By Barry H. Thomas and VŽronique Morisset, Health Canada, Ottawa
Disinfection by-products are a major health concern in drinking water derived from chemically treated surface water sources.
Chlorine, and all other chemical disinfectants, react with natural organic matter dissolved in raw water to produce chemical compounds that have a range of toxic effects, including cancer.
The best known compounds produced are trihalomethanes, which include the common chemical chloroform. The health-based guideline for trihalomethanes was reduced to 100 mg/L in 1993, and this value will be published in the 6th edition of Health Canada's Guidelines for Canadian Drinking Water Quality, which will be available in 1996.
The organically rich waters found in most Prairie dugouts make it difficult to achieve this new guideline without the use of treatment methods more advanced than simple chlorination.
One approach is to reduce the organic content of the raw water by coagulation with alum, followed by sedimentation and filtration. There is growing concern, however, that aluminum in drinking water may also be a health hazard at high levels. Health Canada will be recommending a health-based guideline for aluminum within the next year.
It must be emphasized that the health risks associated with drinking surface water that has not been disinfected are far greater than those associated with disinfection by-products due to the use of chlorine.
Agricultural practices can also add toxic chemicals to drinking water supplies. The most common problems are high levels of nitrate and/or nitrite from fertilizers and defective septic sewage systems. Children and pregnant women are particularly sensitive to nitrate/nitrite, which can cause methaemoglobinaemia (e.g. "blue baby syndrome"). Pesticides and herbicides can also be a problem, but more careful use of safer products has led to this being less frequently observed.
A particularly common problem in Prairie dugouts is the formation of blue green algal, or more correctly, cyanobacterial blooms. These blooms can release some very potent toxins that are only now being studied in detail.
Health Canada will be recommending a guideline for one of the toxins, Microcystin-LR, at the next meeting of the Federal-Provincial Subcommittee on Drinking Water. This substance is very toxic to the liver, and has been detected in many Prairie dugouts. It was also found in the City of Winnipeg's water supply in the summer of 1993. Removal of the toxin can be achieved with activated charcoal, but the best solution to the problem is to manage the dugout so as to prevent bloom formation.
Problems that usually only occur with well water include high levels of metals like manganese and iron, which are considered aesthetic problems, or metals like arsenic and uranium, that are considered health risks.