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Part 2 - Biogeochemistry
Methylation and Demethylation of Mercury Throughout the Idrija River System
Mark E. Hines1, Jadran Faganeli2 & Milena Horvat3
'Department of Biological Sciences, University of Massachusetts Lowell, Lowell, MA 01854, USA, e-mail: mark_hines@uml.edu PMarine Biological Station, National Institute of Biology, Piran, Slovenia QDepartment of Environmental Science, Jozef Stefan Institute, Ljubljana, Slovenia
Abstract: The Idrija Mercury Mine was active for 500 years and Hg has been distributed throughout the system all the way to the northern Adriatic Sea over 100 km away. Total Hg and methylmercury (MeHg) vary greatly throughout the system, and sharp increases in both occur several tens of km for the mine, especially around impoundments and estuarine regions. Radiotracer methods were used to investigate Hg transformations throughout the system and almost every habitat studied actively transformed Hg. Reductive demethylation dominated in riverbanks, but methylation was noted as well. Sediments behind impoundments exhibited rapid methylation near the surface with reductive demethylation gradually replaced by the oxidative path with depth. Methylation was more active in estuarine sediments and demethylation was primarily via the oxida-tive pathway. MeHg concentrations were also much higher in estuarine sediments than in the upstream impoundment deposits. Mercury levels decreased offshore in the Gulf of Trieste, but dissolved species did not. Seasonal changes in redox and bioturbation in the Gulf affected Hg transformation rates and the relative importance of oxidative vs. reductive demethylation. The Idrija system is widespread and dynamic encompassing the gamut of Hg biogeochemistry.
Key words: idrija mine, methylation, demethylation, impoundments, marine sediments
Introduction
Five Hundred years of activity at the Idrija Mercury Mine has left a legacy of contamination along the 100 km route from the mine to the northern Adriatic Sea. Mercury concentrations are elevated in air, waters, sediments, and riverbanks, and mercury (Hg) is transformed in several different types of habitats throughout the system (Horvat et al. 2002). However, mercury concentrations and specia-tion vary greatly along the path to the sea, and in some cases, both total Hg and methylmer-cury (MeHg) levels increase greatly several
km from the mining region (Hines et al. 2000). This is especially noticeable in the middle and lower sections of the Soca/Isonzo River, and in the Isonzo estuary. In addition, the MeHg to total Hg ratio increases toward the sea, suggesting that MeHg formation is very active along the entire path, especially near impoundments and estuarine areas. To investigate the accumulation of MeHg in the Idrija system, Hg transformations were determined in different seasons in riverbank soils, in sediments in the Idrijca and Soca Rivers, and in sediments within the Isonzo estuary and several km into the Gulf of Trieste.
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Materials and methods
Mercury methylation potentials were determined using 203Hg and a toluene extraction procedure. MeHg demethylation potentials were determined using 14C-MeHg and monitoring the production of 14CH4 and 14C02 (Hines et al. 2000). Pore water (Hg, MeHg, nutrients, Fe, S042", £H2S) and solid phase (Hg, MeHg, organic C, N, S, reduced S) chemistry were also determined in many instances (Hines et al. 1997).
Results and discussion
Virtually every habitat studied had the potential to transform Hg to some degree. Sediments were primarily cobble in the 40 km stretch of the Idrija River prior to its confluence with the Soca River, and these deposits yielded little activity. However,
riverbank soils in this reach actively transformed Hg with a dominance of demethyla-tion (kdeg 0.9-5.5 % day"1). Significant methylation was also noted (kmeth 0-0.6 % day"1). The methyl group of MeHg was converted almost exclusively to CHR in these soils indicating that demethylation was catalyzed by an organomercurial lyase enzyme of the bacterial Hg detoxification system (mer) (Barkay et al. 2003; Marvin-DiPasquale et al. 2000).
Sediments behind a hydropower dam impoundment in the Soca River exhibited active methylation and demethylation of Hg, but methylation was restricted primarily to the upper few cm, which also displayed the highest concentrations of dissolved Hg (48 ng/L) and MeHg (6.7 ng/L) (Fig. 1). Demethylation was quite active with CHR accounting for >60 % of the C released at the surface and decreasing to -10 % at 10 cm, suggesting that any contribution of
kmeth (%/daV) kdeq (%/daV) Diss. Hg (ng/L) Diss. MeHg (ng/L) 0 1 2 0 a 5 10 0 300 600 0 10 20
J_I
J Impoundment. J -I-1-1
Estuary
-I-1-
-I-1-
Figure I. Depth profiles of methylation (kmeth) and demethylation (kdeg) rate constants, and dissolved total Hg and MeHg in freshwater sediments behind an impoundment and in estua-rine sediments.
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the reductive path to demethylation was decreasing with depth. The persistence of CHR release was probably also due to the dominance of methanogenesis, which produces some CHr during oxidative demethylation (Marvin-DiPasquale et al. 2000).
Methylation activity in sediments collected in the estuarine region was more rapid than in the impoundment sediments with a subsurface maximum, and the ratio of methylation to demethylation (rate constants) was twice that noted in the freshwater impoundment muds (Fig. I). All C released from MeHg during its degradation was recovered as C02 indicating that demethylation occurred entirely by the oxidative pathway. Dissolved total Hg was 10-20 fold higher in the estuarine pore waters (up to 600 ng/L) than in the freshwater sediments, and MeHg in estuarine pore waters was ~5-fold higher. The estuarine sediments contained a little more than 1.0 mM S042", which decreased
with depth, indicating that S042"-reducing bacteria were probably responsible the oxi-dative demethylation of MeHg as well as its formation. It has been shown that significant, but low concentrations of S042" enhance the formation of MeHg (Gilmour et al. 1992).
Hg and MeHg levels in marine sediments decreased from the mouth of the Isonzo River into the central Gulf of Trieste (Hgtot 45 to 1.0. p,g/g; MeHg 4.0 to 0.2 ng/g), whereas dissolved Hg and MeHg did not vary significantly. Methylation and demethylation activity in marine sediments decreased with depth (Fig. 2), except in summer when a subsurface maximum was noted at the maximum depth of bioturbation (data not shown). The methyl group of MeHg was converted primarily to C02 in marine sediments, except for a slight production of CH4 in surficial sediments in winter; a phenomenon that increased offshore inversely to rates of bacterial sulfate reduction (data not shown).
MeHg Demethylation (% per day)
0369 12 0369 12 0369 12 I_I_I_I_I_ I_I_I_I_I_ I_I_I_I_i_
Hg Methylation (% per day)
Figure 2. Depth profiles of mercury transformation rate constants at three sites in the Gulf of Trieste in March. Site D6 is closest to shore and the river mouth, CZ is most offshore. Black columns represent CHR and white COP production during demethylation of MeHg
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