Click on the photos below for larger images
Marine Biological Station, Piran, Slovenia
Piran at Night
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.
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.
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.
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
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,
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.