Intense, earthy/musty taste and odour (T&O) events occurred for a prolonged period in the late summer of both 1998 and 1999. Previously 1994 had a similar, although less intense, T&O event. These events are serious because the Western Basin of Lake Ontario is the source of drinking water for over 5 million people.

The most commonly reported taste and odour compounds, geosmin and MIB (2-methylisoborneol) are produced in aquatic environments by cyanobacteria (blue-green algae) or mould-like, filamentous bacteria called actinomycetes. Intensive testing of Lake Ontario water during T&O events has confirmed that minute concentrations (measured in parts per trillion) of these compounds create the earthy/musty taste/odour.

Geosmin, the same substance that can be detected when rich soil is turned, is also found in some foods including beets. It can be detected at very low concentrations, with the average person noticing the odour of geosmin at as little as 4 nanograms per litre (ng/L). (A trillion nanograms equal one gram) MIB is usually noticeable at levels of approximately 9 ng/L.

Geosmin appears to be the sole cause of T&O for the water users on the north shore of the western basin of Lake Ontario. While, on the south shore, in the Region of Niagara, minute concentrations of the MIB have also contributed to the somewhat more prolonged T&O events.

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