Sediment quality Atlantic Ocean watershed: chapter 5
The chemical quality of surface sediments in Lake Saint-Pierre has been improving since 1976. Mercury concentrations are now below the TEL for benthic organisms, except in Chenal aux Castors north of Dupas Island, where they are above the TEL.
Spatial distribution of sediment contamination by mercury in lake Saint-Pierre
The figure shows the spatial distribution of the concentration of mercury in the surface sediments of lake Saint-Pierre in 2004. Concentrations vary between 0 and 0.67 micrograms/g. They are highest in the Chenal aux Castors, north of Dupas Island.
The occurrence of chromium along the south shore of the lake corresponds to an erosion zone of postglacial clay. This clay holds fairly high natural concentrations of metals such as chromium, copper, nickel, sodium and magnesium. Despite the fact that we can consider postglacial clay as a source of contamination, the bioavailability of these metals to benthic organisms is actual.
Over ten years, PBDE concentrations have doubled in surface sediments of Lake Saint-Pierre to the point of reaching the highest measured values in the freshwater reaches of the St. Lawrence River, i.e., 62 ng/g in the early 2000’s. Concentrations vary widely and are higher when measured in the zone of the plume from the outfall of the City of Montreal which is considered as one of the major sources of PBDEs.
Dioxins and furans are also contaminants of concern in Lake Saint-Pierre. In 2004, fairly high concentrations were measured in the channels of Sorel, without exceeding the PEL. The occurrence of dioxins and furans can however be more widespread because results rely only upon the analysis of a few collected samples.
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