Résumés
Résumé
La distribution de la grosseur des particules, la composition géochimique et la concentration en métaux (Cu, Cr, Pb, Co, V, Ni, Hg et Zn) furent déterminés dans des carottes datées (à l'aide de matériel radioactif) provenant du lac Saint-Louis et du fleuve Saint-Laurent. Environ 20 cm de sédiments fins se sont accumulés durant les 30 dernières années à deux endroits situés à proximité du centre du lac. Par contre, une couche de sédiment de 20 cm d'épaisseur s'est déposée pendant les cinq dernières années à deux différents endroits de la partie sud du lac. Les profils de concentration des éléments majeurs et de la plupart des métaux présents dans les carottes de sédiment indiquent un apport continu de matériel de nature géochimique semblable aux aires d'échantillonnage. Une distribution uniforme de la grosseur des particules à l'intérieur du profil de sédiment montre que ces aires constituent des zones de dépôt pour des particules de propriétés hydrodynamiques semblables que transportent les eaux du fleuve. Les profils de concentration du mercure (Hg) et du zinc (Zn) dans les sédiments sont significatifs d'une charge provenant d'une source locale située sur le lac Saint-Louis. Les concentrations en métaux mesurées dans les sédiments du lac Saint-Louis dépendent du tri des particules, du transport et du dépôt des particules de la fraction limoneuse (taille comprise entre 2 µg et 63 µg) qui peuvent contenir de plus faibles concentrations en métaux comparativement aux particules de grosseur comparable à l'argile (taille < 2 µg) qui caractérisent les bassins de dépôt du lac Ontario.
Mots-clés:
- Sédiment,
- datation,
- géochimie,
- métaux
Abstract
Particle size distribution, geochemical composition and concentrations of metals (Cu, Cr, Pb, Co, V, Ni, Hg and Zn) were determined in radiodated cores collected from Lake St. Louis, St. Lawrence River, Quebec, Canada, to assess the significance of the lake as a sink of contaminants transported through the St. Lawrence River. The cores were collected at four stations in the depositional areas identified by earlier surveys of Lake St. Louis. Stations Nos. 3 and 9 were located in the northern part of the lake, and stations 17 and 21 along the southern shore. A modified Kajak-Brinkhurst curer was used to collect the sediment cores. The concentrations of major elements and metals were determined by X-ray fluorescence spectrometry. The concentrations of Hg and inorganic C were determined by cold vapour atomic absorption spectrometry and Leco carbon analyzer, respectively. The total Pb-210 and Ra-226 levels in one centimeter sediment core sections were measured by low-energy gamma-ray spectrometry for calculation of sediment age. The effects of mixing on sediment age has been estimated.
About 20 cm of fine grained sediment accumulated during the past 30 years at two sites near the centre of the lake. However, a 20 cm thick sediment layer was deposited over the past five years at two different areas in the southern part of the lake. Calculated sedimentation rate for surface sediments were 207, 296, 1837, and 2912 mg cm-2 y-1 for stations 3, 9, 17, and 21, respectively. The concentration profiles of ten major elements (Si, Al, Fe, Ca, Mg, Na, K, Ti, P, and Mn), organic and inorganic C and most of the investigated metals were similar and consistent in the sediment profiles at all stations, and indicated a continuous input of geochemically similar material at the sampling sites.
A uniform particle size distribution within the sediment profile showed that these areas are settling zones for particles of similar hydrodynamic properties transported by the St. Lawrence River. The concentration profiles of Hg in the cures indicated e continuous input of Hg along the southern shore of the lake. The concentration profile of Hg in the core from the northern part of the lake had a similar pattern to that from Lake St. Clair and Lake Ontario, showing a decrease of Hg in the surficial sediments. This decrease corresponds to implemented contrat of Hg losses by industry in Canada and the USA over the past 15 years. The concentration profiles of Hg and Zn in the sediments indicated an input from local sources along Lake St. Louis.
The concentrations of Cr, V and Co in sediments in the depositional zones were similar to both background and surficial sediment concentrations in depositional basins of Lake Ontario. The concentrations of Cu, Ni and Pb were lower than those in surficial sediments in Lake Ontario depositional basins. Observed concentrations of investigated metals in Lake St. Louis sediments were ascribed to the particle sorting, transport and deposition of silt-size (2 to 63 µm grain size) particles which may contain smaller concentrations of metals than clay-size (< 2 µm grain size) particles in the depositional basins of Lake Ontario. It was concluded that Lake St. Louis provides only a temporal storage for metals associated with clay-size sediment particles which may eventually become resuspended and moved downstream.
Keywords:
- Sediment,
- dating,
- geochemistry,
- metals
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