How does pollution affect water quality in the Great Lakes and St. Lawrence River?
The Great Lakes have been a magnet for industry and urban development, both of which have strained the capacity of the lakes to absorb the resulting pollution and degradation of its waters. Because the lakes are one interconnected system, contaminants starting their journey in Lake Superior will eventually find their way to the St. Lawrence River. A special feature of these impressive “sweetwater seas” is the long retention time of their waters. Pollutants, particularly long-lived toxic chemicals that find their way into the lakes can only be moved very slowly through the system. Lake Superior, which is the largest of the Great Lakes, takes almost two hundred years to flush out. Lake Erie, in contrast, takes less than three years. The lakes are vulnerable to two types of pollution – often described as “point” and “non-point”. Point sources are easily identified – smokestacks, discharge pipes and waste outlets. Non-point sources are harder to identify and to monitor. They include contam
