganeli (Marine Biological Station and Institute of Biology, Slovenia), Milena Horvat (Josef Stefan Institute, Slovenia)


Click on the photos below for larger images

Marine Biological Station, Piran, Slovenia



Piran at Night

Marine Lab at Univ. Trieste, Italy

Coring at Mouth of Isonzo River

Giant Jellyfish in Gulf of Trieste

Core in Lab

Isaac in the Bag Again

Sampling in Grado Lagoon, Italy with Stefano Covelli (right)






The Idrija River system in Slovenia offers a natural laboratory for studying the biogeochemistry of mercury (Hg) in a variety of environmental settings. The riverine system drains cinnabar rich deposits in the Idrija mining district (Figure 1). This area was mined continually for 500 years and is the second largest Hg mine in the world. Of the five million metric tons of Hg ore mined, over 25% of the Hg is thought to have dissipated into the environment. Although the soils in the area are naturally elevated in Hg, the mining operation severely enhanced the mobilization of Hg through smelting activities and the deposition of mine tailings in Idrija. Even though the mine was recently closed, the system still delivers about 1.5 tons of Hg to the sea per year, which is about 100 km away from Idrija.

Figure 1. Locations of sampling sites within the Idrija, Soca nd Isonzo Rivers System. The Idrija Hg Mine is located in Idrija, but Hg-laden material flows the ~100 km distance to the Gulf of Trieste in the northern Adriatic Sea. The Idrija River merges with the larger Soca River, and the Soca's name changes to the Isonzo River when it enters Italy. The "bars" crossing the Soca River are hydropower dams that impound fine sediments and debris. Hg that enters the Gulf of Trieste is carried to the west by Bora winds and these sediments are rich in Hg. MeHg that is bound to fine sediment fractions is carried by a counterclockwise gyre in the Gulf and is deposited in the eastern region of the Gulf.

The Hg transport in the Idrija system is affected by various hydrologic changes and barriers such as the confluence of the Idrija River with the much larger Soca River, and three hydropower dams in the Soca that results in reservoirs with organic-rich deposits trapped behind dams. Moreover, the mixing zone in the Soca estuary greatly alters the chemistry of the river water. Hg-laden riverbank sediments are found throughout the system and the water saturation of these deposits varies with climate. The presence of dynamic riverbanks, reservoir deposits, and estuarine conditions have resulted in profound changes in the speciation of Hg along the 100 km of the system. Most notably, there is an increase in total Hg concentrations in water samples as one moves downstream throughout the lower 50 km of the system, despite the fact that the source of Hg is over 50 km away (Figure 2). In addition, the ratio of methylmercury (MeHg) to total Hg increases throughout the impacted ecosystem, indicating that there are strong sources of MeHg well away from the mercury source, presumably due to methylation in reservoir deposits and in estuarine sediments.

We have been working in the Idrija region for several years, including some studies of Hg transformations throughout the system from the mine into the sea (Bonzongo et al. 2002; Hines et al. 2000; Horvat et al. 2002). In addition, recent studies are beginning to show how dynamic the system is in terms of sources and sinks of Hg and how biogeochemical changes affect Hg speciation, transport, and bioaccumulation. Our Slovene colleagues have been involved in additional studies including effects of Hg contamination on fisherman and miners, and bioaccumulation of Hg in terrestrial mammals and marine phytoplankton, among others.

Figure 2 . Concentrations of total Hg and MeHg in unfiltered water samples from the Idrija/Soca/Isonzo River systems. Site numbers differ from those in Figure 1. Both species increase greatly downstream of the mine. Site 1 is located less than one km upstream of the mine. Note the scale change for total Hg at the mine (site 2). Increases of total Hg and MeHg downstream are likely due to remobilization in impoundments and estuarine areas. The meHg:Total Hg ratio increases greatly after site 4, with the highest ratio occurring at site 8 in the Soca River Impoundments (Hines et al., 2000).

Mercury methylation and demethylation is active in sediments, but is most pronounced behind dams and in the estuary (Figure 3). Fastest methylation rates occur where sulfate reduction occurs (sulfate decrease), but total Hg and MeHg are mobilized most readily in the estuarine areas (Hines et al., 2006). Demethylation occurs primarily via the oxidative pathway, but the C moiety is also recovered partly as methane in the freshwater impoundment sediments since demethylation occurs during methanogenesis in which some of the methyl groups are reduced to methane. Demethylation in the estuarine samples is probably conducted by sulfate-reducing bacteria that completely oxidize methyl group to carbon dioxide.

