A LAKE-ONTARIO GULF OF ST-LAWRENCE DYNAMIC MASS BUDGET FOR MIREX

Mirex, a flame retardant and pesticide, is unique among the organochlo rine contaminants in the Great Lakes as its occurrence is restricted t o Lake Ontario due to its two sources at the Niagara and Oswego Rivers . Because of its high lipophilicity and extreme persistence, mirex has entered the entire Lake Ontario food chain. It is being removed from Lake Ontario (exclusive of sedimentation and burial) primarily by outf lowing water and migrating biota. Mirex concentrations in water and su spended and bottom sediments of Lake Ontario and downstream thereof in the St. Lawrence River and its estuary are useful indicators of its p resent and past movements in this system. We report here new mirex dat a in sediments as far downstream as the Gulf of St. Lawrence and on th e development of a mass budget for this contaminant. This mass budget incorporates results measured and modeled data on the contaminant in t he food chain of the St. Lawrence estuary and a top predator. Our anal ysis showed that over the last 4 decades a total of approximately 2700 kg of mirex has entered Lake Ontario, of which approximately 550 kg h as been removed, mainly by transport to the St. Lawrence estuary.