engleski

A comparative study of mercury distribution and methylation in mudflats from two macrotidal estuaries: The Seine (France) and the Medway (United Kingdom)

Mercury is a contaminant of great concern in the marine environment, particularly in coastal environments where the formation of methylmercury (MeHg) in aquatic sediments, and its subsequent bioaccumulation in edible aquatic organisms (mainly fish), presents a major pathway for human exposure to MeHg. Consequently, it is important to determine the factors controlling MeHg production in sediments, especially in environments exposed to anthropogenic influence. This study investigates some of the main geochemical factors affecting the speciation and distribution of mercury in estuarine sediments from two highly industrialized macrotidal salt marsh/mudflat systems in the Seine estuary, France, and in the Medway estuary, UK. Obtained data revealed that the entire sediments core from a rapidly accreting mudflat in the Seine estuary (the analysed sediment column of 170 cm has accumulated over the last 10-50 years) was contaminated with mercury, at concentrations which are 10-50 times higher than the Hg background for sediments of the Seine basin. In the mudflat in the Medway estuary, characterized by a slow sedimentation rate (400 cm of the sediment column has accumulated over the approximately last 800 a), the near-surface sediments were significantly more contaminated (10-40 times) than sediments at greater depths, characterized with pre-industrial Hg value. Geochemical conditions in the surface sediments of the Medway mudflat, which are characterized with stable redox conditions (about -200 mV), which is generally favourable for Hg methylation, whereas near-surface sediments of the Seine mudflat, is characterized by more oxidizing redox conditions )about +100 mV). Which is generally less conductive to Hg methylation. Consequently, MeHg concentration in the upper 10 cm of the sediment column was about 4 times higher in the Medway than in the Seine mudflat, in spite of the similar total Hg concentrations. In surface sediments, where Hg is actively methylated, MeHg variability was associated with activity of sulphate-reducing microorganisms (SRM) and the presence of acid volatile sulphides (AVS). A strong correlation was observed between MeHg and AVS in sediments of these mudflats, what may be a consequence of the common origin of AVS and MeHg (both produced by microorganism activity), but also can be derived from the ability of Fe monosulfides to adsorb and thus stabilize solid phase MeHg.

mercury; methylation; sediment; mudflat; Seine; Medway

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