Figure 3 . Depth profiles of potential rates of Hg transformations in sediments behind a dam (Impoundment) and in a brackish area upstream of the river mouth (Estuary). Despite the fact that both sites have similar quantities of total MeHg, the amount of dissolved MeHg (and dissolved Hg) is much higher in the Estuary. the methane (CH4) liberated from MeHg in the Impoundment sediments is most likely due to its decomposition by methanogenic bacteria rather than due to a reductive demethylation. (Hines et al., 2006)

Hg is methylated and demethylated in marine sediments in the Gulf of Trieste throughout the year (Figure 4). During summer when conditions are warmer, sediments are more reducing and support more active sulfate reduction. Demethylation occurs by the oxidative path in the summer, and is presumably conducted by sulfate reducing bacteria that produce carbon dioxide from the methyl moiety of MeHg. However, in winter, sediments become somewhat oxidized near the surface as evidenced by minima in sulfate reduction and decreases in reduced sulfur (Hines et al., 2006). An increasing fraction of the MeHg-C is liberated as methane in surficial sediments in winter, which is probably due to an increase in the importance of the reductive demethylation pathway in response to oxidation. Since this pathway reduces Hg to Hg(0) that is not available for remethylation, MeHg loss can occur more readily than in summer. When combined with lower rates of Hg methylation (Hines et al., 2006), the portion of total Hg attributed to MeHg decreases greatly. Since Gulf sediment tend to be more reducing closer to shore, the winter production of methane form MeHg is less pronounced closer to shore and does not occur in sediments at the mouth of the river.

Figure 4. Depth profiles of rates of potential MeHg demethylation and sulfate reduction and percent MeHg data in sediments at site CZ in the Gulf of Trieste (see Fig. 1) in summer and winter. Sulfate reduction is much higher in summer and sediments are more reducing near the surface. In the winter sulfate reduction is lower and sediments are more oxidizing near the surface. Demethylation occurs via the oxidative path in the summer, but occurs partially via the reductive path in surficial sediments in the winter. The presence of the reductive path in the winter may partially explain the decreased %MeHg values since Hg liberated from MeHg cannot be methylated back to MeHg since it is reduced to Hg(0). (Hines et al., 2006)

We have also conducted studies of Hg transformations in sediments and mine tailings and calcines (burned ore) at other Hg mines including the Almaden Mine in Spain (World's largest and oldest mine) and mines in the Big Bend area of Texas (Gray et al., 2004, 2006). Mines wastes actively transform Hg and pose a threat to nearby aquatic and terrestrial biota.


Bonzongo, J. C., W. B. Lyons, M. E. Hines, J. J. Warwick, J. Faganeli, M. Horvat, P. J. Lechler, and J. R. Miller (2002), Mercury in surface waters of three mine-dominated river systems: Idrija River, Slovenia, Carson River, Nevada and Madeira River, Brazilian Amazon, Geochem. Explor. & Environ. Analysis, 2, 111-120.

Gray, J. E., M. E. Hines, and H. Biester (2006), Mercury methylation influenced by areas of past mercury mining in the Terlingua district, Southwest Texas, USA, Applied Geochemistry, 21, 1940-1954.

Gray, J. E., M. E. Hines, P. L. Higueras, A. Adatto, and B. K. Lasorsa (2004), Mercury speciation and microbial transformations in mine wastes, stream sediments, and surface water at the Almaden Mine, Spain, Environ. Sci. Technol., 38, 4285-4292.

Hines, M. E., J. Faganeli, I. Adatto, and M. Horvat (2006), Microbial mercury transformations in marine, estuarine and freshwater sediment downstream of the Idrija Mercury Mine, Slovenia, Applied Geochemistry, 21, 1924-1939.

Hines, M. E., M. Horvat, J. Faganeli, J. C. J. Bonzongo, T. Barkay, E. B. Major, K. J. Scott, E. A. Bailey, J. J. Warwick, and W. B. Lyons (2000), Mercury biogeochemistry in the Idrija River, Slovenia, from above the mine into the Gulf of Trieste, Environ. Res., 83, 129-139.

Horvat, M., V. Jereb, V. Fajon, M. Logar, M. Kotnik, J. Faganeli, M. E. Hines, and J.-C. Bonzongo (2002), Mercury distribution in water, sediment and soil in the Idrijca and Soca river systems, Geochem. Explor. Environ. Analysis, 2, 287-296.