Qualité des sédiments en suspension et de fond du système saint-laurent (Canada)

Abstract

The drainage area of the St Lawrence River isthe seat of several human activities which turn it into an important axis of development. However, this type ofdevelopment usually generates a deterioration of the environment and water contamination both from industrial and municipal wastes and from the contribution of surface runoff. This article deals with suspended and bottom sediments of the water system of the St Lawrence (river, estuary and gulf). In the waters of the St Lawrence River the concentration of suspended solids varies from 4 to 10 mg 1-1 in the region upstream of Lake St.Pierre and from 30 to 50 mg l-1 in the downstream region up to Quebec City. With the exception of the Sorel delta, this portion of the St Lawrence system does not have important sediment traps. The concentrations of SiO2, AI2 O3, Fe90q. CaO and MgO are essentially similar to those in bottom sediments and soils in the region investigated. However, bottom sediments are enriched in phosphorus, polychlorinated-biphenyls, copper and lead. The portion of the St Lawrence system immediately upstream of Orleans island, where the fresh water of the river mixes with the saline water of the gulf, constitutes a zone of high concentrations of suspended solids (up to 300 mg 1-1). In the estuary and the gulf of the St Lawrence, water salinity varies following a double gradient; it increases with the depth and to the east towards the Atlantic Ocean. The flocculation of fine particles in contact with saline waters provides an important settling zone immediately upstream of Orleans island. These fine sediments are rich in phosphorous, organics and polychlorinated-biphenyls. In the region of the estuary and the gulf of the St Lawrence, the suspended sediments profile is, on the whole, characterized with a concentration decreasing from Orleans island (40-80 mg I-1) to the St Lawrence Gulf (1-2 mg l-1). In the St Lawrence Gulf, there exists a correlation between the pattern of surface water circulation and the concentrations of suspended matter. © 1987 Taylor & Francis Group, LLC.