The most contaminated water tends to be found where there are small, rural groundwater treatment facilities. Groundwater can contain gasoline or oil, pesticides, inorganics and the natural radioactive gas radon. Surface water is more likely to contain organic contaminants from bacteria, decayed vegetable matter and even sewage. Toronto drinking water is surface water. In Toronto, four water filtration plants draw water from Lake Ontario and daily treat nearly 2.5 billion litres for distribution.
Water treatment consists primarily of the killing of bacteria and of particulate filtration.
Because of cheap energy and plentiful fresh water, the Niagara Falls area is a popular industrial site. But, as is common with organics, TCDD tends to concentrate in tissues over time because it is not excreted. According to Pollution Probe, a shovelful of dioxin in Lake Ontario would devastate the ecosystem all the way to the St. Lawrence River. Thus far, the threat is more potential than present, although the limestone bedrock at Hyde Park is known to be cracking. Dioxin has not been detected in Toronto drinking water, but it has been detected at St. Catherines and Mississauga.