RESUMO
We analyzed dated sediment cores for evidence of Rhothane (dichlorodiphenyldichloroethane; DDD) applications to the Saint Lawrence River at Montreal, QC, Canada for the World Exposition of 1967 (Expo 67). More than 16,000kg of this pesticide were applied between 1965 and 1967 to abate nuisance shadflies that threatened visitor enjoyment. Concentrations of DDD and DDE in Lake Saint-François, 70km upstream of Expo 67, reached 12.2 and 11.5µg/kg dry weight (dw), respectively, with clear peaks between 1945 and 1970, consistent with historical use patterns; DDT was not detected. In Lake Saint-Pierre, ~100km downstream, DDD and DDE concentrations were 2 to 5 times higher, exceeding sediment quality guidelines, and DDT concentrations were as high as 3.8µg/kg. Once normalized for grain size and organic carbon, peaks of DDD, DDE and DDT were observed between 1945 and 1990 in the sediment record. Ratios of DDD to DDE were 1.0 or less in Lake Saint-François, consistent with their formation as degradation products of DDT. In contrast, ratios exceeded 1.0 in Lake Saint Pierre between 1965 and 1970, coinciding with Rhothane applications at Expo 67. Downstream, subfossil diatom assemblages showed little response to DDD inputs, but the abundance of some chironomid taxa increased while others decreased in tandem with elevated DDD concentrations. Overall, contamination of river sediments and impacts on insect communities by DDD applications at Expo 67 were still evident in sediment records 100km downstream of Montreal.
