The Great Lakes have been the focus of intensive long-term research and monitoring programmes for the past 40 years. Spatial distributions and temporal trends have been determined for a range of environmental compartments, including surface water, sediment and fish. In general, there have been dramatic reductions in contamination by legacy pollutants including polychlorinated biphenyls (PCBs), organochlorine pesticides and metals. Concentrations of PCBs and lead in surface water at the mouth of the Niagara River have decreased by 58 and 54%, respectively, over the period 1986–2007. Correspondingly, concentrations of PCBs and lead in offshore sediments of Lake Ontario have decreased by 37 and 45%, respectively, since peak accumulations in the 1970s. Temporal trends for more modern chemicals, including polybrominated diphenylethers and perfluoroalkyl compounds, showed increases up until 2000 when management actions and heightened stakeholder awareness resulted in a levelling off or decline in the subsequent time period. While legacy issues are largely associated with areas of historical industrial activity, the presence of newer chemicals is generally associated with modern urban/industrial areas that act as diffuse sources.

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