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ECOLOGICAL RISKS OF DIOXINS IN LAKE ONTARIO: A TALE OF TWO SEDIMENT CORES
Cook, P. M. AND J. A. Robbins. ECOLOGICAL RISKS OF DIOXINS IN LAKE ONTARIO: A TALE OF TWO SEDIMENT CORES. Presented at 2002 SETAC Annual Meeting, Salt Lake City, UT, November 16-20, 2002.
Sediment box cores have frequently been used to determine organochlorine chemical loading histories of lakes and reservoirs. 137Cs and 210Pb radionuclide dating techniques are employed synchronously with chemical analyses of the contaminants for thin sections extruded from adjacent sub-cores. Typical profiles for PCBs or DDT show concentrations that increase from around 1940 to1970 and then decline to the present. Applying these data to retrospective ecological risk assessments introduces a need for a higher level of resolution and accuracy, especially if the results are to be used ultimately to support prospective risk assessments. A primary core was chosen from eastern Lake Ontario to determine historic exposures of lake trout embryos to dioxin-like chemicals, and hence early life stage toxicity risks. A second core from the south central portion of the lake was also analyzed to verify predictions based on the primary core. Site-specific BSAFs, adjusted for historic changes in chemical distribution between water and sediments, were used to predict toxicity equivalence concentrations in embryos. Analyses of both cores resulted in predictions of a long period in which lake trout sac fry mortality was severe, followed by improving survival in the 1990s. However, the predictions of when survival was initially impaired and when peak exposures occurred were surprisingly different for the two cores, creating a potential discontinuity with epidemiological data. Successful resolution of the discrepancy as a preventable systematic error, rather than unavoidable data variability, required an intensive examination of the data, analytical methods, and radionuclide dating models. The implications for risk assessments will be described.