Although the parameters regulating mercury methylation in sediments are fairly well known, conditions controlling the formation and mobilization of methylmercury 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 through the analysis of sediment cores. Box-cores taken at coastal, estuarine and riverine locations in the period 1994-1997 were analysed for bulk sediment 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 not show significant temporal or spatial variations. On the contrary, the distribution of methylmercury was distinct from that of its inorganic form since it displayed seasonal and spatial variability. Maximum methylmercury concentrations (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 and a decreasing level in the deeper layers. As total mercury and methylmercury were nor correlated, the temporal and spatial behaviour of methylmercury appeared to reflect variations in salinity, redox potential and the sulphate reduction rate. Low concentrations of reactive mercury, but very high total mercury levels, were measured in the porewaters (10-400 ng l-1), with maximum values at the sediment/water interface. In spite of a high proportion of organically complexed dissolved mercury, methylmercury in porewaters was mainly below the detection limit of the method (2 ng l-1). From mercury 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. |
