Speciation of Mercury in a Fluid Mud Profile of a Highly Turbid Macrotidal Estuary (Gironde, France)

Mercury (Hg) speciation and partitioning have been investigated in a fluid mud profile collected in the high turbidity zone of the Gironde estuary. The formation of the fluid lens generates local and transient oxic−anoxic oscillations following the sedimentation-resuspension tidal cycles under a specific hydrodynamic regime. The total Hg concentration, ranging from 5 to 190 nM, increases with SPM concentration (4−174 g L-1) to a maximum at bottom. Particulate Hg averages 99% of total Hg. Particulate inorganic Hg (IHg(II)P) and monomethyl Hg (MMHgP) exhibit a similar trend: the maximum concentration is observed within the upper layer above the depth of 7 m and the minimum at the bottom layers of the fluid mud. Significant levels of “dissolved” (i.e. filter passing) Hg species (IHg(II)D, Hg°D, MMHgD, DMHgD) are observed within the redox transition interface. In the sub/anoxic fluid mud layer, increasing concentrations of IHg(II)D and MMHgD coincide with decreasing concentrations of IHg(II)P and MMHgP, respectively. The distribution coefficient (log K d) between the “dissolved” and particulate fraction for IHg(II) averages 4.5 ± 0.2. A K d minimum for IHg(II) is observed in the surface layer and at the bottom of the fluid mud and coincides with the maximum levels of dissolved Mn and Fe. Log K d for MMHg averages 3.3 ± 0.9 and presents the highest values (4.3−4.6) in the surface and the lowest (∼2.2) at bottom, corresponding to the particulate carbon profile. These results demonstrate that the fate of IHg(II) and MMHg in the fluid mud system is influenced by the redox cycling of major species such as carbon, Fe, and Mn. It is therefore suggested that the redox oscillations generated by fluid mud formation in the high turbidity zone affect the distribution and transfer of Hg species in macrotidal estuaries.