Dieldrin, heptachlor epoxide, and oxychlordane are highly chlorinated, persistent, bicyclic epoxides that are widely present in the environment. Given the long-standing restrictions on the production and use of these compounds, it is appropriate to wonder if their environmental levels have decreased over time, and if so, how fast. The Integrated Atmospheric Deposition Network (IADN) provides this information by measuring the concentrations of these compounds in the atmosphere at six sites near the Great Lakes once every 12 days. During the period 1990-2021 (inclusive), a total of 4125 samples have been collected. The median concentrations at the six sites are significantly different from one another. The concentrations of all three compounds at Chicago (population: 3,900,000) are significantly higher than those at most other sites, and those at Eagle Harbor (population: 1330) are significantly lower than those at most other sites. This concentration trend reflects the use of these compounds as termiticides and insecticides in cities as opposed to their use in agriculture. Multivariate statistical analyses of the spatially- and time-resolved data indicate that the atmospheric concentrations of these compounds are decreasing by a factor of two every 14 ± 5 years. Although the atmospheric levels of these three highly chlorinated epoxides are decreasing slowly, it is clear that they are environmentally persistent. It is also clear that our earlier reported halving times (1-10 years) for dieldrin were overly optimistic. To determine reliable long term temporal trends, it is essential to have long term concentration measurements.
Keywords: Dieldrin; Halving times; Heptachlor epoxide, oxychlordane; Lake Erie; Lake Michigan; Lake Ontario.
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