Behaviour of anthropogenic mercury in coastal marine sediments

The diagenetic behaviour of anthropogenic mercury accumulated in sediments of the Saguenay Fjord (Canada) was investigated. Box-cores taken along its main axis and in the St. Lawrence estuary were analyzed for bulk sediment and porewater total and methyl-mercury concentrations as well as a number of other chemical variables. Mercury concentrations as high as 10,000 ng g−1 (dry weight) were measured in a core taken at the head of the fjord attesting to the presence of large quantities of mercury discharged by a chlor-alkali plant operated two decades ago along the Saguenay River. Porewater mercury concentrations ranged from 17 to 500 ng 1−1but were not correlated to the Hg content of the solid phase. Most of the mercury appears to be bound to organic matter, part of it is recycled with Mn and/or Fe oxides at the redox boundary whereas some may be adsorbed to or coprecipitated with the anomalously abundant acid volatile sulphides. These sulphides are very susceptible to oxidation and provide a more reactive sink to Hg than would pyrite. Despite the closure of the chlor-alkali plant in 1976 and relatively high sedimentation rates, surficial sediment Hg concentrations remain abnormally high. We investigated whether this observation could be explained by the diagenetic remobilization of Hg from the highly contaminated sediments buried below or resulted from other processes. The remobilization of Hg from deeper layers appears to be too slow to account for the high surficial sediment concentrations. Resuspension of older, contaminated sediments upstream during spring runoff or submarine mass flow may explain these observations. Methylation increases the solubility and mobility of Hg in sulphidic sediments but the CH3Hg(II) flux (0.07 ng cm−2 yr−1) from the contaminated layers to the surface sediment is negligible and accounts for only 0.01% of the present accumulation rate of mercury at the sediment surface. Dissolved and solid CH3Hg(II) profiles also indicate that this species may not diffuse through the thin oxic layer at the sediment-water interface. The estimated flux of Hg to the water column (20 ng cm−2 yr−1), however, could be underestimated since the activity of burrowing organisms would increase the exchange rate with the water column.