Pesticide use at Expo '67: Can we find the evidence 40 years later?
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The applications of the pesticide DDD to the water of the St. Lawrence River prior to and during the World Exhibition of 1967 in Montréal, Québec were examined. The pesticide was applied between 1965 and 1967 in excess of 16 000 kg to abate nuisance shadflies. To determine if the evidence was captured in the environmental media, sediment cores were sampled from a fluvial lake downstream of the Expo ’67 site, Lac Saint-Pierre, Québec, Canada and from an upstream fluvial lake, Lac Saint-François, Ontario and Québec, Canada. Sequential slices of sediment cores from the two lakes were analyzed and compared for pesticide concentrations using gas chromatography and dated using 137Cs isotopes. The percentage of total carbon in the sediments was also analyzed to express pesticide concentrations on a carbon basis. Concentrations of DDD and DDE in Lac Saint-François ranged from below detection to 12.2 ppb and from below detection to 11.5 ppb, respectively. Concentrations of both contaminants were highest approximately between the late 1950’s and early 1960’s, consistent with historical use patterns. DDT was not detected in sediment slices from Lac Saint-François at any depth. Concentrations of total DDD, DDE and DDT in Lac Saint-Pierre ranged from below detection to 26.4 ppb, from below detection to 20.9 ppb, and from below detection to 80.5 ppb, respectively. Contaminant concentration distributions demonstrated very complex and multi-peaked profiles that were not reflective of expected historical use patterns. The influence of sediment grain size and carbon content on the contaminant profiles was examined but did not appreciably explain the complexity. Mixing of sediment due to physical processes in combination with other variables may help explain the complex contaminant and 137Cs activity profiles. Although pesticide concentrations were generally higher in the downstream lake compared to the upstream lake, conclusions of the source of the pesticide cannot be accurately drawn due to the complex signals in Lac Saint-Pierre and the absence of DDT in Lac Saint-François, which prevents the comparison of a DDD:DDT breakdown ratio.