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

Ouddane, B., Mikac, N., Cundy, A.B., Quillet, L. and Fischer, J.C. (2008) A comparative study of mercury distribution and methylation in mudflats from two macrotidal estuaries: The Seine (France) and the Medway (United Kingdom). Applied Geochemistry, 23 (4), 618-631. (doi:10.1016/j.apgeochem.2007.11.001).

Abstract

Mercury is a contaminant of great concern in the marine environment, particularly in coastal environments where the formation of methylmercury (MeHg) in aquatic sediment, 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 sediment, especially in contaminated environments. This study investigates some geochemical factors affecting the speciation and distribution of Hg in estuarine sediment 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 sediment core from a rapidly accreting mudflat in the Seine estuary (170 cm vertical accretion over the last 10–50 a) was contaminated with Hg at concentrations which are 10–50 times higher than the Hg background for sediments of the Seine basin. In the Medway mudflat, characterized by a slow sedimentation rate (400 cm over approximately the last 800 a), near-surface sediment was significantly more contaminated (10–40 times) than sediment at greater depths, which were characterized by pre-industrial Hg concentrations. Geochemical conditions in the surface sediment of the Medway mudflat are characterized by stable anoxic redox conditions (about ?200 mV), which are generally favourable for Hg methylation, whereas near-surface sediment of the Seine mudflat is characterized by more oxidizing redox conditions (about +100 mV), which are generally less conducive to Hg methylation. Consequently, MeHg concentration in the upper 10 cm of the sediment column was about four times higher in the Medway than in the Seine mudflat, in spite of similar total Hg concentrations. In surface sediment, where Hg is actively methylated, MeHg variability was associated with the activity of View the MathML sourceSO42--reducing microorganisms (SRM) and the presence of acid volatile sulphides (AVS). A strong correlation was observed between MeHg and AVS in sediments from these mudflats, which 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.

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Contributors

Author: A.B. Cundy

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