DDT and HCH, two discontinued organochlorine insecticides in the Great Lakes region: Isomer trends and sources

The uses of the insecticides 1,1′-(2,2,2-trichloroethylidene)bis[4-chlorobenzene] (p,p′-DDT) and γ-hexachlorocyclohexane (γ-HCH) have been discontinued for several years, but they are still consistently detected in air samples collected on the shores of the Great Lakes. Although the agricultural uses of DDT have been restricted in the United States since 1972, DDT continued to be used to manufacture the miticide, dicofol, up until 2011. The use of the technical HCH mixture in North America was restricted in the 1970s, when it was replaced by one of its purified conformers, γ-HCH, also known as lindane. In this study, we have focused on isomer-specific data to gain insights on the temporal trends and possible sources of these compounds. In particular, we calculated ratios of the concentrations of p,p′-DDE + p,p′-DDD versus the sum of the concentrations of the three p,p′ isomers. These ratios are about the same at all five of our sampling sites and are about the same as observed globally. We also calculated the ratio of the concentrations of o,p′-DDT versus the sum of concentrations of o,p′-DDT + p,p′-DDT. This ratio has increased significantly at all five sites over the last 15-20 years. We suggest that dicofol, which contained about 11% o,p′-DDT, may now be a significant, additional source of DDT to the Great Lakes. The average ratio of the concentration of γ-HCH (lindane) versus the sum of the concentrations of γ-HCH + α-HCH did not vary significantly with time, but it did show an urban signature, suggesting that cities may be more important sources of these compounds than previously suspected.