by Hans Peterson, Senior Research Scientist, Saskatchewan Research Council, Saskatoon, Saskatchewan
Diquat is used to control weeds in farm dugouts and other surface water reservoirs used for drinking throughout the Canadian prairie. Diquat is registered for use by the federal government in these water reservoirs. The average treatment concentration of diquat is around 1,000 ug per litre of dugout water. The Canadian Drinking Water Guideline for diquat is 70 ug per litre. The federal government restricts consumption of diquat treated water for 24 hours. It is, however, likely that a treatment concentration of 1000 ug per litre has not decreased to 70 ug per litre 24 hours after treatment.
The U.S. Environmental Protection Agency allows only 10 ug of diquat per litre of drinking water. Diquat treated water in the U.S. must not be consumed until 14 days after treatment or if the water is tested it has decreased to a level at or below 10 ug per litre. The European Community is only allowing a residual pesticide level of 0.1 ug per litre (for individual pesticides). Canada used to have the same restrictions as the U.S. on the consumption of diquat treated water; don't consume until 14 days after treatment. In 1967, however, at the request of the Canadian distributor, the federal government reduced the Canadian consumption delay to 24 hours.
As a result of the scientific uncertainties surrounding the disappearance of diquat and, the conservative standards of other countries, in 1988, SRC expressed its concern about the 24 hour consumption delay. Health and Welfare Canada, the federal agency overseeing the Guidelines for Canadian Drinking Water Quality, agreed in 1989 that a revision was appropriate. In fact, in a letter to SRC dated January 11, 1993, Health and Welfare Canada expressed support for the change:
"From the studies, I cannot see sufficient (or in fact any) justification for reducing the reuse period from 14 days to 24 hours" Mrs. Grace Wood, Health and Welfare, Canada, Ottawa.
However, to date, the change has not been made.
Provincial agencies rely on federal expertise in dealing with pesticide contaminants in drinking water. However, in contrast to the federal registration, Alberta Environment, Saskatchewan Environment and Manitoba Environment do not recommend or allow the use of diquat in municipal drinking water reservoirs. In Saskatchewan, a member of the Legislative Assembly, Ms. Pat Lorje (Saskatoon Wildwood) has expressed concern over the private use of diquat to treat drinking water. Diquat can still be used on privately owned drinking water supplies (such as farm dugouts) following the user restrictions on the diquat label. We therefore recommend:
If diquat is used allow a 14-day consumption delay until the federal government has produced new guidelines for the use of this product in drinking water supplies.