Speciation of mercury in sediments of the Seine estuary (France)

Although the parameters regulating mercury methylation in sediments are fai rly well known, conditions controlling the formation and mobilization of me thylmercury in aquatic sediments are still poorly understood. In this work the environmental factors governing the distribution and behaviour of total mercury and methylmercury within the Seine estuary have been assessed thro ugh the analysis of sediment cores. Box-cores taken at coastal, estuarine a nd riverine locations in the period 1994-1997 were analysed for bulk sedime nt and porewater total mercury and methylmercury concentrations, as well as a number of other chemical variables. Levels of total mercury in sediment (300-1000 ng g(-1)) were found to be uniform all over the estuary and did n ot show significant temporal or spatial variations. On the contrary, the di stribution of methylmercury was distinct from that of its inorganic form si nce it displayed seasonal and spatial variability. Maximum methylmercury co ncentrations (up to 6 ng g(-1)) and percentages (up to 2%) were obtained at the estuarine location with peak loading just below the sediment surface a nd a decreasing level in the deeper layers. As total mercury and methylmerc ury were nor correlated, the temporal and spatial behaviour of methylmercur y appeared to reflect variations in salinity, redox potential and the sulph ate reduction rate. Low concentrations of reactive mercury, but very high t otal mercury levels, were measured in the porewaters (10-400 ng 1(-1)), wit h maximum values at the sediment/water interface. In spite of a high propor tion of organically complexed dissolved mercury, methylmercury in porewater s was mainly below the detection limit of the method (2 ng 1-(1)). From mer cury speciation in the solid phase and dependence of its dissolved fraction on iron and manganese in porewater, the source and distribution of mercury in porewaters are discussed, Copyright (C) 1999 John Wiley & Sons, Ltd.