Is there cause for concern about chemical contaminants in drinking water from the St. Lawrence? Does water treatment always eliminate microbes? Which microbes are most resistant to chlorination? What are the health risks of chlorination?

Although the situation has improved significantly in recent years, the St. Lawrence still receives large quantities of wastewater from riverside municipalities, septic tanks, and run-off from agricultural areas. These waters may contain pathogenic microbes which can cause infectious diseases.

Few testing methods are used routinely to detect such microbes, most of which originate from human and animal fecal matter. The presence of indicator organisms of fecal contamination (fecal and total coliforms) in the water can however be evaluated. Under the Quebec’s Drinking Water Regulation, this kind of control is used to ensure the quality of water used for drinking-water purposes.

These indicators are a reliable means of detecting microbial contamination, but some pathogens in the water can go undetected. Recent studies suggest that even treated water that meets current biologic standards may contain pathogens. A 1993-1994 study in the Montreal region established that gastrointestinal illnesses may be attributed to tap water, even if the water conforms to quality standards 74. The poor quality of raw water, failure to maintain optimum treatment conditions, treatment-resistant microorganisms, and the invasion or regrowth of bacteria within a distribution system may explain such contamination.