The available knowledge reveals widespread contamination by a number of chemical products in the waters and sediment of the Estuary and Gulf, with contamination levels highest close to probable sources. The most abundant contaminants appear to be PCBs and other organochlorine pesticides because they do not easily biodegrade.
New contaminants are also regularly introduced into the ecosystem. An example is a chemical used as a flame retardant in clothing, levels of which are now being found in the blubber of Beluga Whales in the St. Lawrence River. Such new contaminants are added to the mixture of contaminants already present and may aggravate the situation of animals that remain for long periods in the Gulf (such as Belugas, seals and other marine mammals).
Proximity to urban and industrial areas influences the concentration of contaminants in the sediments of the St. Lawrence. Heavier particles are deposited close to their source while smaller particles are transported over longer distances and are deposited in places where the currents are weak. This phenomenon results in the global contamination of the Estuary and the Gulf. There are also zones such as the Laurentian Channel and others that are even more contaminated. In shallow waters, physical phenomena, such as tides, storms and ice movement and chemical phenomena, such as precipitation, absorption of chemical products and the degradation of molecules in the sediments, continually redistribute contaminants into the environment.